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2973 Answers 2973

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Rotate / Flip a Unicode Box Drawing

Given a Unicode box drawing, followed by a series of rotate and/or flip commands, output the result of those operations on the drawing. For clarification, a box drawing can be made from the following characters:

─ │ ┌ ┐ └ ┘ ├ ┤ ┬ ┴ ┼
═ ║ ╔ ╗ ╚ ╝ ╠ ╣ ╦ ╩ ╬
    ╒ ╕ ╘ ╛ ╞ ╡ ╤ ╧ ╪
    ╓ ╖ ╙ ╜ ╟ ╢ ╥ ╨ ╫

The rotate and flip commands are also presented using Unicode symbols:

↔  Flip the drawing horizontally
↕  Flip the drawing vertically
↷ Rotate the drawing 90° clockwise
↶ Rotate the drawing 90° counter-clockwise
↯  Convert all single lines to double and vice versa (Optional - 10% bonus)

All other characters should remain unchanged, but moved to fit where they would be in the modified drawing. For example, if given the inputs:

Input   Output          Input   Output
 ┌┴╖     ╓┴┐             ┌┴╖     ╔═╕
 │A║     ║A│             │E║     ╣E├
 ╘╦╝     ╚╦╛             ╘╦╝     ╙─┘
 ↔                       ↔↷

 ┌┴╖     ╒╩╗             ┌┴╖     ┌─╖
 │B║     │B║             │F║     ┤F╠
 ╘╦╝     └┬╜             ╘╦╝     ╘═╝
 ↕                       ↷↔

 ┌┴╖     ╓─┐             ┌┴╖     ╔╩╕
 │C║     ╣C├             │G║     ║G│
 ╘╦╝     ╚═╛             ╘╦╝     ╙┬┘
 ↷                      ↶↶

 ┌┴╖     ╒═╗             ┌┴╖     ╔╩╕
 │D║     ┤D╠             │H║     ║H│
 ╘╦╝     └─╜             ╘╦╝     ╙┬┘
 ↶                      ↔↕

The drawing may be of any size (let's say anywhere from 1×1 to 50×50), and not necessarily square.

The flip and rotate commands will always occur after any drawing to be flipped, and by themselves on one single line. They should be executed from left to right. (Note that order matters - the E and F examples use the same two commands but in reverse order, and produce different results.)

There may be an arbitrary number of flips and rotates, but you'll note that there are only 8 possible end-states for the final drawing. The G and H examples show two different sets of commands that produce the same result (other than the letter in the middle). It is possible that a series of commands will result in simply returning to the original drawing.

Input may be supplied via command line, user input, file I/O, or any other means you see fit. (Though it should obviously support multiple lines of input. You may use \n to represent line breaks if your input mode only supports a single line.) Likewise, output may be to the screen or a file at your discretion.

This is code-golf, so shortest code wins.

Some more complicated examples:

Input    Output        Input    Output
          ┌─┐                    ┌─┐
          │A│                    │B│
┌─╥─┐     ╞═╡          ┌─╥─┐     ╞═╡
│A║B│     │B│          │A║B│     │A│
└─╨─┘     └─┘          └─╨─┘     └─┘
↷                     ↶

   Input           Output
┌─┐╔═╗╒══╓──    ──╖══╕╔═╗┌─┐
├─┘╠═╝│  ║ ╖    ╓ ║  │╚═╣└─┤
│  ║  ╘══╙─╜    ╙─╜══╛  ║  │

┌─┐╔═╗╒══╓──    ╔═╗┌─┐╓──╒══
├─┘╠═╝│  ║ ╖    ╠═╝├─┘║  │ ╕
│  ║  ╘══╙─╜    ║  │  ╙──╘═╛

┌─┐╔═╗╒══╓──    ┌─┐╔═╗╒══╓──
├─┘╠═╝│  ║ ╖    ├─┘╠═╝│  ║ ╖
│  ║  ╘══╙─╜    │  ║  ╘══╙─╜
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  • \$\begingroup\$ You should have examples where characters other than the box move, the box is a different size and with more than one box, if that is allowed. This looks like a good challenge, but one that is likely to be rife with ambiguity. \$\endgroup\$ – FryAmTheEggman Dec 9 '15 at 14:46
  • \$\begingroup\$ @FryAmTheEggman - I chose one starting box which is asymmetrical just as a simple example. I could add a couple more complicated shapes - I was thinking this could be applied to any shape using the box-drawing symbols, not just a simple box. \$\endgroup\$ – Darrel Hoffman Dec 9 '15 at 18:55

Create a spiraling image




Given an image, output the image with a spiraling effect.


This is a , so the submission with the highest number of votes wins!

Test cases

Note that your program should work for images with any size. The output should be a 512×512px image. Also, note that the test cases are examples, you may use any algorithm to produce these images and do not have to match the images below. Use your creativity!

Test case 1

enter image description here


enter image description here

Test case 2

enter image description here


enter image description here

Test case 3

enter image description here


enter image description here

Test case 4

enter image description here


enter image description here

Test case 5

enter image description here


enter image description here


The full size images:

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  • \$\begingroup\$ What counts as the Droste effect does not appear to be clearly defined here, and some of the example outputs are hard to justify as exhibiting the effect. The chat image in particular doesn't seem to have any repetition of the image within itself, only distortion of the scale. \$\endgroup\$ – trichoplax Feb 13 '16 at 23:23
  • \$\begingroup\$ @trichoplax Oh, you're right. That isn't even a Droste image. I should probably remove the "Droste" thing and make something else out of it. Maybe define it as a "spiral" image. \$\endgroup\$ – Adnan Feb 13 '16 at 23:30
  • \$\begingroup\$ Either way it sounds like the too-broad end of the popularity contest spectrum. For a Droste image challenge you could probably make it well defined enough to not need to be a pop con if you also have a template as input, defining one or more places where the image needs to reappear recursively. So for each hole in the template, the image will reappear, including the holes in the template for repeatedly smaller images. \$\endgroup\$ – trichoplax Feb 13 '16 at 23:33
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    \$\begingroup\$ @trichoplax Hmmm, okay. I should give this a lot of thought then. I do think that with the amount of possible ways to approach this problem, this should be a popularity contest. But it's definitely too broad at this stage. \$\endgroup\$ – Adnan Feb 13 '16 at 23:41

Random Physics Golf #1: Net Gravitational Force

Introduction to the Series

Every week or so I will be posting a physics challenge. My goal here is to design challenges that in the end, teach some people some physics. Overall, the challenges will be very basic with little information. All of these challenges will have the minimal information necessary to solve them, and the goal is for users like you to do some research, watch some videos, and understand how these concepts work to teach you how to approach these types of physics problems and explain how they work. Of course, I will also give two optional hints per challenge, which are there if you do not have the time or determination to do the research, or you cannot figure out how to do the problem after researching. The two hints will be "necessary equations for this challenge" and "process to solve the problem". The hints are completely optional to use and it is encourages to not use them, but as stated above to learn the information for yourself. The series will have one main leaderboard. Whoever has the least combined byte count for all of the challenges gets a to be determined prize. Each challenge will range in difficulty, with an upwards trend of difficulty. I wish you all luck and I hope you learn a thing or two!

