# Sandbox for Proposed Challenges

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

## Posting

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.

## Discussion

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
• 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.

If you think one of your posts needs more feedback, but it's been ignored, you can ask for feedback in The Nineteenth Byte. It's not only allowed, but highly recommended!

It is recommended to leave your posts in the sandbox for at least several days, and until it receives upvotes and any feedback has been addressed.

## Other

Search the sandbox / Browse your pending proposals

The sandbox works best if you sort posts by active.

To add an inline tag to a proposal use shortcut link syntax with a prefix: [tag:king-of-the-hill]. To search for posts with a certain tag, include the name in quotes: "king-of-the-hill".

Get the Sandbox Viewer to view the sandbox more easily!

# Generalised multi-dimensional chess knight's moves code-golfcombinatoricschessarray-manipulation

Posted

• You know that the necessary conclusion is we do all the pieces - take my +1 and start the chain. Jun 14 at 6:26
• @StackMeter I don't think most of the pieces would be very interesting. Pawns in combination with details of what pieces are already on the board, maybe. Otherwise, it's just this challenge with some slightly simpler vectors Jun 14 at 6:48

# Write a C++ demangler

• Is _ZN3foo3barE3baz -> foo(bar)::baz valid?
– tsh
Jun 7 at 8:39
• That would be foo::bar::baz. The base identifier is baz, and it is prefixed with the namespace foo::bar. Jun 7 at 14:27
• Won't foo::bar::baz be _ZN3fooEN3barE3baz or _ZNN3fooE3barEbaz?
– tsh
Jun 8 at 6:14
• No, nested namespaces are placed together without a separator. Jun 8 at 12:39
• Decided to remove the "if it doesn't start with _Z, then it is to be printed as-is" as that adds unnecessary complexity. Jun 13 at 23:55
• I've edited this down to a stub now that it's been posted to save space Jun 17 at 20:30

# Create word lightning ascii-artstringtree

Posted

• Trees can be taken in different formats, right?
– Wezl
May 14 at 13:35
• yes, they can be taken in any suitable format. May 14 at 13:47

# Reconstruct a recursively prime-encoded integer

• Looks great! At first glance seems easy but it's actually a little more difficult. I think it's ready to post, although you might want to wait a day or so.
– user100690
Jun 20 at 12:14

# Play Thud

Thud is a game described by Terry Pratchett in his novel, Thud!.

The game simulates a battle between the Dwarfs (in blue) and the Trolls (in green) on an octagonal board with the Thudstone (an impassable space) in the centre of the board.

I have created an environment to play the game and develop game playing code at: https://ajfaraday.github.io/Thud/dist/index.html

The challenge is to write the most successful dwarf or troll player of this game (these will be two separate challenges).

# Rules

Starting with the Dwarfs, players take it in turns to move.

## Dwarf Movement

On the Dwarf player's turn, they can move one dwarf piece either as a walk or a hurl.

Walk: Dwarfs can move as far as they like in any direction until they hit an obstacle (another dwarf, the edge of the board, or a troll). They can only kill a troll by walking if they are only one space away.

Hurl: If two or more dwarfs are in a line (horizontal, vertical or diagonal), they can hurl the dwarf on the end of the line, by the length of the line (e.g. in a line of 3, the dwarf on the end can be hurled 3 spaces). If a dwarf is hurled into a troll, the troll is killed, reducing the trolls score by 4 points.

## Troll Movement

On the Troll player's turn they can move one troll piece, either as a walk or a shove.

Walk: Trolls can move one space in any direction, unless a troll, dwarf or the edge of the board is in the way. Whenever a troll moves, it kills all dwarfs adjacent to it's destination space.

Shove: If two or more trolls are in a line (horizontal, vertical or diagonal) they can shove the troll at the end of the line that number of spaces away, but only if any of the target space's immediate
neighbours contain a dwarf. When a troll is shoved, it kills all dwarfs on or adjacent to it's destination space.

It is not permitted for a troll to land directly on a dwarf by either walk or shove moves.

Each dwarf killed reduces the dwarfs score by 1 point.

## Scores

The score is calculated thus:

• The dwarf player has one point for every dwarf remaining on the board.
• The troll player has four points for every troll remaining on the board.
• The key figure is the difference between these two. This will be used to calculate players' scores in the tournament.

## Ending the game

The game ends when any of these conditions is met:

• There are no dwarfs on the board.
• There are no trolls on the board.
• Both players have declared the game over.
• The game has reached it's cut-off length of 500 moves.

# How to manually play a game

• Hover the mouse over a piece to see it's available moves.
• Safe moves are outlined in green.
• Dangerous moves (which can be killed the next turn) are outlined in orange.
• Killing moves are highlighted in red when the mouse hovers over them.
• Click a piece to select it for the current move.
• Click one of the available moves to move the piece.
• (You can click the relevant 'Make Peace' button to declare the game over according to that player, during their turn)

# How to set up a local instance of the game

You don't have to clone the repository and use it locally to to create an entry, but it helps.

• git clone git@github.com:AJFaraday/Thud.git
• cd Thud
• npm install
• You can then run ./get_answers.sh to get the latest entries from Stack Exchange

If you prefer, you can use the github pages instance at https://ajfaraday.github.io/Thud/dist/index.html

# How to customize a game

• Open /dist/index.html in your browser
• Click 'Customize'
• Select troll and dwarf clients (manual allows direct control)
• Select a turn time in milliseconds (only relevant to non-manual players)
• Click 'Run Game' to see or play the game.
• (Clicking 'Close' will not enact any changes)

# Clients

The game is played by clients, which represent either a troll or a dwarf player. Each is a JavaScript class which must have these three functions:

• constructor(controller) - controller is an object which acts as your interface with the game (see below).
• turn() - This is called whenever it is your players turn to move.
• end_turn() - This is called after your player's turn is over. It can not move pieces, but can make decisions on whether or not to declare the game over.

## Controller

The controller object is your client's interface with the game itself. You can find full documentation for the controller class here: https://github.com/AJFaraday/Thud/blob/main/docs/controller_interface.md

It provides these methods to interrogate the state of the game:

• turn() - Current turn of the game

• scores() - The current score

• spaces() - Every space, and what's in it

• space_info(x, y) - Detailed information on any space on the board.

• dwarves() - The location of every dwarf

• trolls() - The location of every troll

• pieces() - All pieces belonging to the current player (equivalent of dwarves() or trolls())

• indexed_dwarves() - The location of every dwarf with a fixed index

• indexed_trolls() - The location of every troll with a fixed index

• previous_move() - What got moved to where last turn

• killing_moves() - All moves which can kill one or more opponent

• current_space - Currently selected space (not a function)

• clear_space() - Empties currently selected space These methods are used to actually make your move:

• check_space(x, y)- Find out what moves are available from a given space

• select_space(x, y) - The player decides to move a piece at space.

• check_move(x, y) - Find out what will happen if you move to a place

• move(x, y) - The player moves the current piece to the selected space.

These are concerned with ending the game:

• declare(game_over) - Say whether or not your player thinks the game is over.
• opponent_declared() - Has the opponent declared the game over?

# How to write a client

Warning: There is an issue with the project on Firefox (https://github.com/AJFaraday/Thud/issues/3) which prevents editing the code in the browser. This has been confirmed to work in Chrome.

• Open 'dist/index.html' in your browser.
• Click 'Customize'.
• Select 'dwarf/template' as the Dwarf player (or use another client as a starting point).
• Click 'Edit' beside the Dwarf player select.
• Write your client code in the text box provided.
• The Validate button will change colour based on whether or not the client is passes validations (see below).
• When you're happy with it, click 'Apply' (This can be done before it passes validation, but it may not actually work).
• Select a worthy opponent and click 'Run Game' to see the game.

## Validations

In order for a client to work, and therefore be enterable in the challenge, it has to pass these validations:

• It must evaluate as Javascript code.
• The code must return a class, with a constructor which accepts one argument.
• Instances of this class should have functions named turn() and end_turn()
• The client must play a game until it is over (i.e. it must call a valid move during every turn call). The validator will run games against default opponents to determine if this happens.
• Does not have any forbidden terms ** game. - Only interact with the game via controller ** Math.random - Please keep it deterministic ** setTimeout or setInterval - Keep it sequential
** eval, require or import - Just don't

You can open the developer console (F12) to see more detailed information on your client's validation process.

## How to save a client

If you have cloned the git repository, you can save your entry for future tinkering. This step is not required for entry in the challenge, but it may be helpful.

• Edit a client, as above.
• When you're happy with it (preferably if it's passing validation, too), click 'Copy' from the edit interface.
• Create a .js file in /src/clients/dwarf/entry with the name of your entry e.g. /src/clients/dwarf/entrygreat_dwarf_player.js. (This folder will not be wiped by get_clients.js)
• Run node script/get_clients.js from the Thud directory to make your entry available from the Dwarf player select. You only need to do this once to make it avilable.
• npm run build - this will keep watching for changes in your entry and updating the package.

## How to enter your client in the competition

• Decide on the name of your client, your client_name must only have alpha characters and underscores.
• The first line of your answer should be your client's name as a title (with = characters under it on the second line)
• There should be a code block containing the class for your entry (with or without the preceeding module.exports =)
• After that please include a brief explanation of your client's behaviour, and any other information you'd like to include.

Once this is in place, anyone running ./get_answers.sh will see your client available under your username.

The GitHub Pages instance will also be updated periodically. So by making an entry, your code will be added to the repo.

