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3553 Answers 3553

8 9
11 12

Reveal by Halves (in need of a better name)

Inspired by this: http://nolandc.com/smalljs/mouse_reveal/ (source).

A valid answer:

  • Takes a number \$w\$ and (assumed non-negative) integer \$x\$.
  • Outputs an integer list with a length of \$2^w\$, initially filled with zeroes.
  • For each number \$n\$ from \$0\$ to \$w-1\$ (inclusive), divide the list into sub-lists of size \$2^n\$, then increment all of the values in the sub-list that contains the index \$x\$.


(with coordinates from left, 0 indexed, but your answer may have change these)

w=3, x=1

w=2, x=2

w=3, x=5

w=4, x=4

w=2, x=100
Do not need to handle (can do anything) because x is out of bounds

Meta questions

  • Are these tags fitting?
  • Would this be better in one dimension? (like \$3, 2\$ returns 11320000) Edit: I've changed it to one dimension but I can revert if it makes it less interesting.
  • Should \$w\$ or \$2^w\$ be the input?
  • Is this a duplicate?
  • 1
    \$\begingroup\$ My opinions on some meta questions. 1) I think one dimension would be better, the core of the challenge remains the same but the challenge itself becomes more "pure" which, in my opinion, is a good thing. 2) I'm a fan of flexible I/O, so if it were up to me I'd let people choose if they want \$w\$, \$2^w\$ or both as input. If you don't like this, both options are honestly fine. \$\endgroup\$
    – Delfad0r
    May 10 at 22:24

I'm Lazy*: Top-left align my text


  • \$\begingroup\$ Definitely not too trivial for code golf \$\endgroup\$
    – qwr
    Jun 2 at 14:20
  • \$\begingroup\$ I think squash up could be its own challenge which has room for simplification. My thoughts being using a transposed grid of strings, which I guess can work for this challenge too \$\endgroup\$
    – qwr
    Jun 2 at 14:21

Demonstrate some advanced abstract algebra

  • \$\begingroup\$ I think we should be able to define the types and values of S, rather than necessarily using integers. In that case, - and + would not (necessarily) be actual arithmetic negation and addition, so maybe they would have to be renamed to use other symbols (or just use function syntax f(a,b)?) \$\endgroup\$
    – pxeger
    May 31 at 8:50
  • \$\begingroup\$ And do all 9 functions have to operate on the same set S? I think it could be more interesting if they didn't have to, but it might result in cheating/loopholes. Also, what does "uniquely exhibits" mean exactly? Demonstrates exactly one of the 9 properties? \$\endgroup\$
    – pxeger
    May 31 at 8:52
  • \$\begingroup\$ In fact, I think people will just submit "addition, addition, multiplication, subtraction" for the first 4 at least, and I suspect they will almost always be the shortest option in most languages so it might not be very interesting as it is \$\endgroup\$
    – pxeger
    May 31 at 9:04
  • \$\begingroup\$ Writing one program is hard enough. Writing 9 seems like a lot to ask. I think you could make a stripped down challenge using just commutativity and associativity. I barely known any abstract algebra. I think these varieties are called magmas? \$\endgroup\$
    – qwr
    Jun 2 at 14:54
  • \$\begingroup\$ Is this even possible with the surjectivity condition? You should provide an example of each program. \$\endgroup\$
    – qwr
    Jun 2 at 14:57
  • \$\begingroup\$ @qwr I don't have examples for each program, and even if I did, I wouldn't include them as that would just lead to people porting them into their own languages. Yes, I believe magma is the correct term for \$*\$ here. I'm not sure if this is possible, but I'd be surprised if it isn't. I've allowed for an answer to be a proof of impossibility however, on that off-chance. \$\endgroup\$ Jun 2 at 15:04
  • \$\begingroup\$ Well it's more than a magma since you added more two more operators right \$\endgroup\$
    – qwr
    Jun 2 at 15:18
  • \$\begingroup\$ @qwr No, I believe a magma is just a pair, the binary operator and the set its closed on, no matter the additional operators defined on that set \$\endgroup\$ Jun 2 at 15:31
  • \$\begingroup\$ This is a really cool problem. It is hard so I wouldn't be again having a separate "easy" version with just the main three: commutative/associative/distributive. Uniquely exhibiting those is already a nontrivial and neat challenge. I don't know if others would vote a dupe, but I'd def be in favor of having both. As is, I don't think the harder version will have a lot activity. But I do think an easier one would! \$\endgroup\$
    – AviFS
    Jun 13 at 1:18
  • \$\begingroup\$ @AviFS I do actually have an easier version Sandboxed, where I think they're clearly separate enough to not be dupes. \$\endgroup\$ Jun 13 at 1:42

Minimal distinct character quine

  • \$\begingroup\$ This seems well specificed \$\endgroup\$ Jun 16 at 10:04
  • \$\begingroup\$ distinct means different? \$\endgroup\$
    – math
    Jun 19 at 10:01
  • \$\begingroup\$ @math Yes (filler) \$\endgroup\$
    – emanresu A
    Jun 19 at 10:30

Full name quine

  • 1
    \$\begingroup\$ Nice question but seems hard \$\endgroup\$
    – math
    Jun 25 at 18:11
  • 1
    \$\begingroup\$ i assume custom SBCS languages will have to output the full names of their characters when represented in unicode? Or should they output the name of the visual representation? \$\endgroup\$
    – Razetime
    Jun 28 at 6:20
  • \$\begingroup\$ Full names represented in unicode. \$\endgroup\$
    – emanresu A
    Jun 28 at 8:29

r my Vyxal

  • \$\begingroup\$ Needs more test-cases. Also, in the explanation you use spaces as though they're ignored, but that isn't mentioned in the description of the task itself. I'd suggest just saying that "all other characters" will be limited to ASCII letters only, for example?. \$\endgroup\$
    – pxeger
    Jul 4 at 7:47
  • \$\begingroup\$ @pxeger In Vyxal, spaces are NOPs used to seperate stuff. In the subset I'm using, spaces are a function like everything else. I'll make that clearer, and add more testcases. \$\endgroup\$
    – emanresu A
    Jul 4 at 8:15
  • \$\begingroup\$ But I'm suggesting limiting the definition of what are functions to ASCII letters only to provide more golfing opportunities without having to handle edge cases that might occur because of spaces \$\endgroup\$
    – pxeger
    Jul 4 at 8:54
  • \$\begingroup\$ @pxeger Ok. (filler) \$\endgroup\$
    – emanresu A
    Jul 4 at 8:55
  • \$\begingroup\$ It says that we can assume there will always be two values to pop, but one of the testcases is simply “1”. Is that something that we will have to account for? \$\endgroup\$ Jul 4 at 15:09
  • \$\begingroup\$ @AaronMiller What I meant is, wherever there's a function, there will be two. I'll add that. \$\endgroup\$
    – emanresu A
    Jul 4 at 19:58

Extremely small data compressor

In 2014 Jarek Duda at Purdue University wrote a paper containing several ideas for encoding computer data, entitled “Asymmetric numeral systems: entropy coding combining speed of Huffman coding with compression rate of arithmetic coding". The paper is available at Cornell University Library’s ArXiv project: https://arxiv.org/abs/1311.2540

One of the many fascinating things about this paper is that it begins by describing an extremely simple data compression algorithm, using the concept of the "Uniform asymmetric binary systems (uABS)". In fact, it is so simple, that you can implement it in only a few lines of code.

Basically it attempts to interpret a sequence of input symbols as a single Integer, and as each symbol comes in the Integer can be appended with new information. The trick is that the Integer is represented not using a place-value binary number system, but an alternative system. This representation is designed so that sequences of symbols which occur with higher probability will be represented by a smaller amount of space within the Integer's encoding.


You will implement the simple uABS compression algorithm, so that given a sequence of 0s and 1s, your program will compress them into a (usually) smaller sequence of 0s and 1s.


The algorithm in psuedocode is as follows:

  • Begin with an Integer X, and set it to 1. This will be the main Integer that we append during the algorithm.
  • The input data is a sequence of symbols, each 0 or 1, called Input
  • Find the probability P that any given symbol in Input is 1, (the number of 1s divided by the total number of symbols)
  • For each symbol S in Input, set X to the output of the function Encode(x,s,p)
  • After processing all the input symbols, output the final integer X. -- This encoded integer will hopefully have less bits than the input

The Encode function itself can be described as follows:

$$ Encode(x,s,p)= \left\{ \begin{array}{11} \mbox{if } s = 0 & \big\lceil\frac{x+1}{1-p}\big\rceil-1 \\ \mbox{if } s = 1 & \big\lfloor\frac{x}{p}\big\rfloor \end{array} \right. $$


$$ \begin{array}{11} s \text{ is a symbol, either 0 or 1} \\ x \text{ is the Integer} \\ p \text{ is the probability that any symbol in the Input data is 1 } \\ \lceil \rceil \text{ is the mathematical ceiling function } \\ \lfloor \rfloor \text{ is the mathematical floor function } \end{array} $$


  • The input is a sequence of symbols, each symbol being 0 or 1, in any method that is available in your chosen language. Examples include a sequence of ascii characters '0' '1', an array of integers, etc.

  • The output will be a sequence of symbols in the same format as the input sequence. The output sequence represents the compressed version of the input data.

  • Empty input data has undefined behavior.

  • Input data containing only 0s has undefined behavior.

  • Sometimes the encoded Integer might have more bits than the input, not less. This typically happens when the number of 1s and 0s is relatively even. Data with an unbalanced number of 0s and 1s results in better compression.

  • You may assume that the size of Integer will be your language's largest integer type. The test cases outside this range can be ignored for your language.

  • Note that if you are trying to test this by 'decoding' or 'decompressing' the compressed data, and compare it to the original, one would have to store additional information, such as the length of input and probability P, but for simplicity this has been left out of the challenge.

Example Input and Output

Short examples:

Input             Output    
10                101
10010100000       1011101001
1111              1
11111111111       1
10000000          11011
10011111010101    10110000100101    

Longer examples:

Input  11111110110111110111111111011111
Output 11111000011110110

Input  000000000001000000010000000000001100000000001
Output 1110000101100111000011111

Input  000000000001000000010000000000001100000000001000000000000000000000000000000000000000000000000000000000000000001
Output 1010100110111110010111011110110101010


  • The program with the fewest number of characters wins.
  • \$\begingroup\$ 1. IMO the pseudocode could be made clearer by firstly explaining what "machine integer" means (does it mean "unbounded integer" aka "big integer"?) and secondly golfing it a bit: using a "foreach" loop notation for S and eliminating the variable X'. 2. I think it would be helpful to be explicit about how p should be derived from the input. I presume that it means looping over the input twice, once to count and once to compress. 3. IMO restricting the input format to strings of ASCII 0 and 1 detracts from the core challenge. Why not allow arrays/lists of integers? \$\endgroup\$ Jul 7 '18 at 12:13
  • \$\begingroup\$ Thanks, i have revised. \$\endgroup\$
    – don bright
    Jul 7 '18 at 14:08
  • \$\begingroup\$ I really like this one. Something quasi-practical, and yet simple and small enough to be fun. Just to be clear, the output is the binary representation fo the integer X, without any leading zeros, correct? \$\endgroup\$ Jul 8 '18 at 19:02
  • \$\begingroup\$ Also, you mention "input size of at least 128 symbols", but it might be more important to specify output size limit, since many languages have hard bounds on maximum integer size. Since output size varies for the same input length, it might have to be something like "you may assume that the number of symbols in the output is less than or equal to the number of bits in your language's largest integer type". (The last test case would then be optional in languages that can handle only up to 32 bit integers). \$\endgroup\$ Jul 8 '18 at 19:12
  • \$\begingroup\$ yes the output is the binary representation of the final integer X, i believe the leading zeros is correct. do you think 32 bit is the good limit or 64, since modern machines tend to be 64 bit? thanks \$\endgroup\$
    – don bright
    Jul 8 '18 at 20:05
  • \$\begingroup\$ 32 is probably a reasonable limit, one that most languages can handle without need for external libraries. \$\endgroup\$ Jul 14 '18 at 14:22
  • \$\begingroup\$ @sunar thanks, i have updated. \$\endgroup\$
    – don bright
    Dec 29 '18 at 14:21
  • \$\begingroup\$ I assume the intent is for P to be calculated as # of '1' in the input / # of symbols in the input? That seems like it would match the definition given, but it would be helpful if it's described explicitly. \$\endgroup\$ Jan 3 '19 at 22:24
  • \$\begingroup\$ Done, thanks.... \$\endgroup\$
    – don bright
    Jan 3 '19 at 23:32