Challenge #1: Net Gravitational Force

Note 1: this challenge highly requires knowledge of the mathematical vector quantity, if you do not know what that is, I suggest reading this and this before attempting this challenge.

Note 2: this challenge considers gravity in CLASSICAL MECHANICS. Disregard general relativity for this challenge.

Lets start with the definition of a force. A force is a vector quantity, a number with both direction and magnitude. Simply put, force is mass times acceleration. Many mathematicians will know the name of this formula as Newton's Second Law. Now, that is a well known formula, but here is something less known: all forces are classified in one of four categories: weak nuclear, strong nuclear, electromagnetic, and gravitational. These four are called the four fundamental forces of the universe. We will be focusing on the gravitational force in this called.

The gravitational force is then classified as a field force. This means that the force acts on all objects in a certain radius around another object. In this case, gravity pulls down on objects from anywhere in a radius around them. However, I still have not defined where gravity comes from. Well, the simple answer is from mass. An object with more mass has a bigger gravitational pull on objects around it. In case you are wondering, the earth has a gravitational pull of -9.81 m/s^2 (an object will gain 9.8 m/s of downward velocity every second). But here is where it gets fun: because gravity comes from mass, every object with mass has a gravitational pull. This is where you come in. I want you to calculate the net gravitational pull of all the objects surrounding another object. Here is a better explanation:

You will receive co-ordinates and masses of objects in space for input. So an example input could be visualized as this: enter image description here You can easily see each force being applied on the target object by the three larger objects. Objects will smaller masses have a smaller gravitational field (as shown by the orange force arrow). Your job in this challenge is to find the net gravitational force being acted on the target object. To do this, it is a simple vector addition problem with the forces from the other objects. So, the resultant vector (net force), may look something like this: enter image description here This is all of the information that I will give you. It is now your job to find the equations, and research how this all works.

Challenge Specs

  • Input will be several lists of numbers consisting of the x co-ordinate, y co-ordinate, and mass of each object. The numbers could be either integers or decimals. You may take this in any convienent format ([[1 2 3][4 5 6][7 8 9], [(1,2,3),(4,5,6),(7,8,9)] and 1,2,3|4,5,6|7,8,9 are all acceptable. The first list of inputs will always be the target object (red in the pictures above), and the other lists will be the other objects. All inputs will have at least two objects. No objects will have the same co-ordinates.
  • Output will be two numbers, in any human-readable format, in any order. One will be the magnitude of the net force, using your desired unit system (SI, Planck units, Imperial units, etc.) and the other will be the direction in radians OR degrees of the net force. Output must be precise to the least number of significant figures in the input (Input: 2, 3.5, 6.1 -> Output: 200 (232.34 before rounding), note this is not an actual test case). Output may or may not be in scientific notation, its up to you.
  • You may assume input will not cause any error during execution, and you may assume all inputs will be valid.

META NOTE: Help me decide the precision of the output: http://strawpoll.me/6825341

Test Cases

Meta Note: WIP


These hints are for those who do not want to put in the time and effort of research, or those who could not find a solution. So, here are the two hints:

Hint 1: Equations:

You need the following equations for this challenge:

enter image description here

You will also need the standard vector equations for this challenge:

enter image description here

Hint 2: Sample Solution Process:

It would take up too much space to fit it all here, so I made this to aid you for this hint.


Meta Note: blah blah blah, working leaderboard will eventually go here! This leaderboard will contain and combine scores for all of the weekly challenges. It will only be visible on this question, though.

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  • 1
    \$\begingroup\$ The input format currently is cumbersome, requiring string parsing. I would recommend loosening it to allow input as a list of 3-tuples (or similar representations). You should also specify whether the inputs will be integers or decimals. \$\endgroup\$ – user45941 Feb 5 '16 at 3:45
  • \$\begingroup\$ @Mego I took your recommendations. Does that look good? \$\endgroup\$ – GamrCorps Feb 5 '16 at 21:55
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    \$\begingroup\$ "Output must be precise to at least three decimal places" would get a rebuke from every physics teacher I ever had. Surely it should be to a number of significant figures which depends on the s.f. of the input? \$\endgroup\$ – Peter Taylor Feb 6 '16 at 13:45
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    \$\begingroup\$ @PeterTaylor Well, this is a programming challenge, and sig-fig calculations would (IMO) overly complicate the answers. (If your comment was a joke/sarcasm sorry ;) \$\endgroup\$ – J Atkin Feb 13 '16 at 19:56

Temperature in a line of rooms

You have a line of rooms that are different temperatures.

      1       2       3  
 1.2  |  3.5  |  4.0  |  3.7

The doors between adjacent rooms start out closed. When you open a door, the now-connected rooms average out their temperatures. For example, opening door 2 gives

      1       2       3   
 1.2  |  3.75 _  3.75  |  3.7

Then, opening door 1 equalizes the first three rooms to their average (1.2+3.75+3.75)/3 = 2.9

      1       2       3   
 2.9  _  2.9 _  2.9  |  3.7

Finally, if we close door 2 and then open door 3, the last two rooms will average out without affecting the other rooms.

      1       2       3   
 2.9  _  2.9 |  3.3  _  3.3

You can think of the instructions to open and close doors as a sequence of toggles that switch between open and closed, here 2, 1, 2, 3, with the doors starting closed. Given the initial temperatures of the rooms and the sequence of door toggles, output the final temperatures. Fewest bytes wins.


  • A list of initial room temperatures, which are positive reals. There will be at least two rooms.
  • (Optional) The number of rooms n.
  • A list of doors to toggle in order, which range from 1 to n-1. Optionally, these may be zero-indexed.

Output: The list of final room temperatures to some reasonable precision.

TODO: Test cases.

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Senior Prank

We're graduating to a full site soon, and there's only one thing left to do before graduation: pull a senior prank! I think we should do a variation on the classic "fill a hallway with cups of water" gag.


Your program will read in text and output that text, covered in upside-down cups of water. An upside-down cup of water looks like this: /~\
These cups can only be placed in whitespace in the input, and can only be placed so that all three characters of the cup are directly above a non-whitespace character (otherwise the water would spill out!). Cups cannot be stacked on top of other cups. Cups must be placed in every available opening, and it is assumed that every input is surrounded by an infinite field of whitespace.
We need to pull the prank off quickly and without anyone noticing, so fewest bytes in each language wins.

