This "sandbox" is a place where Code Golf users can get feedback on prospective challenges they wish to post to main. This is useful because writing a clear and fully specified challenge on your first try can be difficult, and there is a much better chance of your challenge being well received if you post it in the sandbox first.

Sandbox FAQ


To post to the sandbox, scroll to the bottom of this page and click "Answer This Question". Click "OK" when it asks if you really want to add another answer.

Write your challenge just as you would when actually posting it, though you can optionally add a title at the top. You may also add some notes about specific things you would like to clarify before posting it. Other users will help you improve your challenge by rating and discussing it.

When you think your challenge is ready for the public, go ahead and post it, and replace the post here with a link to the challenge and delete the sandbox post.


The purpose of the sandbox is to give and receive feedback on posts. If you want to, feel free to give feedback to any posts you see here. Important things to comment about can include:

  • Parts of the challenge you found unclear
  • Comments addressing specific points mentioned in the proposal
  • Problems that could make the challenge uninteresting or unfit for the site

You don't need any qualifications to review sandbox posts. The target audience of most of these challenges is code golfers like you, so anything you find unclear will probably be unclear to others.

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4670 Answers 4670

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109 110

Is the line closed?

Given input consisting of only whitespace characters and -/\|, tell if the line created by -\/| is closed.

For this challenge, \ points one character up and to the left, as well as one character down and to the right. / points up and to the right, and down and to the left. | points one to one character above and below it. - points one character to the left and right of it.

# means it is being pointed to.





A line is a bunch of /\-| characters that point to each other.

\    |   |
 \---/   \

The characters - and | cannot validly point to each other. Similarly, / and \ cannot point to each other either.





A closed line is one where each of its characters validly point to another of its characters.

|  |
\--/ = CLOSED

|  |


Your task, as said above, is to write a program that says if a line is closed.

Test cases coming soon.

  • \$\begingroup\$ Your -/ connections violate your own specification. I think the clearest specification would be to enumerate all possible legal pairs of connected line segments. \$\endgroup\$
    – user62131
    May 8, 2017 at 13:47
  • \$\begingroup\$ @ais523 My mistake, I will fix both this and the other. \$\endgroup\$
    – sporkl
    May 8, 2017 at 14:14

Determine if a relation is an equivalence relation

(Content to be added.)



Uncompress a permutation

I have a number of permutations, of which this is an example:

[0] 1 2 3 15 14 26 27 28 29 30 31 32 33 5 4 16 17 18 19 20 21 22 23 35 34 6 7 8 9 10 11 12 13 25 24 36 37 38 39 [40]

(0 is used for 0-indexed permutations and 40 for 1-indexed permutations.) In order to save space, I want to compress the permutation by reducing each ascending run of consecutive numbers to its first element, giving:

0/1 15 14 26 5 4 16 35 34 6 25 24 36

This compression method is invertible, because each missing element from the compressed form must follow its predecessor among the integers.

Your challenge is to write a program or function that takes a compressed permutation as input and returns an uncompressed permutation. Your input will consist of an array or list or other reasonable type representing the compressed permutation, plus an integer number of elements in the uncompressed permutation. Examples:

[0], 2 -> [0, 1] (0-indexed)
[1], 2 -> [1, 2] (1-indexed)
[6, 2, 0], 8 -> [6, 7, 2, 3, 4, 5, 0, 1] (0-indexed)
[7, 4, 1], 9 -> [7, 8, 9, 4, 5, 6, 1, 2, 3] (1-indexed)

In order to make the compression worthwhile I obviously need as short a decompression routine as possible, so this is a challenge.

  • \$\begingroup\$ I don't think there's actually a specification of the decompression routine, and the one which I would reverse engineer from the examples at the end is not compatible with the first example. \$\endgroup\$ May 8, 2017 at 14:34
  • \$\begingroup\$ I believe the specification is just "for each number that doesn't appear in the compressed permutation, place it to the right of that number minus 1". It might be worth specifying that in the question. \$\endgroup\$
    – user62131
    May 8, 2017 at 15:11
  • \$\begingroup\$ @PeterTaylor The specification is "Create a permutation of the first n integers which, when compressed by removing consecutive numbers after the first, results in the input array/list". \$\endgroup\$
    – Neil
    May 8, 2017 at 18:33
  • \$\begingroup\$ I've made an edit which I think makes that clearer. \$\endgroup\$ May 8, 2017 at 21:24

An implementation of a game called Nomic

Nomic is a game where you can change the rules. Humans are biased judges though, so let's have a computer run the rules! (Note that there are many variations of Nomic. Below is just one example.) Since you're minimalists, you want the original ruleset to be as short as possible.

Your program first accepts as input a list of players. You must be able to accept any list of 2 to 16 unique alphanumeric usernames (you may assume that the list of players names are valid according to this criteria).

Next print The name of this game is nomic. The goal is to get the last line of output of this program to be "[username] is the winner!", where [username] is your username.

Now, for each player print [username], do you want to propose a revision to the rules? If so, type the revised code below: where [username] is the player's username. (This input must be able to accept multiple lines.) If the input is empty, proceed to the next player.

If not, for each player print [username], do you want to accept or reject this change to the rules?(y/n), where [username] is the player's username. (You may assume input to this will either be y or n.) If the number of ys are more than the number of ns, execute it (in the language you are writing the implemenation). If not, proceed with the next player.

If the code executes without an error, the program ends. If not print The ruleset proposed by [username] has crashed. The game will proceed with the original ruleset. where [username] is the players username. Then proceed to the next player after them.

Note: After all the players have either proposed a rule or passed, start with the first player again.

This is . The shortest Nomic implementation wins! (You get 0 characters off your score if you show an example game you played with your friends. This bonus is doubled if all the revised rulesets are golfed.)

  • \$\begingroup\$ You say a computer has to "run the rules", so is each rule typed by a user an executable piece of code? Basically, your description of what the implementation needs to look like is extremely vague. Be specific. \$\endgroup\$
    – mbomb007
    May 12, 2017 at 16:55
  • \$\begingroup\$ Having long hardcoded strings in a challenge is generally a bad idea unless it's a compression challenge, as the question will be mostly about compressing the strings rather than about what it's meant to be about. One possibility is to not count the strings against the byte count if they appear literally in the code. \$\endgroup\$
    – user62131
    May 12, 2017 at 17:01
  • \$\begingroup\$ @mbomb007 the rule should be a piece of executable code. If it isn't, the program announces the error. \$\endgroup\$ May 12, 2017 at 17:14
  • \$\begingroup\$ @ais523 that's a little tricky since it involves a little bit of string splicing. Is there a way around that, or should I just shorten them? \$\endgroup\$ May 12, 2017 at 17:15
  • 3
    \$\begingroup\$ Probably best at this point to just use codes for the various situations, e.g. using just V to request someone to vote. \$\endgroup\$
    – user62131
    May 12, 2017 at 18:00

Output the Balmer Series


Write a program which outputs the following image (note, it is an svg):

It is the emission spectrum of hydrogen, caused by the Balmer series: https://en.wikipedia.org/wiki/Hydrogen_spectral_series#5

Your program must not read the image from the internet or a local file.

The image may be to an image file or a graphics window. If not a vector, the image sides must be greater than 400 pixels.

Your image must have the same colours and same ratios of spacing between each line.


Shortest code wins.

  • \$\begingroup\$ What does it mean to output the image "exactly"? It's obviously not pixel-for-pixel for two reasons: firstly, because the image isn't raster, and secondly, because the spec allows for variation in image size. \$\endgroup\$ May 12, 2017 at 22:22

Compute Area of Convex Hull of Roots of Polynomial

Given a nonzero polynomial with integer coefficients, find the area of the convex hull of the complex roots of the polynomial.


The polynomial can be given in any reasonable format, e.g. as a list of coefficients. The area must be correct to 4 significant figures. You can assume that the degree of the polynomial is no larger than 8, and that the coefficients are less than 8^8 = 16777216, (and by Rouché's theorem the roots have therefore an absolute value of less than 8^8 + 1).


The polynomial x^4 - 1 has the roots {1,-1,i,-i} which form the vertices of their convex hull, which is a square of sidelength sqrt(2) so the total area is 2.

The polynomial x^5 - x has the roots {1,-1,i,-i,0}. But the convex hull is again given by the vertices points {1,-1,i,-i}.

The constant polynomial 1 has no roots, and therefore the convex hull of its roots is empty.

The polynomial x^2 - 1 has the roots {1,-1}, so their convex hull is just a line segment, which has no area.

In the following the polynomials are given as a list of coefficient of the monomials in descending degree. (i.e. the constant comes last)

[1,2,3,4,1] 2.0382
[1,2,3,4,5] 3.5830
[1,-1,1,-1] 1.0000
[a,b] 0 (where a != 0 and b arbitrary)
  • \$\begingroup\$ For what range of inputs must the area be correct to 4s.f.? \$\endgroup\$ May 13, 2017 at 14:43
  • \$\begingroup\$ Oh I forgot to add that, thanks for mentioning! \$\endgroup\$
    – flawr
    May 13, 2017 at 14:44

Hidden Strings

This is real rough, I just wanted to get the idea down.