Alphabet Reconstruction

  • 1
    \$\begingroup\$ Fairly similar \$\endgroup\$
    – Dingus
    Oct 17 at 11:00
  • \$\begingroup\$ @Dingus but mine is better :P I'll give this some thought and if I can't think of a new spin to put on this ill just delete it cause its a dupe. Thank you! \$\endgroup\$ Oct 17 at 16:06
  • \$\begingroup\$ @Dingus hope this changes it enough. Flipped it on its head :D \$\endgroup\$ Oct 18 at 13:20
  • 1
    \$\begingroup\$ I certainly like the new story better! I found it a bit confusing that the first example has the unordered letters first in the output; the significance of 'any given wordlist may have more than one proper alphabet' hadn't sunk in. I'd suggest expanding on this point under one of the earlier subheadings. \$\endgroup\$
    – Dingus
    Oct 19 at 0:55
  • 1
    \$\begingroup\$ For the printable characters, perhaps specify ASCII 33-126? But I'd actually suggest restricting it to only alphabetic (A-Za-z) characters instead. Handling ^ or $ (for instance) doesn't really add anything to the challenge but will complicate any regex-based solutions. \$\endgroup\$
    – Dingus
    Oct 19 at 0:55
  • \$\begingroup\$ @Dingus noted! ill think abt how to word the "more than one proper alphabet" bit, and once i figure it out ill make both edits :) thank you for your help \$\endgroup\$ Oct 19 at 2:02
  • \$\begingroup\$ @Dingus does this clarify the multiple valid options thing? also, do you think I should include every valid output for the Cod Com Dy example? \$\endgroup\$ Oct 19 at 13:14
  • 1
    \$\begingroup\$ Nice, it's crystal clear now. The only other suggestion I'd make is to consider deferring to the default I/O formats instead of stipulating space/newline delimited strings. At the least, taking input as a list of strings seems fairly natural, as does outputting a list of characters. \$\endgroup\$
    – Dingus
    Oct 19 at 23:11
  • \$\begingroup\$ @Dingus ah, it was some weird holdover. What exactly should I write, "May take input and output in any reasonable format"? \$\endgroup\$ Oct 20 at 0:05
  • \$\begingroup\$ Yep sounds good. \$\endgroup\$
    – Dingus
    Oct 20 at 1:06
  • 1
    \$\begingroup\$ @Dingus great :) im at +3 so ill ask chat for feedback and unless theres any objections, i think ill post it :] \$\endgroup\$ Oct 20 at 2:15

Play RPS with 3 bits of memory

This is a rough draft for now, the specifics, presentation and title will probably be adjusted

In this game you will be building bots to play rock paper scissors against each other. Of course rock paper scissors is not a very interesting game, just pick one of the three randomly. Can't get better than that?

The first thing here is that, we will play a slight variation on the game which introduces a small amount of strategy.

But more importantly in this version we will be designing very simple bots. Your bot will not be able to pick things randomly, nor will it be able to simulate complex strategies, because your bots will have 3 bits of working memory.

The game

Before we get into exactly how the bots will be made and what exactly it means to have only 3 bits of memory lets cover the game.

For each pair of bots we will play 48 rounds of RPS. In each round both bots will select a choice of Rock, Paper or Scissors. Rock beats scissors, paper beats rock and Scissors beats paper, if the two chose the same move they tie.

When you win you will receive points based on your play. If you win with scissors you get 1 point, if you win with paper you get 3 points, and if you win with rock you get 6 points. If you tie or lose you get 0 points.

Each bot will play every other bot and the bots will be scored on the number of points gained in total.

The bots

Your bot will have 3 bits of working memory, that means at any given time it will have stored a number between 0 and 7. To decide what to play it will know two things

  1. What it has in memory
  2. The last move it's opponent made

Given those it should spit out

  1. What move it wants to make
  2. 3-bits to write into memory

This is so simple you don't actually need to write "code" to represent your bot. Your bot is really just a \$8\times 3\$ lookup table, plus a single move which it will make as it's first move. (We can assume that the starting memory is 0 without loss of generality)

And in fact you will submit your bots in this format as it makes it easy to verify your bot works and doesn't cheat.


I like this challenge because it is

  1. Completely deterministic who wins, to the point where you can, for small bot pool work out with pen and paper the scores.
  2. It is basically language agnostic. No need to bother with JS.
  3. There's basically no way to cheat. It's going to be really hard to exploit a vulnerability in the handler when you can't run arbitrary code.

I am a little concerned though that there might not be a whole lot to do? I'm not sure how much better one bot really can be than others. Obviously you can always take 1 bot and design a bot which plays perfectly against it. But I'm not totally sure how much a carefully arranged bot is going to do better than ones that are just a pile of random connections.

Turning the memory size up could improve this but the larger you make it the more complex each bot gets, and I think the fun is really in being able to hand tune your bot.

However I don't know what I can do to find out other than just post this.

  • \$\begingroup\$ Seems bruteforceable \$\endgroup\$
    – pxeger
    Oct 17 at 18:40
  • 4
    \$\begingroup\$ This is a unique challenge, and I think you could post it. If it doesn't work out, then we'll all know not to do it again (or an improved version could be posted later). If it does work, CGCC'll have a new kind of challenge, which would be great. \$\endgroup\$
    – rues
    Oct 17 at 18:57
  • \$\begingroup\$ @pxeger There are 1333735776850284124449081472843776 machines possible. Brute forcing that would probably mean playing every machine against every other machine. It may be solvable, but I don't think it is feasible to brute force it. \$\endgroup\$
    – Grain Ghost Mod
    Oct 17 at 21:15
  • 1
    \$\begingroup\$ I'd prefer to have rigid I/O (fixed I/O method and format) for KotH purposes. Or you could just say "write down the 8x3+1 possible outputs in a specific format". The barrier to post some bot looks pretty low, so I'd expect a large number of answers in the worst(?) case which would require some kind of automated controller. \$\endgroup\$
    – Bubbler
    Oct 18 at 1:13
  • \$\begingroup\$ @WheatWitch ah, I misread the challenge \$\endgroup\$
    – pxeger
    Oct 18 at 6:59
  • 1
    \$\begingroup\$ @bubbler oh I absolutely will write a controller once the rules are nailed down a bit. Just because you can score this by hand does not mean it would not be very tedious \$\endgroup\$
    – Grain Ghost Mod
    Oct 18 at 7:40

Remove submatrices


Solve the halting problem for ^/a*b*/b*a*/[ab]*$ in ///

///, a.k.a. Slashes is an esoteric programming language with simple two operations. One is to output its source to remove from it. The other is to substitute itself. The language is proven to be Turing-complete, so some programs such as /ab/bbaa/aab won't halt while some such as /ab/bbaa/ab will.

At first I questioned if halting problem for ^/[ab]*/[ab]*/[ab]*$ is solvable, but I learned unlikely.

So I am simplying to ^/a*b*/b*a*/[ab]*$.


Given a slashes program that matches ^/a*b*/b*a*/[ab]*$ in POSIX BRE (i.e. below), determine whether the program halts or not.

Format of program, if you are not familiar with POSIX BRE

program = "/" first "/" second "/" third
first = "" | first "a" | first first.b
first.b = "" | first.b "b"
second = "" | second "b" | second second.a
second.a = "" | second.a "a"
third = "" | third "a" | third "b"


In this problem every program's length is up to 153.

Detailed rules

  • Can be either a full program or a function.
  • Standard i/o apply.
    • Examples of input format
      • a string of program
      • three strings p,q,r when the program is /p/q/r
      • integers p,q,r,s and a string t when the program is /a\{p\}b\{q\}/b\{r\}a\{s\}/t
      • entirely as an integer (think of it by yourself)
    • Examples of output format
  • Standard loopholes apply.
  • This is ; shortest code wins.


Testcase generator 1

My noncompetive solution

///: no
/a//: yes
/ab/bba/aab: yes
/ab/bba/aaab: yes
/ab/bba/aabb: no


  • Were similar things ever done before?
  • I am not even sure if this problem is solvable.
  • Just thought there are answers if I clarify maximum length of input.
  • Should I change the problem's genre to ? Would making a maximum length of the program be boring?
  • 1
    \$\begingroup\$ /// is turing-complete, so this is not possible \$\endgroup\$
    – pxeger
    Apr 25 at 12:33
  • \$\begingroup\$ Should we simplify it more? \$\endgroup\$ Apr 25 at 20:23

Chess move

The Challenge

Write a program that gets a string containing a chessmove and a chessboard as input, and then outputs the chessboard.


The chess move will have this format:

<from square><to square>[<promoted to>]



The chessboard format is not fixed, but there must be a 1 to 1 relation between the board and the string to represent the board. Also the format of the input must bet the same as the format of the output. Two suggestions of what it could look like:


rnbqkbnr pppppppp 00000000 00000000  00000000  00000000 PPPPPPPP RNBQKBNR

It is not required to store anything except the location of the pieces, and validity of moves can be assumed.


Base score is character count (assuming your program can move pieces for all moves)

Bonus multipliers:

  • If the program updates the promoted piece, divide by 2
  • If the program also moves the rook when castling, divide by 2
  • If the program also removes the pawn when capturing en passent, divide by 2

The moves, and castling & en passent in particular are explaned on Wikipedia.

So basically writing a 100 character solution for the base problem gives the same score as an 800 character solution with all bonus multipliers.


If you would choose to use one of the board formats above, your input would look like one of these strings:

e2e4 rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR

e2e4 rnbqkbnr pppppppp 00000000 00000000  00000000  00000000 PPPPPPPP RNBQKBNR

Your corresponding output string would then be one of these:


rnbqkbnr pppppppp 00000000 00000000  0000P000  00000000 PPPP0PPP RNBQKBNR
  • 1
    \$\begingroup\$ Before I get on to more specific criticisms: as presented, without the bonus this is too trivial to be interesting. I suggest removing some flexibility: require Fen notation for the board position and algebraic notation for the move, and making the current bonus options mandatory. On specifics: it's not clear why you talk about storage; and the board position notations you suggest don't include enough information to know whether en passant is possible. \$\endgroup\$ Dec 22 '13 at 23:56
  • \$\begingroup\$ @PeterTaylor I agree that compared to chess programs this may be trivial, but I would like to make it a golf challenge. Compared to the hot code golf questions this is quite elaborate already in its basic form. (For a good solution the board design may need to be changed drastically). It is true that there is no attention to the legality of moves (whether it is possible to capture en passent) but for a mere viewer this is not required so I am not too worried about this. So far the chess questions seem to get very few answers as they tend to be complex and I hope to offer relatively easy entry. \$\endgroup\$ Dec 30 '13 at 11:02
  • 2
    \$\begingroup\$ Your point about en passant is valid - you had said in the spec to not worry about legality. I'll try to convince you of my first point: without the bonus, this reduces to: a) parse first four characters into (col 1, row 1, col 2, row 2); b) take board as a 64-char string; c) board[8*row_2+col_2] := board[8*row_1+col_1]; board[8*row_1+col_1] := ' '; print board. This is trivial compared to any good golf question. (Note that the hot questions at the moment are neither golf questions nor good questions). \$\endgroup\$ Dec 30 '13 at 12:14
  • \$\begingroup\$ This sandbox post has had little activity in a while. Please improve / edit it or delete it to help us clean up the sandbox. Due to community guidelines, if you don't respond to this comment in 7 days I have permission to vote to delete this. \$\endgroup\$
    – user58826
    Jun 9 '17 at 15:40

Black Box

Your task is to analyze a given situation for the game Black Box. Given a sequence of guesses and answers, your program is to either print the solution or suggest the next move.