# Tournament rules

The tournament will pit every available dwarf client (in /src/clients/dwarf/) against every available troll client (in /src/clients/troll/), and each pairing will play exactly one game.

The difference between the two players' scores will then update a running total for each client. The winner will gain the difference, and the loser will lose the difference.

There are two winners in the tournament, the most successful troll player and the most successful dwarf player.

According to the rules, after playing a game, the players swap sides, so please also write an entry on the Troll challenge.

This is now nearly complete (apart from some UI improvments and presenting the tournament results a bit more nicely). There's a working example of the code importer working against these two meta questions:

I could really use someone to attempt an end-to-end run at creating a client and adding it to one of these to check that my instructions are clear and everything works.

• Very well described challenge. Seems nice! Jun 20 at 14:17
• @EliteDaMyth Thank you for taking a look. Glad it looks complete. Jun 20 at 22:43
• Two rule questions: 1. in which direction can be hurled/shoved? 2. It is not permitted for a troll to land directly on a dwarf by either walk or shove moves. but both troll moves have lines like kills dwarfs on … destination/…only if the target space … contains a dwarf.
– xash
Jul 2 at 14:17
• @xash Good questions, I'll update to be clear. The answer is always 8 directions vertical, horizontal and diagonal. Also, I learned about the rule that trolls can't land on anything after writing the rules. I should have updated the steps too Jul 2 at 14:31

# Splinter metagolf

• An example for a short repetitive string would be nice Jul 12 at 9:42
• @pxeger Ok, I'll have a look. Jul 12 at 9:43

## Which character to change (Robbers)

• I think cops should definitely be able to choose what strings they print (they obviously should reveal them to the robbers) Jul 13 at 8:30
• @pxeger that's a good idea
– math
Jul 14 at 13:45
• Minor suggestion: Move the expected outputs to the start, because you have to scroll all the way to the end to see them.
– user
Jul 19 at 23:59
• @user Done, thanks. Jul 21 at 4:36

# Visual Encoding

I want to create a program to randomize certain words, however, I would like all the swapped letters to have the same form factor as the previous one.

#### Challenge

Given a string of only lowercase letters (and no spaces), randomize its letters according to the following groups:

1: acemnorsuvwxz
2: bdfhiklt
3: gpqy


Each letter cannot be transformed into the same letter as it started as. Additionally, choosing the new character must be uniformly random (within codegolf guidelines).

One final thing is that for the letter j, it must be transformed into either group 2 or group 3, and this can be done by either:

Uniformly choosing between each group and then uniformly choosing a letter or Uniformly choosing between any of the letters in both groups

Note that nothing can turn into j itself.

#### Examples

helloworld -> kadfrszmhl
jamaica -> genokac
jamaica -> penokac
abpj -> odyt


This is so the goal is to create the shortest answer in bytes.

# Parse some Husk (WIP)

Husk is a "functional golfing language inspired by Haskell." Its syntax is prefix, albeit with a twist: Husk's functions can be curried: so uses its static typing to determine how many arguments a function should take at a time. For example, Husk can tell that m+2:2;3 should be parsed as m(+2)(:2(;3)) and not, say, m(+2(:2(;3))) or m(+)(2:2;3), which are meaningless.

This challenge involves validating a subset of Husk that has 5 functions and two types: integers 0-9 or lists of those integers. It also does not have parentheses or overloading. Your submission will take a string consisting only of the characters mo;:+0123456789 and determine whether it is a valid program according to the rules below.

In the following descriptions, "unary integer function" refers to a function that takes an integer and outputs another. It's a made-up term, let me know if there's a better one. "list" refers to a list of integers, and "integer" refers to an integer 0-9. You don't need to understand the purposes of each function, just the types of their inputs and outputs.

• 0-9 are values/integers.
• ; is the unary function singleton. Its argument is an integer x, and it returns a list ([x]).
• : is the binary function prepend. Its first argument is an integer x and the second is a list l. It returns x prepended to l ([x, ...l]).
• m is the binary function map. Its first argument is a unary integer function f and the second a list of integers l. It returns [f(l[0]), f(l[1]), ..., f(l[-1])].
• o is the trinary function compose. Its first argument is a unary integer function f and the second a unary integer function g. The third is an integer x. It returns f(g(x)).

Here is what their types might look like in Haskell:

(;) :: Int -> [Int
(:) :: Int -> [Int] -> [Int]
m :: (Int -> Int) -> [Int] -> [Int]
o :: (Int -> Int) -> (Int -> Int) -> Int -> Int


Here is pseudo-pseudo-pseudo-not-even-BNF-anymore:

<int> ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | + <int> <int>
<list> ::= ; <int> | : <int> <list> | m <unary-int-int> <list>
<unary-int-int> ::= o <unary-int-int> <unary-int-int>
<valid-husk-program> ::= <int> | <list>


# Questions for Meta:

• Is this collection of functions okay? Should I add more or replace/remove some?
• Is this challenge interesting?
• Is this a dupe?
• Is the explanation good enough? How can I make it clearer?
• This currently doesn't have a lot of variety in the currying. Should the functions given to map/compose also be allowed to input/output lists? (and if so, would lists be allowed to be nested?)
• Can Perl regex do this? I'm making this challenge hoping that it can't.
• You haven't really specified what the output is. The closest is saying the task is to "validate", but what do we actually need to do? Aug 20 at 16:10
• are we validating or executing(akin to the gelatin challenge)? Sep 3 at 7:52

# Distances between keys on a QWERTY keyboard

• fro kolmogorov i suggest keeping everything in the same unit. Aug 19 at 3:58
• All jokes aside, this is a well specified challenge. Aug 19 at 14:05

# Fastest untyped lambda calculus evaluator fastest-codefunctional-programminglambda-calculusinterpreter

## Challenge

What it says on the tin. Mainly because googling "fastest untyped lambda calculus" gives almost zero meaningful results.

Each submission is expected to take a lambda term from STDIN and print its normal form to STDOUT. The lambda term is represented using de Bruijn indexes, and we will use prefix notation for this challenge. Since a de Bruijn index may have multiple digits, each token will be separated by a single space. The input will have no surrounding whitespace, but you may output any amount of whitespace before and after the formatted lambda term.

LambdaChar = "\"             // single backslash
DeBruijnIndex = [1-9][0-9]*  // a positive integer
ApplyChar = "@"
Term = DeBruijnIndex | LambdaChar " " Term | ApplyChar " " Term " " Term


For example, \ \ @ 1 @ 2 1 represents lambda x. lambda y. y (x y).

The evaluation semantics to implement is normal order beta-reduction (no eta-reduction).

The test cases will be hand-crafted so that it takes significantly more time to evaluate the expression than to parse the input and format the output. Also, they will involve various kinds of Church- and Scott-encoded terms, so optimizing for any specifically encoded data (hopefully) has less effect than optimizing for general improvement. It is guaranteed that the test cases have a normal form and do not contain free variables.

Good starting points include this PEPM '17 paper and my Haskell implementation which was modified from the paper's algorithm to actually return the normal form. Other notable keywords: graph reduction, supercombinators, G-machine, TIGRE, STG (spineless tagless G-machine). Note that, if your submission has separate compilation and execution phases, both phases count towards the total execution time (which may negatively impact your score).

The submissions will be scored within WSL (Ubuntu 20.04) on my Windows 10 PC, which has Intel Core i7-6700 CPU (3.40GHz) and 8GB of RAM. The score is the sum of the timings measured for all the test cases. Lowest score wins.

## Meta

• Todo: write example and actual test cases.
• Should I include a description about how the "normal order beta reduction" works for de Bruijn indexes?
• are you planning to actually test with >9 levels of lambda nesting?
– ngn
May 26 at 2:48
• @ngn Depends on what I come up with. May 26 at 3:04

# Light-Cycle KotH

Basically the light-cycle game as a KotH competition.

## The Game

Players will write bots that compete in a light-cycle arena. A round of light-cycle starts with bots randomly spaced out along the edge of a (tentatively 10x20) rectangular play area:

+--------------------+
|    @          @    |
|                    |
|                    |
|                    |
|                    |
|                    |
|                    |
|                    |
|                    |
|    @          @    |
+--------------------+


Each turn, each bot simultaneously must choose one of 4 directions to step (N/E/S/W). A space that was once occupied by the "head" of the bot becomes part of the "body" of the bot.

+--------------------+
|    #         @#    |
|    @               |
|                    |
|                    |
|                    |
|                    |
|                    |
|                    |
|               @    |
|   @#          #    |
+--------------------+


The round ends as soon as one or more bots crash into a wall, or into the head or body of another bot (including its own head/body).

         *CRASH*
+-----------@--------+
|    #      #####    |
|    ###             |
|      #             |
|      @             |
|                    |
|   @                |
|   #                |
|   #            @#  |
|   #           ###  |
|   ##          #    |
+--------------------+


The bot(s) that crashed into the wall will be removed, and the game will start again with new starting positions. (If multiple bots crash on the same turn, the most-recently submitted bot will be eliminated and the others will survive) (Update: If multiple bots crash on the same turn, they will all be eliminated)

Bots are eliminated each round, until only 1 bot remains. The leaderboard rankings are based on the order that the bots are eliminated.

## Code / API

For ease of judging and creating the online platform, submissions will be restricted to javascript.