The strings utility that comes on many GNU/Linux distros finds the strings in executables and lists them. The goal of this challenge is to write a program that has a null-terminated string in the compiled executable (compiled languages only, sorry) but that string does not appear in the source. The scoring is the ratio of the length of the hidden string to the size of your source code.

The following constraints apply:

The string must be entirely dictionary words from todo dictionary with length greater than 2 concatenated together without spaces and without punctuation. If xyz and abc are words in the dictionary, the following is a valid string: xyzabcxyzabc, but the following is not xyabcz (assuming that it is not a concatenation of other strings in the dictionary). For those of you who care, if E is the dictionary of words of length greater than 2, then strings must be in E* where * is the Kleene star.

"Hidden" means that the string and its substrings may not appear as strings or characters in your source. This definition needs a bunch of work.

This challenge was supposed to be something like "Hey, how would you hide a funny string so that it doesn't appear obviously in the source, but does appear when someone looks at the executable". I welcome all suggestions because as I have it currently stated, I think there are a lot of problems.

  • \$\begingroup\$ I'm guessing that a minimal C program already contains a certain number of strings in the binary? Does this come down to creative you get with compiler flags, asking it to statically link as many libraries as possible? \$\endgroup\$ May 15, 2017 at 4:28
  • \$\begingroup\$ I suggest specifying a particular string that has to be hidden, in order to prevent solutions based on the default behaviour of the toolchain. (This would also simplify the question somewhat) Additionally, that string should probably be banned from thing like the filename and the current directory. \$\endgroup\$
    – user62131
    May 15, 2017 at 17:58

Write a TIO bot

Write a program/function which:

  1. Takes three strings Language, Code, and Input
  2. Do one of the following:

    1. Assume access to a browser open on https://tio.run/nexus/; click [data-id="Language"
  3. Go to https://tio.run/nexus/Language/

  4. Enters Code in the <textarea id="code">
  5. Enters Input in the <textarea id="input">
  6. Clicks the <span id="run">
  7. Waits for any of these things to happen:
    1. <textarea class="output"> changes
    2. <textarea id="debug"> changes
    3. 60 seconds pass
  8. Returns/prints the content of <textarea class="output">

Alternatively, you may use the TIO API.

  • 1
    \$\begingroup\$ I'd allow people to use the API, instead of using the HTML. \$\endgroup\$ May 15, 2017 at 15:58
  • \$\begingroup\$ @NathanMerrill Added. But good luck finding out what it is. \$\endgroup\$
    – Adám
    May 15, 2017 at 16:23
  • \$\begingroup\$ The problem with the API is that it's bound to change until it becomes stable. Incidentally, this is why I'm not documenting it at this point, as nobody should use it while it's still in flux. \$\endgroup\$
    – Dennis
    May 15, 2017 at 18:20

Convert ambiguous dates to "the correct way" (YYYY-MM-DD)

A problem I run into often with spreadsheets of data is dealing with dates of unknown ordering.

XKCD says it best

Your task is to take a series of dates that are in one of the following three formats, and output them as YYYY-MM-DD:


No other date formats are acceptable.

Since many dates are ambiguous, you must read all the dates to determine whether the set is MM/DD or DD/MM before outputting anything.

Your program must also detect the following error conditions and return an error code instead (with no other output):

  • -1: Unable to determine date ordering. (All dates provided could be DD/MM or MM/DD)
  • -2: Inconsistent date ordering. (There are dates provided that are definitely DD/MM and others that are definitely MM/DD, within the same set).
  • -3: Bad data. (There are things included that are not valid dates, as defined above, including things like 31/04/2017 and 12/1/1980).

For the purposes of this challenge, February always has 28 days, April, June, September and November have 30 days, and all others have 31 days. All years, from 0001 to 9999 are equivalent.

Sample input/output

['11/05/1988','12/12/1960'] => -1
['15/05/1988','03/18/2001'] => -2
['15/05/1988','31/06/1989'] => -3
['15/05/1988','2014-02-02', '18/03/2001'] => ['1988-05-15','2014-02-02', '2001-03-18']
['05/15/1988','03/18/2001'] => ['1988-05-15','2001-03-18']


You can receive data in whatever way is normal for your language (line-separated stdin, array of strings, whatever). You may assume that the data you receive is of the correct type (ie, your array won't contain integers, nulls etc), but may contain empty strings.

If outputting to standard output, errors can be sent to stderr, or as program termination code. If implementing a function, you can return an integer instead of an array, throw an error, or whatever seems natural.


The score for your program is L - 2Y + 20D where:

  • L is the length of your code, in bytes
  • Y is 2017 minus the year in which the first version of the language you're using was published.
  • D is the number of times the letters 'date', in any case, appear consecutively in your code.

For the purpose of Y, all versions of Lisp are 59, C is 39, C++ is 32, all EcmaScripts are 20 etc. Languages are considered the same if they have the same name (other than the version number). (No cheating by using some golf language that happens to be called C though.)


(what else?)

  • 1
    \$\begingroup\$ I think your scoring mechanism isn't going to work. It's probably best you stick to plain old shortest code. \$\endgroup\$
    – Okx
    May 16, 2017 at 11:33
  • \$\begingroup\$ Any particular reason why? \$\endgroup\$ May 16, 2017 at 22:11
  • \$\begingroup\$ Possible dup \$\endgroup\$ May 16, 2017 at 23:17
  • \$\begingroup\$ @SteveBennett If your score decreases by 20 each time 'date' is in the code, then if someone fits an infinite amount of 'date' into a comment, they win. \$\endgroup\$
    – Okx
    May 17, 2017 at 9:28
  • \$\begingroup\$ It increases, not decreases...? \$\endgroup\$ May 17, 2017 at 11:21
  • \$\begingroup\$ @DigitalTrauma They're both in the same ballpark but a lot of differences, including fundamental task (reformatting vs outputting a single probability), input formats (strict mm/dd/yyyy vs m-d-yy) etc. \$\endgroup\$ May 17, 2017 at 11:24

Be the Richest Mammut!

The Game

This game is an adaptation of a board game called Mammut, and basically an optimization problem.

Each match contains five players and five rounds, in which every player participates. Every round starts with a bunch of tiles being flipped at random on one of their sides, and put in the middle stack The tile composition will be fixed, but has not been completely decided yet There will be 30-35 of them, and each of them will have two different sides, with exactly 15 different types of tiles, and about half of those only having one single instance available. A random player is chosen as the starting player for the first round, and performance will decide the starter for the following four rounds. In order, each player that doesn't own any tiles will take an action, and this will keep going on until every player has at least one tile. The actions available are:

  1. Take an arbitrary number of tiles from the middle stack.
  2. Take all of the tiles owned by another player, and put one of them back in the middle stack.

Obviously, you can take the entire middle stack, but if there is at least one other player without tiles and you take too many, he will most likely steal your tiles right after.


Every player starts with 10 points, and if they end a scoring phase under zero, they will be given enough free points to go back to zero.

When everyone has tiles, the round is over, and we move on to scoring that round. Since this is played by bots and not humans, an infinite loop is possible, so after 1000 actions, the round will be stopped by force, and the players participating in the infinite loop who ended up empty ended will naturally end this round with a terrible score.

Scoring happens all at once, and you get (or lose) points according to the tiles you have, and the tiles the other players have. When scoring is done, we move on to the next round, or declare the winner if that was the fifth round.

The Tiles

There are different types of tiles, and you job is basically to find the best balance between them while not having enough to make your stack worth stealing.

  • Fire | The player who has the fewest fire tiles loses 5 points
  • Meat | The three players owning the most meat tiles get 8, 5 and 2 points respectively
  • Fur | Every fur tile gives you onw "fur point". During every round after the first, the player with the most fur points plays first. At the end of each round, the player with the most fur tiles gets the round number in points (1-5), while the one who has the least loses the round number in points.
  • Ivory Tile | Every Ivory tile gives you two points.
  • Axe Tile | (This one should be discussed) Every axe tile gives you one point, and every axe Tile obtained in the last round is Worth 3 points
  • Animal Tiles | There are 7 different animals (Bat, Opossum, Bunny, Buffalo, Squirrel, Beaver and Duck), and only one of each in the game. When you end the round with one, you "obtain" this animal, and getting the same one again will do nothing. At the end of the game, depending on the number of animals you have obtained, you get points. (0: 0, 1:0, 2:0, 3:4, 4:9, 5:15, 6:22, 7:30)
  • Mystery Tile | If you get a mystery tile, it will be turned around to reveal its other side during the scoring phase. The other side can't be another mystery tile or an animal tile.
  • Tie-Breaker Tile | This tile serves as a tie breaker. One side is the "clockwise" side, and the other side is the "counter-clockwise", so every round will contain a tie-breaker tile. During any and all ties during scoring, the tie-breaker tile decides who wins or loses.


