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
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  • Problems that could make the challenge uninteresting or unfit for the site

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4667 Answers 4667

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140 141

I'm Lost!

Lost is a 2d esolang where the instruction pointer starts at a random location. In this challenge, we'll replicate that for every language.

Your task is to write a program, which can be shifted any number of characters (these will be moved from the start to the end), and will still work. The program should print the string Which way is , followed any consistent string of your choice which is at least four characters, and a question mark.


Say my program is:

import print

print("Which way is Boston?")

What actually gets run might be:

port print

print("Which way is Boston?")im

(assuming there's no trailing newline)


print("Which way is Boston?")import print


ay is Boston?")import print

print("Which w

All of these must print the same, correct string.

Sandbox: I'm considering making the challenge to print how many characters your program was shifted by, but I'm worried that'd be impossible

  • 1
    \$\begingroup\$ Maybe you can add to the score the number of shifts that are impossible? That would allow languages to compete where this would otherwise be impossible \$\endgroup\$
    – mousetail
    Apr 25, 2023 at 14:12
  • \$\begingroup\$ @mousetail Yeah, true. Ooh, what if I went with the print-the-amount-shifted-by thing, and factored the number of shift positions where it doesn't work into the score? \$\endgroup\$ Apr 25, 2023 at 16:57
  • \$\begingroup\$ Your challenge may be inspired by Lost, but the shifting of the entire program in this challenge has nothing to do with the random starting position & direction of a Lost program.. The "In this challenge, we'll replicate that for every language." is basically false. Maybe change that sentence to something along the lines of "Making a Lost program deterministic, meaning it will have the same output regardless of where it starts or travels, can be quite tricky. In this challenge, we'll do something similar by having the same output for each rotation of your program, written in any language." \$\endgroup\$ May 15, 2023 at 10:15
  • \$\begingroup\$ @KevinCruijssen Well I mean, since most programming languages go from beginning to end, shifting it randomly does kinda emulate the random starting position. I do see what you mean tho, and I'll change that if I come back to this challenge at some point. \$\endgroup\$ May 15, 2023 at 13:33

Compute Pi using pure arithmetic

This is a sequel to the challenge Primality testing formula.

Given a positive integer \$ n \$ as input, write a Python 3 function or program that outputs the first \$ n \$ digits of Pi as an \$ n \$-digit decimal integer using pure arithmetic. This means that your source code can only contain:

  • operators =, +, -, *, **, //, %, <<, >>, &, |, ^, ~, <, >, <=, >=, ==, !=
  • variables
  • integer constants
  • parentheses ()
  • semi-colons/newlines (to separate statements)

Unlike with the other challenge, you may use = if it helps speed up or simplify your code. However you may not use loops, built-ins, or anything else that's not stated above. Yes, it's possible.


The winning criterion for this challenge is . You will be scored on the highest \$ n = 10^b \$ your code can compute in one minute. The time taken for that run will be used to break ties.

Hardcoding to prioritize certain test cases is not allowed, and the code must work in theory for all positive values of \$ n \$.

Test Cases

  n | output
  1 | 3
 10 | 3141592653
100 | 3141592653589793238462643383279502884197169399375105820974944592307816406286208998628034825342117067

Let's start with a fairly innocuous definition of a polygon:

A polygon is a set of points \$P\$ in Euclidean space and set of edges each having two distinct endpoints in \$P\$, such that the edges form a single cycle.

We will define a regular polygon then as:

Take each edge of the polygon and divide it into two halves of equal length. The polygon is regular if for any pair of half edges, \$A\$ and \$B\$, there is an isometry of ambient space that maps every half edge to another and maps \$A\$ to \$B\$.

This can informally be phrased as "no two edges or faces are distinguishable by anything other than absolute position". You may also hear the common phrasing "all edges are the same length and all angles are equal".

Now the astute among you will be quick to point out that while this allows for what we normally consider polygons it also includes so called "star-polygons" such as the pentagram:

A pentagram!

These edges intersect each other, but we didn't forbid this. So by the definition this is a valid 5-sided polygon.

These sort of edge cases are sort of common knowledge. However the even more astute among you may have noticed a much stranger loophole. I said "Euclidean space", not "2-dimensional Euclidean space".

Skew polygons

So can we make new polygons in higher dimensional spaces? Yes, for example here's a regular 6-sided polygon in 3D space:

Skew hexagon Image curteousy of Polytope wiki user Sycamore916, CC-BY-SA-4.0

It turns out for example we can take any even sided polygon and raise half of its vertices up out of the plane to get a "skew" version of it that is 3D. And in 4D we can do even stranger things.

Challenge statement

The challenge is going to be to count these skew polygons. You are going to take an integer > 2 and output a list \$a\$ where \$a_d\$ represents the number of distinct regular polygons in \$d\$ dimensions.

This is so the goal is to minimize the size of your source code as measured in bytes.


In order for this to work we must add 2 more definitions, the first is what makes two polygons distinct and the second is the dimension of a polygon.

A polygon in \$d\$ dimensions is a polygon such that the affine span of its vertices is a \$d\$-dimensional subspace.

For example the following vertices form a triangle:

$$ (1,0,0)\\ (0,1,0)\\ (0,0,1) $$

Each coordinate is given in 3D, however the polygon itself is 2D since all of its vertices lie on the same plane.

Two polygons \$A\$ and \$B\$ are the same if there is an invertible linear transform taking \$A\$ to \$B\$. Two polygons are distinct if they are not the same.

For example the skew hexagon discussed earlier can be stretched vertically by any positive factor. These do not count as distinct polygons since stretching is an invertible linear transform. However the skew hexagon is distinct from the normal planar hexagon since the stretch factor would be 0, and that linear transformation is not invertable (you cannot divide by 0).

Very nicely these two definitions coincide so that if two polygons have different dimensions they are automatically distinct, so there is no ambiguity as to how to count them.

Test cases

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


Here's the part where I would explain where the magic numbers come from.

  • \$\begingroup\$ Your 6-sided polygon looks like a Cyclohexane... \$\endgroup\$
    – tsh
    Jun 8, 2023 at 7:53

Monopoly, But its in Creative Mode (sort of)

Disjointed Monopoly (?)