A bot is a javascript function with the following API:

(state) => {
// bots can use the state to make decisions
// bots return one of "N", "E", "S", "W"
// e.g.:
return "N";
}


The state variable contains the following API calls:

• state.getDim(): Returns a [w, h] tuple of the width and height of the arena. e.g. [20, 10]
• state.getSelf(): Returns a [x, y] tuple of the current bot's head. e.g. [3, 7]
• state.get(x, y): Given integers x and y, returns a string corresponding to what is currently on the map at (x, y)
• "#" - body of a bot, or out-of bounds of the map ("wall")
• "@" - head of a bot
• " " - empty cell

(Coordinate system: The bottom-left-most cell of the play-field is (0, 0), with x increasing to the left and y upwards)

## Code submission

I plan on making a GitHub repository containing the source code for the judge. The repo will include an HTML file so that it is possible.

Submissions will be made by making a PR to the repo to add their bot's source code (in addition to posting the submission as an answer, of course!)

• I'd recommend removing all bots that crash in the same turn. Basing it off of external factors like submission dates is a bit unfair. Sep 13 at 17:04
• My justification for removing only 1 per turn is that it simplifies the logic of generating a leaderboard and removes some edge cases, otherwise things like 3-way ties for first place might be possible if all the bots crash at the same time. Sep 13 at 20:51
• I feel like biasing the rules against newer submissions is fair. After all, new submissions inherently have the advantage that they're able to see the older submissions and can strategize around these existing submissions. Sep 13 at 20:55
• The issue is that the oldest submissions functionally move first, meaning that being the first bot posted is a significant advantage. What about edits? If a bot is changed, does it drop to the bottom? I agree that removing all bots which crash is the right call. Sep 14 at 18:36
• Hmm... I was going to disallow edits, so that anyone who wanted to update their bot must instead post it as a new bot. @Spitemaster Are edits typically allowed for KotH submission? Sep 14 at 20:27
• Frequently; not always. But you've still got the problem of earlier entries effectively moving first. If you really want to do it this way, it doesn't make sense to have simultaneous movement (because it's not really simultaneous). Sep 14 at 20:34
• Ok, that seems fair, I will update post to allow for simultaneous movement Sep 14 at 20:49
• If I were you, I'd consider sizing the arena based on number of bots. Sep 14 at 20:56
• That seems like a reasonable suggestion, what kind of scaling do you think would be good? I'm thinking maybe something like a square with side-length 2n+6, for n players. Sep 14 at 21:02

# Metagolf: Catlike Piet

The goal of this is to write a catlike program, which would be executed (in a Unix environment, though you needn't stick to that) by the following:

yourprogram < file > output
piet output


where piet output writes the contents of file to stdout. That is, you're to generate a Piet program which prints the input to yourprogram.

One-liners

Straight line programs can be written in Piet... in straight lines. If you're willing to take a hit to your score, your output can take the form of a string of commands:

=  none (continue color block)
|  push
^  pop
-  subtract
*  multiply
/  divide
%  mod
~  not
>  greater
.  pointer
\  switch
:  duplicate
@  roll
$input number ? input character # output number ! output character  which is trivial to convert to a Piet program with the following (partially golfed) Python code: def P(s): h=v=0;l=len(s)+1;R="P3 %i 2 255 192 0 0 "%(l+2) C=[1,3,2,6,4,5];V=[0,192,192,255,0,255] for x in map("=|^+-*/%~>.,:@$?#!".find,s):
C=C[x//3:]+C[:x//3];V=V[x%3*2:]+V[:x%3*2]
for i in [1,2,4]:R+="%i "%V[(C[0]//i)%2]
return R+"255 "*4+"0 0 "+"255 "*l*3+"255 0 0 "*2


The dimension of said program is (n+3) x 2 if there are n characters in the string.

Scoring

Your code will be judged on the maximum dimension of the images that it outputs.

• Part 1: Take the maximum score taken over all ascii codes (that is, single-character inputs), discounting EOF.

• Part 2: Take the score for the input "Hello. My name is Inigo Montoya. You killed my father. Prepare to die."

Your score is the product of the scores in part 1 and part 2.

Punishment: Double your score if you write one-liners as above (that is, if you don't output an image).

Bonus: If your program is written in Piet, take the square root of your score above.

• It took me a while to understand the task as "Write a program taking INPUT which produces as output a piet program that takes no input but produces INPUT." I think it is a interesting and challenging, but it's reception will depend entirely on how many people are willing to learn/futz-around-in/deal-with piet. And I have no feel for how many that is. Jul 7 '11 at 3:12
• @dmckee; would it be better if I just used a reduced instruction set, and only ask for the instruction stream? I think this is still challenging with {push 1,duplicate,add,subtract,multiply,output}. Come to think of it, if I restrict to {push 1,duplicate,add,output}, there's a reduction to some awesome algorithms. Jul 7 '11 at 4:48
• I did this in piet some time ago: craigoclock.blogspot.com/2011/05/metaprogramming-in-piet.html May 21 '12 at 18:31
• 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.)
– user58826
Jun 9 '17 at 15:22

## Play Simple 2-Dimensional Minecraft

Recently I found this video of "HansLemurson" showing a computer that was built in minecraft, which runs minecraft. He is playing minecraft on a computer that was built in minecraft that is running on his computer. To be specific, it is a two dimensional version with an 8x8 grid of cells. There is gravity, block placement, and even jumping. It is worth noting that the computer is single purpose. The same person has built programmable computers, but making them single purpose allows the computer to be much smaller.

## Details

The minecraft world is an 8x8 grid (one horizontal and one vertical dimension). The grid is comprised of either Xs (representing blocks) or empty spaces. The player is an X that is blinking on and off about once every second.

There are two modes in the game, controlled by a toggle switch. The first mode is movement. This is controlled by a WASD-like button arrangement. If the player chooses to move left/right/down, the computer checks to see if the space immediately in that direction is empty. If so, then the player moves into that space.

If the player chooses to move up, then the computer checks that the block underneath the player is solid. If so, then the player moves upward two units. Notice that this can propel the player into a solid block. If this happens, the player is obscured by the solid block, but can still move to an empty block next to him. When the player is inside on a solid block, the game continues as if the block isn't there, although the block is still there once the player leaves it.

After each move, the player falls down one unit if there is empty space there. This simulates gravity. This is also why moving up moves up two units, so that the gravity makes a net movement of up one unit. Gravity does not cause the player to fall all of the way to the ground, just one unit.

The second mode is block placement. In this mode, the same exact WASD buttons are used. Instead of moving the player, they toggle the state of the block in that direction. If the player presses "left" and there is a block there, then the block is destroyed. If there is not a block there, then a block is placed. Again after this move, the player is again subject to gravity. The blocks are not subject to falling.

Toggling the toggle switch does not count as a move, and does not invoke gravity.

The game board is a torus, so all actions (movement, block creation) can wrap around the board. The board does not scroll with the player. The player moves, and the blocks stay in the same place.

## The challenge

You challenge is to write the shortest program that simulates this game. Your program should display and update the map correctly (with Xs as blocks, and with the blinking player). It should accept input from a button that toggles the state and four buttons for movement and actions. This is code golf.

There are imaginary bonus points for adding more features (block types, game size, etc) to your game.

## Suggestions?

• With more complicated challenges I find that it helps to do a reference implementation so that you have a very concrete idea of how much work is involved. Aside from that, I like it. Jun 3 '12 at 20:11
• Is the blink rate selected to fit with the ANSI escape sequence? Either way I would explicitly allow that, because it's the obvious way to do it on compatible terminals. Jun 5 '12 at 7:14
• The blink rate wasn't selected to be anything specific. I think that I will broaden the restriction. Maybe any blink rate between 3 blinks per second to 1 blink every 2 seconds. Jun 5 '12 at 20:21
• @programmer5000 No, for two main reasons: First, challenges can go extended periods of time in the sandbox before they are posted and/or adopted. In the past I've posted challenges after not touching them for 4 years. Second, deleting this answer will not reduce lag, as deleted answers are still present, simply not visible. Users with sufficient rep will see all 4040 answers in the sandbox, and you will too once you earn the "view deleted answers" privilege. Apr 13 '17 at 18:15

Overview:

Let's say you've decided to operate your computer using voice recognition software, but unfortunately you did a horrible job researching the various products out there and chose a package that does not recognize numbers as numerals, only words. (i.e. "one" (spoken) == "one" (typed), not "1".) Rather than spend more money to get another option, you decide to make do. Now you want to use the computer's calculator, but this poses a problem, since your machine doesn't know how to add "one plus one".

Objective:

Implement a basic calculator that will read in a string of the written-out equation, perform the correct calculations, then return the result in its text form. Your code should be as short as possible; this is code golf.

Rules/Constraints:

• Input/output will be using your preferred method (STDIN, ARGV, etc.).
• Your calculator must be able to handle input and output within the billions (non-inclusive) -1,000,000,000 < i < 1,000,000,000, but you may expand to more if you wish.
• Decimal values and/or parts must be accepted (0 < i < 1) up to 3 places/digits.
• When calculating answers, proper rounding must be used, so "three point one four one five nine two six" must be returned as "three point one four two".
• Basic calculator functions required:
• "Add"/"Plus"/"Sum"/"And" (+)
• "Subtract"/"Minus"/"Remove" (-)
• "Multiply"/"Times" (*)
• "Divide"/"Divided"/"Divide by"/"Divided by" (/)
• "Raise"/"Exponent"/"Power"/"To the power of" (^)
• "<Base>Root"/"<Base>Radical" (√)
• "Point"/"Decimal" (.)
• "Pi" (π)
• All strings in the list above must be accounted for in your code, capitalization does not matter.
• Numbers may be presented as their full value ("one thousand") or by digit (one zero zero zero).
• Negative numbers may be assigned using "Minus" or "Negative".
• The string "Minus" bust be accounted for as an operator and identifier. (see example)
• "And" is only acceptable as an operator, not as part of a named number.
• "one hundred and one"
• "one hundred one"
• "a" or the absence of a number does not equate to any number; all numbers will be explicitly accounted for in the program input.
• "a hundred" does not equate to "one hundred" and is not a valid input.
• No more than 2 terms will be used.
• "one plus one minus one" will not be implemented.
• If an invalid input is supplied, your function/program should handle the error and exit gracefully with an error description.