At the start of the game, every player will be given a random ID between 1 and 5. The player with ID 1 will start the game. Then, for the following rounds, the most fur points decides who starts, and the ID decides the order. Example: ID 3 has the most furs, then the order will be [3, 4, 5, 1, 2].

This is considered "clockwise". The counter-clockwise order would be [3, 2, 1, 5, 4]. This is important, because when dealing with ties, the tie-breaker tile takes the order into account.

If player 4 owns the clockwise tie-breaker tiles, and has 3 meats. Player 1 and 2 also have three meats. The first, second and third place in meat then go to players ID 4, 1 and 2 respectively. If everyone has 3 fires, except player 3 and 5 who have 1, then player 3 is the one to lose points. Having the tie-breaker tiles means you are first in the tie-breaking order, and that tile's visible side decide the rest of the order.

Challenge specifications

You must make a C# class that inherits from a "Player" class. You must also override the method

List<int> ChooseTiles(Dictionary<int, List<Tile>> otherPlayers, List<Tile> center)

This method takes a dictionary of player IDs and List of tiles (could be empty), that are each player's tiles at the moment, plus a list of tiles in the middle. You must return a list of integer, which are the IDs of the tiles you want. If you try to get tiles illegally (not following the instructions for taking an action, which are either any number of tiles from the middle or all tiles except one from a player), you will be asked to choose one more time, and if you fail to follow the rules, you will be counted as "attempting to cheat" and removed from the "playing" pool during this round, so you will get scored for an empty hand. The same thing goes for throwing exceptions and infinite loops (Execution lasting more than 5 seconds). You are given one more chance then given an empty hadn for that round.

Here is the class Player you will be inheriting from:

public class Player
    public int ID;
    public int currentPoints;
    public int currentFurs;
    public List<Tile> currentTiles;
    public List<TileType> animalsGotten;

    public Player(int id)
        ID = id;
        currentPoints = 10;
        currentFurs = 0;
        currentTiles = new List<Tile>();
        animauxGotten = new List<TileType>();

    public virtual List<int> ChooseTiles(Dictionary<int, List<Tile>> otherPlayers, List<Tile> center)
        throw new Exception("Fake players don't get to play!");

    public virtual string GetName()
        return "Player";

You should also override the GetName() function, but it is not required. You are allowed to create as many private functions or attributes as you want to store data during a game, but each match will create a new instance of your player.

The Tournament

tournament will be played in a round-robin fashion. You are not garanteed a match with every single combination of four opponents available, but you are garanteed at least one match in common with every other bot, and everyone will play the same number of matches. You will also not play twice with the same 5 players. The total number of matches will depend on the number of participants. For each match, the first player will get 5 points, and the others will get respectively 3, 2, 1 and zero points depending on their points earned during the match. Furthermore, and ties in points will not be resolved, and the points of the occupied positions split between tied players. This means that if the points are (40, 37, 37, 37, 35), the points are (5, 2, 2, 2, 0)

  • \$\begingroup\$ People usually don't like KotH challenges that are restricted to one language. See some other KotH challenges for ways to get around that. \$\endgroup\$
    – Stephen
    May 16, 2017 at 13:47

Extension of MIME-Type

Your task, given this list of extension and MIME-types, is to choose exactly 20 of them and create a program that takes in either e, the extension, or m, the MIME-Type, and outputs its counterpart.

Example (if my set was [.pdf, .doc]):

Cases to handle:

f('.pdf') = application/pdf
f('application/pdf') = .pdf
f('application/msword') = .doc
f('.doc') = application/msword

Cases you don't have to touch:

f('Nonsense') = [Error, null, false, whatever]

I & O

  • Input will be a single string, so will output.
  • You must handle 20 mime-type combinations and mention which ones you are handling.
  • Trailing spaces and newlines are fine.
  • Extensions always include the preceding period.
  • When choosing a long extension like .cryptocode, if you return the MIME-type of .cryptocode for an input of .cry it will be considered an invalid submission, remember this.


  • 3
    \$\begingroup\$ kolmogorolofvlfoao-complexity??? \$\endgroup\$
    – user58826
    May 17, 2017 at 0:06
  • \$\begingroup\$ Be sure to include the list in the question itself when you post this on main. We want questions to be self-contained. \$\endgroup\$ May 17, 2017 at 3:53
  • 2
    \$\begingroup\$ "When choosing a long extension like .cryptocode, if you return the MIME-type of .cryptocode for an input of .cry it will be considered an invalid submission, remember this." - why? Do I have to detect if the input is one of my set? This will reduce my compression ratio significantly. \$\endgroup\$ May 17, 2017 at 3:56
  • \$\begingroup\$ @JanDvorak it's too big for posting here, and if pastebin goes does a lot of the internet would lose a lot of data. \$\endgroup\$ May 25, 2017 at 17:04
  • \$\begingroup\$ @programmer5000 you can post it if you want. \$\endgroup\$ Aug 27, 2017 at 2:59
  • \$\begingroup\$ Concerning the "exactly 20" requirement: Are we allowed to output anything we want if fed something not in that 20 or does it need to output a falsey value? The addition of the word "whatever" added confusion for me. \$\endgroup\$ Mar 13, 2018 at 14:22
  • \$\begingroup\$ @programmer5000 Tab-separated version for OP's benefit. \$\endgroup\$ Mar 13, 2018 at 14:31

I have a large collection of fine art. My toddler is learning to use scissors and glue; lately she has started playing with my art collection. Fortunately she is really quite good with the scissors and cuts things up into perfect squares. She then uses glue to randomly tile the cut-up squares back into a new piece of art. For example, she reinterpreted my Mona Lisa (which wikipedia kindly hosts for me) as follows:

enter image description here

The following python script simulates what my toddler has done:


import sys
import random
from PIL import Image

origname = sys.argv[1]

im = Image.open(origname)
width, height = im.size
width = (int(width + 99) / 100) * 100
height = (int(height + 99) / 100) * 100

im = im.crop((0, 0, width, height))

im2 = Image.new("RGB", (width, height), "black")

blocks = []
for x in range(width / 100):
    for y in range(height / 100):
        blocks.append(im.crop((x * 100, y * 100, (x + 1) * 100, (y + 1) * 100)))


im2.save("shuf" + origname)

Please excuse python skills - I'm still learning, but was happy to see how quick and easy it was to write this script. Polite code-reviews will be graciously accepted ;-)

It does the following:

  • loads the image whose file name was given as a command-line parameter
  • pads that image with black pixels such that the width and height are exactly divisible by 100
  • divides the image into 100x100 pixel blocks
  • randomly shuffles the blocks
  • reassembles the randomly arranged blocks back into a new image with the same size attributes as the (padded) original
  • saves the new image using the original filename prefixed with shuf

Your task is to write a program that takes the output of this script, analyses the edges of each 100x100 block and reassembles them back to the original picture.


  • an image filename. This may be passed at the commandline, via STDIN or even hard-coded, whichever is most convenient.


  • either output the a singular rearranged image to a differnent file, or display the singular rearranged output to the screen.

Input and output specs are intended to be lenient here, as filename-parsing is a non-goal of this question.

Other rules

  • The program must be able to correctly reassemble any random arrangement of the wikipedia Mona Lisa by the python script. Hard-coding of block transformations of the example image above is strictly not allowed.

  • It is understood that for some degenerate cases (e.g. chequerboard of 100x100) blocks it is impossible to correctly rearrange the image. In those cases it is acceptable to produce incorrect/undefined output. However I think in general for photo images and almost all "famous" artworks, reassembly should be possible. Works by Mark Rothko are a possible exception.

  • Common image manipulation libraries may be used (e.g. Python's PIL), but any APIs expressly designed for this purpose are strictly banned.

  • Standard “loopholes” which are no longer funny

Now my toddler got a hold of The Scream. This is the result that needs to be fixed:

enter image description here

Special thanks to Digital Trauma!

He wrote the original challenge and gave it up original post here


I need to update the images and the script to not have rotations and to not have black bars. Any ideas?