Starting Grid: 8x8 grid:

|XX|  |  |  |  |  |  |XX|
|  |+1|  |  |  |  |+1|  |
|  |  |+2|+1|+1|+2|  |  |
|  |  |+1|x2|x2|+1|  |  |
|  |  |+1|x2|x2|+1|  |  |
|  |  |+2|+1|+1|+2|  |  |
|  |+1|  |  |  |  |+1|  |
|XX|  |  |  |  |  |  |XX|

For each group of four players (grouped randomly), we run the game above with the four players starting at the +1 squares near the corner with 20 dollars each.

To ensure that no player is left over, each entry will be run four times. It is possible that two or more will compete against each other, but the grouping is random. (The order is also random.)

Objective: Eliminate all other players.

All players that has to pay more money than they can are eliminated.

(For all random numbers / choices, assume equal probability.)

We choose a random position and a random number of steps 1 ~ 4, and the bot moves in that directions. The map wraps around. Then, if the bot steps on...

  • a Player: That player is eliminated instantly, and you do not execute the current square action.
  • someone else's Property: Pay that someone the ammount equal to 2^rank dollars. rank is the rank of the property.
  • your own Property: Choose whether to upgrade the property rank by 1 costing you 2*2^rank dollars. rank is the current rank of the property.
  • any Utility station: get 2*rank dollars, and the utility station automatically upgrades its rank by 1.
  • corner of Board (XX): Lose 20% of your money, rounded down.
  • otherwise: Do an Action.

When you step on an empty cell (a cell not qualifying any of the above), you can do an Action. You can either:

  • Build your own Property: Use 10 dollars to build a Property right here with a random integer rank between 1 and 4.
  • Build your own Utility: Use 2 dollars to build a Utility station right here with a random integer rank between 1 and 4.
  • Demolish a Square's structure. This will not demolish the bonus on the structure (for instance *2 and +1) and can apply to XX squares. This action has 25% probability of successing.

Demolish cost (paid even if failed):

  • Property: 2*rank dollars. (Even if it is yours.)
  • Utility: rank dollars.
  • Other (including XX): no cost.


Squares marked with +1, +2, and *2 have added bonus. The operations are applied to the rank when a structure is built on the square the bonus occupies. For instance, if a ranked 3 Utility is built on a *2 square, it will start at rank 6 instead. Everything else proceeds normally.

Meta: Gimme a better name for the challenge

  • \$\begingroup\$ Better name: Disjointed Monopoly \$\endgroup\$ May 24, 2023 at 13:57
  • \$\begingroup\$ I think something's wrong with the title that says Objective \$\endgroup\$ May 24, 2023 at 15:14
  • 1
    \$\begingroup\$ What does "disjointed" have to do with this? \$\endgroup\$ May 24, 2023 at 15:22
  • \$\begingroup\$ @Hippopotomonstrosesquipedalian fixed \$\endgroup\$ May 24, 2023 at 23:56
  • \$\begingroup\$ @RydwolfPrograms hence searching for better name \$\endgroup\$ May 24, 2023 at 23:56



Tripples (BGG) is a board game from the 1970s.

It is played on an 8x8 grid, and each tile on that grid (except the corner starting tiles, and four blank tiles) contain three arrows. The three arrows each point in one of eight directions (up, down, left, right; and diagonally between those). There are therefore 56 tiles, and 56 unique combinations of arrows - so each tile is unique when placed in the correct orientation.

Tiles are placed on the board, and depending on the variant of the rules being played this can either happen before the game starts (face up, either randomly or players take turns to draw and place tiles as they choose), before the game starts (face down, randomly), or over the course of the game as an alternative player action, instead of moving their playing piece.

Players start in adjacent corners and attempt to cross the board to the diagonally-opposite corner before their opponents can do so.

Players take turns, and may move their playing piece one square per turn. The interesting thing about this game is that the direction they can move is limited to the directions indicated by the arrows underneath their opponent's playing piece.

The rules are ambivalent regarding how the blank pieces are treated - either you can't go there at all, you get a second move if you can go there, or (house rule) your opponent can choose to move in any direction if you land there.

The challenge

(sandbox note: I discovered this game recently and thought it could be the basis for an interesting challenge - what the challenge is, though, I'm flexible on; so I'm happy to take suggestions. What is below is just one idea)

Competitors write bots to play against another bot, in a Trippples competition.

*(sandbox query: which variant of the game rules is most interesting?)

A server program will provide the board state, and the competitor bot's function should return its next move.


Longest Tiles Combo

The New York Times has a puzzle game called Tiles on their website. This game consists of a grid of tiles, each made of multiple visual elements which are shared between tiles. The player first selects two tiles which share any visual element, removing those tiles from play. If the player then selects a third tile which matches some visual element with the second, their combo increases; they can repeat this process multiple times to increase their combo.

For example, take the following row:

enter image description here

The player could begin by selecting the middle and left tiles, which share the black middle square; they then could select the right tile, because the left and right tile share a purple square and a light blue background.

In this challenge, the grid of tiles will be represented by a list of tuples, where each tuple contains the same number of symbols; each symbol represents a visual element on the tile. For example, the above could be encoded as

[("light blue BG", "purple square", "small black square"), 
 ("orange BG", "purple circle", "small black square"),
 ("light blue BG", "purple square", "small black diamond")]

Note that for all of the tuples, the elements will be described in the same order -- e.g. the background will always be the first item in the tuple.

You can decide the set of symbols which are used; in the examples below, I'll use integers instead of strings, e.g.

[(1, 2, 3), (4, 5, 3), (1, 2, 6)]

The input format is flexible; for example, since order doesn't matter, you could use a set instead of a list, and instead of tuples you could use lists, strings, or sets.

Given the input as above, your goal is to output an integer indicating the largest possible combo length possible. You can assume there will be at least one matching pair of tiles -- i.e. the max combo will be at least one.