Example I/O:

• "one add one" --> "two"
• "five thousand thirty four subtract ten thousand six hundred" --> "negative five thousand five hundred sixty six"
• Alternatively: "five zero three four subtract one zero six zero zero"
• "three root twenty seven" --> "three"
• "ten minus minus ten" --> "twenty"
• Alternatively: "ten subtract negative ten"

## Sandbox Questions:

1. Is this too basic/complicated? (I'm assuming some languages will handle this much more simply than the method I have in my head...)
2. Does the title fit?
3. Are there any constraints that should be added/lifted?
4. Are any more examples needed for clarification?

• Not everyone says numbers the same way. Does the parser have to treat the following as equivalent? "negative one hundred five", "minus one hundred five", "negative one hundred and five", "minus one hundred and five", "negative a hundred five", "negative a hundred and five", ...? Jun 15 '12 at 15:12
• @PeterTaylor I had had a similar thought re: operators. ("plus" versus "add", etc.) I think it would be more interesting to account for all, but given the wide variety of possible inputs, it may generally be better to limit the options to a specifically defined set (which I have yet to define). Jun 15 '12 at 15:18
• @PeterTaylor I've added some of these details. Please let me know if there's anything unclear about them. Jun 15 '12 at 16:10
• I don't spot any ambiguities in the parser. There is still an ambiguity relating to decimals, though. What precision should be used? Also, I notice now that there's no winning condition. Is this intended to be code-golf? (Ugh - tons of strings which will have to be hard-coded in most languages. I expect Perl has a suitable parser already in CPAN, though...) Jun 19 '12 at 9:03
• @PeterTaylor I don't know where I went... I've updated the spec. re: decimal places and objective. Jun 29 '12 at 13:24
• @PeterTaylor metacpan.org/pod/Lingua::EN::Words2Nums Apr 27 '16 at 20:37

# Huffman Decoding

Write a programm which takes two strings as input and prints a text.

The first argument is a Huffman Tree, serialized in the following format:

• every ascii character except ~ is always a leaf, if ~ is the first characater it is also a leaf.
• <tree0><tree1>~ is a tree where <tree0> is the left subtree and <tree1> is the right subtree.

Example: ab~cde~~~ generates this tree:

 ┌─┴─┐
┌┴┐ ┌┴─┐
a b c ┌┴┐
d e


where a would have the key 00, b 01, c 10, d 110 and e the key 111.

The second argument is a text that has been compressed with with the Huffman code that is defined by the first parameter. This bit-string can contain any bit sequence (also null-bytes and non-printable characters) and is not byte aligned, therefore it has been encoded with a variation of the standard Base64 encoding:

• the characters used for the encoding are the standard base64 characters: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/
• the bitstring is broken up into 6-bit chunks and mapped to this characters
• if the last chunk is smaller than 6 bits, a character with this prefix is used, and padding characters are added to the string:
• - : the last chunk was five bits long
• = : the last chunk was four bits long
• =- : the last chunk was three bits long
• == : the last chunk was two bits long
• ==- : the last chunk was one bit long

Example:

bits:       1 1 1 1 0 1 1 0 1 0 0 1 1 1 0 1 0 1 0 0 0 1 1 0 1
chunks:    |1 1 1 1 0 1|1 0 1 0 0 1|1 1 0 1 0 1|0 0 0 1 1 0|1[0 0 0 0 0]|
characters:       9           p           1           G           g
base64:     9p1Gg==-


Your programm has to decode the text encoded in the second parameter and print it to stdout.

You have to provide your source code encoded in the way described above. The length of your encoded source code + the length of your serialized huffman tree will be the winning criterion.

TODO: example input

• It would be helpful to explicitly state the 64 characters used in the encoding. I presume they're A-Za-z0-9+/ but (especially if you're expecting people to implement that part explicitly) it's best to make the problem self-contained. Oct 8 '12 at 16:23
• 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.)
– user58826
Jun 9 '17 at 15:30

## Code golfing problem: Surface classification

The task: Given a surface-word reply with the classification of what surface it is.

Example 1: Input: aba'b' ----> Output: 1T

Example 2: Input: aabcb'c' ----> Output: 3P

Bounds on the problem: Since there are only 26 letters, there will never be more than that many labels. Additionally output should be in the form S,nT,mP for n,m positive integers.

Background: In the study of algebraic topology students are often presented with diagrams such as the one below. The represent instructions for how to assemble a surface. The assembly is prescribed as: if there are two edges labeled with the letter x then glue them together so that the arrows point the same direction. To make our job easy, topologists have discovered an algorithmic way to classify surfaces using 'words' assembled from these 'plane gluing-diagrams'.

Choosing a corner arbitrarily (top right) and orientation (ccw) we read off the labels on the edges where an inverse appears wherever the arrow points against the orientation. In this case the 'word' that represents this plane model is given as abab.

A surface word is a string that contains the letters a,b,...,@ up to some letter @ and each letter is contained in it exactly twice. In the two occurrences of each letter: 0, 1, or 2 of them may be postfixed by a ' which I am considering using to represent 'inverse' (opposite orientation).

If in a surface word all letters appear twice: once without the ' and once with it (f.ex. ba'b'a) then we say that the surface the word represents is orientable. If a surface is orientable then it is necessarily the direct sum of n Tori for some non-negative integer n. If this condition doesn't hold (like in aab'b) then the surface represented is non-orientable: in this case it is the direct sum of m Projective Planes for some positive integer m.

Once you have found out if the reduced word is orientable or not, the final answer is given as follows. If orientable and number of unique letters in the reduced word is 1 then output should be S. Otherwise if the number of unique letters in an orientable word is n (it will be even) then the output should be sT where s = n/2. If the word is non-orientable then the output should be mP where m is the number of distinct letters in the reduced word.

The goal is to take as input some surface word, reduce it via reduction rules 1-6 and then classify it as a sphere, some number of connected tori, or some number of connected projective planes. Here are the 6 reduction rules where ~ represents 'reduces to':

Let M,A,B,C,D be surface words, x be a single letter, and juxtaposition represents concatenation:

1. Cycle Rule: If M = AB then M ~ BA
2. Flip Rule: M ~ M'
3. Sphere Rule: Axx'B ~ AB
4. Block Rule: ABC ~ ADC if B is a surface word and B ~ D by 1 or 2
5. Cylinder Rule: If M = AxBCx'D, then M ~ AxCBx'D
6. Möbius Rule: If M = AxBxC then M ~ AxxB'C ~ AB'xxC

I am looking for input on:

• should this be code-golf or programming-challenge?
• how would scoring work?
• ???

If I feel satisfied with the question in a few days I'll post it to the site.

• If, for each input, there is only one correct output, then it should probably be code-golf. The scoring criteria would then be source code length. Jun 8 '13 at 14:33
• Yes, this is the case. In general however there is not a unique series of applications of the reduction rules for any given instance.
– Kaya
Jun 8 '13 at 16:21
• I don't think the order of explanation is correct. You should explain reduction before talking about "the reduced word". And "reduce it via reduction rules" doesn't entirely make sense, because the rules are presented as equivalences rather than reductions, and most of them don't have a "natural" direction. Jun 10 '13 at 8:49
• It's also occurred to me that you haven't defined the notation M'. Does it just consist of toggling the orientation of each token, or does it also reverse the entire string? And do you have test cases which between them force implementation of all of the reduction rules? Jun 11 '13 at 8:32
• Good call on the string inverse, yes you have the right idea and I will make it clear. I have a lot of test cases from when I did a number of these computations by hand in a university course and (anecdotal experience) I am pretty sure that it is possible to force the use of all the reduction rules (except maybe 4 which is really just a meta-rule for convenience when doing proofs). Additionally you have alerted me to some concerns regarding the form of the proper output: it's definitely underspecified. I'll put some work into this today.
– Kaya
Jun 11 '13 at 14:04

I have no idea how to create a good code golf question!

See this description of a ray tracer with source code that fits on a business card. The author stopped when the code size was 1337 bytes.

Achieving identical output, optimise for minimum code size. Execution time is not relevant.