Test Cases


    /   ___    /   ___    /   ______/   ________/
   /   /__/   /   /__/   /   /     /   /_______
  /   _______/   _______/   /     /   //__    /
 /   /      /   /      /   /_____/   /___/   /
/___/      /___/      /_________/___________/


    /   ___    /   ___    /   ______/   ________/
   /   /__//~\/   /__//~\/   /     /   /_______
  /   _______/   _______/   //~\  /   //__    /
 //~\/      //~\/      //~\/_____//~\/___//~\/
/___/      /___/      /_________/___________/


 L\\        ____I____
    ========    |  |[\


 L   /~\/~\/~\/~\/~\/~\/~\
 L\\/~\/~\  ____I____
    ========/~\ |  |[\
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  • \$\begingroup\$ @TimmyD Initially that gap was two wide. I must have added the space after making the output. \$\endgroup\$ – Mike Bufardeci Mar 3 '16 at 18:19
  • \$\begingroup\$ @FryAmTheEggman Yeah, that's a typo. I should really double check when I edit on my phone. \$\endgroup\$ – Mike Bufardeci Mar 4 '16 at 4:39

City Life

A cellular automation war game.

In this game, each player will control group of cites on a grid. Each city takes up one cell, and all cells with no city are "wilderness", and have no owner. The game will consist of a series of rounds, called "generations". Play continues until a player gets 1000 points, or 200 rounds, whichever happens first.


The board will start with one city controlled by each player. It will be square with sides length ceil(sqrt(25*n)) for an n player game. Cities will be placed randomly in such a way that no two cities will see each other the first round.

Phase 1: give orders

At the start of each generation, each city gets n actions where n = # of adjacent wilderness spaces + 2. So a city surrounded by wilderness gets 10 actions while a city surrounded by cites gets only 2. The Actions will be divided into these three categories:

Attack/Spread : used to Attack Cites or spread into the wilderness.

  • Takes a direction as a parameter. Will add one "Attacker" to that cell, even if the cell is a city with the same owner. (See resolve attacks)

Defend : used to protect your city.

  • Will add one to the defender count of the city performing this action. (If no defend actions are used, the city will become wilderness.)

Score : used to win.

  • Adds one to the score the cites owner.

Phase 2: resolve attacks

After all cites have put in orders, all cells are checked for takeover.

  • A wilderness cell will become a city if at least three attackers are there. The new city's owner will be determined randomly from among the attackers.(for example, if player A sent 2 attackers and player B sent 3, than A has a 2/5 chance of owning the new city)

  • A city will become wilderness if the number of attackers is equal to the number of defenders (even if both are 0).

  • A city is taken over if their are more attackers than defenders. The city's new owner is determined randomly from among the attackers, as above.

After the round is complete, all attackers and defenders are reset.

To enter the competition, you must create a bot to perform the "give orders" step. All bots will be written in JavaScript.


You will provide a character to represent your city and a function that takes as parameters:

  1. Your vision. each city can see a 5x5 square with the city as its center. It will be represented as an array of arrays of characters, " " representing wilderness, and each players character to represent their cites.


if you have a map like This ("Y" represents you)

|AA  B|    N
|A    |    ▲
|  YY | W< O >E
|C   C|    V
|   C |    S

Your sight parameter will be:

[["A","A"," "," ","B"],["A"," "," "," "," "],[" "," ","Y","Y"," "],["C"," "," "," ","C"],[" "," "," ","C"," "]]`
  1. The number of actions you can perform this turn. (which can be calculated, but I will give it to you as most bots will need it.)

You must return an array of strings, each string representing an order.

"N" - Attack North

"NE"- Attack North east

"W" - Attack West


"D" - Defend

"$" - Score

If you return more moves than you have actions allotted, the moves at the end of the array will be ignored. If you have less, extra moves will be set to "D".

I have made a controller that is reasonable, although I would like add to it and finalize the rules before publishing. If you have any advice or criticism, please comment below.

Example Answer:

Random Bot [?]

function(map, moveCount){
    var allMoves = ["N","S","E","W","NE","NW","SE","SW","D","$"];
    var orders = [];
    while(orders.length < moveCount){
    return orders;

This bot will just assign a random move for each action. Its cites are represented by ?.

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  • \$\begingroup\$ I think you left off a row in the map. You should also clarify whether attackers and defenders are persistent or must be re-assigned each turn. \$\endgroup\$ – ballesta25 Jan 18 '16 at 20:15
  • \$\begingroup\$ "A city can become wilderness if the number of attackers is equal to the number of defenders" implies to me that there's a random element. Is that so? If not, I suggest replacing can with will. \$\endgroup\$ – Peter Taylor Jan 23 '16 at 19:08
  • \$\begingroup\$ @PeterTaylor Thank you, fixed. \$\endgroup\$ – MegaTom Jan 23 '16 at 20:52
  • \$\begingroup\$ I've been thinking more. It would be good to state the initial density (or, in other words, how the size of the world varies with the number of players). In the interests of fairness, probably also worth guaranteeing that the initial cities will have a certain buffer region (and in particular, that two of them won't start next to each other and a third in a nice open space); and that the topology will be toroidal (so the board wraps both horizontally and vertically). Finally, I would make it explicit that you can attack your own city, but not "support" it (i.e. lend it defenders). \$\endgroup\$ – Peter Taylor Jan 23 '16 at 22:25

Super Smash Bots

This is an idea for a KOTH based off of the Super Smash Bros video game series by Nintendo.

The basic mechanics of this KOTH would be an every-man-for-himself battle between a large number of players simultaneously. Players can execute a variety of moves, like short/medium/long-ranged attacks or blocking. As opposed to most combat-based games, players do not have a health bar, but rather die when knocked off of the stage (the arena). When an attack hits a player, it deals damage to that player but also causes knockback. The amount of knockback a player experiences is proportional to the total amount of damage he's received so far in the game.

The Arena

The game takes place on a vertical stage with gravity. There will be several fixed platforms surrounded by empty space. A player too far from a platform is killed. More ideas about the design of the stage are covered in the Stage Design section.

Character Selection?

The actual game offers players the selection of different characters, each of which have different abilities, strengths, and weaknesses. If I am to include character selection, that will be a way for players to pick the strategy they think works best and choose a bot that has those strengths. Another KOTH which had this feature was the Pokemon-themed KOTH (citation needed).

On the other hand, balancing stuff can be hard.


  • Move. Simple as that. Well, not exactly. I think this could make a very good contender for a continuous-surface (not a grid) area. On a surface, the character would walk, while airborne, the character's movement doesn't respond as quickly.
  • Jump. Is like a move, but vertical. More specifically, this gives the player an upward velocity. Double jumping might be possible.
  • Short Range. This deals damage to an immediately adjacent player.
  • Medium Range. This deals damage to players within a certain range. It would likely involve your player physically moving as well.
  • Long Range. This creates a projectile, which can deal damage to a player in the specified direction, no matter the distance.
  • Area of Effect. All players within a given range take a little damage.