Test cases

Input Output
[(0), (0)] 1
[(0), (0), (1)] 1
[(2, 4), (2, 3), (2, 5)] 2
[(1, 2, 3), (4, 5, 3), (1, 2, 6)] 2
[(9, 1, 14), (9, 17, 5), (3, 10, 14), (0, 13, 11), (16, 13, 6)] 2
[(4, 3, 1), (8, 0, 2), (7, 6, 2), (8, 6, 5), (4, 6, 2)] 4
[(16, 8), (12, 8), (15, 17), (18, 5), (12, 6), (0, 5)] 2
[(5, 1), (2, 9), (3, 15), (11, 0), (5, 9), (2, 14)] 3

Here's some very inefficient code to generate your own test cases.

Standard loopholes are forbidden. Since this is , the shortest program wins.


What is possible with all these blocks?

In this challenge, you will write code (in any way) that outputs the number of permutations of all sub-tuples of a tuple of length \$n\$ (also A000522(n) ). Here’s an example:

You have three blocks. You can start by counting how many arrangements of three blocks are possible (6). Then, work out how many ways there are to arrange two blocks (2) and multiply that by the number of unique pairs of blocks you can pick from the three blocks (3) to get the sub-answer (6). Then, work out how many ways you can arrange 1 object (1) and multiply that by the number of unique blocks you can pick from the three blocks (3) to get the sub-answer (3). Then, remember the number of empty combinations (1). Add those together to get the answer (16).

This is code golf, so shortest answer wins!



Make a 0-byte metagolfscript solution that outputs its name.

Shortest name wins.

  • \$\begingroup\$ I feel like this will be closed as a duplicate of the output your program’s name challenge. \$\endgroup\$ Jul 14, 2023 at 7:35
  • \$\begingroup\$ @Iamkindofalanguagedev No. In that question metagolfscript is banned due to standard loophole. In this only metagolfscript allowed \$\endgroup\$
    – l4m2
    Jul 14, 2023 at 23:54
  • \$\begingroup\$ This still seems like an underspecified challenge for some reason... \$\endgroup\$ Jul 15, 2023 at 7:37

Landmine Number V


Compute 0.1+0.2+0.3+...+0.8+0.9+0.10+0.11+0.12+...+0.[n]

Test cases

10  -> 4.6
25  -> 7.3
64  -> 24.85
256 -> 81.496

Sandbox Note


  • \$\begingroup\$ What will the maximum n be, and what kind of precision are you looking for? \$\endgroup\$
    – Adám
    Jul 17, 2023 at 7:35
  • \$\begingroup\$ @Adám Accuracy shouldn't be a problem if only 5 significant digits when n=256 \$\endgroup\$
    – l4m2
    Jul 17, 2023 at 7:45
  • \$\begingroup\$ The number of significant digits is of course ⌊log *n*⌋ which means hitting 64-bit float issues when n approaches 10¹⁷. \$\endgroup\$
    – Adám
    Jul 17, 2023 at 8:44

JSON Data? ASCII is better!

Write a function that prints JSON data using ASCII art, and takes a dictionary/object as input.


    "columns": ["firstname", "lastname"],
    "data": [
        {"firstname": "John", "lastname": "Doe"},
        {"firstname": "Jack", "lastname": "Barrock"},
        {"firstname": "John", "lastname": "Skeet"},


|  firstname  |  lastname   |
|     John    |     Doe     |
|     Jack    |   Barrock   |
| ...                       |

This is , so fewest bytes win.

Note: The input will always have a column attribute, and will always have a data attribute. Think of it as SQL.

  • \$\begingroup\$ Thanks for posting here. Some feedback: 1. Challenge should be fully specified before the test cases. Specifically, how the table is drawn, centered text, what characters are used for table formatting etc. 2. You need more test cases. At least 5 if you can cover all edge cases that way \$\endgroup\$
    – mousetail
    Jul 31, 2023 at 13:12
  • 2
    \$\begingroup\$ I'm not sure if you fix those that it would be sufficienly different from this challenge Arnould mentioned. Note challenges don't need to be identical to be duplicates, closely related challenges can also be closed if they have the same general structure for solutions. \$\endgroup\$
    – mousetail
    Jul 31, 2023 at 13:14

Is it a valid Go type?


Separate two points in a topological space


2D Percolation Model

Per Wolfram MathWorld: "Percolation theory deals with fluid flow (or any other similar process) in random media."

The model is a 2 dimensional lattice whose edges are either "open" or "closed" with probability p in [0,1]. At a percolation probability P, each edge will be evaluated to be open or closed based on if P>p. Connected vertices compose a "cluster". For display purposes, clusters are often colored or otherwise uniquely denoted. An example of a percolation model is shown below:

3x3 Lattice

       Lattice                 Lattice with random 
                                edge values shown

 o ------ o ------ o           o -0.12- o -0.99- o
 |        |        |           |        |        |
 |        |        |          0.75     0.09     0.52
 |        |        |           |        |        |
 o ------ o ------ o           o -0.46- o -0.23- o
 |        |        |           |        |        |
 |        |        |          0.39     0.12     0.85
 |        |        |           |        |        |
 o ------ o ------ o           o -0.97- o -0.23- o

Percolation at P = 0.1, 0.5, and 0.9 yields 4 clusters

       P = 0.1                    P = 0.5                   P = 0.9

 o ------ o ------ o        o        o ------ o        o        o ------ o
 |                 |        |                 |                           
 |                 |        |                 |                             
 |                 |        |                 |                           
 o ------ o ------ o        o        o        o        o        o        o
 |        |        |                          |                        
 |        |        |                          |                        
 |        |        |                          |                        
 o ------ o ------ o        o ------ o        o        o ------ o        o

Vertices named according to cluster

P = 0.1                      P = 0.5                    P = 0.9

A A A                        A B B                      A B B
A A A                        A C B                      C D E
A A A                        D D B                      F F G


Given a side length n and percolation probability P, create a percolation model of lattice size n x n with vertices displayed according to the cluster they belong to. The shortest code wins!


The model shall take two inputs: n, P

  1. n is the side length of the lattice
  2. P is the percolation probability


The model shall return an n x n display of vertices. Its a model so readability is important! List of lists, arrays, or pretty printing is allowed so long as there are n rows and n columns. Vertices shall have alphanumeric names of the same length to ensure readability. Edges need not be shown.