• I think what you have here is a straight ahead golf. All languages. You need only define the requirements. Do you want identical output or do you want "good output encompassing <list of features>"? Oct 6 '13 at 17:22
• For a minimum feature list I'd suggest something like (1) it is ray tracer (2) supports point-like lights and shadow + ambient light (3) supports mirrored (implies reflection) and matte surfaces (3) all objects are sphere and overlaps are allowed. With no requirement for (a) anti-aliasing; (2) finite sized light sources; (c) atmosphere effect or (d) depth of field; or (e) tiling and gradients. Notice however, that the example supports at least (b), (d) and (e). Oct 6 '13 at 17:29
• BTW--The one you linked can get a little bit more with #define Q return (R was already taken for the rand wrapper) and #define O operator. Oct 6 '13 at 17:33
• I suggest reading the Teapot question in the sandbox Mk IV and the comments - it's not the same question, but some of the same issues are relevant, and it might give you ideas for improvements to the spec. Oct 6 '13 at 22:48
• Yes. Read the teapot question for guidance. Ultimately I decided that one was too big, but we did get into some pertinent details. Dec 1 '13 at 9:48
• This sandbox post has had little activity in a while and little positive reception from the community. Please improve / edit it or delete it to help us clean up the sandbox.
– user58826
Jun 9 '17 at 15:32

# Count unique characters in text.

Given a string for input, output the unique non-whitespace characters in that string along with a count of their occurrences. The list should be sorted in ascending order of ASCII code.

Examples

Input:

Hello, World!


Output:

Character    Count
!            1
,            1
H            1
W            1
d            1
e            1
l            3
o            2
r            1


Input:

The quick brown fox jumps over the lazy dog.


Output:

Character    Count
.            1
T            1
a            1
b            1
c            1
d            1
e            3
f            1
g            1
h            2
i            1
j            1
k            1
l            1
m            1
n            1
o            4
p            1
q            1
r            1
s            1
t            1
u            1
v            1
w            1
x            1
y            1
z            1


The actual formatting (headers, spacing, etc) of the on-screen output is up to you. The only conditions are that it must be sorted in ascending order by ASCII code, and it must be easy to tell what represents a character from the string and what represents a count of a given character. (For example, given a string of 99999999, the output should be explicit so that it is not confused as saying I have 9 8s.)

Ultimate challenge (taken from here):

JKqdJg+oJgiowgyIJgkS+gyxJdeS+gyxJ4yoJdybJdioJdqIJ4kS+KwFJ4QS+gzYJg+ow4vIJ4yxvd+IJgy=+dv=JdQx+gzbJrzx24zYJgkxJ4qLJKQxJ4yxJKqx+KqdJKqdJg+oJgiowgyIJgkS+gyxJdeo24yxJm+xJdybJdioJdqIJKi=J4wF+dvS+gzYJg+ow4zYJ4yxvdy=J4i=+Kv=JdQo+KqxJrzdJKzYJgkxJ4qLJgkxJ4yxJKvSJ4qbJKqdJg+oJgiowgyIJgkdJgyxJdeo24yxJm+xJdybJd+oJd+S+dz=J4wF+dvS+g+SJg+ow4vIJ4yxJ4voJgy=+dv=+dzdJgqxJrzdJKzYJgkS+dweJKQxJ4yxJKvSJ4qbJKq=24yYJgiowgyIJgkdJgzdJryo24yxJm+d24zxJd+oJdqIJ4kS+KwFJ4QS+gzYJ4y=2gzYJ4yxJ4voJgy=+dv=+dzdJgqxJrzx24zYJgkS+dweJKQxJ4fK+dQSJ4qbJKq=24yYJgiowgyIJgkS+gzdJryS+gyxJ4yoJdybJd+oJd+S+dz=J4wF+dvS+gzYJ4y=2gvIJ4yxJ4voJgy=+dv=JdQo+KqxJrzx24zY+dzS+dweJKQxJ4yxJKqx+KqbJKq=24vbJdyowgyIJgkdJgzdJryS+gyxJm+d24zxJdioJd+S+dz=J4wF+dvS+gzYJg+ow4vIJ4yxJ4voJgy=+Kv=JdQx+gzbJrzx24zYJgkS+dweJgkxJ4yxJKvSJ4qdJKq=24yYJgiowgyIJgkdJgzdJryS+gyxJ4yoJdybJd+oJdqIJKi=J4wF+dvS+gzYJg+ow4vIJ4yxJ4v=J4i=+Kv=+dzdJgqxJrzx24zYJgkS+dweJgkxJ4fKJ4qx+KqdJKqdJg+SJdyowg+oJgkS+gyxJdeS+gyxJ4yoJdybJd+oJdqIJ4kS+KwFJ4QS+g+SJ4y=2gzYJ4yxJ4v=J4i=+Kv=JdQo+KqxJrzx24zY+dzS+dweJKQxJ4yxJKvSJ4qbJKqdJg+oJgiowg+oJgkS+gzdJryo24yxJ4yoJdybJdioJdqIJ4kS+KwFJ4QS+g+SJg+ow4vIJ4yxvd+IJgy=+dv=JdQo+KqxJrzdJKzY+dzxJ4qLJKQxJ4yxJKqx+KqdJKq=24vbJdyowg+oJgkS+gzdJryo24yxJ4yoJdybJdioJd+S+dz=J4wFJ4QS+gzYJg+ow4zYJ4yxvd+IJgy=+Kv=+dzdJgqxJrzdJKzYJgkxJ4qLJgkxJ4yxJKvSJ4qbJKq=24vbJdyowgyIJgkdJgyxJdeo24yxJm+xJdybJd+oJdqIJKi=J4wF+dvS+g+SJ4y=2gvIJ4yxvd+IJgy=+dv=+dzdJKzbJrzdJKzY+dzS+dweJgkxJ4yxJKvSJ4qbJKq=24yYJgiowg+oJgkS+gyxJdeo24yxJ4yoJKzxJd+oJdqIJKi=J4wF+dvS+gzYJg+ow4vIJ4yxJ4v=J4i=+dv=+dzdJgqxJrzx24zYJgkxJ4qLJKQxJ4fKJ4qx+KqdJKqdJg+oJgiowgyIJgkS+gzdJryS+gyxJm+d24zxJd+oJdqIJKi=J4wFJ4QS+gzYJ4y=2gzYJ4yxvdy=J4i=+Kv=+dzdJKzbJrzx24zY+dzxJ4qLJKQxJ4yxJKqx+KqdJKqdJg+SJdyowg+oJgkdJgzdJryo24yxJm+d24zxJd+5


• This isn't really an interesting problem. The shortest answer is almost certainly going to be fewer than 10 characters. Dec 11 '13 at 12:19
• @PeterTaylor While I mostly agree with your comment - already the header line may contain more than 10 characters. Dec 12 '13 at 6:15
• "The quick brown fox jumps over the lazy dog." contains "e" three times. Dec 12 '13 at 6:16
• @Howard Thanks. I must be blind - it took me about five times of reading your comment to find it. Also, do remember that the header is optional to a certain degree - you just need to make sure the output is unambiguous as to which items are characters from the string, and which are character counts.
– Iszi
Dec 12 '13 at 7:02
• My brain instantly went into bash mode. wc and uniq practically solve half of this, but not in any particularly short manner.
– Rob
Dec 17 '13 at 20:31

## Golf a random Human Genome fragment with non-random features

A totally random genome fragment is easy enough: just spit out the letters ATCG in random order, and you're done. So let's try something a little less random and more useful to science.

• Accept an argument from the user for number of base pairs (20bp-10000bp must be supported, more if you wish)

• Accept an argument from the user for GC content. This indicates how frequently the generated sequence should contain the G and C bases as a percentage of total sequence length.

• Include at least one complete gene in every request of 500bp or more, where a gene is defined as an otherwise random sequence that begins with a start codon triplet (ATG) and ends with the first stop codon triplet it encounters (TAG, TGA, or TAA). The distance between the start codon and the stop codon does not have to be a multiple of 3.

• Vary gene content (the portion of the fragment that is "gene", inclusive of the gene's start and stop codons) linearly with respect to GC content (when sequence >= 500bp). At the extremes, when GC content is 0%, gene content is 10%; when GC content is 100%, gene content is 60%.

• Output a single-strand sequence that complies with the above specs and the user's given parameters. (i.e. a single row of letters will suffice since it is trivial to deduce the complementary strand of the DNA given the sequence of one strand)

• Calculate the actual GC content %, actual number of genes, and actual gene content % in the resulting fragment, and output a status line below the sequence conforming to the example format below. Percentages may be rounded to one decimal place. Actual values may deviate by +/- 3% from the expected outcome based on user's input.

GC content: 42.1% | Genes: 3 | Gene content: 32.1%

• Use any Internet, library, or built-in gene sequence generation functions or databases. Roll your own.

Sufficient randomness:

• For the purposes of this challenge, any built-in random/pseudo-random number generator function, GUID generator, well-seeded cryptographic hash function, etc. is considered an acceptable source of randomness.

What-ifs:

• What if another start codon occurs before the stop codon? E.g. ATGXXXATGXXXXXXXXXXXXTAG. This is acceptable, but the "gene" length in this case is calculated from the most proximal start codon to the stop codon.
• What if another stop codon occurs after a stop codon? E.g. ATGXXXXXXXXXXXXXTAGXXXXXXTAG This is also acceptable, but likewise the "gene" length is calculated from the start to the most proximal stop.
• What if both of these things happen? E.g. ATGXXXATGXXXXXXXXXXXXTAGXXXTGA. Here again, the "most proximal" principle applies and the gene content is demarcated by the innermost start and the innermost stop.
• Do "orphaned" start and stop codons that do not demarcate a gene count as gene content? No.

This challenge is code golf, so shortest valid code wins.

Post example output from a 500-bp request with GC content between 35% and 65%, and have fun!