Is is often typical that players have three lives, and thus must be killed three times to be eliminated. By respawning players, the variance of each match outcome should be reduced.

Game Ticks

If I'm doing a continuous-field, then I would want something the emulates continuous movement. In order to give fair processing time, I can't really have all of the entrant programs running at once in an asynchronous fashion. Some possible solutions are as follows.

  1. Priority Queue
    • Each action taken creates a certain time delay before the player can move again. Standing still is shortest delay, moving is second-shortest, and long-ranged attacks have the longest delay.
    • Turn order is determined by a priority queue. From the list of players, the player with the least delay is selected. Then, the game's physics are simulated for that amount of time ("virtual" time, not "true" time) and that much time is subtracted from everyone's delay. The selected player picks an action, and he is put back into the queue with that move's delay.
    • This creates a period of vulnerability after an attack which may be game-mechanically interesting.
  2. Random Time-Steps
    • Play occurs in a semi-random order, with random, non-uniform time-steps between moves. This makes it so that the player cannot predict exactly what the world will be like in the future, or guarantee the exact timings of any moves.
    • After each player has taken a single move, the order of players would be scrambled.
    • Time-step duration would be approximately constant with a random variation of +/-20% or something like that.


This would be really cool to watch, but I don't think I could possibly animate this by myself.

Stage Design

Stage design will be important, because a large chunk of the entrant programs will be tailored the stage. I don't really have any clue what I'm doing here.

One idea I've had is to crowd-source the stage design.

Maybe something with a few ledges, like this? (scaled down for ASCII-artability)


I could add some more features like so:


The slants represent ramps that the player could walk up.

Player Navigation

I foresee one of the most difficult things for entrants to do is to navigate the stage. Each player would definitely receive a copy of the current stage and players as input every turn (or as arguments/parameters, more details on that below).

I may choose to offload a bunch of pathfinding stuff onto the controller so that entrants, if they so desire, can give the destination and have their character move there. Given that the stage would be constant, this should not be difficult for the controller to do. On the other hand, the continuous-field design can make pathfinding more complicated.

One thing to consider during stage design is the ease of pathfinding.

Vertical or Horizontal?

Pretty much the whole proposal has been assuming a vertical map. I could change this to horizontal to allow a larger number of people to fight in one match.

Classes or Full Programs, and Language?

Personally, I think this would be easiest to do as a Java KOTH with classes. It will run quick(-er than several other methods) and I could give entrants access to a variety of methods that give information about the stage.


Literally no work has been done yet.

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Golf all the 16 logic gates with 2 inputs and 1 output!

This question asked for 16 independent functions. I would like the opposite: a single function that takes an additional parameter that specifies which of the 16 logic gates is required using an integer from 0 to 15. If you don't want to use a 0-based index of the list in the linked question then you should specify which integers map to which logic gate (but they should still be 0 to 15).


 0,0,0  falsey
 1,0,1  falsey
 2,1,0  truthy
 3,1,1  truthy
 4,1,1  falsey
 5,0,1  truthy
 6,1,0  truthy
 7,0,0  falsey
 8,0,0  truthy
 9,0,1  falsey
10,1,0  truthy
11,1,1  truthy
12,1,1  falsey
13,0,1  truthy
14,1,0  truthy
15,0,0  truthy

This is , so the shortest answer in bytes wins.

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  • \$\begingroup\$ Here you go \$\endgroup\$ – Peter Taylor Jun 15 '16 at 10:37
  • 2
    \$\begingroup\$ @PeterTaylor That one is a kolmogorov-complexity. \$\endgroup\$ – Leaky Nun Jun 15 '16 at 10:47
  • \$\begingroup\$ @Mego How is generating 0101010101010101\n0011001100110011\n0000111100001111\n0000000011111111 the same as golfing 16 logic gates? \$\endgroup\$ – Leaky Nun Jun 21 '16 at 9:48
  • \$\begingroup\$ You may want to ban builtins. J has a two-byte solution, b. \$\endgroup\$ – Conor O'Brien Jun 25 '16 at 22:18
  • \$\begingroup\$ @CᴏɴᴏʀO'Bʀɪᴇɴ How do I go about looking up what b. means? \$\endgroup\$ – Neil Jun 25 '16 at 23:53
  • \$\begingroup\$ @Neil Voila \$\endgroup\$ – Conor O'Brien Jun 25 '16 at 23:54
  • \$\begingroup\$ @CᴏɴᴏʀO'Bʀɪᴇɴ Thanks, I can see that excluding builtins will be necessary. (Strictly speaking, I asked how to look it up, rather than for a direct link, which wouldn't help me look up any other J code should I need to for any reason.) \$\endgroup\$ – Neil Jun 26 '16 at 0:03
  • 1
    \$\begingroup\$ @Neil J is a hard language reverse. I would postix your search on google with site:jsoftware.com \$\endgroup\$ – Conor O'Brien Jun 26 '16 at 0:05


Make me some fireworks !
And since we are super-late for the 4th make them as quick (read short) as possible !

2 integer, the fuse (any value equal or bigger than 5) and the radius (1 - 2 - 3).

The fuse define the lenght of the tail, the last character of the tail is the center of the explosion. The tail must be centered with the explosion.
The radius define the explosion.

No need for exception handling, the input will be a valid one.
You may or may not padd your firework, the choice is up to you.
Input, Output and the choice beetwen full program or function is, once again, up To you and your lenguage of choice.
Standard loophole rules apply.
Hopefully no built-in (i'm looking at you mathematica) exist.


fuse 5, radius 1  
     * *    
    * * *

fuse 10, radius 3  

   *     *
    *   *
     * *    
* * * * * * *
    * | *
   *  |  *

while stretching the fuse is not a big deal, ence no limit to it, I found interesting see if it's gonna be cheaper to have hardcoded the strings for the part where the firework cross the fuse or is gonna be cheaper some fancy algorithm, ence I' ve set a limited number of alternative for the radius.

I'm really sorry if my english is bad, I usually can get my idea trought but more than often i stumble with some verbs.

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  • 1
    \$\begingroup\$ Welcome to PPCG and thanks for using the sandbox! "Number" isn't very descriptive, I assume that the fuse and radius should be positive integers (i.e. not including zero)? What happens if the radius is larger than the fuse? \$\endgroup\$ – FryAmTheEggman Jul 13 '16 at 17:00
  • \$\begingroup\$ thanks for the welcome, i'm actually lurking since a bit, but never had something new \ better golfed to add. anyway 2 time my bad :/ . First with the number (i'm used to deal alot with database where number make sense) and then i must have forgot about min and max value while rewriting my question. edited ! \$\endgroup\$ – Jackyz Jul 13 '16 at 17:29

Peter Piper and the Peck of Pickled Peppers


Without an introduction, output the following tongue twister:

Peter Piper picked a peck of pickled peppers.
A peck of pickled peppers Peter Piper picked.
If Peter Piper picked a peck of pickled peppers,
Where's the peck of pickled peppers Peter Piper picked?

with or without a trailing newline.