  1. Cluster names shall be unique
  2. The model shall be able to compute any percolation phase state from P = 0 to P = 1

Compressed UTF-8


Worst algorithm for anything

Now, any coder worth their salt will know how to sort an array in-place in nlogntime. Most can probably figure out a way to do it in n!n time and factorial memory usage. Let's see an algorithm that tops those numbers; can you come up with a way to sort an array in up-arrow time? Can you come up with an algorithm whose O-notation contains the Graham's number series?


  • The algorithm must solve a problem that isn't just an obfuscation of 'do a ridiculous amount of NOPs".
  • Highest O-notation in either memory usage or time wins, tie broken by the other one and then by a golfed implementation.


I'm not sure whether I should limit this to solving a specific problem. For that matter, I don't know if this would be an interesting challenge.


Periodicity of the family of sequences s(n) such that n(x) is the first number co-prime with the previous n elements

The sequence of all numbers coprime to the previous is all natural numbers. This is OEIS 000027:

1, 2, 3, 4, 5, 6, 7, 8...

The sequence starting with 1, 2, and where the n-th term is the first number coprime to the previous 2 in the sequence, is periodic with period 6 (2x3). This is OEIS 047255

1, 2, 3, 5, 7, 8, 9, 11, 13, 14, 15, 17, 19, 20, 21, 23

If you take the first number coprime with the previous 3 terms, you get a sequence periodic with a period of 210 (235*7). This is OEIS 062062

1, 2, 3, 5, 7, 8, 9, 11, 13, 14, 15, 17, 19, 22, 23, 25, 27, 28, 29, 31, 33, 34, 35, 37, 39, 41, 43

Your task is, given a number N output the periodicity of the co-prime sequence taking the last N terms.

As stated, the first elements of this sequence are:

1, 6, 210

The fourth is unknown, yet your program, given enough time, should in theory calculate it. It is known to be at least 245,589. It should not be extremely hard to calculate, just nobody has tried yet.

MathJax test, please ignore: \$abc\$

  • \$\begingroup\$ Probably worth showing at least one example of n(x) for clarity's sake \$\endgroup\$ Aug 21, 2023 at 19:26

Complex logarithm

We have challenges for the regular real logarithm and the matrix logarithm, but we do not yet have a challenge for computing the logarithm of a complex number.


I will add a challenge body if there is interest



  • You may chose which logarithm of the import you return as long as the imaginary parts for any two returned values differ by at most \$2\pi\$
  • Please add built-in solutions to the community-Wiki post
  • ... More rules will follow
  • This is the shortest solution in bytes wins


  • Is this different enough from the other two challenges ?

    As far as I can tell most (no built-in) algorithms used in these challenges break when applied to complex inputs

  • Would there be interest in solving this challenge ?

  • \$\begingroup\$ Isn't the complex logarithm not unique? Which value(s) should answers output? \$\endgroup\$
    – Bbrk24
    Aug 22, 2023 at 14:20
  • \$\begingroup\$ To bsoelch: I find this challenge boring (I’ll abstain from voting because it isn’t terrible either). @Bbrk24 The term is principal natural logarithm. Real part ≔ ln | x |; and imaginary part ≔ arg(x) ∩ (−π, +π] in radians. \$\endgroup\$ Aug 31, 2023 at 18:30

When is My Holiday?


I recently learned of a holiday, that occurred on August 20, 2023 and August 12, 2022. This got me wondering how on earth they were determining which day this holiday falls on.

The Challenge

In this challenge you will be given two dates which are from the same month but different years as input. You must construct the simplest possible description which applies to both dates.

Descriptions must be structured in a particular way. First, there are types of days

type cost
day 1
Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday 1
weekday, weekend day 1
day which is a multiple of \$N\$ \$N\$

For the following we'll let \$D\$ be some type of day. We then have modifiers and constants.

modifier constant
the nth \$D\$ after the nth \$D\$
the nth \$D\$ before the nth to last \$D\$
the nearest \$D\$ to

A valid description consists of 0 or more modifiers followed by a single constant. The simplest description is the one with the lowest cost, which is the sum of the cost of each \$D\$ which appears in the description.

For the purposes of this challenge we define "nearest" to mean "nearer than all others" so if there is a tie, then neither day is nearest and the description is not valid. "Nearest" may include the day itself. "Before" and "after" do not include the day itself.


Standard I/O rules apply, the dates may be taken in any reasonable format. And since the challenge is not about supposed to be about date calculation you may choose to take in data about the month surrounding each day. Some examples of valid input formatting are as follows. A program would take two such inputs.




[[xx, xx, 01, 02, 03, 04, 05]
 [06, 07, 08, 09, 10, 11, 12]
 [13, 14, 15, 16, 17, 18, 19]
 [20, 21, 22, 23, 24, 25, 26]
 [27, 28, 29, 30, 31, xx, xx]], 
[3, 2]

You may assume any input given will have a solution. Your description need not apply to all years, just the ones in the input (eg. August 2023 has no 5th monday, but "the 5th monday" is still correct for 2021-08-30, 2022-08-29). If two or more descriptions tie you may output any or all.

Output the elements as seen in the tables above separated by spaces.

Test Cases

These test cases follow the YYYY-MM-DD input format

2022-07-04, 2023-07-04 -> the 4th day
2021-08-30, 2022-08-29 -> the 5th monday
2022-08-12, 2023-08-20 -> the 1st day which is a multiple of 4 before the 3rd Monday

How clear is this?

Should I be more lenient on output? (allow encoding schemes)

Yes I'm going to add more test cases.


Calculate equal, winning or losing trades in chess

For this challenge, assume you are playing white

Given two arrays containing all white and black chess piece positions on a chessboard, output if a given square is a equal, losing or winning trade. If there is no trade for a giving square, output something else.

  • An equal trade happens when both black and white can lose the same amount of material in a given square

  • A winning trade happens when white loses less material than black in a given square.

  • A losing trade happens when white loses more material than black in a given square.