  • \$\begingroup\$ The way to make this objective is to have a large list of images (like 50 or so), and require 100% accuracy. You should also reserve the right to regenerate the test cases if anybody is hardcoding the input. \$\endgroup\$ May 16, 2017 at 16:01
  • \$\begingroup\$ Please credit the question that the inspiration (or direct copy ?!?) came from. Also, please note the comments to that question very carefully. In its current form, there significant problems (up to and including impossible to get 100% accuracy). \$\endgroup\$ May 16, 2017 at 22:52
  • \$\begingroup\$ @DigitalTrauma Sorry. I forgot to add that. It was a copy with edits based on you giving it up to gift exchange \$\endgroup\$
    – user63187
    May 17, 2017 at 0:38
  • \$\begingroup\$ Somebody suggested using lossless image formats, which would help a lot in terms of edges now being exact matches. That might be less interesting though. Nathan suggested a large list of images, which could give it the test-battery tag perhaps? My two suggestions I believe are at odds. \$\endgroup\$
    – nmjcman101
    Jun 20, 2017 at 19:25

Make some arrays

A recent post on The Daily WTF concerned the creation of four two-dimensional arrays of height n. Please write a function to create these arrays for me. Here is an example for n=4:

[[1] [[1 0 0] [[0] [[0 0 0]
 [0]  [0 1 0]  [0]  [1 0 0]
 [0]  [0 0 1]  [0]  [0 1 0]
 [0]] [0 0 0]] [1]] [0 0 1]]


  • All four arrays must be two-dimensional. Arrays which have been flattened in any way are not acceptable.
  • You can write separate functions for each array, in which case your score is the total of the scores for each function.
  • Your functions can call each other, provided you include the name in your score. Otherwise unnamed functions are acceptable.
  • Rather than returning the array(s), you can be passed them in as an input parameter (e.g. using int** in C).
  • If your language has a builtin for inputting arbitrary data structures, then you can write a full program that outputs using the input format that the builtin would recognise.
  • Alternatively, you can write a full program that outputs source code for the arrays in your language.
  • All standard loopholes are banned.
  • This is , so the shortest function wins.
  • \$\begingroup\$ Hmmm, I wonder if you can reconstruct these four arrays if you just evaluate that code block as is. If so, that might be a fun challenge too. :) \$\endgroup\$ May 17, 2017 at 12:08

Parse a 4-dimensional array

A 2D array is fairly easy to represent in ASCII art, something like this:

1  2
6 24

One approach for a 3D array is to have a number of 2D arrays, with an extra blank line between each array:

1  2
6 24

1  2
4  8

Things get a little more complicated for 4D arrays though. The best you can do is to arrange a number of 3D arrays side-by-side, but with at least two spaces between each array:

1  2   1  2
6 24   3  4

1  2   1  4
4  8  27 64

So that I don't have to ban full programs, the challenge will be to input 4 (numeric) indices along with a 4D array which will be as some sort of string in the above format, and output the (numeric) 4D array element at those indices. For example, if the 1-indexed indices were 1, 2, 1, 2 then the result would be 4. Rules:

  • You must at a minimum support integers from 0 to 255, written in decimal.
  • The array will not be ragged, but you will not be given the size of the array. Any of the dimensions may be 1, so 0 0 0 0 0 is a valid input in 0-based indexing.
  • Numbers in the array will be consistently justified (please state whether you support left or right, or decimal justification if you support floating-point numbers), but the output must not contain any whitespace except for an optional trailing newline.
  • You may choose 0 or 1-based indexing.
  • You can choose row-major format instead of column-major format, as long as both outer and inner arrays have the same format. The first two indices will however always refer to the outer array and the last two indices to the inner array indexed by the first two indices.
  • You can assume that the indices will always be valid.
  • All standard loopholes are banned.
  • This is , so the shortest program or function wins.
  • \$\begingroup\$ some sort of string in the above format so no JSON? \$\endgroup\$
    – Stephen
    May 17, 2017 at 13:24
  • \$\begingroup\$ @StephenS Correct, you won't be given an actual array, you'll be given an ASCII-art printout of the array. \$\endgroup\$
    – Neil
    May 17, 2017 at 13:26

Generating strings of Rs and spaces

If a task is to make a string of 16 characters using 2 different characters, it can be done in 2^16=65536 ways. Let's generate them all!

Write a program or function which takes a number in range [0, 65535] as input and outputs a string of 16 characters consisting of R and  (space). One number corresponds to one string and must generate the same string each time the code is run.


Suppose that a program/function takes a binary representation of the number and replaces zeros with spaces and ones with Rs to make the string. The conversion from input to output will look as follows:

0     -> "                "
1     -> "               R"
2     -> "              R "
3     -> "              RR"

Sandbox notes

  • This challenge probably needs a better name. If you have an idea, please mention it.
  • I know that this challenge will be boring if making the string from binary representation is the shortest way in all languages. I hope that this isn't the case.
  • \$\begingroup\$ The only reason I can think of that making it from the binary representation would not be the shortest way is if there's a shorter way with a Cartesian product built-in, but that's still boring. \$\endgroup\$ May 18, 2017 at 8:23


N boxes are lined up in a sequence (1 ≤ N ≤ 20). You have A red balls and B blue balls (0 ≤ A ≤ 15, 0 ≤ B ≤ 15). The red balls (and the blue ones) are exactly the same. You can place the balls in the boxes. It is allowed to put in a box, balls of the two kinds, or only from one kind. You can also leave some of the boxes empty. It's not necessary to place all the balls in the boxes.


Write a program, which finds the number of different ways to place the balls in the boxes in the described way.


Input contains one line with three integers N, A and B separated by space.


The result of your program must be an integer written on the only line of output.

Sample I/O

2 1 1

Simplify ordinal expression



Split the atom

Input: 0 <= X <= 2³² - 1

Output: List of numbers in decimal, after recursive splitting in binary format.


Example 1:

Input: 255

Current Output is 255.

Binary representation of 255 is 1111 1111. Splitting it, we get 1111 and 1111, which in decimal are 15 and 15.

Continuing the Output, we will have 255 15 15.

Now the numbers 15 and 15 will serve as inputs and these numbers are to be split.

Continuing the Output, we will have 255 15 15 3 3 3 3.

Continuing the logic, final Output will be 255 15 15 3 3 3 3 1 1 1 1 1 1 1 1 . And since 1 can no longer be split, the output stops.

Example 2:

Input: 225

Current Output is 225.

Binary representation of 225 is 1110 0001. Splitting it, we get 1110 and 0001, which in decimal are 14 and 1.

Continuing the Output, we will have 225 14 1.

Now the numbers 14 and 1 will serve as inputs and these numbers are to be split.

Since 1 is no longer split-able, the Output will be 225 14 1 3 2.

Final Output will be 225 14 1 3 2 1 1 1 0.

Example 3:

Input: 32

Output: 32 4 0 1 0.


  1. If the number of binary digits are odd, the first number will have one fewer binary digit than the next one. Example, 20 (10100) will be split as 10 and 100, with decimal output being 2 and 4.
  2. Standard loophole rules apply.
  3. 0s and 1s do not propagate further.
  4. Program crashing for trying to display too many numbers is a valid exit condition.
  • 1
    \$\begingroup\$ Surely you reversed the inequality signs. \$\endgroup\$
    – Leaky Nun
    May 18, 2017 at 11:16
  • \$\begingroup\$ s/Splitting it/Splitting it in half/ \$\endgroup\$
    – Leaky Nun
    May 18, 2017 at 11:17
  • \$\begingroup\$ @LeakyNun Yes, yes I did :D \$\endgroup\$ May 18, 2017 at 11:20
  • \$\begingroup\$ @trichoplax I updated the conditions. Is that clearer? \$\endgroup\$ May 18, 2017 at 11:34
  • \$\begingroup\$ @trichoplax I hope 3 examples clears everything up. \$\endgroup\$ May 18, 2017 at 11:53
  • \$\begingroup\$ I think this is a duplicate of an earlier question, but I haven't found it by searching for binary split. Perhaps someone else will be able to find it. \$\endgroup\$ May 18, 2017 at 11:58
  • \$\begingroup\$ @ctrl-shift-esc yes that clears up the treatment of the zeroes \$\endgroup\$ May 18, 2017 at 14:05

Pi - The Movie!


So we've had the musical version The Sound of Pi, now its time for the movie.


Your challenge is to write a full program or function that outputs an animated version of pi to 100 decimal places.

For your convenience, here's Pi to 100 decimal places


Input and Output

Input: No input required

Output: A single gif file


  • Only one digit or decimal point should be shown at a time
  • Each digit or decimal point should be shown for for between 0.25 and 1.5 seconds (your choice).
  • The font should be clearly visible (different colour to background)
  • The characters should be at least 100 pixels high and have at least a 1 pixel boarder
  • You can use built in values of pi as long as they're correct to 100 dp
  • Standard loopholes apply
  • This is , so the submission with the least amount of bytes wins!

Sandbox questions

  • Is the time constraint realistic?
  • What tags should I use?
  • \$\begingroup\$ I'd VTC this as "too broad". \$\endgroup\$ May 18, 2017 at 13:56
  • \$\begingroup\$ @EriktheOutgolfer could you explain why? a VTC on its own doesn't help me improve it :) \$\endgroup\$
    – Notts90
    May 18, 2017 at 14:01
  • \$\begingroup\$ Basically there are many ways you could make a "video file" (file formats), the font size could be invisibly small, the background color may be the same as the foreground color, digits might not fully appear (i.e. be cut) etc. \$\endgroup\$ May 18, 2017 at 14:05
  • \$\begingroup\$ i think a gif with fixed size would be better \$\endgroup\$ May 18, 2017 at 14:06
  • \$\begingroup\$ @EriktheOutgolfer added size and colour constraints. I've already specified the format should suitable for vlc or wmp. \$\endgroup\$
    – Notts90
    May 18, 2017 at 14:18
  • \$\begingroup\$ @Notts90 saying it's suitable for vlc or wmp does not say much, something that runs in my vlc may not run in yours (installed codecs and stuff) \$\endgroup\$ May 18, 2017 at 14:48
  • \$\begingroup\$ I'll go with the gif option then! \$\endgroup\$
    – Notts90
    May 18, 2017 at 16:32


Wardialing was a very interesting way to try to hack people back in the '80s and '90s. When everyone used dial-up, people would dial huge amounts of numbers to search for BBS's, computers, or fax machines. If it was answered by a human or answering machine, it hung up and forgot the number. If it was answered by a modem or a fax machine, it would make note of the number.