This is , so shortest code wins.

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  • \$\begingroup\$ I did some searching for duplicates, I came up with: this and this. There were a couple others that were also rather similar (slim shady and old macdonald), but they had a source restriction or some input as well. I'm not sure if any of them are duplicates, but these seem awfully close. \$\endgroup\$ – FryAmTheEggman Jul 26 '16 at 21:05
  • 3
    \$\begingroup\$ I wish the words didn't come in the same chunks. The substrings "Peter Piper picked" and "peck of pickled peppers" are most of the text, and the rest has little structure: X a Y. A Y X. If X a Y, Where's the Y X? Is there another tongue-twister where the words are permuted more? \$\endgroup\$ – xnor Jul 27 '16 at 1:21

Transpose a Ragged Array

Given an array of arrays of integers where the rows may not be of equal length, pad those rows with nulls, and transpose the array.


  • Use any sane input for the array.
  • Specify which null(s) you are using for this function.
  • The output should be a transposed array, printed in whatever way is sane for your language.
  • This is code golf. Aim for the shortest code possible.

Test cases

I: [[1, 2], [3], [4, 5]]          # Padding with nil here
O: [[1, 3, 4], [2, nil, 5]]

I: [[1], [2, 3], [4, 5]]
O: [[1, 2, 4], [nil, 3, 5]]

I: [[1, 4, 5], [8, 3, 2], [1, 7, 9, 6]]
O: [[1, 8, 1], [4, 3, 7], [5, 2, 9], [nil, nil, 6]]

I: [[1], [2]]
O: [[1, 2]]

I: [[1, 2]]
O: [[1], [2]]

I: [[4, 5, 6, 7], [8, 9]]                  # Padding with spaces here
O: [[4, 8], [5, 9], [6, ' '], [7, ' ']]    # as an example of a different null

As always, if the problem is unclear, please let me know. Good luck and good golfing!

| |
  • \$\begingroup\$ Having arrays that contain both integers and strings seems odd (and unrelated to the challenge) to me. \$\endgroup\$ – Nathan Merrill Aug 19 '16 at 6:29
  • \$\begingroup\$ @NathanMerrill In my head, I was trying to allow as many nulls as possible by restricting what data the arrays would have. You're right, though, and I have removed the reference to numbers and strings. \$\endgroup\$ – Sherlock9 Aug 19 '16 at 6:32
  • \$\begingroup\$ There's still a test case with it :) \$\endgroup\$ – Nathan Merrill Aug 19 '16 at 13:54
  • \$\begingroup\$ @NathanMerrill That's because I still want arrays that can contain any data. \$\endgroup\$ – Sherlock9 Aug 19 '16 at 13:59
  • \$\begingroup\$ @NathanMerrill Alright I rewrote the test case \$\endgroup\$ – Sherlock9 Aug 19 '16 at 17:00
  • \$\begingroup\$ Perhaps "ragged" is a better term for "uneven" \$\endgroup\$ – Conor O'Brien Sep 10 '16 at 15:54
  • \$\begingroup\$ Does the "null" have to be constant or can it depend on the input? \$\endgroup\$ – Dennis Sep 16 '16 at 6:20
  • \$\begingroup\$ @Dennis, I think the nulls should be constant for all inputs. \$\endgroup\$ – Sherlock9 Sep 16 '16 at 10:01

Play a 1D chess variant

in this challenge, you must create a bot that plays a 1d variant I created, featuring all leaper pieces, and a 15 long board.

Esolangs are encouraged to participate

Game Rules


To capture the opponents royal (king/queen).


The board is 15 squares long

Player _______________________________ Player
 one   |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|   two
       player1 side       player2 side

Each player has their side of the board. (note that while playing, you will always be on the left side of the board). You have six squares that belong to you, the opponent has six of their squares, and three squares in the middle are unowned.

Setting up

At the start of the game, you will place your pieces into a configuration of your choice. The pieces will be set independently of each other; this game does not have perfect information. When setting up, you will pick where to place your four pieces, and whether they will be reversed (see next section). You may only place your pieces in your owned squares.


There are 4 / 8 pieces; there are 4 distinct pieces, which you can have reversed, and unreversed. I have given them distinct names. Their movements are detailed here.


reversed means that all the moves of these pieces are reversed in their direction; 2 forward becomes 2 back, three back becomes three forward.

Capture means to move to a space occupied by an enemy piece, and remove that piece from the game. Move means to move to a space occupied by no pieces.

Ascii "diagrams" have been provided. P denotes the piece being showed, $ denotes a space the piece can move to, X a place the piece can capture on, # a piece can move and capture on. Diagrams are the same for both pieces in a pair, because one simply executes the moves backwards.

You must have exactly 4 pieces on the board, and have exactly one from each of the pairs below.

Footman (f)/Coward (c)

The footman may move to an empty space one (1) forward, and capture two (2) forward.

The coward may move to an empty space one (1) backward, and capture two (2) backward.

Horse (h)/spider (s)

The horse may move to an empty space two (2) or three (3) forward, and move or capture one (1) backward.

The spider may move to an empty space two (2) or three (3) backward, and move or capture one (1) forward.

#P $$
Archer (a)/Trickster (t)

(I doubt anyone will use trickster unironically)

The archer may move two (2) squares backward, one (1) forward, and may capture four (4) steps forward

The trickster may move two (2) squares forward, one (1) backward, and may capture four (4) steps backward

I have included some black and white squares for clarity in distance

# # # #
$ P$  X
King (k)/Queen (q)

Both Royal pieces will end the game when captured.

The king may move or capture one (1) backward

The queen may move or capture one (1) forward



Check does not exist: Royal pieces may be left en prise, capture of them results in a win for the capturing side.

There is no (pawn) promotion of any kind

A piece may not attempt to move of the board; the board is a fixed size and does not wrap.


Programs will take input. When they take input of "0", or a specific input of your choosing, they will output a setup for the game. when they take input "1", or a specific input of your choosing, they will then receive input of the board, and output a move. (the program will be run multiple times)


To move, you will output the square that the piece is currently on, and the space where you wish to move the piece to, or have the piece capture on. The format is as so (in regex):


The board is zero indexed, a piece on the first square is on square 0.

There should be exactly 2 non-newline characters in the output


when it is your turn to move, you will receive input. The input you receive will represent the pieces in the playing field. The input will fit the regex here:

[fFcChHsSaAtTkKqQ ]{15}

Caps represent enemy pieces, lowercase represent your pieces.

when you receive the board positions, you will always have the perspective of player one

WIP, will do more later


What should happen if bots enter an infinite loop? I could just make a draw on a time limit, but that seems not so great, since it is hard for bots to know when they are doing this.