The game

The board consists of 8×8 cells, with edges labeled like this:

I'll probably create nice images here, particularly to make sure that the squares of the board are really square.

i        I
j        J
k        K
l        L
m        M
n        N
o        O
p        P

The player shoots rays into the interior of the box, where they might get deflected, reflected or absorbed. He is told the position where the ray leaves the black box again, and from that has to deduce the positions of 4 atoms inside the black box.

I'll have to include more of the game rules here, but for now see Wikipedia.

Input and output

Input is a sequence of line, each consisting of two characters. The first denotes the point where the ray of light enters the black box, the second the place where it comes out again. In the case of a reflection, both characters will be equal. In the case of a hit, the second character will be -.

If the input is enough to fully determine the locations of the atoms, then output should be four lines giving the coordinates of each atom. The lines should be two lower case characters each, the first giving the row and the second giving the column of the found solution. The atom positions must be printed in lexicographical order.

If the input is consistent with more than one set of atom positions, then the output should consist of a single line containing a single character, which is the location where the next ray should be shot. That location has to be chosen in such a way that it can help find the solution. This is the case unless all of the atom positions consistent with the input so far would produce the same output for this next ray as well.

Your output has to be terminated by a newline character.


Let's take the atom configuration the Wikipedia article uses as an example as well:

i        I
j        J
k O    O K
l        L
m        M
n   O    N
o        O
p      O P

If the input were


then the output should be


but if the input were only


then the output might be for example



This is code golf, so shortest answer wins. However, I'll only accept answers which are practical in so far as they compute their result in reasonable time. I'd say no more than five minutes on my system where I'll evaluate the answers, and I'll simply hope that correct solutions will be much faster and incorrect ones much slower, so that the speed of my system doesn't make a difference. A submission which gives a wrong answer for one of my test cases will be disqualified until it gets fixed. I will probably point out the problem in a comment to that post.


Create a program with "exact repetition" in its source code

The task is to create a program, with the following restrictions placed on the printable ASCII characters in the source code: choose some k > 0.

  • Every non-alphabetic character has to appear exactly k times.
  • Every alphabetic character has to appear at most k times.
    • This rule differs from the former in order to avoid boring dummy identifiers while still making it a challenge to choose good library functions to call.

Character set definitions used:

  • Non-alphabetic characters are !"#$%&'()*+,-./0123456789:;<=>?@[\]^_{|}~ and '`' (backtick).
  • Alphabetic characters are ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz.

Note that no restriction is placed on characters outside of the range of printable ASCII characters (including control codes, tabs, newlines, higher unicode codepoints, etc).

What the program does is up to you; be creative. Some general guidelines:

  • Programs that do something interesting might have better chances, although more impressive code structure (i.e. fewer comments) is also beneficial.
  • Stuffing excess characters in comments is boring, and should be avoided/is discouraged.
  • Dead/no-op code isn't terribly interesting either, but is probably unavoidable and at least has to conform to the language's grammar.

This is : whatever has the most upvotes at Feb 1, 2014 gets accepted as the winner.

Example answer (C)

/*$$@``*/_[]={9.};main() {printf("He%clo \

Prints "Hello world!" (adapted from an answer to another question). Probably wouldn't score a lot (since what it does isn't terribly interesting). Each of the non-alphabetic characters appear exactly twice, and no alphabetic character appears more than twice.

For meta: I want to post this, but I'm worrying that "do something interesting" might give too little guidance and the question won't receive many answers.. thoughts? Is it good as-is, or should I come up with some task that one should be required to implement (and possibly change the ruling to code-challenge, with length + 2^(characters-in-comments) as the score)?


This is my first try at writing a challenge. Please let me know how I can improve it.

Roman Calculator

Create a basic calculator for Roman numerals.


  • Supports +,-,*,/
  • Input and output should expect only one prefix per symbol (i.e. 3 can't be IIV because there is two I's before V)
  • Input and output should be left to right in order of value, starting with the largest (i.e. 19 = XIX not IXX, 10 is larger than 9)
  • Left to right, no operator precedence, as if you were using a hand calculator.
  • Supports whole positive numbers input/output between 1-4999 (no need for V̅)
  • No libraries that do roman numeral conversion for you

For you to decide

  • Case sensitivity
  • Spaces or no spaces on input
  • What happens if you get a decimal output. Truncate, no answer, error, etc..
  • What to do for output that you can't handle. Negatives or numbers to large to be printed.

Extra Credit

  • -20 - Handle up to 99999 or larger (numbers with a vinculum)

Sample input/output

XIX + LXXX                 (19+80)

XCIX + I / L * D + IV      (99+1/50*500+7)

The shortest code wins.

  • 1
    \$\begingroup\$ You might want to be explicit about which variants of Roman numerals need to be supported. For example, do I have to understand IV as 4, or can I require that it be written as IIII? And what about, say, writing 8 as IIX instead of VIII, 19 as IXX or XVIV instead of XIX, or 99 as IC instead of XCIX? (All these variants have, AFAIK, been used classically.) \$\endgroup\$ Feb 9 '14 at 22:36
  • \$\begingroup\$ @IlmariKaronen thanks. I modified the question to be slightly more specific about that. \$\endgroup\$
    – Danny
    Feb 10 '14 at 14:09
  • 1
    \$\begingroup\$ I think that using IV, IX, IC, XC, etc. should be alright, but only allow one prefix. Also, 19 should be written XIX, not IXX. One other thing, can we assume that the operators will be separated by a space, or no? \$\endgroup\$
    – user10766
    Feb 12 '14 at 0:32
  • 1
    \$\begingroup\$ Hello! This looks like a good but abandoned meta post, would you be willing to offer it for adoption? (If you want to, you can still post to main.) \$\endgroup\$
    – user58826
    Jun 9 '17 at 16:06
  • \$\begingroup\$ 1. I don't need to handle I/III but need to handle I/III+II/III? 2. For the extra can I output maybe [V] for 5000? \$\endgroup\$
    – l4m2
    Apr 12 '18 at 15:05
  • \$\begingroup\$ @programmer5000 it was posted to main awhile ago. codegolf.stackexchange.com/questions/20670/… \$\endgroup\$
    – Danny
    Apr 26 '18 at 11:58

4 and 20 baked in a π

While some might describe π as a string of seemingly random numbers, one can also look at it in a way similar to a monkey with a typewriter. Eventually, it should calculate out to something more interesting. For example, the sequence 1337 shows up 4,814 places to the right of the decimal. At 700,731 places right of the decimal, you'll find the sequence 160151, which is "pi" represented as ASCII (although you'll find a 'pointer' to it much faster, as the sequence 700731 begins at 29,830 digits to the right).

So, your task is to make a program to find things in π. Your program will accept a positive integer and output the number of places right of the decimal point that number appears. To keep the run times down, input can be limited to numbers in the range of 0 to 1000 (without leading zeros).

Example: Using 415 as the input, the output should be 2:



  • You can not use any precalculated values of π, including language constants, built in functions that return π or digits of π, or any resource outside the code itself (such as files or websites).
  • You can not use any trig functions to calculate π.

Bonus points if you find the sequence 072 101 108 108 111 044 032 087 111 114 108 100 033.

This is code golf, so lowest score wins.

  • 1
    \$\begingroup\$ It's not clear to me whether you require answers to support leading zeroes. Also: program, named function or snippet? And how indexed? (Giving 415 as a test case would be a good way to answer the last question) \$\endgroup\$ Mar 11 '14 at 6:58
  • 3
    \$\begingroup\$ Isn't this just Calculate 500 digits of pi with a search function tagged on at the end? By the way, your bonus points are quite safe — even if you searched a trillion trillion trillion digits of pi, your chance of finding an arbitrary 39-digit sequence would still be less than 0.1%. \$\endgroup\$
    – r3mainer
    Mar 11 '14 at 14:59
  • \$\begingroup\$ Edited to clarify leading zeros and indexing. @squeamishossifrage - Yes and no. The number of digits to find the answer depends on the input, which both limits the choice of algorithm to generate the search space and gives more ample room to golf the integration of the search function. The worst case is under 10000 digits for n between 0 and 1000. I suppose I could put in a time limit of a couple minutes and expand the range of n to 10000 (worst case is just under 390k), but that seems obnoxious. Thoughts? \$\endgroup\$
    – Comintern
    Mar 11 '14 at 17:20
  • 1
    \$\begingroup\$ @AlexA. - Not a drug reference. \$\endgroup\$
    – Comintern
    Apr 1 '15 at 22:34
  • \$\begingroup\$ Hello! This looks like a good but abandoned meta post, would you be willing to offer it for adoption? (If you want to, you can still post to main.) Due to community guidelines, if you don't respond to this comment in 7 days I have permission to adopt this. \$\endgroup\$
    – user58826
    Jun 9 '17 at 16:15

Create a calendar

We all know HDD-space is precious and bandwidth is expensive, therefore it is best to reduce the size of your executables. Let's start with your calendar:

Your task is to build a calendar app in at most 512 bytes. The calendar must at least support the following features, but additional features may gain you additional upvotes:

  • It must be able to show the current month with the current day highlighted
  • The user must be able to find out the week day of each day


  • Maximum code length is 512 bytes (counted as UTF-8 without BOM)
  • You may subtract the bootstrapping code (i.e. int main(int argc, char **argv) in C or <?php in PHP) and imports from the final size to allow for more verbose languages to be in
  • You may use standard time / date functions of your programming language, as long as they don't allow you to output a ready to use calendar
  • No network access (I said bandwidth is expensive!)
  • Voters decide on the amount of features / look and feel / creativity

This needs a tag for the size restriction, any suggestions?

  • 1
    \$\begingroup\$ "bandwidth is expensive" <sup>[citation needed]</sup> \$\endgroup\$ Mar 22 '14 at 5:27
  • 1
    \$\begingroup\$ Seems rather close to Output: Calendar Month \$\endgroup\$ Mar 22 '14 at 5:33
  • \$\begingroup\$ Who decides what counts as bootstrapping code? It seems odd to arbitrarily exclude code like that, and the examples you gave can be golfed a lot: they're more or less equivalent to main(){ and <? respectively. \$\endgroup\$ Mar 24 '14 at 20:49
  • \$\begingroup\$ @WanderNauta Bootstrapping code is the code that's essentiell to get a working noop program. \$\endgroup\$
    – TimWolla
    Mar 24 '14 at 21:00
  • \$\begingroup\$ @TimWolla That definition won't fly. A zero-byte file is a working noop PHP script, for example. \$\endgroup\$ Mar 24 '14 at 21:01
  • 1
    \$\begingroup\$ @WanderNauta A zero byte file is a working noop in every language. \$\endgroup\$
    – TimWolla
    Mar 24 '14 at 22:12
  • \$\begingroup\$ So what's bootstrapping code then? :) \$\endgroup\$ Mar 24 '14 at 22:53
  • 1
    \$\begingroup\$ for the limit I'd say code-shuffleboard or restricted-source \$\endgroup\$
    – Einacio
    Mar 26 '14 at 15:57
  • \$\begingroup\$ This sandbox post has had little activity in a while. Please improve / edit it or delete it to help us clean up the sandbox. Due to community guidelines, if you don't respond to this comment in 7 days I have permission to vote to delete this. \$\endgroup\$
    – user58826
    Jun 9 '17 at 16:28

ASCII ART edge detection

As the title says, I was thinking to contest in which one must detect edges of an ASCII art.

The code should accept a B/W ASCII art as input. A B/W ASCII art is defined as (by me) an ASCII art with only one kind of non-white-spaces character (in our case: an asteriks *). And as output produce a standard ASCII art (all ASCII characters are accepted) which should remember the contourn of the first.