Your job is to make a URL wardialer. Something that tests and checks if it's a valid website from one letter of the alphabet.


  • Program must take user input. This input has to be a letter of the alphabet, no numbers. Just one letter of the alphabet and form multiple URLs that start with the letter.
  • Standard loopholes apply.
  • You must make 8 URLs from 1 letter, and test to see if it is a valid site.
  • If you hit an error (not a response code), instead of leaving it blank, go ahead and return a 404
  • If you hit a redirect (3xx), return a 200 instead.
  • You may output the results in any reasonable format, as long as it includes the website name, status codes for all the websites and the redirects
  • This is Code Golf, so shortest amount of bytes wins.

What counts as a URL for this challenge?

http://{domain-name}.{com or net or org}

For this challenge, the domain name is should only be 4 letters long, no more, no less.

What should I test?

For each 4 letter domain name, test it against three top-level domains (.com, .net, .org). Record all the response codes from each URL, remember from the constraints that any (3xx) should return 200 and be recorded as a redirect in the output and any error getting to the website should result in a 404.




| Website | .com | .net | .org | Redirects? |
| ajoe    | 200  | 200  | 200  | .com, .net |
| aqiz    | 200  | 404  | 404  | no         |
| amnx    | 200  | 503  | 404  | .com       |
| abcd    | 200  | 404  | 200  | .com       |
| ajmx    | 200  | 503  | 404  | no         |
| aole    | 200  | 200  | 200  | .com       |
| apop    | 404  | 200  | 200  | .net       |
| akkk    | 200  | 200  | 200  | .com       |
  • 2
    \$\begingroup\$ I don't like the formatting aspect; it adds a whole unnecessary layer to the meat of challenge. I also don't like the fact that you have to go through .com, .net, and .org for the same reason. But I do like the concept, and I think if you stay closer to the actual challenge ("generate a few websites and check if they exist") it could be well-received on main. \$\endgroup\$ May 22, 2017 at 19:43
  • \$\begingroup\$ but the formatting looks sooooo good \$\endgroup\$
    – kuantum
    May 22, 2017 at 19:44
  • \$\begingroup\$ Well then, maybe you should post a challenge idea to format a table like that ;-) I think we may have already had that challenge though. \$\endgroup\$ May 22, 2017 at 19:45
  • \$\begingroup\$ ehhh, nah. what should i replace it with \$\endgroup\$
    – kuantum
    May 22, 2017 at 19:47
  • \$\begingroup\$ I'd allow users to output in any reasonable format they like: an array of arrays, a newline-separated string, etc. A table like that would still be allowed, though it probably wouldn't be the shortest. \$\endgroup\$ May 22, 2017 at 19:49
  • \$\begingroup\$ URLs for websites all start with the letter h. If you want to make an argument about relative URLs, I will concede that they can also start with the non-letter /. But abusing a clearly defined term like URL to the extent to which it's abused in this draft only serves to confuse people. The task is: given an input of one letter of the alphabet, generate 8 alphabetic labels, from each of those labels to generate three domain names, and for each of those names test the response code of the URL http://{name}/ \$\endgroup\$ May 22, 2017 at 21:46
  • \$\begingroup\$ Also, in "Something has more than 4 letters in it", more than is exclusive, so this is not consistent with the example. \$\endgroup\$ May 22, 2017 at 21:47
  • \$\begingroup\$ There. I think this edit helps @PeterTaylor \$\endgroup\$
    – kuantum
    May 22, 2017 at 22:17

Program Equilibrium Iterated Prisoner's Dilemma Tournament

This is an iterated prisoners dilemma tournament. You are to make a bot that plays the prisoners dilemma against the other contestants, but with a twist- you can perfectly simulate your opponent!

Iterated Prisoner's Dilemma

The prisoner's dilemma is a two-player game in which players can choose to either "Cooperate" or "Defect" against their opponent. The payoff matrix for the prisoner's dilemma looks like this:

                           Player 2
                       C              D

                |              |               |
            C   |    (3, 3)    |    (0, 5)     |
                |              |               |
Player 1        |------------------------------|
                |              |               |
            D   |    (5, 0)    |    (1, 1)     |
                |              |               |

In the iterated prisoner's dilemma, multiple rounds are played in succession, and players can use their knowledge of the previous rounds when making a decision.

The twist

Bots are also allowed to run their opponent and examine the output before making a move. This allows for significantly more complex strategies in which bots can simulate their opponents to determine if it is safe to cooperate with them, as well as attempt to exploit less sophisticated players.


There will be a program available in the path called simulate that you can use to simulate what your opponent would do against you

simulate [instruction to your own bot] [fake history to the opponent]
simulate cooperate 00 10 01 11 01 10

All arguments are optional, and you may also use underscore _ for no argument. If you do not explicitly state the history, the real history will be used.

Example output from the simulate program:


The first number is you, the second is your opponent. 0 means defect, 1 means cooperate.

The instruction

It wouldn't be much use to simulate what your opponent would do against you, because he'd probably simulate you right back, and then you'd be stuck in an infinite loop.
The instruction is your way to simulate what the opponent would do against another agent. You could for instance have a cooperate instruction that causes your bot to cooperate mindlessly, to see what the opponent would do if you were to cooperate.


Your program will be given the following input at the beginning of each round:

Instruction (if any)
Your score
Your opponents score
Current round number
Complete history of past interactions with this bot. Space separated: the first number is you, the second is your opponent


00 10


0 for defect or 1 for cooperate, with any number of leading characters. Trailing characters (including 0 or 1's) will be ignored.



The tournament

The tournament will be round-robin elimination: Each bot will play one match against all other bots, where a match consists of 100 rounds of the prisoner's dilemma. At the end of the round-robin round, the lower-scoring half of the tournament pool will be eliminated. This process will be repeated until only one bot remains, or there is a tie.


  • Your program has to run on Linux. Please provide a brief description of how to run your program if possible
  • Your program has to give an answer within 5 seconds. Failing to do so will be interpreted as defect.
  • Your program may not perform multiple simulations at the same time
  • You may only enter a single bot, this is to prevent people from entering multiple bots that cooperate
  • You may not read or write any files. If this is required by your language, please state so in your answer.
  • You may not in any way store state between rounds. This is to prevent bots from figuring out whether they are being simulated.
  • If you exploit another loophole to figure out whether you are being simulated, you will be disqualified. Guessing (or strategical testing) is obviously allowed

About the host computer

Arch Linux 4.10.13-1
Intel(R) Core(TM) i7-7500U CPU @ 2.70GHz, 4 cores
I will disconnect from the internet while running the tournament


I got the idea as well as some of the text from this Github repository. I have not directly asked the author for permission to use the challenge, but he said that "if you want to run this tournament on your own, you are more than welcome to" in a blogpost.


I will update this periodically whenever new bots enter.


How do I best discourage/prevent players from intentionally using every bit of their time limit to sabotage other players trying to simulate them? I am thinking random time limits and/or penalties

  • 1
    \$\begingroup\$ Have you ever run a KotH? If not, I think you are about to learn how incredibly slow a round can be even if the programs have very simple logic. With the simulate instruction I anticipate taking hours to run a single round. \$\endgroup\$ May 22, 2017 at 21:50
  • \$\begingroup\$ @PeterTaylor That's good advice; I might not want to run this on my main computer \$\endgroup\$
    – BlackCap
    May 22, 2017 at 22:54

Identify the variable case

Your task is to write a function which determines which casing convention a provided variable name belongs to:

  • camelCase
  • kebab-case
  • Train-Case
  • snake_case
  • PascalCase
  • Ada_Case (not a real term, but apparently the standard casing used in Ada)

Your function should return one of the above strings, either exactly as above, or as spaced title-case: "Train case", "Screaming snake case".

Casings that include - or _ may include leading or trailing -s or _s (eg, __MY_SNAKE__ or -webkit-kebab).

Multiple consecutive upper case letters are allowed in mixed casings, as long as they follow a lower case letter (MyXMLDocument, This-CSS-Crap).

Where an input fits multiple schemes (eg, F or foo), you may return any valid classification.

If an input does not fit one of the above schemes (eg stu-pid_case), you must return something different. You may also throw an error.

The input string is guaranteed to be at least one character, and begin with a character matching /[a-zA-Z_-]/. Every other character will match /[a-zA-Z0-9$-]/

This is , so shortest length in bytes wins. Standard loopholes forbidden. Standard input and output methods.