Black -> ["Ra8", "Bc8", "Qd8", "Rf8", "Kg8", "Nd7", "Ne7", "Bg7", "a6", "b5", ...
White -> ["Rd1", "Rf1", ...
Target -> "d5"

Output: Equal Trade

Visually would be:

enter image description here


  • Would this be interesting?
  • I'm not sure If I should allowed dynamic calculations or no. For example in the previous image, for square f5, there would be an equal trade since pawn attacks f5 square and if it takes, bishop will now be able to attack f5 square

I will add more examples later...



Construct a Turing machine with tape alphabet {0, 1} that, starting on a tape filled with 0, it halts with at least TREE(3) 1's. Least states wins.


As math scat mentioned, this is asked in math stackexchange, but such a question may fit better here LOL


Return every possible program with positive probability

Write a program or function a (subset of a) Program language of your choice that returns every possible Program in the same Language with a positive probability


  • You are allowed to take a stream of random bits as input
  • Each program in your language has to have a positive probability to appear
  • All returned programs have to be syntactically correct (META: might need further clarification)
  • If in your language every string satisfies the above condition, consider using a different language


  • Would this be an interesting challenge/ is this a duplicate?

  • Should I add minimum language requirements (Turing completeness...)?

  • Is "syntactically correct" clear

  • Possible variant with more clear definition:
    "Generate every python program"

  • \$\begingroup\$ Regarding whether "syntactically correct" is clear, I'm not sure. In standard languages it's fairly clear - its compilation doesn't terminate with a syntax error (although even that might be vague if the compilation can halt earlier due to another, valid, factor). But in a lot of esolangs every finite string of characters is a well formed program, so it might give them a very very significant advantage. You should also specify whether the language your code is in must be the same as the one it outputs. \$\endgroup\$ Sep 8, 2023 at 14:56
  • \$\begingroup\$ This seems like it would be incredibly difficult for any language with moderately complicated syntax (e.g. Python), so limiting it to a language like that wouldn't likely result in any valid answers IMO \$\endgroup\$ Sep 19, 2023 at 14:46

Simulate Feedback Scheduling on a Uniprocessor

Alternatively, exploiting university course content for code golf reputation

Like round robin scheduling, Feedback Scheduling is a way to schedule ready processes in an operating system. And like round robin scheduling, it runs processes for a certain amount of time (quantum), interrupted after that time (if not yet finished), and then added back to a ready queue for another round.

However, the key difference is that as a process repeatedly goes through the ready queue, it gets a longer quantum1. This can be compared to having multiple ready queues and processors daisy-chained as so:

enter image description here

In the diagram, the first ready queue sends its processes to the first process to run for some time \$k\$. If the process finishes in this time, it is released. Otherwise, it is sent to the second ready queue, which has a quantum of \$2k\$.2 The second ready queue sends its processes to the second processor to run for some time \$2k\$. Once again, if the process finishes in this time, the process is released. Otherwise, it is sent to the third ready queue. This generalises all the way down to the nth ready queue, which has interrupted processes sent back to itself. That is, it gets readded to the nth ready queue in a loop.

As this is a uniprocessor simulation, there's only one processor, so the concept of daisy-chaining multiple processors doesn't really work3. However, this can be simulated by using process priorities. Indeed, a higher priority process would be in one of the higher ready queues (closer to the first ready queue). A lower priority process would be in one of the lower ready queues (closer to the nth ready queue). A process with priority \$n\$ will be executed for \$n k\$, where \$k\$ is the quantum.

The priority only changes how long a process is executed. It does not change the selection order of the ready queue - that's still a first-come first-serve selection method. Processes arrive with a priority of 1, so as to not have any processes running for 0 time units.

The Challenge

Given a list of [int, int] (both > 0), as well as a base quantum, and a maximum priority, return a list of int representing the order that processes were executed. Assume that all the processes arrive at the same time.

The list of [int, int] represents a (simplified) list of processes. The two ints are the process id and service time (the time the process needs to fully run). (The priority is to be handled by the program, as all processes start with priority 1).

For example:

[[1, 5], [2, 10], [3, 3], [4, 12]]

Represents the following processes

Process ID Service Time
1 5
2 10
3 3
4 12

The base quantum is how long each priority runs, before priority multiplication. The maximum priority represents the priority at which processes loop back into the same quantum.

Worked Example

Using the above process list ([[1, 5], [2, 10], [3, 3], [4, 12]]), a quantum of 4, and a maximum priority of 4, the process execution order will be:

TODO: Worked example at a time that isn't 11:39pm


  • Input can be taken as list[list[int, int]], int, int, list[int], list[int], int, int, or some other reasonable input method.
  • The processes, quantum and maximum priority can be taken in any order.
  • For example:
list[int: processIds]
list[int: serviceTimes]
int: quantum
int: maximumPriority


int: maximumPriority
list[int: serviceTimes]
int: quantum
list[int: processIds]

or any other combination.

  • Output can be list[int], str (joined on newlines, or spaces, or any constant delimiter) or some other reasonable output method.
  • There will always be at least one process.
  • The process ids list will always be a permutation of the range [1, number of processes].
  • The process ids can be 0-indexed if you want it to be for some reason. Using 0-indexing, the ids list will always be a permutation of the range [0, number of processes].
  • The order of the process ids list is important - processes are executed in the order they arrive (in a first come first serve manner). That is how the ready queue in an actual FB simulation works after all (FCFS selection strategy, with pre-emption on time quantums).

Test Cases

TODO: Write

This is , so the shortest answer in each language wins.

Sandbox Meta

  • Does the description I've provided make sense? It's not verbatim how FB scheduling works, but it's a simplification to make code golfing it not as tedious as it could be.
  • Because I don't have a worked example/test cases yet, some parts might not make sense.

1 Some variants of Feedback Scheduling use constant time slices for all quantums. However, this challenge will assume a variable time quantum, so as to provide fairness to longer processes.

2 The factor of 2 is arbitrary. It could be any integer > 1.

3 At least, not with the simplification provided for this challenge. In practice, it might still work.


Roll a ball down an array

You are in a frictionless environment in a vacuum. Obligatory XKCD

You are given a inertialess, sizeless point mass which you have to roll down a slope. The ball starts with 0 speed.