Also just other feedback in general

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  • \$\begingroup\$ Maybe put the ASCII characters used to represent the pieces next to each description so we know what each piece is? It looks pretty vague right now. Also, the king and queen together seems redundant - maybe have it one or the other? \$\endgroup\$ – clismique Oct 4 '16 at 5:47
  • \$\begingroup\$ 1. The solution to infinite loops would seem to be automatic draw on three-fold repetition. 2. Is the Nash equilibrium pure or mixed? \$\endgroup\$ – Peter Taylor Oct 4 '16 at 7:33
  • \$\begingroup\$ @PeterTaylor 1. Three-fold repetition== one repetition of position (if bots have no RNG/are determistic, and I don't want non-RNG bots to suffer), and the bots don't know when they are repeating, because they have no memory. I was kind of thinking of maybe removing a square or something, but that would kind of mess up some stuff... I kind of want to introduce some aspects that will change the position, like adding a piece or something. 2. I imagine you are talking about the starting positions, given perfect play. I have no idea, but it's probably an intricate Rock Paper Scissors cycle \$\endgroup\$ – Destructible Lemon Oct 4 '16 at 7:47
  • \$\begingroup\$ When I asked about the Nash equilibrium I'd calculated that the number of starting positions is only a bit over 5000 each, so I thought it would be practical to brute-force. But the number of board positions is quite a bit higher, so it's probably not very practical without spending a lot of money. I'm not going to try writing a game tree searcher tonight to verify that. Re repetition: normally in koth you want to use persistent processes where possible, because otherwise the overhead of launching the program for every single move makes it really slow to score. \$\endgroup\$ – Peter Taylor Oct 4 '16 at 20:57
  • \$\begingroup\$ I was also thinking that maybe I would make the time it takes to win factor into the score, so that riskier strategies can do as well as a perfectly planned program might; It would make a more diverse series of strategies \$\endgroup\$ – Destructible Lemon Oct 4 '16 at 22:46
  • \$\begingroup\$ Is there a limit on how long a turn can take? IMO the obvious solution is a minimax tree, so we'd need to know about how many turns we can look ahead. \$\endgroup\$ – Riley Oct 12 '16 at 15:40

(-: Emotional Programming ;-)

Write the most emotional program you can, i.e. which consists of emoticons as much as possible.

The program should receive a word and print an appropriate emoticon.

Scoring - I'm really not sure about this, and the question will be worthless without a good scoring algorithm. I want to:
1. Avoid giving an advantage to very short programs (e.g. (-:, is 100% emotional).
2. Avoid a meaningless help of emoticons - print '(-:' #(-:(-:(-: and such.
3. Prefer a variety of different emoticons.

1. Count characters of code.
2. For each emoticon in the code, reduce a character. 2. For N different emoticons used, reduce further 2*N^2 characters.

What's an emoticon? Anything that somewhat resembles a face, or a closed list?

And should I ban Emoticon?

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  • \$\begingroup\$ My suggestion to fix scoring would be to add restrictions to the submissions' output, and have scoring be a function of their code and their output. \$\endgroup\$ – jwrush Aug 31 '13 at 16:32
  • \$\begingroup\$ Thanks @jwrush, Scoring based on output seems to complicate things, and I'm already unclear about my scoring. I changed the required behavior to something more closed. Still, I don't feel the question is good enough. \$\endgroup\$ – ugoren Sep 1 '13 at 4:45
  • 2
    \$\begingroup\$ This seems to have a strong bias towards GolfScript, which produces lots of emoticons naturally. It also seems hard to tighten to an objective spec. \$\endgroup\$ – Peter Taylor Sep 1 '13 at 18:43
  • \$\begingroup\$ @PeterTaylor, I don't like a bias towards GolfScript, but if that was a reason not to post questions, this site would be much smaller. The requirement can be very tight - I can provide a list of words and emoticons, and require translation between them. I'm not sure I want to tighten this way though. \$\endgroup\$ – ugoren Sep 2 '13 at 4:27
  • \$\begingroup\$ I disagree on the first point. I see a difference between GS having an advantage in code golf because it's designed to be terse, and GS having an advantage in a challenge because the challenge actively rewards a property which GS has as a side-effect of its design. Also note that I probably wouldn't be the loudest protestor against a pro-GS bias, but I think it's fair to warn you that other people might protest ;) \$\endgroup\$ – Peter Taylor Sep 2 '13 at 10:53
  • \$\begingroup\$ @PeterTaylor, I wish that was the problem. I don't really how to make this a question that triggers interesting solutions, so it looks like it stays in the sandbox. \$\endgroup\$ – ugoren Sep 8 '13 at 6:54
  • \$\begingroup\$ Might want to forbid comments? \$\endgroup\$ – Justin Nov 28 '13 at 22:22
  • \$\begingroup\$ I think this would only work as a popcon \$\endgroup\$ – Destructible Lemon Nov 3 '16 at 10:19

Create an Autostereogram (Optical Illusion w/ Hidden 3D Shape)

An autostereogram is a type of optical illusion requiring the viewer to, simply put, change distance at which your eyes are trying to view the image. In order to see the hidden image rather than just a nonsensical two-dimensional image, the viewer must either focus their eyes in front of the image (cross-eyed), or behind the image (wall-eyed). Depending on its type, autostereograms may be viewed either way, only cross-eyed, or only wall-eyed. Wall-eyed are the most common.

The illusion below taken from the linked Wikipedia article may only be viewed successfully using the wall-eyed technique. Viewing the full-sized image may help.


The hidden image is:

A shark

I didn't know they were called autostereograms until today, but I always liked this type of optical illusion, since I found the hidden image easy to spot using the wall-eyed technique.

Your goal is to take a depth map and either:

  1. Take in an image to modify with the map, then return/display the resulting image
  2. Create a random dot autostereogram

The quality of the image must be such that the image is hidden and can be viewed using one of the techniques listed above.