• "Use hardcoded fragments for anything other than the start and stop codons." - why not? Specifying criteria for what counts as enough randomness should make these useless in any case. Speaking of which, you need to specify criteria for what counts as enough randomness. Feb 28 '14 at 5:54
• The only partial output example given flagrantly violates the spec. If the GC content is 42.1%, the gene content should be 31.05%, not 22.0%. The definition of "gene" is also imprecise: in the sequence AUGCCAUGCCUAGCUAA, which is the gene? Feb 28 '14 at 12:02
• @PeterTaylor AUG starts the gene, then come the CCA, UGC, CUA and GCU triplets, none of which terminate the gene. Now if there were three C's instead of two, then UAA would be the terminating triplet and the whole sequence would form a gene. I agree the definition is imprecise, though. Feb 28 '14 at 12:11
• @JanDvorak, (part of) the point of that example is that there are two AUG substrings. Feb 28 '14 at 12:30
• Good points. I was hoping to avoid having too much text, but that came at the expense of less clarity than the challenge demands. Edit forthcoming. Feb 28 '14 at 13:58
• Also, I've muddied the waters with RNA encoding and DNA encoding, (U vs T), which we can chalk up to a late night. Feb 28 '14 at 14:00
• Revised accordingly, although I remain open to suggestions on how best to frame the standards for acceptable randomness. I want something that won't be exploited by answers making no effort at randomness, but that doesn't have the pain-in-the-butt factor of generating 10mb+ of data and running a Diehard test battery. Feb 28 '14 at 17:20
• " This is acceptable, but the "gene" length in this case is calculated from the most proximal start codon to the stop codon. " - wait, what? In nature, the first one is the start codon, and the rest encode methionine. Under your scheme, methionine (which is an essential amino-acid) would be impossible to include into proteins. Your scheme would also be much harder to splice. Also, what happens to AUG substrings that are not triplet-aligned to previous AUG substrings? Mar 1 '14 at 9:25
• In nature, the first ATG encodes the start of a protein coding region and defines a reading frame (triplet boundary), the rest encode methionine and the first triplet aligned stop codon encodes the end of the protein coding region (and no amino-acid). Mar 1 '14 at 9:29
• As for the randomness, I'm not worried about the source of randomness (whatever native library is available is assumed to be good enough) but rather how the source of randomness is used (can we just start the sequence with a start codon and insert an end codon at just the right spot if it doesn't occur naturally sooner, then fill in with more random codons while avoiding ATG subsequences? Your "sufficient randomness" places constraints on the RNG (useless) but no constraints on how it's used (or that it needs to be used at all) Mar 1 '14 at 9:34
• My true random number sequence generator was sitting there watching silently as I typed away the sequence ACACACACACACAC.... It's all okay. The TRNG was capable of producing something better - it just didn't really get to it. Mar 1 '14 at 9:38
• In fact, the 3% tolerance for the CG content leaves no room for randomness when there are only 20 base pairs. I can shuffle the pairs and turn some A<->T or C<->G, but that's it. In fact, if the CG content is set to zero, the task is impossible: we want a gene content of 2 base pairs (which is itself impossible), but the start codon contains a G, and a single G in a 2bp sequence means a 5% CG content, 2% than is the limit. Not including a gene means that we are 7% under the gene content lower limit. Similarly, it's not possible to start or stop a gene with nothing but Cs and Gs. Mar 1 '14 at 9:45
• Yeah, the 20bp starting point is a bad idea. The problem with start codons is that I considered introducing the idea of promoters and decided that would make the whole thing too complex. So in the absence of promoters there has to be some way to determine which Met is the start codon vs an amino acid and the easiest simplification is to have no Mets in the gene. Likewise, for "not triplet aligned", I'm trying to avoid having to go into explanations of frameshift mutations (even though a Frameshift% would be a cool parameter). Mar 1 '14 at 14:29
• I am starting to think that all of these complexities should be included (this proposal stems from me noticing that most of the extant random DNA generators are pretty weak) and this should just be a popularity contest instead of a golf. Link a couple of good articles on the structure of the genetic code and let people add as many features as they wish. Making it a golf seems to be a catch-22 between too many compromises or a too-impenetrable wall of rules and conditions that will dissuade participation. Mar 1 '14 at 14:33
• Perhaps a code-challenge where people earn x points for each complexity implemented?
– user10766
Mar 2 '14 at 5:52

## DIM, the DIM Integer Machine

The DIM Integer Machine is an engine for producing integer sequences.

It has one major problem: To put it mildly, it's kind of...dim.

After producing a single number, it immediately forgets what sequence it was working on. The only thing it remembers is the last number it produced and the current direction of the search, either ascending or descending. (And of course, it remembers the methodology for finding numbers according to the commands it understands).

Consequently, the user is free to change their mind after each number by issuing a new command.

Suppose the DIM has just produced an integer square: 81

• User inputs P and submits the input.
• DIM understands that P is requesting the next prime number after 81
• DIM computes and returns 83.
• DIM forgets what it was doing.
• User inputs O.
• DIM understands that O is requesting the next odious number and returns 84.
• DIM forgets what it was doing.

The DIM functions only for numbers between 1 and 1,000,000. If the DIM reaches either extreme while performing a search it will reverse direction and continue searching.

(For example: If searching in ascending order for a prime when the last number was 999,999, it will encounter 1,000,000 which is not a prime, then switch to descending order and continue searching for the "next" prime by moving downward - 999,999...999,998, etc.)

The DIM remembers the last number as 1 when it is first activated for a searching session.

This is the full list of commands that the DIM understands:

• P - Next prime number
• S - Next square number
• F - Next Fibonacci number
• O - Next odious number
• W - Next wasteful number
• U - Next undulating number
• K - Next katadrome
• R - Reverse direction immediately; the next command will proceed in the new direction

Because the DIM is so...dim, it absolutely DOES NOT precompute lookup tables of numbers in these sequences. It is far too forgetful for that to work. The DIM also has no Internet connection, so it is unable to consult the Online Encyclopedia of Integer Sequences or other such sites. It also has a sense of pride, so it does not make use of built-in Fibonacci functions or NextPrime / PrimeIndex / PrimeTest type functions.

Given the parameters it knows - a starting number, a search direction, the type of number to find - it simply computes the next number by some means other than mere data retrieval.

The DIM may accept input interactively, or from a newline-terminated text file, or from a pre-initialized array. You may not pack extraneous data other than the command sequence into the input - play fair!

This is a code golf, so least number of bytes wins. Submit your program with output results for the following search sessions:

1. P O U R F O R U S O U R P R O W S
2. W O R K F O R P O O R F O R K S K O O P S R O O K S F O U R W O W S
3. P O O P O O P O O P P O O P P R O P S P R O W S P O R K S

It is assumed that you know what prime, square, and Fibonacci numbers are. A brief explanation of the other integer sequences follows.

Odious - a nonnegative number which has an odd number of 1s in its binary expansion. The first few odious numbers are 1, 2, 4, 7, 8, 11, 13, 14, 16, 19

Wasteful - a natural number that has fewer digits than the number of digits in its prime factorization (including the exponents). The first few are 4, 6, 8, 9, 12, 18, 20, 22

Undulating - has alternating digits of the form aba, abab, ababa, etc. Assume all U numbers are non-trivial, i.e. 3 digits or more. The first few: 101, 121, 131, 141, 151, 161, 171, 181, 191, 202, 212

Katadrome - A number whose hexadecimal digits are in strict descending order. The first few are 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 32, 33, 48, 49

• When I post the question, I'll also include external links to MathWorld or OEIS for those who need more detail on the less familiar sequences, but the explanations above should be sufficient for most, I think. Mar 6 '14 at 23:28
• Your definition of "undulating" isn't the one I'm familiar with, which just requires that the digits alternately increase and decrease. Also, it would be better to include expected answers for the test cases, so that submitters can use them as test cases rather than them serving just for you to say "No, this is buggy". Mar 6 '14 at 23:57
• Yes, that's my plan, I just haven't finished double checking my results for the test cases yet. OEIS and Mathworld have the strict 2-digit definition of undulating, but I'll make sure to make the definition here more prominent so it is clear which interpretation is meant. Mar 7 '14 at 0:04
• 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.)
– user58826
Jun 9 '17 at 16:09

# Unified format patcher

Write the shortest program that will take a patch file in the unified format from stdin and apply that patch. No external tools that do the process for you can be used.

### Clarifications

• Extra documentation about the unified format can be found here
• All file paths will be relative
• Only one file will be modified per patch
• Timestamps can be ignored
• The patch file will be valid and will apply cleanly to the file specified (it will not lie about line numbers, etc..)
• Assume all files being patched already exist, you don't need to create/delete files

### Extra

• -35 - Take an argument that allows you to unpatch a patch

### Example

/test/a.cpp

#include <iostream>
using namespace std;

int main() {
cout << "Hello world!";
return 0;
}


patch.txt

--- a/test/a.cpp
+++ b/test/a.cpp
@@ -1,7 +1,8 @@
#include <iostream>
+#include <vector>
using namespace std;

int main() {
-    cout << "Hello world!";
+    cout << "Goodbye world!";
return 0;
}


Run patch

patch.exe patch.txt


/test/a.cpp

#include <iostream>
#include <vector>
using namespace std;

int main() {
cout << "Goodbye world!";
return 0;
}

• Can the program assume that the @@ lines contain the correct line numbers? Mar 6 '14 at 17:52
• A good explanation of the patch file format is needed. If not too long, include it in the question. Else, provide a link. Mar 6 '14 at 17:53
• You forgot the obvious "no external tools" disclaimer. You don't want the patch \$1 answer. Mar 6 '14 at 17:55
• @ugoren thanks for the comments, I added some further clarifications. Mar 6 '14 at 18:38
• Does "The patch file will be valid (it will not lie about line numbers)" also mean that it will apply cleanly? Mar 6 '14 at 19:24
• @PeterTaylor yes, updated question. Mar 6 '14 at 19:51
• "The shorted program" should say "the shortest program", but other than that I think it's ready to go. Of course, no-one's actually going to do more than filter out the lines starting -, remove the first char from each line, and parse the line-numbers to work out how to splice the resulting text in. Mar 7 '14 at 0:01
• 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.
– user58826
Jun 9 '17 at 16:10

# Efficient Testing for Armstrong Numbers

An Armstrong Number (also known by different names, including Narcissistic Number; see Wikipedia for more information) is a non-negative number (for our purposes represented in base 10) that is equal to the sum of the individual digits of the number each raised to the power of the number of digits. For example:

2. The individual digits are 4, 0, & 7.
3. Since it is a three digit number, we raise each digit to the third power: 64 (4^3), 0 (0^3), & 343 (7^3).
4. The sum of those values is 407 (64 + 0 + 343).
5. Because the final sum is equal to the original number, it is an Armstrong Number.