Test cases

myFunkyVariable => camelCase
no-op => kebab-case
__c => snake_case
XML-Madness => Error
Get_Input => Ada_Case
i => camelCase (or snake_case)
char2int_factory => snake_case
  • \$\begingroup\$ You should specify exactly what each of the cases requires. (e.g. train case is separated by underscores with only the first character capital) Because this is a competition there should be no room for interpretation. \$\endgroup\$
    – Wheat Wizard Mod
    May 26, 2017 at 4:56
  • \$\begingroup\$ What ambiguity have I missed? \$\endgroup\$ May 26, 2017 at 5:19
  • \$\begingroup\$ You should define what each of the 7 cases entails. Currently you just list 7 examples, which is certainly helpful, but I don't think sufficient. \$\endgroup\$
    – Wheat Wizard Mod
    May 26, 2017 at 5:30
  • \$\begingroup\$ You really don't think they're sufficiently self-explanatory? I mean, I can add explanation, but I'm curious if you have an example of a variable name that you don't know how to classify? \$\endgroup\$ May 26, 2017 at 5:35
  • \$\begingroup\$ I think I get it based on the examples and I think any reasonable person would be able to jump to the correct conclusion, but we should strive to make questions as concrete as possible. Some people might not be aware of casing in the first place and definitions could help. It may seem like I'm making a fuss over nothing, and perhaps I am, but I think it could only benefit to add specific definitions of the terms, since the question is about identifying them. As far as examples go, I believe E_S_S is ambiguous between screaming snake and Ada. Definitions might help clear that up. \$\endgroup\$
    – Wheat Wizard Mod
    May 26, 2017 at 5:46
  • \$\begingroup\$ That one is ambiguous, which, as stated, means that either answer is ok. But I'll add an introductory paragraph about casing. \$\endgroup\$ May 26, 2017 at 5:58

21 = (1 + 3 + 5 + 7 + 9) - (1 + 3)

Sandbox question:
Is this different enough from the original challenge?


  • This is a sequel to my previous challenge, but this time with a slightly more interesting practical application (the factorization of the input).

  • It is based on 1) the fact that any odd integer n can be expressed as the difference of two squares a² - b² and 2) the well known formula:


The task

You're given a positive odd integer n. Your task is to determine how many iterations of the following algorithm need to be processed to reach n.

  • Initialization: find the highest integer k such that k² ≤ n. Build the list L = [ 1, 3, 5, ..., 2k-1 ].
  • Iteration:
    • Step #1: If the sum of the terms of L equals n, stop here.
    • Step #2: Increment k and append 2k-1 to L.
    • Step #3: While the sum of the terms of L is greater than n, remove the first term from the list. Go on with step #1.

All iterations (either full or partial) must be counted. In other words, you must return the number of times step #1 was executed.

Example #1

This is a trivial example for the perfect square n = 9:

  • Initialization: k = 3 because 3² ≤ 9 < 4², which leads to L = [ 1, 3, 5 ].
  • Step #1: 1 + 3 + 5 = 9. We're done.

We went through one iteration. So the expected result is 1.

Example #2

Here is what we get for n = 21:

  • Initialization: k = 4 because 4² ≤ 21 < 5², which leads to L = [ 1, 3, 5, 7 ].
  • Step #1: 1 + 3 + 5 + 7 = 16, which does not equal n.
  • Step #2: We increment k and we add 2k + 1 = 2 x 5 - 1 = 9 to the list: L = [ 1, 3, 5, 7, 9 ].
  • Step #3: We remove the first term from the list: L = [ 3, 5, 7, 9 ]. The sum of the terms is now 24, which is still greater than n. So we remove another term: L = [ 5, 7, 9 ].
  • Step #1 : 5 + 7 + 9 = 21. We're done.

We went through two iterations. So the expected result is 2.

Example #3

Below is a summary of all steps for a slightly more complex example with n = 145.

Initialization: k = 12 because 12² ≤ 145 < 13².

Iteration | Step | k  | L                                 | Sum
    1     |  1   | 12 | 1,3,5,7,9,11,13,15,17,19,21,23    | 144
          |  2   | 13 | 1,3,5,7,9,11,13,15,17,19,21,23,25 | 169
          |  3   | 13 | 11,13,15,17,19,21,23,25           | 144
    2     |  1   | 13 | 11,13,15,17,19,21,23,25           | 144
          |  2   | 14 | 11,13,15,17,19,21,23,25,27        | 171
          |  3   | 14 | 17,19,21,23,25,27                 | 132
    3     |  1   | 14 | 17,19,21,23,25,27                 | 132
          |  2   | 15 | 17,19,21,23,25,27,29              | 161
          |  3   | 15 | 19,21,23,25,27,29                 | 144
    4     |  1   | 15 | 19,21,23,25,27,29                 | 144
          |  2   | 16 | 19,21,23,25,27,29,31              | 175
          |  3   | 16 | 23,25,27,29,31                    | 135
    5     |  1   | 16 | 23,25,27,29,31                    | 135
          |  2   | 17 | 23,25,27,29,31,33                 | 168
          |  3   | 17 | 25,27,29,31,33                    | 145
    6     |  1   | 17 | 25,27,29,31,33                    | 145

We went through 6 iterations. So the expected result is 6.

Factorization of the input

This paragraph is for illustration purposes only. It describes how the factorization of the input can be deduced from the result of the algorithm.

Factorization of 21:

  • 21 = 5 + 7 + 9 = (1 + 3 + 5 + 7 + 9) - (1 + 3)
  • 21 = 5² - 2²
  • 21 = (5 + 2)(5 - 2)
  • 21 = 7 x 3

Rules and clarifications

  • The input is guaranteed to be a positive odd integer. It may be either a composite integer or a prime.
  • You're required to process any odd n in [ 1, ..., 65535 ] in less than one minute on mid-range hardware.
  • Given enough time, your program/function should theoretically work for any value of n that is natively supported by your language. If it doesn't, please explain why in your answer.
  • You may use any other method instead of the described one as long as it returns the correct result.
  • This is code golf, so the shortest answer in bytes wins!

Test cases

To be completed

  • \$\begingroup\$ Typo: you wrote "k²=n", which can't possibly be correct (and contradicts the rest of the post). \$\endgroup\$
    – user62131
    May 26, 2017 at 3:30
  • \$\begingroup\$ @ais523 Thanks for catching that out. (I believe all my "less than or equal to" signs where turned into equal signs when I copy/pasted through Notepad++.) \$\endgroup\$
    – Arnauld
    May 26, 2017 at 7:10
  • \$\begingroup\$ 1. There's also an extra = in the LaTeXified image. 2. You say it's a variant on an earlier question of yours, but it seems so similar that I don't see why it wouldn't be a dupe. If it isn't, I definitely think it's a dupe of codegolf.stackexchange.com/q/18349/194 : the two questions are essentially "Find the factorisation of n into two parts which are as close as possible to sqrt(n). \$\endgroup\$ May 26, 2017 at 8:12
  • \$\begingroup\$ @PeterTaylor Thanks for your feedback. Yes, I think it's a dupe indeed. \$\endgroup\$
    – Arnauld
    May 26, 2017 at 8:16

Dungeon of Botdom

Bots will play a variant of this game:


(Our version will be played by one bot versus another bot, and there will only be one character, which may or may not be in the actual game, and it will be repeated a bunch.)

Essentially, there is a deck of cards, with enemies on each card. players take turns either passing, or drawing a card and either a) placing it in the initially empty dungeon or b) removing a piece of gear and discarding the enemy. enemies have different hit points, which affects the damage done to a bot traversing the dungeon. When everyone but one player has passed, they enter the dungeon. when a bot is traversing the dungeon, they will encounter the enemies placed in the dungeon in LIFO order (the card on top is taken first) (if I take out some items this may be irrelevant). enemies are instantly defeated if the player has a corresponding item, otherwise the player takes damage equal to the enemies number. if a player makes it through the dungeon, they get one point. get two points to win the match. if they fall while traversing the dungeon, they lose one chance. fail twice, and the opponent wins the match.

What this means, is that you might want to do one of two things: make the dungeon too dangerous to survive without your opponent thinking it is, and passing, or making the opponent bot believe the dungeon is too dangerous, when it is not, and running through the dungeon and succeeding.

We'll have a good ol' round robin of a 1000 rounds each or some such. the nice thing about this game is that the rounds are short, because once you finish the deck, a player automatically enters the dungeon.

  • 2
    \$\begingroup\$ You may want to include a better description, not just the link. Describe the various cards, the game mechanics, the format of the bots, and include the code for testing bots. KOTH challenges tend to have lots of text. \$\endgroup\$
    – Gryphon
    May 26, 2017 at 13:29
  • \$\begingroup\$ @gryphon did I not do that? Also I haven't decided on the exact deck yet, and exact items, and exact round formats. I have described the basics of the game anyway \$\endgroup\$ May 27, 2017 at 0:43
  • \$\begingroup\$ @Gryphon also I haven't made the controller yet. we can't all get away with straight posting to main because of our incredible talent for Koths and making the controllers immediately \$\endgroup\$ May 27, 2017 at 0:54
  • \$\begingroup\$ OK, sorry. I would just make sure I have the controller before I actually posted the question. \$\endgroup\$
    – Gryphon
    May 29, 2017 at 17:27
  • \$\begingroup\$ I have the controller finished here \$\endgroup\$ May 30, 2017 at 8:24
  • \$\begingroup\$ Nice Poem, @DestructibleLemon! \$\endgroup\$
    – Gryphon
    May 30, 2017 at 10:28

Can my favourite team still become Football Champion?