The purpose of using more than one character in the output is to make some edges ssmoother. For instance, one could let this input


could became:

    _/   ) 
  _/    /
 /      |
|      /
|      \
 \      |
  `\     |

The input \n separated string as input. Each line has a maximum of 80 characters. The number of rows is not specified.

I'd put it as a popularity-contest since, beside my simple code, I'd like to see more "round" edge detections which use more than one character in smooth edges.

Also, I don't want to tag it as code-golf since I'm quite sure one can do this job using aplay (with ASCII art renderer) and command line GIMP (to apply edge detection).

As a popularity contest, there are no strict rules on how the output should be..just use your fantasy!

This is my sample program:

import fileinput as f
import re as r
import copy as c
a,s,p='*',' ','+'
def read(n):
    s=[list(' '*n)]
    for l in f.input():
        k=list(r.sub('[^ ^\%c]'%a,'',' '+l+' '))
        s.append(k+[' ']*(n-len(k)))
    s.append([' ']*n)
    return s
def np(s):
    for l in s[1:-1]:
        for w in l[1:-1]: print(w,end='')
def grow(i):
    for x in range(1,len(o)-1):
        for y in range(1,len(o[x])-1):
            if(i[x][y]==a): o[x-1][y-1]=o[x-1][y+1]=o[x-1][y]=o[x+1][y]=o[x+1][y-1]=o[x+1][y+1]=o[x][y+1]=o[x][y-1]=a

    return o
def diff(i,o):
    for x in range(0,len(i)):
        for y in range(0,len(i[x])):
            if(i[x][y]==a and o[x][y]==s): l.append(p)
            else: l.append(s)
    return c

Here below I put both input of the programs. It is an 80x70 ASCII ART. It means it has 70 lines of 80 characters, each separated by \n.

                                          *****          *****                   
                                     ******                  ***                 
                                    ***                         ****             
                             *********                             **            
                          ***********                               **           
                     ******   *******                                **          
                 *****       *******      ***                         **         
              ****          ********     *****                          *        
             **            *********     *****                    *****  *       
           ***            *********     *******                  ******  **      
          **             **********     *******                  ******   **     
         **              **********    *******                  ********   *     
        *               ***********   ******                    ********   *     
       **              ************   *****                     ********    *    
       *               ************    ***                       ********   *    
      *               *************                               ******    *    
     *                *************                                 ***     *    
    **                *************                                         *    
    *                **************                                         *    
   **                *************                                         **    
   *                 *************                                         **    
  **                *************                                          ***   
 ***                *************                                          ****  
 **                 ************                                           ****  
 **                *************                                           ****  
 **                *************           *****                           ****  
 **                *************          **   **          **              ****  
 **                 ************          *     *         ** **            ****  
 *                  ************          **   **        **   **           ****  
 *                  *************        *******         **   ***          ****  
 *                  ************          *****           *******          ****  
 *                   ************         ***               *****          ****  
**     *             *************                          ****          *****  
**    ***            **************                                      *****   
*    *****            *************                                     ******   
** *******             **************                                  *******   
**********             ***************              *                *********   
**********              *****************          ***             ***********   
***********              *******************                    **************   
***********               **********************            ******************   
************              *****************     **     ***********************   
*************             ******************      ****     *******************   
**************            ******************              ********************   
****************           ******************              *******************   
***************           *******************              *******************   
****************           ******************              ******************    
******************         ******************             *******************    
*******************         *****************             *******************    
*********************      ******************           ********************     
*********************************************          *********************     
**********************************************       ***********************     
************************     *****************      ************************     
 **********************       ******************* **************************     
 *********************        *********************************************      
 *********************        ****************************  ***************      
 ********************         **************************    ***************      
 ********************         *********************         ***************      
 *******************          ********************         ****************      
 ******************           *****************            ****************      
 *****************             ****************            ***************       
 *****************             ****************            ***************       
 *****************             *****************           ***************       
  ****************             *****************           ***************       
   **************              ******************          ***************       
                                 ****************          ****************      
                                  **************            ***************      

A possible output could be:

                                         +++++             ++++
                                    ++++++     ++++++++++     +++
                                   ++      +++++        +++++   +++++
                            ++++++++   +++++                ++++    ++
                         ++++         ++                       ++++  ++
                    ++++++           ++                           ++  ++
                +++++      +++       +   +++++                     ++  ++
             ++++     +++++++       ++  ++   ++                     ++  ++
            ++    +++++   ++        +   +     +                  +++++++ ++
          +++  ++++      ++         +  ++     ++                ++     ++ ++
         ++   ++        ++         ++  +       +                +      ++  ++
        ++  +++         +          +  ++       +               ++      +++  +
       ++  ++          ++          + ++       ++               +        +++ +
      ++ +++          ++           + +      +++                +        + + ++
      +  +            +            + +     ++                  +        ++++ +
     ++ ++           ++            + ++   ++                   ++        + + +
    ++ ++            +             +  +++++                     ++      ++ + +
   ++ ++             +             +                             +++   ++  + +
   +  +             ++             +                               +++++   + +
  ++ ++             +              +                                      ++ +
  +  +              +             ++                                      +  +
 ++ ++             ++             +                                       +  ++
++  +              +             ++                                       +   ++
+   +              +             +                                        +    +
+  ++             ++            ++                                        +    +
+  +              +             +         +++++++                         +    +
+  +              +             +        ++     ++        ++++            +    +
+  +              +             +        +  +++  +       ++  +++          +    +
+  +              ++            +        + ++ ++ +      ++  +  ++         +    +
+ ++               +            ++      ++  +++  +      +  +++  ++        +    +
+ +                +             +      +       ++      +  +++   +        +    +
+ +                +            ++      ++     ++       ++       +        +    +
+ +   +++          ++            ++      +   +++         +++     +       ++    +
  +  ++ ++          +             ++     +++++             +    ++      ++     +
  + ++   ++         +              +                       ++++++      ++     ++
 ++++     +         ++             +++                                ++      +
  +       +          ++              ++            +++              +++       +
          +           +               ++++        ++ ++           +++         +
          ++          ++                 ++++     +   +        ++++           +
           +           ++                   +++++ +++++    +++++              +
           ++           ++                      +++   ++++++                  +
            ++           +                 +++++  +++++                       +
             ++          +                  +  +++    +++++                   +
              +++        +                  ++   ++++++  +                    +
                +        ++                  +           ++                   +
               ++        +                   +            +                   +
                +++      ++                  +           ++                  ++
                  ++      +                  +           +                   +
                   +++    ++                 +         +++                   +
                     ++++++                  +        ++                    ++
                                             ++     +++                     +
                                              +    ++                       +
                        +++++                 ++++++                        +
+                      ++   ++                   +                          +
+                     ++     +                                             ++
+                     +      +                            ++               +
+                    ++      +                          ++++               +
+                    +       +                     ++++++ ++               +
+                   ++       +                    ++      +                +
+                  ++        +                 ++++       +                +
+                 ++         ++                +          +               ++
+                 +           +                ++         +               +
+                 +           +                 +         +               +
++                +           +                 ++        +               +
 ++              ++           +                  +        +               ++
  ++++++++++++++++            +++                +        +                +
                                ++              ++        ++               +
                                 ++++++++++++++++          ++              +
                                                            ++            ++

This is also the output produced by the script above. Of course it is not the best output and I'm sure one can easily produce a smoother one.

  • 1
    \$\begingroup\$ It would be useful to be more precise about which characters should be non-blank in the output: characters which were non-blank in the input but adjacent to blanks, or characters which were blank in the input but adjacent to non-blanks? \$\endgroup\$ Apr 4 '14 at 9:50
  • \$\begingroup\$ Thanks for pointing. I re-written the phrase in the answer. You can use every ASCII character in the output (as usual ASCII art). E.g. I used only + symbol, but one could makes round edges using symbols like \ or / etc.. \$\endgroup\$ Apr 4 '14 at 9:55
  • \$\begingroup\$ edited again... \$\endgroup\$ Apr 4 '14 at 10:09
  • \$\begingroup\$ Can you define the input that will be used by all the participants? It's necessary to have only one input to compare the outputs of the different answers. The first example is too simple and the last one is too long. So I suggest to use something between these 2 examples. \$\endgroup\$
    – A.L
    Apr 4 '14 at 17:29
  • \$\begingroup\$ Thanks, I chosen a cute panda as input. \$\endgroup\$ Apr 6 '14 at 8:09
  • 1
    \$\begingroup\$ one could let this input (…) could became → try something like "this input (…) could become" outpuit → output \$\endgroup\$ Apr 7 '14 at 13:31
  • \$\begingroup\$ I edited it now, so do you people thinks it is a good question? \$\endgroup\$ Apr 9 '14 at 17:15
  • \$\begingroup\$ Hello! This looks like a good but abandoned meta post, would you be willing to offer it for adoption? (If you want to, you can still post to main.) Due to community guidelines, if you don't respond to this comment in 7 days I have permission to adopt this. \$\endgroup\$
    – user58826
    Jun 9 '17 at 16:32
  • \$\begingroup\$ Hi @programmer5000 , I already asked such a question. Do you mean to re-use it again? See: codegolf.stackexchange.com/questions/26139/… \$\endgroup\$ Jun 12 '17 at 13:39

Hi, first time golf questioner, hopefully I'm doing it right!

Maths Trade Calculator

A maths trade (or "math" trade if you prefer) is a way of calculating complex trades of arbitrary items in a circle of participants where not all participants want all items.

X participants have an item they would like to trade. Each participant is assigned a unique number, and provides a list of (numbers identifying) the items they would willingly trade their item for. They may provide an empty list (i.e. they would rather not trade).


X lines, one for each participant, comprising a unique number identifying them, followed by a colon, then a comma-separated-list of numbers identifying other items that they would trade for. e.g.:


The numbers identifying the participants will not necessary be in order, nor will they necessarily be 1 to X. You may assume that they will be numeric.

This string can be in STDIN, or an argument to a function, or similar and can be followed by a new-line or not, whatever the coder prefers.


One or more trade loops in which all participants are making trades they're happy with. Each loop should be on a new line and comprise a participant number, followed by "->", followed by the participant they should give their item to, then another "->", and another participant number etc, until the loop is closed and the last participant number matches the first one. Another line is added with the number of completed trades. e.g.:


Participants for which no valid trade is possible are omitted.

Output can be via STDOUT, or returned as a string, or something else, with an optional final new-line.

Trade rules

  1. A participant may not be involved in more than one trade
  2. A participant may not receive an item that they didn't want
  3. All loops must be closed
  4. Maximum number of possible trades should be completed (i.e. no submitting a zero-trade output and claiming it's valid). If there are multiple permutations, pick whichever you prefer.

This is a code golf challenge, so shortest working code wins.

Some more example inputs and possible outputs




For instance, in this trade: 9 stated that he would accept 1's item in a trade, 10 stated that he would accept 9's item, 3 would accept 10's and 1 would accept 3's. In the second loop, 2 receives 7's item, 5 receives 2's, 6 receives 5 and 7 receives 6's. (Other outputs are possible from this input.)







1->9->1 is also valid in this case, but both cannot be completed. Either is acceptable.

Thanks for reading guys! Let me know if there are any improvements I can make.