By contrast:

2. The individual digits are 4 & 7.
3. A two digit number, so raise each digit to the second power: 16 (4^2) & 49 (7^2).
4. The sum of those values is 65 (16 + 49).
5. The final sum of 65 is not the original number, so it is not an Armstrong Number.

Your mission, should you decide to accept it: Write a program in any programming language (using only standard language features and libraries) implementing the most efficient algorithm possible to test the numbers from 1 through 18,446,744,073,709,551,615 (264-1) inclusive for "Armstrongness", generating a list of Armstrong Numbers, and only Armstrong Numbers, as output.

While any language is acceptable, it should be obvious that interpreted scripting languages will be at a disadvantage in the efficiency department. That being said, a superior algorithm in an interpreted scripting language can beat the pants off an inefficient algorithm in hand tuned assembly language.

## Winning Criteria

The algorithm that can check all possible candidate numbers for "Armstrongness" in the least amount of time on a reference computer will be the winner. The reference computer will have the following specifications: {approximately an AMD Phenom class computer with 8 GB RAM running Windows 7 Ultimate 64 bit}

• I don't know that this would belong in the (already very long, maybe too long) problem statement above, but other historical background. The class was for Fortran 77, and I was in a friendly competition with my TA to write the shortest version. I never could win that one, so I decided to write the most efficient version instead. Hence: I prefer efficiency puzzles to code golf (though code golf is fun too). Feb 20 '14 at 8:30
• This doesn't seem to have an objective winning criterion. You do list "criteria I'll be using to judge this", but a) it mixes specification with winning criteria; b) it combines factors without indicating their weight. Feb 20 '14 at 11:51
• The question also seems to be about twice as long as it needs to be. If you use the [link text](url) link notation you can shorten it slightly; you can also lose paragraphs by cutting the worked example and brute-force code (link to the existing question on narcissistic numbers instead); cutting the waffling about which languages you think have advantages; and simplifying the motivation. Feb 20 '14 at 11:57
• I think efficiency problems are not well suited to code-golf. The efficiency of an algorithm depends on too many factors. You could perhaps require the lowest number of power operations. Feb 20 '14 at 12:43
• @ugoren, 0 is easily obtained. Feb 20 '14 at 12:57
• @PeterTaylor, You're right. Still, trying to replace a time measurement with the number of operations of a certain type sometimes helps define the problem better. Feb 20 '14 at 15:12
• @PeterTaylor: I agree it is quite long, and will consider revisions to it. Feb 20 '14 at 21:43
• @PeterTaylor: I'm open to better phrasing of the "objective winning criteria" but really, it is pretty objective already. One, no wrong answers allowed in the winner. Two, how efficient is the algorithm (based on the range of numbers tested and time taken to test them). For example, an algorithm that tests all numbers through 9 digits in 100 seconds is faster than an algorithm that takes 20 seconds to test all numbers through 8 digits (10 times larger interval in only 5 times the time). How might you suggest integration of this with the problem statement? Feb 20 '14 at 21:48
• @PeterTaylor: Glad I included the disclaimer about failing eyesight, given that I searched for narcissistic numbers and came up with nothing. I either searched the wrong portion of PCG (meta) or I made a typo when spelling narcissistic. Feb 20 '14 at 21:49
• @ugoren: efficiency may not be suited to code golf, but my understanding was that this 'forum' was about "programming puzzles" and "code golf". I certainly would consider finding a more efficient algorithm to be like solving a puzzle, though maybe I'm alone in that, in which case no biggie. Feb 20 '14 at 21:51
• Edited the problem statement (which is still admittedly quite long, still considering other edits) by removing the final PPS paragraph and replacing the existing links as suggested. Feb 20 '14 at 21:59
• The winning criterion is still too imprecise IMO. (NB Of the judging criteria you list, the first is part of the spec, so it's an acceptability criterion rather than a winning criterion). A genuinely objective winning criterion allows me to calculate my score before I submit my answer. Mar 12 '14 at 8:47
• It should be much shorter in order to not discourage people from approaching your challenge. Almost all the text after the definition doesn't add anything to the challenge - beside "don't print wrong numbers" which is of course relevant. I also think that a more precise criterion should be given instead. Mar 12 '14 at 9:03
• I've posted a "radical" update to it. I suspect the new winning criteria will not be acceptable either, since it involves a "reference computer" for final timing. Very open to suggestions on how to restate it so that a crappy algorithm on fast hardware doesn't beat an efficient algorithm on slow hardware. Mar 12 '14 at 20:17
• The possibility that processor architecture or available memory affects the results is a tricky issue with fastest-code questions, but there isn't really a better way of comparing speed of programs than measuring on a large test case. I can at least measure how my program compares to someone else's on my computer, and know whether it's close or not. Mar 12 '14 at 21:23

## Amino Acids Matcher

In genetics, a codon is a set of three nucleotides, the most basic form of nucleic acids. A codon "codes" (no pun intended, that's the actual term used) for a specific amino acid. Given a string of DNA, it is converted into RNA form by taking the opposite complementary pair.

DNA    RNA
A      U (T changes to U)
T      A
C      G
G      C


You will be given a String of unknown length that contains multiple codons. You must convert them to RNA form and print out the amino acid for each. See here for a chart: http://en.wikipedia.org/wiki/RNA_codon_table#RNA_codon_table

## Sample Input

TACTCGGATACT

Is split into

TAC, TCG, GAT, ACT

We now change each letter to its reciprocal

AUG, AGC, CUA, UGA

And print out the amino acids

Methionine, Serine, Leucine, Stop

This would probably be

I know that this is most likely not sufficiently explained and might be too complicated. Additional, tell me if there is any incorrect information above.

• So basically this is a challenge to compress a lookup table. You should probably specify that the string will be a multiple of three characters (or specify what to do otherwise); and it would seem sensible to inline the lookup table so that a) the question doesn't rely on the external page remaining intact; b) you save everyone who wants to answer the question the hassle of calculating it. Mar 17 '14 at 12:42
• Thanks for the feedback. I'll update accordingly later today. Mar 17 '14 at 15:48

# Find words in word square solver

On social media I often see images with letters and in them are some positive words for people to find. I challenge you to write a program that finds all words in the puzzle that matches a input dictionary. An example of such puzzle is this one:

An ASCII representation I made of this:

XCUALOVEYKBWSNG
DUAWKCBEAUTYRJV
YOUTHFSMGNEZLPR
MHJREYWDKZLUSTJ
FSUCCESSDHEALTH
ENMQXPTIMELMSAQ
VEXPERIENCEGHBW
GHUMOURLOYMONEY
SYZPOPULARITYNA
AMKCFUNBXHUZYIX
CWIHYSHAPPINESS
HONESTYCFRIENDS
KPYJAETWPOWERQC
BTYACFREEDOMJMO
RIWINTELLIGENCE


Now I imagine we can find words horizontally, vertical and diagonal and all of the mentioned in reverse. The program must be able to take a square and a dictionary like this one and print all the matching words.

As a test case I give custom dictionary:

bar
bid
dir
dog
fed
foo
god
man
mod
set
sun


And a test square:

OGFIR
DOMAN
ODBID
OPGES
OGFIR


Your code should be able to print all but the two last words in the dictionary. For diversity you should specify how the cube and the dictionary is bo be entered.

This is so shortest code wins.