As a fan of an at most moderately successful footballBE team, towards the end of the season I often wonder whether my favourite team still has any theoretical chance left of becoming champion. Your task in this challenge is to answer that question for me.


You will recieve three inputs: the current table, the list of remaining matches, and the current position of the team we are interested in.

Input 1: The current table, a sequence of numbers were the i-th number are the points gained by team i so far. For example, the input [93, 86, 78, 76, 75] encodes the following table (only the last column is of importance):

premier league table

Input 2: The remaining matches, a sequence of tuples where each tuple (i,j) stands for a remaining match between team i and j. In the above example, a second input of [(1,2), (4,3), (2,3), (3,2), (1,2)] would mean that the remaining matches are:

Chelsea vs Tottenham, Liverpool vs Man. City, Tottenham vs Man. City, Man. City vs Tottenham, Chelsea vs Tottenham

Input 3: The current position of the team we are interested in. For example, an input of 2 for the above example would mean that we'd like to know whether Tottenham can still become champion.


For each remaining match of the form (i,j), there are three possible outcomes:

  • Team i wins: Team i gets 3 points, team j gets 0 points
  • Team j wins: Team i gets 0 points, team j gets 3 points
  • Draw: Team i and j both get 1 point

You must output a truthy value if there is some outcome for all remaining games such that at the end, no other team has more points than the team specified in the 3rd input. Otherwise, output a falsy value.

Example: Consider the exemplary input from the above section:

Input 1 = [93, 86, 78, 76, 75], Input 2 = [(1,2), (4,3), (2,3), (3,2), (1,2)], Input 3 = 2

If team 2 wins all its remaining matches (i.e. (1,2), (2,3), (3,2), (1,2)), it gets 4*3 = 12 additional points; none of the other teams gets any points from these matches. Let's say the other remaining match (i.e. (4,3)) is a draw. Then the final scores would be:

 Team 1: 93, Team 2: 86 + 12 = 98, Team 3: 78 + 1 = 79, Team 4: 76 + 1 = 77, Team 5: 75

This means that we have already found some outcome for the remaining matches such that no other team has more points than team 2, so the output for this input must be truthy.


  • You may assume the first input to be an ordered sequence, i.e. for i < j, the i-th entry is equal to or greater than the j-th entry. The first input may be taken as a list, a string or the like.
  • You may take the second input as a string, a list of tuples or the like. Alternatively, you may take it as a two-dimensional array a where a[i][j] is the number of entries of the form (i,j) in the list of remaining matches. For example, a[1][2] = 2, a[2][3] = 1, a[3][2] = 1, a[4][3] = 1 corresponds to [(1,2), (4,3), (2,3), (3,2), (1,2)].
  • For the second and third input, you may assume 0-indexing instead of 1-indexing.
  • You may take the three inputs in any order.

Please specify the exact input format you chose in your answer.

Side node: The problem underlying this challenge was shown to be NP-complete in "Football Elimination is Hard to Decide Under the 3-Point-Rule". Interestingly, if only two points are awarded for a win, the problem becomes solvable in polynomial time.

Test Cases

All test cases are in the format Input1, Input2, Input3.


  • [93, 86, 78, 76, 75], [(1,2), (4,3), (2,3), (3,2), (1,2)], 2
  • [50], [], 1
  • [10, 10, 10], [], 3
  • [15, 10, 8], [(2,3), (1,3), (1,3), (3,1), (2,1)], 2


  • [10, 9, 8], [], 2
  • [10, 9, 9], [(2,3), (3,2)], 1
  • [21, 12, 11], [(2,1), (1,2), (2,3), (1,3), (1,3), (3,1), (3,1)], 2


This is , so the shortest correct answer (in bytes) wins. The winner will be chosen one week after the first correct answer is posted.


Closely related to Words from periodic table of elements (but that one is closed due to unclear specification?).

Closely related to Find the Chemistry of a name (probably a dupe, slightly different requirements though).

Closely related to [Br]eaking Code Golf [Ba]d (allows strings to be not expressible as solely a sequence of element abbreviations).

May I get a community consensus, whether this is better specified and/or sufficiently different to not be immediately closed as a dupe?

Elementize a string

Convert an input string to a concatenation of chemical element abbreviations.

Write a program/function/procedure etc. which will take as input a string/array of characters/pointer to a string etc. and return/print/display the same string expressed as a concatenation of chemical element abbreviations.

For example, takagi can be expressed as TaKAgI (i.e. the abbreviations for Tantalum(Ta), Potassium(K), Silver(Ag), Iodine(I)).

For this challenge you must use the following element name abbreviations:

{"H", "He", "Li", "Be", "B", "C", "N", "O", "F", "Ne", "Na", "Mg",
"Al", "Si", "P", "S", "Cl", "Ar", "K", "Ca", "Sc", "Ti", "V", "Cr",
"Mn", "Fe", "Co", "Ni", "Cu", "Zn", "Ga", "Ge", "As", "Se", "Br",
"Kr", "Rb", "Sr", "Y", "Zr", "Nb", "Mo", "Tc", "Ru", "Rh", "Pd",
"Ag", "Cd", "In", "Sn", "Sb", "Te", "I", "Xe", "Cs", "Ba", "La",
"Ce", "Pr", "Nd", "Pm", "Sm", "Eu", "Gd", "Tb", "Dy", "Ho", "Er",
"Tm", "Yb", "Lu", "Hf", "Ta", "W", "Re", "Os", "Ir", "Pt", "Au",
"Hg", "Tl", "Pb", "Bi", "Po", "At", "Rn", "Fr", "Ra", "Ac", "Th",
"Pa", "U", "Np", "Pu", "Am", "Cm", "Bk", "Cf", "Es", "Fm", "Md",
"No", "Lr", "Rf", "Db", "Sg", "Bh", "Hs", "Mt", "Ds", "Rg", "Cn"}

These are the elements with numbers 1 through 112. You may optionally also use the abbreviations for elements 113 through 118:

{"Nh", "Fl", "Mc", "Lv", "Ts", "Og"}

You may not, however, use the placeholder three-letter abbreviations for not yet named elements, such as "Uuo" (Ununoctium).

If the input string cannot be expressed by the above abbreviations, you shall return one of the following:

  • a falsey value (clearly distinct from element names, in other words A would not be valid, even though there is no element "A"; something like 0, null, false, newline is fine)
  • an empty string
  • exit without output
  • exit with an error
  • something similarly unambiguous signifying failure and not returning some incorrect output that could be accidentally misinterpreted as an answer (suggestions to make this specification clearer?)

Possible output, using example input takagi:

  • an appropriately capitalized string: TaKAgI.
  • a flat array of characters {"T", "a", "K", "A", "g", "I"} with appropriate capitalization.
  • a list of strings (capitalized or not) separated by newlines or as separate members of an array, etc., e.g. ta \n k \n ag \n i.

The rule of thumb is that the division into separate elements must be clear. Please comment if additional clarification is needed!

You may assume the input to be a single word consisting only of letters.

Compression is not the intent of this challenge! Boiler-plate code that fetches the list of elements from somewhere, hard-coding the lists of abbreviations, assuming the list of abbreviations to be stored in a variable or passed as a second argument to your function is OK and should not be included in the byte count.

Sample input -> sample output
no -> No             // as in Nobelium, alternatively see next line
no -> NO             // Nitrogen-Oxygen, either one is valid
helium -> falsey output
heliam -> HeLiAm
fog -> FOg           // If using 118 elements, falsey otherwise.
ppcg -> falsey       // I'm really, really sorry

Suggestions of further test cases are appreciated.

This is code-golf, shortest code wins.

  • \$\begingroup\$ Potassium is K, not P. \$\endgroup\$ Jun 1, 2017 at 2:44
  • \$\begingroup\$ @calculator brain-fart, fixed, thanks. \$\endgroup\$
    – LLlAMnYP
    Jun 1, 2017 at 4:35
  • \$\begingroup\$ I think it makes more sense to work on improving the existing closed question than to post a duplicate. \$\endgroup\$ Jun 1, 2017 at 6:30
  • \$\begingroup\$ @Peter I should have realized sooner, that a [chemistry] tag exists here. I'm not sure I'm at ease with modifying someone else' challenge (distorting their intent?) and potentially invalidating others' answers... \$\endgroup\$
    – LLlAMnYP
    Jun 1, 2017 at 6:37

How many substitutions till palindromization?

Given a string, find the minimum number of character substitutions needed so that the string is a palindrome.