  • \$\begingroup\$ "can be followed by a new-line or not, whatever the coder prefers." How flexible is this? For instance, can I use trailing commas, like 1:2,4,7, if it makes my code shorter? \$\endgroup\$ May 2 '14 at 17:28
  • 2
    \$\begingroup\$ Will the participants always be numbered 1 to n and their input lines provided in order? If so, state it. If not, include a test case which fails if an implementation decides to ignore everything before the : in each input line. \$\endgroup\$ May 5 '14 at 10:02
  • \$\begingroup\$ @m.buettner I would say a trailing comma is not acceptable, on the end of any line, or the end of the input/output. \$\endgroup\$
    – Johno
    May 6 '14 at 8:55
  • \$\begingroup\$ @PeterTaylor Good tip. I'll correct the question to state that you can't assume that the numbers will be 1 to n, in order. \$\endgroup\$
    – Johno
    May 6 '14 at 8:57
  • \$\begingroup\$ Hello! This looks like a good but abandoned meta post, would you be willing to offer it for adoption? (If you want to, you can still post to main.) Due to community guidelines, if you don't respond to this comment in 7 days I have permission to adopt this. \$\endgroup\$
    – user58826
    Jun 9 '17 at 16:39

Design and Solve a Maze

(this question on hold while the details are ironed out)

Your task is to play the roles of both characters in this scene from Inception. In it, Cobb gives Ariadne a challenge:

You have two minutes to design a maze that takes one minute to solve.

Some liberties will be taken on that description. Most importantly, this challenge is not time-based, rather scores are based on the effectiveness of your mazes and maze-solvers.

I apologize for the many edits to this challenge as we iterate towards an easy and fair format..

Part I: Maze format

All mazes are square. A cell in the maze is represented as a zero-indexed tuple row column.

Walls are represented by two binary strings: one for horizontal walls (which block movement between rows) and vertical walls (vice versa). On an NxN maze, there are Nx(N-1) possible walls of each type. Let's take a 3x3 example where the cells are labelled:

A   B | C
D | E   F
G   H | I

all possible vertical walls are: AB BC DE EF GH HI. Translated into a string, the walls shown are 011001 for vertical walls and 010010 for horizontal walls. Also, by "binary string" I mean "the characters '0' and '1'".

The full maze format is a string which contains, in this order:

  • width
  • start cell tuple
  • end cell tuple
  • horizontal walls
  • vertical walls

For example, this maze:

   0 1 2 3 4
0 | |  E|  _|
1 |  _|_|_  |
2 |_ _ _  | |
3 |  _ _  | |
4 |____S|___|

is formatted to this:

4 2
0 2

Part II: The Architect

The Architect program creates the maze. It must play by the rules and provide a valid maze (one where a solution exists, and the end is not on top of the start).

input via stdin: Two positive integers:

size [random seed]

Where size will be in [15, 50]. You are encouraged to make use of the random seed so that matches can be replayed, although it is not required.

output to stdout: A valid size x size (square) maze using the format described in Part I. "valid" means that a solution exists, and the start cell is not equal to the end cell.

The score of an Architect on a given maze is

   # steps taken to solve
max(dist(start,end),(# walls))

So architects are rewarded for complex mazes, but penalized for each wall built (this is a substitute for Ariadne's time restriction). The dist() function ensures that a maze with no walls does not get an infinite score. The outside borders of the maze do not contribute to the wall count.

Part III: The Solver

The Solver attempts to solve mazes generated by others' architects. There is a sort of fog-of-war: only walls adjacent to visited cells are included (all others are replaced with '?')

input via stdin: the same maze format, but with '?' where walls are unknown, an extra line for the current location, and a comma-separated list of valid choices from this location. (This is a big edit that is meant to make it simpler to write a maze-parsing function)

example (same as the above 5x5 maze after taking one step left)

4 2
0 2
4 1
4 0,4 2

Which corresponds something like this, where ? is fog:

   0 1 2 3 4
0 |????E????|
1 |?????????|
2 |?????????|
3 | ?_?_????|
4 |__C_S|_?_|

output to stdout: One of the tuples from the list of valid choices

Each Solver's score is the inverse of the Architect's score.

Part IV: King of the Hill

Architects and Solvers are given separate scores, so there could potentially be two winners.

Each pair of architects and solvers will have many chances to outwit each other. Scores will be averaged over all tests and opponents. Contrary to code golf conventions, highest average score wins!

I intend for this to be ongoing, but I can't guarantee continued testing forever! Let's say for now that a winner will be declared in one week.

Part V: Testing

I have written a Python testing kit which includes a Maze class for parsing and writing in the proper formats, as well as an example architect/solver pair: Daedalus and the Minotaur

Available on both Dropbox and GitHub

Part VI: Submitting

  • I maintain veto power over all submissions - cleverness is encouraged, but not if it breaks the competition or my computer! (If I can't tell what your code does, I will probably veto it)
  • Come up with a name for your Architect/Solver pair. Post your code along with instructions on how to run it.
  • \$\begingroup\$ I suppose input is via STDIN? You might want to mention that explicitly, because at least the architect could just as well take the input via command-line arguments. \$\endgroup\$ May 15 '14 at 15:34
  • \$\begingroup\$ updated. I have a driver/referee program which will handle I/O; I'll update it to use stdin/stdout since that will no doubt be the easiest standard. \$\endgroup\$
    – wrongu
    May 15 '14 at 16:04
  • \$\begingroup\$ @m.buettner before de-sandboxing this, would you be willing to try the test kit? \$\endgroup\$
    – wrongu
    May 15 '14 at 18:20
  • \$\begingroup\$ I'd love to, but I'm afraid I'm too busy this week. Try ask for help in the chatroom. \$\endgroup\$ May 15 '14 at 18:35
  • \$\begingroup\$ Possible architect issue: With this scoring method (steps/walls), you can get a minimum score of 3 by simply putting the start/finish right next to each other with a single wall between. It takes three steps to go around. Most actual mazes I've seen have too many walls to make a score of 3 likely, much less guaranteed. \$\endgroup\$
    – Geobits
    May 16 '14 at 13:51
  • \$\begingroup\$ Thats a problem. What if the dist function was shortest path? Then only mazes which cause detours could get a score > 1 \$\endgroup\$
    – wrongu
    May 16 '14 at 15:09
  • \$\begingroup\$ That would probably be better. That way it's scored on best vs actual. It would take away the incentive to figure out how to build hard mazes with few walls, though, which was interesting itself. \$\endgroup\$
    – Geobits
    May 17 '14 at 3:01
  • \$\begingroup\$ Hey rangu... not sure if you're still planning to do this thing, but overactor just said something in chat which reminded me of your challenge and might be a neat way to avoid the combined score: split this up into two code-challenges, one for maze generation and one for maze solving. Each code-challenge's benchmark set (to determine the scores) would be the outputs of the other challenge's participants. Then you could just pick a best solver and a best generator independently. \$\endgroup\$ Aug 1 '14 at 11:18

Author note: I was thinking about new genres today, and I had an idea. What if there could be a challenge that encourages people to write good code, instead of the code-golf gibberish we all know? Here's a challenge that attempts to do that. (This could even possibly be a , which would be great because it would bring in a greater high quality question volume to the site, but I'm terrible at coming up with names. Feel free to suggest something in the comments.)

Build your own image editor


For this challenge, you will create the best GUI image editor that can perform the most tasks that you possibly can... from scratch.

Tasks and scoring

Here are the features / tasks used to score your program. Each task is worth a certain amount of points, which is specified in brackets before the task description. For convenience, each task will also be prefixed by an ID string so that you can refer to them when describing your program.

  • [1 A] Brush tool: Simple, click and drag the mouse to draw freestyle doodles. Must draw a contiguous path.
    • [1 A1] Ability to change the brush size.
  • {TODO: etc., add more}


Your editor must conform to the following requirements:

  • Must accept input via the mouse. Tools (brush, flood fill, etc.) can be switched and configured with keyboard shortcuts, by clicking icons with the mouse, through a menu, or however you would like.
  • You may not use a single built-in function to accomplish one or more of the tasks. For example, if your language has a built-in image flood fill function, you may not use it and must build your flood fill from scratch.

Final score and voting

This is the syntax you should use to describe your score in your answer:

# {language}, {your score} score
<sup>(features implemented: {A, A1, ...})</sup>

    {your code here}

{description, comments, other notes, etc. here}

The amount of votes your post has (upvotes minus downvotes) will be multiplied by {TODO: figure out a good number} and added to your score. (Do not add this to the score in your post, since votes change constantly; I will add them manually.) Voters, please vote according to the following criteria:

  • elegance and readability of the source code
  • ease of use of the image editor and how powerful it is
  • remember to sort by "active" so that you're voting for new answers too, and not just the top voted ones!
  • 3
    \$\begingroup\$ Your questions should be reasonably scoped. If you can imagine an entire book that answers your question, you’re asking too much. A really good answer to this would run into millions of lines of code. \$\endgroup\$ Jun 4 '14 at 15:52
  • \$\begingroup\$ @PeterTaylor Yes, I was a little worried about that, but if it's not broad enough, it will be easy to just implement all the features. Any suggestions for fixing this? I was thinking of adding a "brevity" criterion in the voting section, but that doesn't seem like an ideal solution. \$\endgroup\$
    – Doorknob Mod
    Jun 4 '14 at 16:00
  • 3
    \$\begingroup\$ To be honest, the site for good code is Code Review. They already have a monthly challenge, for which they post snippets for review. I don't see a need to copy them. \$\endgroup\$ Jun 4 '14 at 16:12
  • \$\begingroup\$ @PeterTaylor Wait, isn't Code Review for questions and answers, not challenges and contests? In any case, is there any reason for that to prevent us from posting challenges like we always have? \$\endgroup\$
    – Doorknob Mod
    Jun 4 '14 at 16:17
  • \$\begingroup\$ meta.codereview.stackexchange.com/questions/tagged/… . Surprised you don't know about it, given how dedicated you are to spying on them ;) In general, if a question is on topic for multiple stacks then there's no obligation to do the sensible thing and post it on the one which it best fits, but you should expect people to ask why you're not doing the sensible thing. I think you're going to have to work hard to turn this into a question which fits this site, whereas it's already a good fit for CR's challenge programme. \$\endgroup\$ Jun 4 '14 at 16:31
  • \$\begingroup\$ @PeterTaylor Hmm, that's strange. Wouldn't that be more on-topic here? (And I only occasionally pop in to their chat/meta to see what they're up to. :-P) \$\endgroup\$
    – Doorknob Mod
    Jun 4 '14 at 16:32

Help Joe Bloggs with his password hash

Joe was confidently using "password1" as his main password to all his accounts until one day he received an e-mail from fBay. His account has been compromised and he must change his password immediately. Yet worse, the attacker had access to all Joe's accounts. Being an engineer, Joe thought: What if I could hash somehow my password using a keyword? I wouldn't need to remember any passwords and I would have a different one for each account.

Joe then creates an algorithm - he takes the domain name as a key and creates the password for each of his account consisting of:

1. (<consonants><vowels>)(alternating case: lower, capital, lower...)
2. <number of consonants><number of vowels>
3. <sum of consonants and vowels numbers converted to a character on US Qwerty Keyboard>

Joe then opens an account on SO to create a new code golf challenge. He uses stackoverflow as a key to generate password:

1. sTcKvRfLwAoEo - consonants and vowels in alternating case
2. 94 - 9 consonants, 4 vowels
3. 9+4=13, 1+3=4, Shift+4=$

Therefore, Joe's password for stackoverflow is: sTcKvRfLwAoEo94$


Create a shortest function to generate a password given the rules above. The code should accept a string type parameter d and return/display the generated password.