• What should be output? Only the matched words? Their positions? And directions? Apr 3 '14 at 15:47
• @JanDvorak Just print the words found. Do you think coordinates and direction can be given a bonus? Apr 3 '14 at 15:51
• Cube? I'm only seeing two dimensions. On a more general note, perhaps for questions of this sort it would be OK to assume the availability of a standard dictionary file like /usr/share/dict, and discount the characters used to access this file? What do people think? Apr 3 '14 at 15:55
• @squeamishossifrage OMG You're right. I meant square of course :-) I think people can choose. eg. The question is open for diversity like cat square.txt dic.txt | solver now, but I'm open for change that does not discriminate. Apr 3 '14 at 16:03
• How does the program know where the wordsearch ends and the dictionary starts? Apr 3 '14 at 21:39
• @PeterTaylor By mistake I made the test a rectagle, but I fixed that. The length of the first line would be the number of lines in the square. Anyway how the input is done I thought should be up to the solver so that they can choose to open files, read stdin or maybe more disturebing ways to get input in... Apr 3 '14 at 21:47
• 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.
– user58826
Jun 9 '17 at 16:30
• @programmer5000 It only got two upvotes so I let it be. Feel free to post it if you'd like. Jun 12 '17 at 15:27

## Collatz ...something

The Collatz conjecture states that every natural number n leads to the number 1 if the recursive function f(n) is applied to it defined as

f(n)=n/2    if n is even
=3n+1   if n is odd


Let "ai" be the value of f applied to n recursively i times so that a0 = n , a1 = f(n) , a2 = f(f(n)) ... ai = f(ai-1)

Let A be the set {a0, a1, ..., 1}

Thus, for n=10, we get the sequence

a0 = 10 --> a1 = 5 --> a2 = 16 --> a3 = 8 --> a4 = 4 --> a5 = 2 --> a6 = 1

and the set A as A = {10,5,16,8,4,2,1}

Your task is to write a function/program that will accept a set of naturals say I. You must output a set of numbers say C such that I is a subset of the union of the sets A for all numbers in C.

### Rules

• Network access is forbidden
• Any of the standard loopholes are forbidden
• Your program must end in less than 200 seconds. You may assume that all the input terms are less than 2^(45); however note that the individual terms of the collatz sequence can go higher.

### Input

• List/array of naturals in I as an argument to a function
• , or space or \n separated naturals in I on STDIN

### Output

• return a list/array/set of all naturals in C
• print all the naturals in C separated by \n

### Scoring

( ( (10)^(number of elements in C) ) * (sum of all elements in C) ) + ceil( 100*log(total number of bytes of your code) )


log() is the natural logarithm

Lowest score wins.

### Examples

Input:

I = { 16 , 32 , 40 }


Possible outputs along with the score

C=                   Score

{ 16 , 32 , 40 }     ((10)^(3))*(16 + 32 + 40) = 8000   + constant
{ 32 , 40 }          ((10)^(2))*(32 + 40)      = 7200   + constant
{ 32 , 13 }          ((10)^(2))*(32 + 13)      = 4500   + constant --> most optimal
{ 1024 , 320 }       ((10)^(2))*(1024 + 320)   = 134400 + constant
... Infinitely many higher numbers


where constant is ceil(100*log(code length))

In this case, the answer { 32 , 13 } is the most optimal.

Note: This is NOT code-golf even though the length of your program is considered. Please also provide a readable version.

I'm being flexible with the I/O so that the more verbose languages might get some benefit. You can write a complete program or a function or a lambda function. It is not required that your function(if you choose to write one) returns. Using a function for input while printing the output is fine if that makes the code shorter.

This will be tagged as

## Sandbox feedback

• Can anyone suggest a better title?

## TODO

• Scoring needs specific test cases. Perhaps the final score could be the average of all scores of the test cases.

• Needs a proper title.

• The timing constraint is not reasonable unless you also provide constraints on the number and size of the inputs. For any input for which the constraint is reasonable at all, I think that the first point of the spec is unnecessary: if a counterexample exists, it's right at the edge of what fits in a 64-bit number. The second point of the spec is currently quite difficult to understand. Apr 3 '14 at 9:47
• @PeterTaylor Is it OK now? Apr 4 '14 at 16:15
• Looking around a bit at the standard terminology, I think that it might be best introduced with something like "Each positive integer n generates a Collatz sequence by repetition of the map f(n) = n % 2 == 0 ? n/2 : 3*n+1. Define the orbit of n as the set containing the integers in its Collatz sequence, and the orbit of a set {n_i} as the union of the individual elements' orbits. Your task is to find an optimal set under the constraint that its orbit contain a specified subset." That then leads into the example. Apr 4 '14 at 16:48
• I'm not sure that it's justifiable to claim that for your example {I2, C5, C10} is "(not the most ideal)". Whether or not it is depends on which arrows are /2 and which are *3+1, which isn't shown in the example. It's also occurred to me, which I missed earlier, that your scoring system requires a bit more of a test suite: at present, you have no way of distinguishing between answers which get the optimal solution to one test case. And I suggest a title, based on my previous comment: "Optimal Collatz orbits". Apr 4 '14 at 16:52
• I suggest you to add a link describing what is a collatz sequence. As a non-mathematician, I find it hard to understand. There is extra whitespaces after  in your first code block.
– A.L
Apr 4 '14 at 17:12
• @PeterTaylor Edited a lot. Are you sure it is called an orbit? I couldn't find that term anywhere. Apr 6 '14 at 16:39
• It occurs 4 times in the Wikipedia page on the Collatz conjecture, and Google gives over 6 million hits for collatz orbit. Apr 6 '14 at 22:08
• 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.
– user58826
Jun 9 '17 at 16:31

# Filter out repetitive lines

Google Suggest doesn't show any results if a string contains more than 4 repetitions of a substring. More specifically, if a substring is repeated 4 times in a row, followed by the first character of that substring (i.e. abcabcabcabca or x x x x x), nothing is suggested. This rule changes slightly if the substring is all the same digit - a digit may be repeated 5 times in a row, but no more. This is probably to allow searching for ZIP codes like 22222. (This doesn't extend to strings like 1010101010, though.)

Let's simulate this behavior! Write a program that takes lines on standard input and echoes those lines back on standard output, unless the line fits the criteria for repetitiveness, in which case it's silently discarded.

Sample input:

a simple query
nananananananana
ffffgggghhhh
48719999936
abc abc abc abc asdf
xyzzzzzyx
122333444455555666666
repetitiverepetitiverepetitiverepetitive
erepetitiverepetitiverepetitiverepetitive
101010101
55555 zzzzz


Output:

a simple query
ffffgggghhhh
48719999936
repetitiverepetitiverepetitiverepetitive


(Google's behavior is actually quite a bit more complicated than this; there are a few exceptions to all of these rules, but let's just ignore those for this challenge.)

There was a similar challenge posted awhile ago (Recognizing Repetition in strings), but it was closed due to vagueness. I think the criteria proposed above are more than thorough enough.

• The current exceptions make it complicated enough to track what you're looking for: basically you're asking for grep -v ((.).+)\2{3}\1|([^0-9])\3{4}? Apr 19 '14 at 19:05
• @PeterTaylor I would like to try to solve it without regex, though. Apr 19 '14 at 19:10
• I had thought about regex, but I didn't think it would be that simple. Would adding more restrictions or banning regex help? Apr 20 '14 at 1:18
• @Fraxtil, my opinion is that as a general rule if you need to ban the obvious way of doing something then you might as well just abandon the question. (With the exception, obviously, of banning libraries which are specifically designed to solve the same problem. Regex being a general tool rather than something designed for this specific problem don't fall into that exception). Apr 21 '14 at 8:52
• @PeterTaylor, that's a good point. Maybe I'll revisit the idea later if I can find a way to make it more interesting. Apr 21 '14 at 18:56
• I did make a decent question out of doing a basic regex problem without the use of regex (I should have, in hind sight, banned basic pattern matching as well as regexes...Bash shouldn't almost beat APL in sheer character count in a code golf). Apr 21 '14 at 21:06
• @impinball "Bash shouldn't almost beat APL in sheer character count in a code golf" -- why? Apr 24 '14 at 12:53
• Or at least in that context (tr is a pattern matching replace algorithm with regex like functionality). I would be a little more likely to accept Bash's builtin pattern matching expansion than tr. Apr 25 '14 at 21:49

# Am I a Matroid?

## Input:

A list I that is a subset of the powerset of E={1,2,...,n} which represents the independent sets of elements of the purported matroid M=(E,I). Note that the cardinality of the ground set may be for the purposes of this question ignored. Any elements of E that appear in none of the elements of I cannot contribute (i.e. if M=(E,I) is a matroid then M=(E union K,I) is a matroid for any set K.

Input may be in whatever list format you desire, be it as simple as no separators but spaces (using 0 for the empty set): 0 1 2 3 12 13 or as complicated as whatever list literals are in your favorite language (such as python's: [[],[1],[2],[3],[1,2],[1,3]]).

## Output:

A variation on 1/0, true/false, yes/no answering the question: is M a matroid?

## Definition:

M=(E,I) is a matroid if:

1. I is not the empty set
2. If J is in I and K is a subset of J, then K is in I
3. If J,K are in I and |K|<|J| then there exists an element x that is in the set difference J-K such that K union {x} is in I.

There are equivalent formulations of condition 1 and 3, also there are conditions on the bases (maximal elements of I w.r.t. cardinality) that are equivalent to these. If people want I can post those too or leave them as optional research.

## Examples:

I={{},{1},{2},{1,2}} is a matroid.

I={} is not a matroid because it is empty (by axiom 1).

I={{},{1},{1,3}} is not a matroid because if it has {1,3} independent then it must have {3} independent (by axiom 2).

I={{},{1},{2},{3},{1,2}} is not a matroid because if it has {1,2} and {3} independent then it must have either {1,3} or {2,3} independent (by axiom 3).

I={{}} is always a matroid, as is I=powerset([1,2,...,n]) for any n>0 as they both trivially satisfy the axioms.

## Specs:

Submission is either a program taking input from standard input or command line argument or a function that takes I as input (as a string) and returns the specified binary answer. No upperbound on the size of input should be hardcoded.

I would intend for this to be a code-golf challenge.

• Rather than provide alternative definitions, just link the first mention of the word matroid to the Wikipedia page. May 5 '14 at 11:59
• 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.
– user58826
Jun 9 '17 at 16:38