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  • \$\begingroup\$ I've also seen reddit.com/r/crossview which is a similar idea but uses 2 images next to each other to make it look 3d. \$\endgroup\$ – Faraz Masroor Apr 7 '16 at 14:42
  • \$\begingroup\$ I'd like input on whether people would prefer a [popularity-contest] or [code-golf] for this. I'm not sure if answers would be unique enough for a popularity contest. How much quality will users drop for saving bytes? \$\endgroup\$ – mbomb007 Apr 7 '16 at 15:11
  • 1
    \$\begingroup\$ I would clearly go for code-golf and restrict it to random dot autostereogram (which e.g. also needs e.g. the number of points as input). This can be defined quite well. (In a popularity contest people would probably just vote what they can see easily. Furthermore the there would be the question about a validity criterion.) I totally like the idea! \$\endgroup\$ – flawr Apr 11 '16 at 20:19
  • \$\begingroup\$ @flawr The thing is, I actually think random dot ones are much harder when trying to view the hidden image. \$\endgroup\$ – mbomb007 Apr 11 '16 at 20:55
  • 1
    \$\begingroup\$ @mbomb007 Harder to view or harder to produce? (PS: tx.technion.ac.il/~yonie/stereogram.txt) What do you think about ASCII stereograms? It would be quite a bit easier to make a validity criterion. \$\endgroup\$ – flawr Apr 11 '16 at 21:01
  • \$\begingroup\$ From chat, it seems like the consensus is that this question should be narrowed. This could potentially become 3 different questions about autostereograms: using random dots, using an image, and using ASCII. I think I'd like to start with using an image, since its output is easily viewable. \$\endgroup\$ – mbomb007 Apr 12 '16 at 13:49
  • \$\begingroup\$ @flawr Do you think other languages would be able to do what Mathematica can? mathematica.stackexchange.com/a/57108/35531 \$\endgroup\$ – mbomb007 Nov 2 '16 at 16:35
  • \$\begingroup\$ Probably, but the mathemtica version is already quite long, and most other languages will probably need way more code to do the same, which, so I'm afraid, could deter many people from participating. \$\endgroup\$ – flawr Nov 2 '16 at 21:31
  • \$\begingroup\$ @flawr Well, it's not golfed yet. \$\endgroup\$ – mbomb007 Nov 2 '16 at 21:34

Generate a Call Tree

Given a C program, generate a graphical call tree.

A call tree is a tree where the nodes represent stack frames, and the connections represent nested function calls. For example, here is a simple C program and its call tree:

#include <stdio.h>

int foo() {
    return 4;

int bar(int a) {
    return a + 2;

int baz(int a, int b) {
    return a + foo() + bar(a)*bar(b);

int main() {
    int a = baz(1, 2);
    printf("%d\n", a);
    return 0;

call tree


The layout of a call tree is as follows:

  • The top function is always main().
  • On the next level below are all of the functions that main calls, in order of calling (top-to-bottom, left-to-right).
  • On the next level are all of the functions that those functions call, and so on.
  • The leaves of the tree are functions which do not call other functions, or are standard library functions (which are treated as black boxes).
  • Each function (with its argument list) is in a rectangle, large enough that the function name and argument list isn't touching the rectangle's borders.
  • Rectangles must not touch other rectangles.
  • The lines drawn between the rectangles must not touch or cross any line or rectangle other than the two rectangles they are connecting.
  • The text must be a monospaced 14-point font.
  • The colors of the text, rectangles, lines, and background are not important, so long as all text is one color, all rectangles are one color, all lines are one color, the background is a single color, and everything can be clearly distinguished from the background.

Note that the example image above does not perfectly follow these rules.


  • You may assume that the C program is a valid, standalone program (all functions called are either defined in the input source code or exist in an #included header, and no input is taken from any source).
  • Any functions not defined in the input source code are assumed to not call any other functions (this is not the case in reality, but it allows for simplification of the call trees, namely by avoiding implementation-specific details)
  • All function calls in the call tree must include the arguments passed.
  • main is assumed to not have any arguments.
  • There will be no function pointers, gotos, setjmp/jonglmp calls, dynamic memory allocation/deallocation (so no malloc, calloc, realloc or free), or preprocessor macros in the input source code.
  • There will be no infinite loops or infinite recursion (all programs are guaranteed to terminate and thus have a finite call tree).
  • All arguments will be ints (to simplify matters, since the types don't really matter for this challenge). This means that the example program above would not be a valid input, because of the printf call.
  • All functions will either return an int or will be void (non-returning) functions.
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  • \$\begingroup\$ What about recursion? Do we have to support the full complexity of the C grammar? \$\endgroup\$ – orlp Dec 1 '16 at 7:07
  • \$\begingroup\$ @orlp Clarified on recursion as well as loops. Are there any parts of the C grammar that would be problematic, in your opinion? \$\endgroup\$ – user45941 Dec 1 '16 at 7:09
  • \$\begingroup\$ Mainly that the actual grammar is quite large. \$\endgroup\$ – orlp Dec 1 '16 at 7:21
  • \$\begingroup\$ @orlp For the most part, a lot of it can be ignored, because it won't have anything to do with function calls. Solutions will still be larger than typical code golf solutions, but that's fine - not every code golf challenge needs a 10-or-less-bytes solution. \$\endgroup\$ – user45941 Dec 1 '16 at 7:24
  • \$\begingroup\$ It is implied that the text in the nodes should include a representation of the arguments, but what should that representation be when they aren't "nice" ones like integers or strings? Obvious nasty cases are structs, unions, and pointers. \$\endgroup\$ – Peter Taylor Dec 1 '16 at 8:27
  • \$\begingroup\$ @PeterTaylor Good point. I'll restrict it to ints. \$\endgroup\$ – user45941 Dec 1 '16 at 8:29
  • \$\begingroup\$ Is it possible that the source will contain function declarations (as opposed to function definitions)? Those can look a lot like function calls (especially if they appear inside functions, which is legal in C). \$\endgroup\$ – user62131 Dec 1 '16 at 10:59
  • \$\begingroup\$ @ais523 Yes, they are allowed. They don't look that similar to function calls - function calls don't have a type signature prefixed (foo(); versus int foo();). \$\endgroup\$ – user45941 Dec 1 '16 at 22:35
  • \$\begingroup\$ What about code of the form a * foo();? That's either a function call or a function declaration depending on whether a is a type or a variable. (You could probably get around this by banning pointers full stop, not just function pointers.) \$\endgroup\$ – user62131 Dec 1 '16 at 22:40
  • \$\begingroup\$ @ais523 I clarified that all functions will either return int or void, so that won't be an issue. Besides, you can't declare a variable that has the same name as a type, so that situation could be deterministically resolved without the restriction. \$\endgroup\$ – user45941 Dec 1 '16 at 22:42
  • \$\begingroup\$ I like the idea of the challenge a lot, but I'm not convince that the "graphical output" part adds something... I think just printing it one call per line, and increasing the indentation level would be enough. \$\endgroup\$ – Dada Dec 3 '16 at 16:04
  • \$\begingroup\$ Also, what about if there is a function call like f(f(n)) ? Should we consider that a call to f(1) is done first, then a call to f(result of f(1))? (maybe adding a line about it would be nice) \$\endgroup\$ – Dada Dec 3 '16 at 16:06
  • \$\begingroup\$ @Dada Re: graphical-output: it's a lot harder to have an unambiguous, readable tree in text format versus graphical format. RE; nested calls, yes, nested calls are evaluated left-to-right, starting with the innermost nest and working outwards. For example, int main() { f(a(), b(c())); return 0;} would be main [ a, c, b, f ] (see what I mean about unambiguous and readble format for text?). \$\endgroup\$ – user45941 Dec 5 '16 at 7:42
  • \$\begingroup\$ @Mego If you ask for a text format version based on indentation, I don't think there can be any ambiguity. But if you prefer graphical-output, why not (I just think that most of the code will be about formatting the graphical output, while I find the "genrate the call tree" part more interesting, but that's only my personal opinion, I like the challenge either way). \$\endgroup\$ – Dada Dec 12 '16 at 17:33

Regex Golf Generators



The cops must post a 150 byte or less program in any language that outputs between 20 and 200 strings of printable ASCII (this excludes newlines), half of them "match" strings and half "don't match" strings. You can't output an odd number of strings -- there must be one don't match for each match.