If a ball has 0 speed and is touching the ground, it gains 1 speed in the direction of the lowest adjacent point. If both adjacent heights are the same, add 1 speed in the direction last traveled. When a ball changes direction, subtract 1 speed after the change. (This ensures that no loops occur)

The ball gains or loses speed based on the difference between the ball's current height, and the end height. A ball moves to the lowest possible square where the speed obeys the formula. $$(speed+1)^2 < (h-newh)^2 + (idx-newidx)^2$$

Given an array of integer heights, return the steps for the ball to settle/escape.

Ball Speed-move diagram

     . 4 4 4 .
   4 4 3 3 3 4 4
 . 4 3 2 2 2 3 4 . 
 4 3 2 1 1 1 2 3 4
 4 3 2 1 O 1 2 3 4
 4 3 2 1 1 1 2 3 4
 . 4 3 2 2 2 3 4 .
   4 4 3 3 3 4 4
     . 4 4 4 .


[5,3,2,1,2,4] -> 5

Diagram:     Ball SPD = 1 Ball SPD = 2 Ball SPD = 3 BALL SPD = 1 Ball ESCAPED!
0            1            2            3            4            5
()           x\
55           55()         55 x         55           55        () 55         x
55        44 55        44 55()      44 55 x`\,   44 55        44 55        44
5533      44 5533      44 5533      44 5533  ()  44 5533   x  44 5533      44
553322  2244 553322  2244 553322  2244 553322  2244 553322  2244 553322  2244
553322112244 553322112244 553322112244 553322112244 553322112244 553322112244

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

          SPEED 1   SPEED 2   SPEED 4   SPEED 7   BALL ESCAPED
0         1         2         3         4         5
@         x
9         9@        9x        9         9         9         
98        98        98 @      98 x      98        98        
987       987       987       987       987       987       
9876      9876      9876      9876 @    9876 x    9876      
98765     98765     98765     98765     98765     98765     
987654    987654    987654    987654    987654    987654    
9876543   9876543   9876543   9876543   9876543 @ 9876543 x 
98765432  98765432  98765432  98765432  98765432  98765432  
977654321 977654321 977654321 977654321 977654321 977654321

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

 0 1 2
[2,1,2] -> 3

 0 1
   0 (Loses 1 speed due to the direction change)

[2,1,3] -> 4

Questions for sandbox:

  • How many more test cases should I add?
  • Are there any inconsistencies or rules that need clarification?

Digits of Infinity (computing A206636)

In this PDF, https://www.vixra.org outlines a way to find the last \$ n \$1, 3 digits of infinity and a method to do so.

Let us define the infinity sequence2, taking the function \$ f(n) \$.

\$ f(1) = 2^2 = 4\$
\$ f(n) = 2^{f(n-1)}\$

Starting from \$ f(3) \$, the last \$ n - 2 \$ digits are the same for all \$ f(n) \$ numbers with \$ n \gt 2 \$ in this sequence.

Your task: given an integer \$ n \$ through standard I/O, return the last \$ n \$ digits of infinity.


n, an integer (or the closest to one in your language), \$ \ge 1 \$ (if 1-indexed; you can also take a zero-indexed input, but the number of digits outputted will then have to be n+1). Most programs will probably compute \$ f(n+2) \$ for the output, which is permissible (but not compulsory; I'd love to see any other creative way for large n, though no bonus points). Input will be less than or equal to the highest number n such that your program can compute in under 2 minutes.


The last n (or n+1 if 0-indexed) digits of infinity (through accepted I/O).

Test Cases:

Input -> Output

1 -> 6
2 -> 36
3 -> 736
10 -> 3432948736
22 -> 8098615075353432948736

This is , so shortest answer wins!

Note: my bet is that most programs that are well-golfed will not be able to calculate these digits easily; \$ f(5) \$ has over 19000 digits! However, if your program, given infinite time, infinite memory, and unbounded integer types, can compute this, it's valid.

1. Apparently, the digits of infinity are infinite.
2. This is OEIS A206636.
3. I do not believe in this method, I just picked it out because I thought it would make for a good challenge.

  • 3
    \$\begingroup\$ Infinity is not a number. Therefore, it cannot have digits. The paper seems a bit sketchy to my knowledge of different infinities. \$\endgroup\$
    – Seggan
    Sep 14, 2023 at 15:32
  • 2
    \$\begingroup\$ What are the differences between viXra.org and arXiv.org? "Warranted or not, it has a reputation of being an alternative to arXiv for cranks and to host a lot of junk science, fake proofs or even outright nonsense" \$\endgroup\$ Sep 14, 2023 at 23:32
  • 5
    \$\begingroup\$ I would suggest it be named differently, like "A206636". This removes the dependency on viXtra. I also agree with the above, infinity is a concept, not a number, unless they are talking about p-adics, although that's unlikely. \$\endgroup\$ Sep 15, 2023 at 1:01
  • 2
    \$\begingroup\$ I'd upvote because this would be a challenge that really punishes naive approaches, due to the tetrative growth. Also, is there a limit on the inputs? Because -1 is also an integer, and so is 2^127-1 \$\endgroup\$ Sep 15, 2023 at 1:05
  • 2
    \$\begingroup\$ This paper shows such a violent misunderstanding of infinity it's comical. \$\endgroup\$
    – ATaco
    Sep 15, 2023 at 1:16
  • \$\begingroup\$ Alright, everyone; I don't actually believe that the digits of infinity can be calculated with such a simple sequence; I just chose this because I just thought the sequence was an interesting one. And yes, there is a limit; only from 1 may the inputs be passed. \$\endgroup\$ Sep 15, 2023 at 4:04
  • 1
    \$\begingroup\$ I my opinion allowing solutions that compute f(n+2) and then take the last n digits, makes the challenge less interesting. I would suggest some requirement for reasonable running time (e.g. "the program has to be able to compute the result for n=10 in less than a minute on a modern computer"). \$\endgroup\$
    – bsoelch
    Sep 15, 2023 at 11:23
  • \$\begingroup\$ Would it be allowed that the program only works up to n=18 (largest result that fits in a signed 64-bit integer), or do you require big-int support \$\endgroup\$
    – bsoelch
    Sep 15, 2023 at 17:14
  • \$\begingroup\$ @bsoelch about your earlier comment; it's a mistake with my framing: I meant that most trivial programs will do this, and it's not compulsory (that would be a non-observable requirement), and the second comment, I believe my last part makes the ruling on that clear (if your code *practically* can compute up till n=18, and theoretically can compute for any n, then it's valid) \$\endgroup\$ Sep 17, 2023 at 9:36
  • \$\begingroup\$ If infinity did indeed have digits, it would have to be an infinite stream of 0s, since it has to be a multiple of every power of 10. \$\endgroup\$ Sep 25, 2023 at 7:44
  • \$\begingroup\$ False; I could make a bigger infinity by adding 1. \$\endgroup\$ Sep 25, 2023 at 9:05
  • \$\begingroup\$ @UndoneStudios but that can’t be an infinity, because it is not divisible by 10. Infinity must be an “integer” multiple of every single number. \$\endgroup\$ Sep 25, 2023 at 10:21
  • \$\begingroup\$ You are assuming a value of infinity as a value; it is not; but the point is it cannot be an infinite stream, because that would make it indivisible by... it's a complicated topic, and that's the reason why we don't consider *infinity* to be a value, which I already explained in my earlier comment. Also, such a definition would be recursive and would include itself as well, causing issues in calculation. \$\endgroup\$ Sep 25, 2023 at 13:02
  • \$\begingroup\$ Your definition is one of multiple definitions, but it is absolutely not the definition I hold when thinking of infinity as having digits. \$\endgroup\$ Sep 25, 2023 at 13:03