E.g. the string abchefa needs 2 substitutions, so it can take any of the following paths:

abchefa -> afchefa -> afehefa
                   -> afchcfa
        -> abehefa -> afehefa
                   -> abeheba
        -> abchcfa -> afchcfa
                   -> abchcba
        -> abcheba -> abeheba
                   -> abchcba

You must return a minimal number, e.g. abc can use 3 substitutions (abc -> dbc -> ddc -> ddd) but it really only needs 1 (abc -> aba) so you must return 1.

The string will only contain printable ASCII, no newlines.

Additional testcases:










  • You can take the string as a string or as a list of chars, but not as a list of strings
  • You must return a minimal value in a generally accepted output format.
  • \$\begingroup\$ I thought there was a near-dupe somewhere, but I'm pretty sure this is the one I was thinking of... \$\endgroup\$ Jun 1, 2017 at 23:38
  • \$\begingroup\$ Actually this might be a dupe of this: perform any answer for all indexes in the string, sum, divide by two. \$\endgroup\$ Jun 1, 2017 at 23:41
  • \$\begingroup\$ @ETHproductions You spoiled the hidden way to do it... \$\endgroup\$ Jun 2, 2017 at 5:39
  • \$\begingroup\$ I'm pretty sure I've seen an exact dupe, although it might still be in the sandbox; codegolf.stackexchange.com/q/95343/194 is a generalisation. \$\endgroup\$ Jun 2, 2017 at 9:00

Write machine code that also works when left rotated one byte

Write machine code for a CPU that runs and does something. The twist is that your code should run and do something else, without errors, if it is left rotated one byte.

Suggestions? Ideas? Thoughts? Let me know.

  • 4
    \$\begingroup\$ So is this "any code that doesn't crash if rotated"? Generally it's best to give a specific task, not just "do something", or else it's quite broad. \$\endgroup\$
    – Geobits
    Oct 26, 2016 at 0:15
  • 2
    \$\begingroup\$ This is completely uninteresting in any instruction set which has one-byte instructions. E.g. in x86 the byte that's rotated away can be INC, and then the "something else" is just the same function with an offset to one of the arguments. \$\endgroup\$ Oct 26, 2016 at 7:53
  • 1
    \$\begingroup\$ Far too broad, IMO. Also very, very easy in machine code dialects which have single-byte jump instructions, or any instruction that has a single-byte opcode plus X bytes of data, where X is no shorter than the length of the jump instruction; this describes almost every machine code dialect in existence. \$\endgroup\$
    – user62131
    Jun 2, 2017 at 15:03
  • \$\begingroup\$ What about reversing it? \$\endgroup\$
    – anna328p
    Jun 2, 2017 at 16:49

Reverse divmod


The divmod function is one that is included in many standard libraries and is defined as follows:

enter image description here

Or, in other words, divmod(a,b) returns a list containing the integer quotient of a/b (i.e. floor(a/b)) and mod(a,b) (i.e. the remainder in the division of a/b).

Your challenge is not to implement the divmodfunction, but rather reverse it.


You will take two integers x and y as input, in any reasonable input format (two integers, list, etc.). Both x and y are guaranteed to be in the range -2^31 to 2^31-1, inclusive.


The output of your code shall be two integers, in any reasonable output format (two integers, list, etc.). Note that you must output the two integers in such a way that a human is able to distinguish one from another. These two integers should be an a and b such that divmod(a,b)==(x,y) (i.e. floor(a/b)==x && mod(a,b)==y). Please specify in your answer how these two integers are outputted if it is not immediately clear (for example, if you output b before a).

Remember that this is , so shortest solution in bytes wins!

  • \$\begingroup\$ Note to self: add I/O examples \$\endgroup\$
    – GamrCorps
    Jun 3, 2017 at 2:10
  • 1
    \$\begingroup\$ So: given x and y output x*b+y, b for any b > y if y is positive or b < y if y is negative? Or have I missed something? \$\endgroup\$ Jun 3, 2017 at 12:59
  • \$\begingroup\$ @PeterTaylor No you haven't, I realized that the challenge is much simpler than I originally thought (because of the method you mentioned). I am going to try and increase the difficulty later. \$\endgroup\$
    – GamrCorps
    Jun 3, 2017 at 13:05

Help me golf my numbers! (Part 2)

Thanks for all your help in Part 1! I recently just found out that my language also supports expressions, and we can use those to make our numbers even shorter!


Write a full program that takes in a list of integers less than 2^53-1. For each integer, rewrite it in the shortest way possible using expressions, and output the result.

Allowed operators

The operators below are given in order of precedence from highest to lowest, with groups separated by empty lines. (These precedence levels are the same as Python.) All binary operators are left-associative, except exponentiation, which is right-associative. Note that some operators are two bytes in length.

**: Exponentiation

* : Multiplication
/ : Integer Division
% : Modulus

+ : Addition
- : Subtraction
~ : Bitwise NOT

<<: Left shift
>>: Right shift

& : Bitwise AND

^ : Bitwise XOR

| : Bitwise OR

You may use decimal, hexadecimal, or scientific notation to represent integers in your output (the answers to Part 1 will help you choose the shortest representation for each integer). You may also use parentheses to group subexpressions.


I will post a list of 1000 integers to be used as the test battery. A program's score will be the size of the output for the provided test, where the lowest score wins. A solutions is invalid if any of the expressions do not evaluate to the given integer. There will also be an execution time limit of 20 minutes for all 1000 inputs (roughly 1 second per input) in order to discourage brute-force solutions.

Numbers for the test battery will be chosen according to the following algorithm:

def choose():
    msb = randint(0, 52)
    return randint(0, 1 << msb)

with duplicates filtered out.


  • Is there anything here that can be clarified?
  • Anything else that can make this challenge more fun?
  • \$\begingroup\$ Sandbox #2 - I don't think bitwise operators will make it too broad. #3 The community has generally agreed that penalties do not make the challenge any more interesting. General advice- make some more rules. Clarify what you mean by "shorten them as much as possible", I had to read that three times to understand. And what do you mean by takes a list of integers? How does one shorten a list using arithmetic? \$\endgroup\$
    – MD XF
    Jun 2, 2017 at 22:32
  • \$\begingroup\$ You should specify how your expressions handle precedence and associativity. I assume expressions can use any number literals? \$\endgroup\$
    – xnor
    Jun 2, 2017 at 22:38
  • \$\begingroup\$ I think it would be good to say something about you're choosing numbers for the test battery. If they are random, heuristic solutions will be effective (or maybe just hardcoding?). Or, if you intentionally choose inputs that benefit from rarely-useful operations like `%, that would be useful to optimize for. \$\endgroup\$
    – xnor
    Jun 2, 2017 at 22:41
  • \$\begingroup\$ @xnor Yes, they may. I'll add information about precedence and associativity as well. \$\endgroup\$ Jun 2, 2017 at 22:41
  • \$\begingroup\$ @xnor Good point. My plan was to choose randomly, do you think this is a good idea? \$\endgroup\$ Jun 2, 2017 at 22:42
  • \$\begingroup\$ I think random would be good. Maybe with some bias towards shorter numbers so they're not all 50-ish bits. It might hard though to beat hardcoding, though the bitwise operations might help. Perhaps you could try to hand-optimizing some random numbers and see how well you do. I think you also need something to rule out brute-force solutions, like a run-time bound. \$\endgroup\$
    – xnor
    Jun 2, 2017 at 22:46
  • \$\begingroup\$ I feel like this might be a dupe. \$\endgroup\$
    – Shaggy
    Jun 3, 2017 at 10:54
  • \$\begingroup\$ @Shaggy could you link to the duplicate please? \$\endgroup\$ Jun 3, 2017 at 12:12
  • \$\begingroup\$ I haven't figured out the right search terms to get me there yet. \$\endgroup\$
    – Shaggy
    Jun 3, 2017 at 12:29
  • \$\begingroup\$ @xnor I added a possible algorithm for choosing numbers as well as a time bound. Do you think this is ready for posting? \$\endgroup\$ Jun 3, 2017 at 18:39
  • \$\begingroup\$ @musicman523 Having thought about it a bit more, I worry hardcoding the number is too effective. It seems really unlikely a random number allows a shorter arithmetic expression than its digit length. I'm not even sure such a number would appear in 1000 samples. \$\endgroup\$
    – xnor
    Jun 3, 2017 at 19:14
  • \$\begingroup\$ I agree. I previously entered 'there can be only one' ( codegolf.stackexchange.com/questions/92962/there-can-be-only-1 ), which was simliar but only allowed using '1'. I don't believe the operators will be useful for almost all numbers, and if you require an operator is used people will just use '+1' most of the time. While a break from your original plan, you could (for example) require using scientific notation, with one number before and after the 'e', so things like '2e5' and '3e7'. That would make operators required and useful. \$\endgroup\$ Jun 4, 2017 at 21:12
  • \$\begingroup\$ Maybe make operators cost less than digits. e.g each operator is 2 and each digit is 5 in order to discourage hardcoding \$\endgroup\$
    – H.PWiz
    Jul 9, 2017 at 15:53
  • \$\begingroup\$ Not a bad idea. I'll probably revive this at some point. Thanks guys! \$\endgroup\$ Jul 9, 2017 at 18:43
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