  1. Only Latin letters from the input should be used. Any other characters should be ignored.
  2. Minimum input length is 1 letter. (guys at q.com need passwords as well!)
  3. Assume Y is a vowel
  4. If vowels or consonants are missing, use 0 accordingly. E.g. input a would result in a01!
  5. Shortest code wins

List of vowels and consonants

US qwerty keyboard

  • \$\begingroup\$ Thanks for the feedback @m.buettner. I meant to say, without using any libraries. The problem is, that people become lazy to think sometimes and just dive straight away to use Linq where a bit of thought will do \$\endgroup\$
    – mai
    May 28 '14 at 13:14
  • \$\begingroup\$ Well actually you can, I'm just checking now. You can do a lot of manipulations on strings without libraries. \$\endgroup\$
    – mai
    May 28 '14 at 13:18
  • \$\begingroup\$ Looping over string characters, concatenation work perfectly. Nevertheless, I've updated the challenge. If a function to depend on a library, it must be included in the character count. \$\endgroup\$
    – mai
    May 28 '14 at 13:21
  • 3
    \$\begingroup\$ 1. Strictly speaking, in .Net you don't have strings without libraries. The string keyword is syntactic sugar for a class in mscorlib. 2. As things currently stand, your rule 1 strictly prohibits something and then says what to do if you ignore that prohibition. This is illogical. It's also unclear what "that" in "please inlcude that in characters count" means. Does it mean that each submission should be a program as opposed to a code snippet? If so, state it explicitly. \$\endgroup\$ May 28 '14 at 13:32
  • \$\begingroup\$ Hmm.. I don't know how to write it the best way. mscorlib is included by default so that is permissible. I don't want the code to use other libraries as Linq as it's less fun. \$\endgroup\$
    – mai
    May 28 '14 at 13:47
  • \$\begingroup\$ @m.buettner I agree with you. Nevertheless, there will solutions provided in other languages as well (there always are). And I would like the authors of those solutions to think about the best approach in their language of choice without depending on libraries like Linq. \$\endgroup\$
    – mai
    May 28 '14 at 14:00
  • \$\begingroup\$ Does Rule 2 mean ONLY vowels/consonants to be used from input? What about symbols *@#$ etc. Depending on that answer, potentially clarify Rule 5 regarding symbol input. As for Step 3 in the hash, should that progress further, similar to my Appended Numbers game so 103 consonants and 5 vowels would follow as 103+5 = 108, 1+0+8/10+8, etc.? \$\endgroup\$
    – Matt
    Jun 4 '14 at 2:35
  • 1
    \$\begingroup\$ @Matt, clarified - only Latin letters are used from the input. If consonants or vowels are missing, use 0 instead. The sum should progress, until it's <=9. E.g. 103 consonants, 5 vowels: 103+5=108, 1+0+8=9. Then, Shift+9='(' \$\endgroup\$
    – mai
    Jun 18 '14 at 10:36


Note for Sandbox: I have not finished (or really started) the control program for this game, because I wanted to see if there was interest in it before I dedicated too much time to the project. that means that the rule are still up to be tweaked, so please leave a comment if you have a suggestion, and comment or vote if you are interested in seeing this happen.

Diplomacy is a complex strategy game, with a very entertaining combat system. This challenge will be to write a bot to compete in a simplified version of diplomacy combat.



Countries (bots) will begin the game with 10 health, representing their remaining will to fight. The goal is to eliminate all other Nations by attacking them until they have 0 health.

The game will consist of several rounds. On the first round, all bots will receive 2 numbers as command line arguments: The first will be the total number of countries fighting, and the second will be their number in the list. Each following round, bots will receive a command line arguments containing the actions taken by each player last round and a list of all bots and their remaining health separated by commas, like so

 1:A2,2:S3,3:A4,4:A3 1:10,2:7,3:7,4:1

Each bot must then output a desired action, which is one two commands

  1. Attack a player. This is done by printing the letter A followed by the number of the player you with to attack. For instance, A3
  2. Support a player. This will give the player you support a boosted attack.

Resolving combat

After player have sent in their moves, attack scores will be calculated thus:

  1. All players start with a strength of 1, and one point is added for every player supporting them. For instance, if the moves are 1:A3,2:S1,3:A2,4:S2 then bot 1 has strength 2, bot 2 has strength 2, bot three has strength 1, and bot 4 has strength 1.
  2. After strength has been calculated, bots will deal damage based on their strength. The formula for damage is (Attacker's strength + 1) - (Defender's Strength) In the above situation, player 3 would take 2 damage and player 2 would take 0 damage. Note that, unlike regular diplomacy, attacking a supporter does not cut support.
  3. All attack take place simultaneously and independently. This means that if player 1 and 2 both attack player 4, then they each deal 1 damage. If player 3 were to support player 4, then player 4 would take no damage.

Round Ends

After combat has been resolved, countries that have 0 health will no longer be able to attack or support. However, they still will be listed in the input with an health of 0. When a bot is eliminated, all remaining bots will receive a single point.

Ending the game

The game ends when either 100 turns have elapsed or only 2 or less players remain. At this point, the player with the highest remaining health is the winner and receives 1 point. In case of ties, all tied bots will revive 1 point. If all bots die on the same turn, this is not a tied victory, but mutually assured destruction, and all bots will receive 0 points.


The control program will run 100 rounds of the game. The winner will be the country with the most points at the end of 100 rounds.


You may write in any language I can reasonably compile. I will make an effort to compile odd languages, but make no promises as to my ability to do so. Please provide your source code, an explanation, and a command line command to run your program.

  • You are allowed to write to a file. In fact, you are encouraged to do so.

  • Because this is a game where cooperation is paramount, you are allowed to write bots that work together, with the following restriction:

    • Only two bots can be written by a single player to work together at a time.
  • Standard Loopholes apply. You are not allowed to change the way the control program runs. If you provide invalid input to the control program, the program will just skip your turn. However, you are allowed to spy on other countries files, and all bot programs will be in the same folder at runtime. This is war, after all!

  • I reserve the right to disqualify any country that takes more than about a second to run, or that tries a loophole not mention within. That being said, if it is sufficiently clever I will probably let it go.

I will have source code up soon for a sample country that will be competing, and will post the control program when I finish it.

  • \$\begingroup\$ "In case of ties, all tied bots will revive 1 point". Is that supposed to say "receive"? "If all bots die on the same turn, ... all bots will receive 0 points." If there are two bots left, each of which has received 1 point from the earlier death of a third bot, and the two bots destroy each other on the same turn, what's the final score for the round? I'm not sure whether it's 0-0-0 or 1-1-0. "You are allowed to write bots that work together": but how can they identify each other? Do they have to use their moves as a covert channel? \$\endgroup\$ Aug 29 '14 at 14:28
  • \$\begingroup\$ "Support a player. This will give the player you support a boosted attack." Or defence. Might be clearer to say "boost that player's strength for the turn". Should also state whether or not it's possible to support yourself. \$\endgroup\$ Aug 29 '14 at 14:29

Check GenericScript source code for compiler errors

Given the source code for a GenericScript program as input, parse the source code to check that it conforms to the syntax rules for the language. The syntax definition for GenericScript is below. If a part of the source code is found to be invalid, the program should output "Invalid syntax", otherwise it should output "Valid syntax".

Win Criteria

This is code golf. Shortest code wins.


Source code will be considered to be valid if it matches the rule for "Program" below.

Program             = Sequence
Sequence            = Statement [Sequence]
Statement           = SequenceBlock | Assignment | If | While | Output
SequenceBlock       = "{" Sequence "}"
Assignment          = Identifier "=" (String | Bool); 
If                  = "if(" Bool ")" Statement ["else" Statement]
While               = "while(" Bool ")" Statement
Output              = "print(" String ");"
Identifier          = {Any sequence of alphanumeric characters prefixed with "var" }
Bool                = StringEquals | Identifier
StringEquals        = String "==" String
String              = StringConstant | OperatorConcat | Input | Identifier
StringConstant      = "'"StringContent"'"
StringContent       = Character [StringContent]
Character           = {Any character except for "'"}
OperatorConcat      = String "&" String
Input               = "read()"

Whitespace is defined as any sequence of the ascii characters 9, 10, 13 and 32. Whitespace characters are allowed between tokens but are not required.


  1. The answer should be a complete program
  2. Standard input/output allowed
  3. Standard loopholes apply
  4. Universally testable answers only

Test Input

Valid syntax:

print('What is your name?');
varInput = read();
print('Hello ' & varInput);

Invalid syntax:

if(read() == 'DoTask1')
  print('Executing you'r command');

Text Adventure Game


Your goal is to develop a complete text-based adventure game with the shortest code possible. The player navigates in a dungeon composed of rooms. The game objectives are to find the treasure, slain the dragon and rescue the princess.


A room description is as follows:

You are in <description>.
You can go <exits>
You see <object>      (optional)
  • exits can be "north", "east", "west", "south".
  • description can be "a adjective cavern", "a adjective room", "a adjective corridor", "a adjective hall", "a cell", "the dragon's lair".
  • adjective can be "dark", "murky", "small", "large", "narrow", "gloomy", "huge", "strange", "tiny", "broad", "old".
  • object can be "the princess", "the dragon", "a troll", "a goblin", "a sword", "gold", "a key", "a trunk".

Exit list must be comma-separated and end with "and". If there is no object in the room, the last line is omitted.

Example of valid description:

You are in a murky room.
You can go north, east and south.
You see a goblin.

The game accepts the following commands (case is ignored) :

  • GO direction : direction can be NORTH, EAST, WEST, SOUTH
  • TAKE item : item can be SWORD, GOLD, KEY
  • KILL monster : monster can be DRAGON, TROLL, GOBLIN. The DRAGON and the TROLL can be killed only if the user has the SWORD. If he hasn't, he loses the game. The weak GOBLIN can be killed with bare hands. When a monster dies, he disappears from the room. When the GOBLIN dies, he drops a SWORD. When the TROLL dies, he drops a KEY.
  • KISS person : person can be PRINCESS, DRAGON, TROLL, GOBLIN. Kissing the princess validates one of the objective of the game, and the princess disappears from the room. Kissing a monster results in player death.
  • OPEN object : object can be TRUNK. If the player has the KEY, the TRUNK object disappears and is replaced with GOLD.

The player can perform an action on an object only if the object is in current room. A room can contain only one object ; a given object can be found in only one room. At the beginning of the game, only the following objects are placed in the map : PRINCESS, DRAGON, TROLL, GOBLIN, TRUNK. Other objects are not yet created.


  • If an action cannot be performed (e.g. GO NORTH where there is no exit to the north, or TAKE DRAGON, or DANCE GANGNAM STYLE), the message "Sorry, I can't do that" must be displayed.
  • If an action can be performed, the message "OK" and the current room description should be displayed.
  • You can read game commands from console or as a program parameter, as you wish.

The dungeon should have at least 30 rooms. The dungeon should not contains a series of more than 5 exits in the same direction. The exits between rooms must be consistent, e.g. if you go north from room #1 to room #2, there must a south exit in room #2 leading back to room #1. Every room name should be unique. There must be at least one room of each kind (hall, cavern, corridor...)

  • A hall has at least 3 exits.
  • A corridor can have only 2 exits.
  • The cell has only one exit.
  • There is only one dragon's lair and only one cell, containing respectively the dragon and the princess.

The game ends when the player has been killed, or when he has taken the gold, slain the dragon and kissed the princess.

  • If the player dies, the message "You have been killed by X !" is displayed, with X being the name of the monster.
  • If the player wins, the message "Well done adventurer ! you've conquered the dungeon." is displayed.

Player should not be able to win the game in less than 40 turns.


You are in a murky room.
You can go north, east and south.
You see a goblin.
You are in a murky room.
You can go north, east and south.
You see a sword.
You are in a murky room.
You can go north, east and south.
You are in a narrow corridor.
You can go south and east.  


The shortest code wins.