really shitty sloppy wip but i just want to start this post so that i can be past the first hurdle next time i look at this. feel free to ask anything just know this is like step 0;

implement this weird algorithm which will be given a more descriptive name

here is the algorithm in js:

f = x => x.reduce((a, b) => [b, ...f(a)], [])

implement this program in your language of choice;


it has to run in faster time complexity than this naive implementation, which is like, exponential i think.

shortest code wins

notes: i might make it so you only have to output the result of running this on range lists like [0 1 2 3 ... n], so only the permutation that the algorithm results in, rather than for arbitrary input, since generating the permutation is one thing and then applying it is another


Find an eigenvalue of a 3x3 Matrix

Write a program of function that given a 3x3 matrix as input outputs an Eigenvalue of that matrix


[[1,0,0],[0,1,0],[0,0,1]]    -> 1.0
[[1,2,3],[4,5,6],[7,8,7]]    -> -0.22165260583979401
[[5,3,4],[2,6,7],[0,4,10]]   -> 1.887934888738847
[[-3,2,5],[0,6,7],[0,0,-10]] -> -3.0
[[0.2915131378594483, 0.41257765061649787, 0.9253019986284902], [0.21696836678486353, 0.5331738150906348, 0.3247753676328128], [0.20672257620794932, 0.3453811902047663, 0.4771215769253173]] -> -0.03135020654472091
[[0.2447175622890705, 0.38693057299215594, 0.8576673327257548], [0.47607062999761074, 0.1126298515845301, 0.9707452917395663], [0.7524371727807047, 0.502675518344781, 0.6725321940186385]] -> -0.2768104753672565


  • You can take the matrix in any convenient format
  • The input matrix will be 3x3, the entries will be in the interval [-10,10]
  • The error of your result should be at most 10^-5
  • Your program may fail for some inputs as long as the probability of failure for a Matrix with uniformly distributed entries is 0
  • You can choose any eigenvalue as output, and may return different values on different calls
  • You are not allowed to use built-in functions that directly compute a eigenvalue/eigenvector of a matrix
  • You are allowed to use other matrix functions but are encouraged to solve the problem without matrix built-ins

Example solution in Python (non-golfed)

Uses a derivative-free variation of Newtons algorithm on the determinant

def det(M):
  return (M[0][0]*M[1][1]*M[2][2]+M[0][1]*M[1][2]*M[2][0]+M[0][2]*M[1][0]*M[2][1]-

def copy(M):
  return [[a for a in R]for R in M]

def subtX(M,x):
  for i in range(len(M)):
  return N

def eigenValue(M):
  while abs(y0)>1e-5:
  return x0

Attempt This Online!


  • Is this duplicate/ and interesting question ?
  • Is my explanation clear ?

Interpret a Turing Complete (strict) Subset of your Language

  • Write a program which can interpret some programs exactly the same way as your language, where these can be used to model any computation.
  • Demonstrate how to do this.
  • Your Interpreter may have features your language doesn't have, but your demonstration may not use them.
  • You may not use a built-in which interprets the source language, like eval()

The feature set does not need to be made of characters or something, you just need to say what it is.

It's [code-golf] until i can think of something better. how do you do tags in markdown?

Example: a javascript interpreter which only allows String.replace, functions, and the ternary conditional thing ? : may be able to be used as a convoluted slashes emulator.


  • This challenge was originally intended to make all languages equally interesting, but I think it still favors turing tarpits and golfing languages.
  • As with many Challenges which already exist, this one also gives actually interpreted languages kind of a leg up.
  • Making it illegal for turing completeness to arise from interpreter quirks is propably not that important. (like, ACE in my brainfuck interpreter?)
  • Banning eval might introduce rules-lawyering on what eval actually is, like, are Regexes allowed? is a built-in which evaluates arithmetic allowed? I'd say yes. It might also further favor turing tarpits, though it would nerf golfing languages which propably have an eval construct. It might also make the program more laborious to write, obviously, but
  • Allowing eval might make writing the program boring as well, as that's an established technique.
  • Making this [code-golf] is boring, maybe i should make it about the conciseness of an example program? It wouldn't devolve into HQ9+, but its discriminatory, like how is slashes supposed to calculate a factorial?
  • 1
    \$\begingroup\$ Welcome to cgcc, and interesting idea. Can you define "useful" more (maybe even formally)? Also, don't forget to add a winning criterion (such as code-golf) \$\endgroup\$ Sep 26, 2023 at 21:11

This is my first time here, as this was originally a pure math question that I figured would be more likely to be answered on this site.

Chaitin's incompleteness theorem states that for any theory whose axioms can be computed by a computer program (e.g. Peano arithmetic, ZFC, etc.), there exists a explicit number such that the theory cannot prove that any specific sequence of bits has Kolmogorov complexity larger than that number if the theory is consistent. That is, if the theory is consistent, one cannot prove in this theory that any program which computes this explicit sequence of bits must be of length at larger than this number.

One doesn't really have to know too much about computation theory to do the golfing exercise - the proof of Chaitin's incompleteness comes from the computer program which computes the shortest proof that some explicit number has complexity larger than N. If N is larger then the length of this program, then one has a length at most N program which computes a number which provably cannot be computed by a program with length at most N.

The task I am proposing is to design a Turing tarpit (which doesn't explicitly refer to Peano arithmetic) such that Peano arithmetic cannot prove that any output takes more than a N bit program to compute, for the smallest N. I'm pretty sure N cannot be more than a couple thousand bits, and it's probably the case that it's on the order of a couple hundred bits.

(Note that Peano is implied by second order Peano, which can be formulated with a finite number of axioms, so you don't have to worry about making programs to generate all of the infinite axioms of Peano arithmetic)

  • \$\begingroup\$ Welcome to CGCC! While this is fine for the sandbox, before posting you should take a look at some challenges and make your formatting more alike to them. Regarding the challenge itself - I believe (although I don't know enough logic to prove) that there are programs which PA can't prove don't output any particular output. What prevents if code=="A": run such a program else: run code as python, which gives \$N=1\$? \$\endgroup\$ Oct 16, 2023 at 3:29
  • \$\begingroup\$ This doesn't quite work, as this doesn't give a witness to PA not proving that any program has Kolmogorov complexity larger than 1. After all, PA does prove that some machine has Kolmogorov complexity is larger than 1 since PA should decide what the first two programs in basically any language output. Sufficient conditions for such a witness is that the program, if it terminates, outputs a number, you can prove PA proves this number has complexity larger than N, the program must halt whenever there is such a proof for any number, and the program is of size at most N. \$\endgroup\$ Oct 16, 2023 at 3:57
  • \$\begingroup\$ But if there's a program which PA can't prove anything about its output (which is hardcoded as part of the language, so a single byte A runs it) than it couldn't prove any string has a larger complexity, because that implies A doesn't output it which PA can't prove \$\endgroup\$ Oct 16, 2023 at 4:15
  • \$\begingroup\$ Ah ok, yeah that's what I meant about the language not referring to PA or any other specific theory explicitly. \$\endgroup\$ Oct 16, 2023 at 4:43
  • \$\begingroup\$ This seems hard to define properly, because there are problems (like Goodstein's sequences) which don't refer to PA but PA still can't prove, and it seems likely there are problems like that of the format we want. \$\endgroup\$ Oct 16, 2023 at 4:49
  • \$\begingroup\$ I wanted to use PA for concreteness, but a version without a naturality requirement would be one where the program adjustable for any theory encoded by a first-order sentence. I think the alternative of requiring an actual witness program of the form of the second comment also allows dropping naturality since encoding the meaning of 'A' into arithmetic would be complicated enough to waste bits. \$\endgroup\$ Oct 16, 2023 at 5:00
  • \$\begingroup\$ In particular, since a programming language must be computable, the only way PA might not prove anything about the output of 'A' is if the program assigned to 'A' doesn't terminate. In this particular case, it must halt whenever PA is inconsistent, so it's some explicit function applied to the first proof of inconsistency. Thus, we have a explicit function of the first inconsistency of PA such that it's provable in the base theory (PA or Q) that if PA can prove that this function is not any explicit number, then PA is inconsistent. This might be impossible, and surely can't happen naturally. \$\endgroup\$ Oct 16, 2023 at 6:13

Seat people as far as possible

Imagine there are \$n\$ people \$\{a_1, a_2, \ldots, a_n\}\$ who enter a room in order and sit down in \$n\$ seats, arranged in a row. However, all of these people hate social contact, so they want to sit as far away from each other as possible; specifically, they sit in the set which maximizes the minimum distance from anyone who is already seated. They break ties by sitting in the seat furthest to the left. For example, suppose \$n=5\$. Then the people will sit down in the following order:

_ _ _ _ _ 
1 _ _ _ _ 
1 _ _ _ 2
1 _ 3 _ 2
1 4 3 _ 2
1 4 3 5 2

Your challenge is, given a positive integer \$n\$, output the final arrangement of \$n\$ people in \$n\$ seats as described above. You can start the numbering of the people from 0 or 1.

Test Cases

(In these examples, numbering starts at 1.)

0 []
1 [1]
2 [1, 2]
3 [1, 3, 2]
4 [1, 4, 3, 2]
5 [1, 4, 3, 5, 2]
6 [1, 4, 3, 5, 6, 2]

Standard loopholes are forbidden. As this is , shortest program wins.


Has this been done before? It feels like would have been (or it reduces to some problem which has been done before), but I don't know how to find out.

  • \$\begingroup\$ “Imagine there are n people {a1,a2,…,an} who enter a room in order and sit down in n seats, arranged in a row.” Wouldn’t that mean that they all have to sit next to each other, if there are exactly n seats in a row? Am I reading this wrong? \$\endgroup\$
    – noodle man
    Oct 31, 2023 at 17:11
  • \$\begingroup\$ @noodleman Eventually, all the seats will be filled, but the i-th person doesn't necessarily sit in the i-th seat -- they sit in whatever seat in the row maximizes the min distance. \$\endgroup\$ Oct 31, 2023 at 17:19
  • \$\begingroup\$ Oh, I see. So this is a slight variant of the “urinal problem”? There are a few challenges related to it, maybe there’s a dupe under a different title \$\endgroup\$
    – noodle man
    Oct 31, 2023 at 17:22
  • \$\begingroup\$ Very closely related: codegolf.stackexchange.com/q/47952/108687 This is like an easier version though so maybe it’s not a dupe \$\endgroup\$
    – noodle man
    Oct 31, 2023 at 17:26
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