  • \$\begingroup\$ @Martin Thanks for your comments! I've updated the question. \$\endgroup\$
    – Arnaud
    Aug 21 '14 at 7:59
  • \$\begingroup\$ Provided the player ignores the troll and goblin (i.e. doesn't try to kiss or kill them), they don't do anything? \$\endgroup\$ Aug 21 '14 at 8:30
  • \$\begingroup\$ @Peter you're right. Maybe the player should kill (with bare hands) the goblin in order to get the sword, and then kill the troll (with the sword) to get the key. \$\endgroup\$
    – Arnaud
    Aug 21 '14 at 8:50
  • \$\begingroup\$ "The map must be spatially coherent" still doesn't disallow always going left without ending up in the same room twice, unless you specify that the rooms are all meant to be square (which is what I think you had in mind). Also, I still think that "at least" 30 rooms is unnecessary. Who would implement 8 additional rooms if they don't have to. It will definitely be shorter if I omit the two longest adjectives and just use all available combinations of the remaining ones (giving 30 unique rooms). So you can omit two adjectives and the "at least" right away, I'd say. \$\endgroup\$ Aug 21 '14 at 9:19
  • \$\begingroup\$ I think it's fine to keep "at least" there for flavour, same with additional adjectives. Also, someone might figure out a way to make the code shorter with a longer adjective (for that reason, having a few more adjectives might be nice) \$\endgroup\$
    – FireFly
    Aug 21 '14 at 9:38
  • \$\begingroup\$ @Martin I've added a criteria "Player should not be able to win in less than 30 turns", to force the golfer to implement more rooms. \$\endgroup\$
    – Arnaud
    Aug 21 '14 at 9:39
  • \$\begingroup\$ @SuperChafouin That doesn't force it though. I just need to place the goblin at the end, troll at the beginning, trunk at the end, so that you need to traverse the map 3 times. \$\endgroup\$ Aug 21 '14 at 9:51
  • \$\begingroup\$ @Martin It's also here to prevent the dungeon to be too straightforward to solve, e.g. if all the objects are in 5 adjacent rooms near the player start location. \$\endgroup\$
    – Arnaud
    Aug 21 '14 at 10:51
  • \$\begingroup\$ +1 for golfing in Inform 7. \$\endgroup\$
    – Lopsy
    Sep 20 '14 at 2:56
  • \$\begingroup\$ Hello! This looks like a good but abandoned meta post, would you be willing to offer it for adoption? (If you want to, you can still post to main.) Due to community guidelines, if you don't respond to this comment in 7 days I have permission to adopt this. \$\endgroup\$
    – user58826
    Jun 9 '17 at 16:58
  • \$\begingroup\$ @programmer5000 Yes no problem :-) \$\endgroup\$
    – Arnaud
    Jun 10 '17 at 1:07

Simulate a Quantum Circuit

Work-in-progress until I can make sure I know what I am doing and can finish the spec. or maybe

Quantum computers are the way of the future! Why wait, when you can simulate one now?

Your mission is to determine the output of a quantum circuit given its input and a diagram of logic gates.


You will simulate a single quantum register and apply a series of quantum logic gates to it. A quantum register is a group of qubits. The state of a register is described by a vector of 2^N complex numbers, where N is the number of qubits in the register.


Above is a representation of a 3-qubit register. Each letter (a b c etc.) represents a complex number. There is an addition restriction that:

|a|^2 + |b|^2 + |c|^2 + |d|^2 + |e|^2 + |f|^2 + |g|^2 + |h|^2 = 1

Quantum gates

Gates are represented by a 2^N x 2^N square unitary matrix, where N is the number of input qubits. All quantum gates have the same number of outputs and inputs, since they neither create nor destroy qubits, they modify them.

A common quantum gate is called the Hadamard gate and acts on a single qubit. The matrix [H] looks like this:

1/Sqrt(2)  1/Sqrt(2)
1/Sqrt(2)  -1/Sqrt(2)

If we let [R] represent the following 1-qubit register:


Then the application of the gate is represented by [H][R] and gives the following result:


It is still true that the sum of the squares of the absolute values is equal to 1.

(TODO: explain how to apply gates to larger registers)


Measurement collapses the state of the quantum register.

(Todo: Explain how measurement works)





The goal of this challenge is to implement an AI for the game of BS, also known as Bull Shit, Cheat, Bluff, and numerous other names.

The game is outlined in this wikipedia article.

The Rules of the Game

For the purposes of this challenge, the game will work like this:

  1. A standard 52-card deck is dealt out to the players
  2. The current rank is set to Ace
  3. The play order is randomized
  4. The player holding the Ace of Hearts goes first
  5. On each player's turn:
    1. The current player plays some number of cards
    2. The current player states how many of what rank they played
    3. Other players may declare 'BS'.
    4. If any player declares 'BS':
      1. All players are notified of which players declared 'BS'.
      2. The played cards are revealed to all players.
      3. If the played cards are inconsistent with the current player's statement:
        • The current player adds the played cards and all cards in the pile to their hand
      4. If the played cards are consistant with the current player's statement:
        • The last player to declare 'BS' that round adds the played cards and pile to their hand.
    5. If no player declares 'BS':
      1. The played cards are added to the pile, without revealing them.
      2. If the played cards were inconsistant with the current player's statement, the current player may declare 'Peanut Butter'.
    6. If the current player has no cards in their hand, the current player wins.
    7. The current rank is incremented. (If the current rank is King, it becomes Ace.)

The Messaging Protocol

Play will be conducted via messages passed to the standard input and received from the standard output of each program. Each message will be terminated with a single newline character.


Card ranks are represented as one of A, 2, 3, 4, 5, 6, 7, 8, 9, T, J, Q, or K. Card suits will be represented as one of S, C, H, or D. Cards are represented as the rank, followed immediately by a suit. For instance, the Ten of Clubs would be represented by TC, and the Three of Hearts would be represented by 3H.

A hand of cards will be represented as a space-delimited sequence of cards. For instance, a hand containing the Queen of Spades and the Six of Diamonds could be represented as QS 6D or 6D QS.

Player Identification

A player will be represented by their nickname, followed by a number from 0 to 32768, in parenthesis, formatted as an integer. This number is guaranteed to be unique within a particular game. A player's nickname must have at least one character, can have up to 32 characters, and may only include letters, numbers, and underscores. For instance, a player with nickname ExampleAI and ID number 16480 would be identified in the game as ExampleAI(16480).

When the game begins, each program will recieve a message containing their unique ID:

Unique ID: uniqueID

Each player will reply with their desired nickname:

Nickname: name

Names may contain only alphanumeric characters and underscores.

After all players have responded with their nickname, the standard play sequence begins.

Standard Play Sequence

When a player's turn begins, each player will receive a be given a list of the players and their card counts, in order of play:

Players: player[count], player[count], ... player[count]

Each player will be informed of the contents of their hands:

Hand: initial_hand

The current the player will then receive this message:

Your turn: current_rank 

The current player will reply with a space-separated list of of cards:

Play: list_of_cards

Once they have submitted their play, all players will receive the number of cards, formatted as an integer, along with the current rank:

Player player plays: nunber_of_cards x current_rank

Each other player may then declare BS on that play by sending any message up to 32 characters, containing the capital letters B and S, and otherwise only contains lowercase letters and spaces. So any of Bull Shit, Bananna Split or Bacon Sandwich would be acceptable.

During this period, the current player may declare Peanut Butter by sending any message up to 32 characters, as long as it contains the capital letters P and B, and otherwise only contains lowercase letters and spaces. So any of Peanut Butter, Pancake Batter or Polish Bacon would be acceptable.

In order to allow the game to move faster, if a player does not wish to declare either of these things, they must instead send:


After all players have responded, all players will receive a list of players who called BS, in the order they called it:

Called BS: player, player ... player

If no player called BS, this message will still be sent --- it just won't have any players listed. If any player did call BS, then all players will recieve:

Player player had played: list_of_cards

If they were bluffing, all players recieve:

Player player was bluffing.

And the current player receives:

Your bluff was called: list_of_cards_recieved

If they were not bluffing, all players recieve:

Player player was not bluffing.
Player last_player receives the pile.

The last player who called BS recieves this message:

You misjudged: list_of_cards_received

The list of cards received will contain, in reverse chronological order, the contents of each play since the last call. (Separate plays will not be delimited in the list.)

If no player declared BS, and the current player was bluffing and declared Peanut Butter, then all players recieve the message:

Player player was bluffing.

If the current player has no cards left in their hand, all players receive this message, and the game terminates:

Player player won!

Otherwise, the next player's turn begins.

Example Game

The following might be considered a typical (abbreviated) message transcript:

Unique ID: 16481
> Nickname: Alice
Players: Alice(16481)[18], Bob(16479)[17], Charlie(16480)[17]
Hand: 2D 7S AS TC 5S JS JC 3C 8H 9D 5D AH 7C 6C 4D KC KH KS
Your turn: A
> Play: AS 2D AH
Player Alice(16481) plays: 3 x A
> PB
Called BS:
Player Alice(16481) was bluffing.
Players: Bob(16479)[17], Charlie(16480)[17], Alice(16481)[15]
Hand: 7S TC 5S JS JC 3C 8H 9D 5D 7C 6C 4D KC KH KS
Player Bob(16479) plays: 2 x 2
> BS
Called BS: Alice(16481)
Player Bob(16479) had played: 2H 2C
Player Bob(16479) was not bluffing.
Player Alice(16481) takes the pile.
You misjudged: 2H 2C AS 2D AH
Players: Charlie(16480)[17], Alice(16481)[20], Bob(16479)[15]
Hand: 7S TC 5S JS JC 3C 8H 9D 5D 7C 6C 4D KC KH KS 2H 2C AS 2D AH
Players: Alice(16481)[3], Bob(16479)[41], Charlie(16480)[8]
Hand: KC KH KS
Your turn: K
> Play: KC KH KS
Called BS: Charlie(16480), Bob(16479)
Player Alice(16481) was not bluffing.
Player Bob(16479) receives the pile.
Player Alice(16481) won!

Your implementation may be written in any language, provided that you, upon request, provide a link to a suitable free-as-in-freedom compiler or interpreter that I can download and run at no cost. You also need to provide a UNIX command that can start your program.

Sandbox Questions

I want to gauge the community's interest in my problem before finalizing the spec and writing the control program.

I also need to get some idea of what sort of time-limiting scheme would be reasonable. In order to be able to to a lot of runs, I will need to be able to ensure that each AI doesn't take too much time to make its decisions, or prevent a stuck AI from holding up a game. I also need to be able to ensure that there is no motivation to deliberately stall a game. For example, if an AI determines that it is very unlikely to win, it might stall in order to prevent the game from finishing.

I would also like feedback on the messaging protocol:

  • Are there any additional messages that you think should be passed?
  • Would it be more convenient/clear if one or more of them were formatted differently?
    • Would it be better to use a different format for the plays message?
    • Would it be better to use different words to help distinguish the plays and played messages?
  • \$\begingroup\$ It looks like quite a tough challenge, but should be enjoyable! \$\endgroup\$ Sep 4 '14 at 16:54
  • \$\begingroup\$ @PopeyGilbert By the way, there was one thing I accidentally left out that I need feedback on. Specifically, time limits - to deal with intentional stalling, getting stuck, or taking too long to decide. \$\endgroup\$ Sep 4 '14 at 17:05
  • 1
    \$\begingroup\$ A question and a feedback. Does our program run as "stop and run" or must receive feedback continuously? And for feedback. I honestly think that the whole username things is kinda confusing. Maybe if you just use only unique id? (Like just simple 0,1,2,3 instead of username) \$\endgroup\$
    – Realdeo
    Sep 5 '14 at 8:33
  • \$\begingroup\$ Oh! More things! I also realize that suit doesn't really matter, right? (We only use suit for deciding who goes first), so fmpov, you can ditch the communication protocol for the suit. (No need for S,C,D,H) We can just use simple random from the computer. Question: What will happened if everybody make infinity loop of pass. For time limit, I prefer 1 second. If no response, make it auto pass. (KOTH chess time limit is 2 seconds. That's why 1 second is good enough) \$\endgroup\$
    – Realdeo
    Sep 5 '14 at 8:38
  • \$\begingroup\$ @Realdeo Card suit is also used to distinguish between separate instances of a card. If Alice plays 3x2 (2S 2D 2C), is not revealed, and Bob gets the pile later, and then Bob plays 3x2 (2S 2C 2H), and this is revealed, it is important for Alice that she knows all four Twoes have passed through Bob's hand. There are other ways that can be used as well. \$\endgroup\$ Sep 5 '14 at 13:29
  • \$\begingroup\$ @Realdeo I am not sure what you mean by "Does our program run as "stop and run" or must receive feedback continuously?". If you mean, "Does an AI program halt in the periods where no response is expected from it?" the answer is no. \$\endgroup\$ Sep 5 '14 at 13:32
  • \$\begingroup\$ @Realdeo If everybody makes an infinity loop of pass, then eventually someone will run out of cards, since you are required to play at least one card each turn. \$\endgroup\$ Sep 5 '14 at 13:33
  • \$\begingroup\$ @Realdeo Which is why making a automatic pass after a timeout not work when waiting for a player to decide their play. Perhaps a simple rule like 'if you take more than 1 second to decide what to play, four cards are selected at random from your hand'. \$\endgroup\$ Sep 5 '14 at 13:36
  • \$\begingroup\$ And if a player has less then 4 cards? I think in some AI website, like aigames.com, they're like forced to give up that hand? You really want to test your entry before put them in the arena(like vsing a bot dummy?) Either way, this is a good challenge =) \$\endgroup\$
    – Realdeo
    Sep 5 '14 at 13:40
  • \$\begingroup\$ @Realdeo Also note that you can actually play more than four cards in one play. A case where you might wish to do this is when: the next player is very close to winning and some other player is close to winning and you believe(all opponents believe(your next opponent will bluff) and the next opponent will not bluff and the next opponent believes(the other opponent close to winning will call BS against them)). A little convoluted, but could happen. \$\endgroup\$ Sep 5 '14 at 13:42
  • \$\begingroup\$ @Realdeo Just to explain what I mean, is that there are two people close to winning, each of which would like to dump a large stack of cards on top of the other. Because of this, they both let your obvious bluff slide because they believe that will let them dump a large stack on the other. \$\endgroup\$ Sep 5 '14 at 13:47
  • \$\begingroup\$ Don't worry I understand. This is a really famous high school game in my country. It just... a little bit too complex for CR. When I saw chess KOTH, I was kinda pessimist. This one? This may deserve it's own AI website. #seriously. I'm just trying to simplify this game =) \$\endgroup\$
    – Realdeo
    Sep 5 '14 at 13:50
  • \$\begingroup\$ Let us continue this discussion in chat. \$\endgroup\$ Sep 5 '14 at 13:50
  • \$\begingroup\$ RE the messaging service, I think the other players should be able to see how many cards the other players have. Also, card counting should be prohibited because that would make the game too easy. \$\endgroup\$
    – Beta Decay
    Sep 10 '14 at 17:31
  • \$\begingroup\$ @BetaDecay First off, according to the protocol, every player is informed of every other player's hand size at the beginning of every round. \$\endgroup\$ Sep 10 '14 at 18:27

Let's talk about this...

I am planning on hosting a King of the Hill challenge in which bots will have to coordinate each other in order to be successful. The idea is to play a Diplomacy-like game between bots. The engine (still in development) will start the bots and communicate with them via stdin/stdout. There will be three phases:

0. Initialization

Well, this is not a recurring phase, it is just the engine telling each bot his id, the total number of bots participating and a seed, which can be used for generation of pseudo-random numbers (bots need to be deterministic).

1. Talking Phase (10s)

In the Talking phase, bots can send messages to each other (via engine) in order to coordinate their actions. To this end, a common language is necessary. This language should be able to express any ideas, plans and opinions a bot could have. However, not every bot is forced to be able to understand everything. Simpler bots might just ignore messages they do not understand.

Since I would like each player to be able to submit more than one bot, it is forbidden to implement a "secret handshake" by which bots recognize each other and from then on work together unconditionally.

2. Planning Phase (2s)

In this phase, bots submit what they want to do this turn. Each bot has a certain amount of supply (initially five), and can command one action per supply point. There are three possible actions:

  1. Attack another bot
  2. Support another bot's attack against a third bot
  3. Defend another bot

There are some restrictions:

  • Per opponent, you can either attack or defend them, and only once
  • You cannot support an attack against a bot you also defend
  • You cannot attack, defend, support yourself or a dead bot, and neither can you support attacks against yourself

3. Resolution Phase (as short as possible)

After all orders have been submitted, the engine will resolve them simultaneously in the following way:

The defending strength of each bot is the number of bots defending that bot. The attacking strength of each attack is the number of support orders for that attack. Each attack with an attacking strength greater than the defending strength of the attacked bot results in the supply counter of the attacked bot being reduced by one, and the supply counter of the attacker being increased by one.

Support orders which support a non-existent attack do nothing.

Then, all bots with supply of zero or less will be shut down by the engine: they died.

Afterwards, all remaining bots are informed about the decisions of other bots, and a new turn begins with its Talking Phase.

Further Rules

A game will consist of ten plus random number turns, so that "last turn betrayals" are not possible. The supply count of each bot will count towards their total score. I plan an ensemble of about 100 games. The bot with the highest total score wins. Tie-breaker will be the popularity (number of votes).

I am interested in your opinion: do you think that this challenge is too complex? I imagine that the code of a decent bot would be too long to fit in a post. So people would have to use github or pastebin or similar to submit their entries. The main problem imo is the interpretation of the (yet to be determined) common language.

  • \$\begingroup\$ I like it a lot. One possible variation would be to make the "secret handshakes" a feature. To do this, you could allow multiple instances of the same bot. Then part of the challenge is to recognise your own kin and mutually support them; and a viable strategy is to try and work out other players' secret handshakes and imitate them. If you're ok with emphasising this aspect of it, then you can make the shared language pretty unrestricted, e.g. bots can just send arbitrary strings to each other. (I realise this is not what you have in mind, I just thought I'd mention the idea.) \$\endgroup\$
    – Nathaniel
    Oct 31 '14 at 15:13
  • \$\begingroup\$ @Nathaniel, what you propose is a battle of obfuscation/cryptography. What I would like to see is a battle of diplomacy. \$\endgroup\$
    – M.Herzkamp
    Nov 2 '14 at 14:27
  • \$\begingroup\$ Fair enough - I just thought I'd say it in case it sparked any interesting thoughts for you, but I knew it was probably too different from what you want to see. If I have any other ideas about your challenge I'll let you know. Designing the language really seems to be the hard part. \$\endgroup\$
    – Nathaniel
    Nov 3 '14 at 0:06
  • \$\begingroup\$ A diplomatic KOTH is something I've been wanting to see for a while. Working out the specifics of the "diplomatic language" is going to be the most difficult part. My proposal is that each message can either 1) state an intention to another bot, or 2) request an action from another bot. Each message would be formatted in a way similar to how a final command would be. \$\endgroup\$
    – PhiNotPi
    Nov 9 '14 at 14:45
  • \$\begingroup\$ @PhiNotPhi: Thanks for the Feedback! I also imagined something similar with the ability to link atomic statements in a boolean fashion. \$\endgroup\$
    – M.Herzkamp
    Nov 11 '14 at 9:23

Happy Holidays!


With the holidays upon us, I decided to make an appropriately themed challenge. You are provided with a list of holidays and their respective date ranges, and given a date, you have to output a holiday greeting or the time remaining until the next holiday as appropriate.


The list of holidays is below. You have to include it in your program (so no using a library or other external resource for this). Feel free to use any convenient format.

Start  | End    | Name
------ | ------ | -------------------
Dec 6  | Dec 7  | Saint Nicholas' Day
Dec 13 | Dec 14 | Saint Lucy's Day
Dec 24 | Dec 27 | Christmas
Jan 1  | Jan 2  | New Year
Jan 6  | Jan 7  | Epiphany
Feb 14 | Feb 15 | Valentine's Day

You are given a date as input (STDIN, function argument, or anything convenient) in YYYY-mm-dd HH:MM:SS format (e.g.: 2014-12-30 11:15:00).

You may assume that the time zone is either UTC or the system's time zone. The holiday lasts from 00:00:00 on the start date (inclusive) to 00:00:00 on the end date (exclusive).

If the date falls within the range of the holiday, you must output Happy <holiday>!, except if it's Christmas, in which case you must output Merry Christmas!.

If it doesn't, but another holiday is coming at most a week in the future, you must output:

<time> left until <holiday>.

where <time> is in the following format:

<days>d <hours>h <minutes>m <seconds>s

You can't use a library for converting the time to that format.

If there are no whole days, hours, minutes or seconds remaining, omit the number entirely. For example, 1d 0h 3m 4s should be printed as 1d 3m 4s.

If there are no upcoming holidays, you must output (no pun intended):

There are no upcoming holidays.

A trailing newline is optional, but be consistent in your program—don't add a trailing newline in one case and omit it in another.

Standard loopholes are obviously forbidden.

Test cases

Date                | Output
------------------- | ----------------------------------
2014-12-05 23:59:59 | 1s left until Saint Nicholas' Day.
2014-12-06 00:00:00 | Happy Saint Nicholas' Day!
2014-12-06 12:00:00 | Happy Saint Nicholas' Day!
2014-12-06 23:59:59 | Happy Saint Nicholas' Day!
2014-12-07 00:00:00 | 6d left until Saint Lucy's Day.
2014-12-14 00:00:00 | There are no upcoming holidays.
2014-12-24 00:00:00 | Merry Christmas!

Note that your program must work for any year, not just 2014.


This is code golf, so the submission with the fewest number of bytes wins. An answer will be accepted after a week, but I'll be happy to change the accepted answer if a new valid submission beats the previous high score.

  • \$\begingroup\$ How do you expect people to test the test cases? It would probably be better to take input than to use the current time, because then it actually makes sense to talk about test cases. You should check date for duplicates, and if there are none you should add that tag. \$\endgroup\$ Dec 29 '14 at 14:51
  • \$\begingroup\$ @PeterTaylor You're right, I'll do that. \$\endgroup\$
    – nyuszika7h
    Dec 29 '14 at 15:37
  • \$\begingroup\$ @PeterTaylor I couldn't find any exact duplicates, only two holiday-themed questions, both of which ask for much less than my challenge. \$\endgroup\$
    – nyuszika7h
    Dec 30 '14 at 11:28

Find the Minimum Width of a Set of Points

Given a set of points in 2D space, you're to find the direction along which those points occupy the shortest width.

More formally, consider a set of n points P = {p1, ..., pn}, where pi = (xi,yi), and a unit vector d = (xd, yd). Now K is the set of lengths obtained from orthogonal projection of P onto d. In particular, ki = xixd + yiyd. The width L of P along d is defined as max ki - min ki. Your task is to find the d along which L is minimal.

To keep things interesting, your algorithm's time complexity must not exceed O(n log n).

You may write a program or function, taking input via STDIN, command-line argument or function argument. The result may be printed to STDOUT or returned.

You can expect the input P to be in any convenient list or string format, but the input must not be pre-processed (e.g. sorted by coordinates). You may assume that the input contains at least 2 points and that no two points coincide.

The output must be correct to 10 significant (decimal) digits. Of course, d is only unique up to the relative sign of the coordinates, so there are two correct answers for each input. You may return either of those.

You must not use built-in functions related to this problem, like finding the minimum width of a polygon, or computing the convex hull of a set of points. You may use built-in vector/matrix types and operations.

Sandbox Notes

  • I'll write my own solution at some point next week, and use it to provide a number of test cases.
  • I'm also planning to add a handful of diagrams to clarify the definitions.
  • The challenge was inspired by this proposal from Calvin's Hobbies, I think they are sufficiently different, as this problem here is only one approach to tackling his challenge (and even then it's only a subproblem). But if people think, they are too similar, and posting this one would make his a duplicate in the future, I'll retract this challenge (as I'd really like to see his posted some time).
  • \$\begingroup\$ I hope you're still planning on posting this. Just a few notes: (a) The minimal direction is not generally unique (e.g., if we have a regular polygon, or just a single point). You'd might want to make the actual (scalar) width the output instead. (b) I assume the input is never empty? (c) Can it contain duplicates? \$\endgroup\$
    – Ell
    Jan 3 '15 at 16:01
  • \$\begingroup\$ @Ell yes I still want to post this. Just didn't get around to writing the reference yet. a) good point. I'll think about asking for the width vs asking for any minimal direction. b) yes, will clarify. c) I'll think about that. Probably not. \$\endgroup\$ Jan 3 '15 at 16:07
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