The strings can be output as two lists of strings, or one list with a fixed delimiter between the "match" and "don't match" sections. The "match" and "don't match" lists can come in any order.

The following special characters are not allowed in the strings: ()[]*+?.\|^$.

Note that the program must be deterministic, and the language must be revealed.


The robbers must pick a cop answer and submit a regex in any flavor that matches the "match" strings but does not match the "don't match" strings.

The regex must be shorter than min(<length of all the match strings> + <number of match strings>-1 + 4, <length of all the don't match strings> + <number of don't match strings>-1 + 10), as this is the length of the regex that simply hardcodes it: ^(<match string 1>|<match string 2>|...)$ or ^(?!(<don't match string 1>|<don't match string 2>|...)$).*.

The shortest regex posted for that submission wins (note that there can and should be multiple competing cracks for one submission).


The robber's score is simply their number of wins (posted the shortest regex for a given submission) -- higher is better.

The cop's score is max(byte count of winning regex - byte count of submission - 4*(number of match strings - 10) for each submission) -- again, higher is better (you should be maximizing the length of the cracks and minimizing the length of your code). The byte count of the winning regex for an uncracked submission is the length of the hardcoded regex. Self cracks are permitted but must be marked as non-competing and will not count toward your cop or robber score.

The winning cop and robber will be announced 2 weeks after the posting of this challenge. Submissions will be allowed after that, but will not count towards your score.

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  • \$\begingroup\$ Can they be a list of lists, i.e. [['string1',truthy],['string2',falsy]...]? With truthy/falsy being match/don't match. \$\endgroup\$ – Rɪᴋᴇʀ Feb 15 '17 at 17:41
  • 2
    \$\begingroup\$ I've said this in chat, but I'm listing it here for reference: 1. No need to have a fixed number of match strings. 2. No need to remove regex special characters. Everybody has the same benefit of using them. 3. No need to have a maximum regex length. It's like having a maximum bytes on a code-golf. 4. I don't see why you want to aggregate the cops' scores. Simply make it a standard maximum-score-wins. Otherwise, a person who posted the best scoring submission may lose to another who only posted a single decent submission. 5. Disallow self-cracks. It's too abusive for robbers. \$\endgroup\$ – Nathan Merrill Feb 16 '17 at 1:42
  • 2
    \$\begingroup\$ What's the point of allowing a "Retina flavor" of regex? Wouldn't that be identical to .NET? \$\endgroup\$ – feersum Feb 16 '17 at 8:23

Machines learning arithmetic

Note: Feel free to take and use this challenge, either the entire challenge as is, or just parts of it.

I've made a complete rewrite of this question. I figured the original version was more complex than it had to be. The task is essentially the same. The original challenge text had 4 upvotes and can be found in the edit history.

You will receive 30 lists of integers. Those lists are the result of a polynomial expression y = p(x) = a*x^4 + b*x^3 + c*x^2 + d*x + e, for x in the inclusive range [-1e5, 1e5]. Let's call those lists L1, L2 ....

I reserve the right to make changes to the lists by changing constants and the order if solutions seem to be custom made for those 30 lists.


Your task is to figure out what the constants a, b, c, d, e are for each of those 30 lists.

You will write a code that pulls numbers from each list (one list at a time). It must ask for the y-values for individual x-values, as many as you want, but one at a time. When you think you have enough information, you'll attempt to guess the value of the constants.

You'll do this for all 30 lists.


The lowest score wins!

  • You get 1 point for every number you pull from the list
  • You get 10 points for every attempt to crack the code (guess a, b, c, d, e)
  • The scores for all lists will be added up.

If no submission cracks all lists successfully then the one that cracked the most will win. Tie breaker #1 will be fewest points, tie breaker #2 will be time of submission.

Rules and clarifications:

You can assume all constants to be integers

The lists will be formatted in this way (suppose the expression for L1 is: 2*x). I'm using MATLAB/Octave syntax for a cell array. You can change this to fit your needs (language).

L1 = {[-200000, -199998, -199996, ... -4, -2, 0, 2, 4, ... 199996, 199998, 200000], [0 0 0 2 0]};

You can change the format to fit your needs, but you must not mix the list and the values for a, b, c, d, e.


You ask for y for four different input values, and get the results:

L1(0) = 0
L1(1) = 0
L1(2) = 2
L1(3) = 12

Your function guesses (for some reason) that this is x^3-2x^2+1, and attempts to crack it:

L1([0, 1, -2, 0, 1]

You have tried 4 values, and attempted to crack it once. This gives you 4 + 10 = 14 points.

You try a few more values:

L1(-9) = 6480
L1(-7) = 2352
L1(7)  = 2352
L1(9)  = 6480
L1(100)= 99990000

You're now confident that this has to be: x^4-x^2, and attempt to crack it again:

f([1 0 -1 0 0])

You have successfully guessed the constants a, b, c, d, e, and get a score of `14 + (5 + 10) = 29 points.

You have 29 functions left.

  • I'll have to post the lists on some suitable place (where)?
  • Can I ban builtin interpolation functions somehow without risking the "unobservable requirements", "x without y" pitfalls?
  • Anything else?
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  • \$\begingroup\$ This isn't yet 100% clear to me, but I've upvoted because it looks like an interesting idea that will make a great challenge once fine tuned. \$\endgroup\$ – trichoplax Jan 18 '17 at 10:58
  • \$\begingroup\$ :) Anything in particular that isn't clear, or everything in general? \$\endgroup\$ – Stewie Griffin Jan 18 '17 at 11:00
  • \$\begingroup\$ I've edited... It was never intentional to have the number of bytes mixed in here... I'm probably just so used to writing it that I didn't even notice it :P \$\endgroup\$ – Stewie Griffin Jan 18 '17 at 11:03
  • \$\begingroup\$ Will it be done like: 1. "start writing your function" 2. "end of function writing period" 3. "release black box function for scoring submissions"? To avoid learning the expressions and tuning the functions to fit. \$\endgroup\$ – Emigna Jan 18 '17 at 11:04
  • \$\begingroup\$ I guess you could have a controller that generates new random test cases each time to allow the competition to be open ended, but rescoring the old entries might reorder them in some cases. Might need to average over a number of different sets of test cases. \$\endgroup\$ – trichoplax Jan 18 '17 at 11:07
  • \$\begingroup\$ The given example of h(x) = ((3*x*x*x)/(x+1))+1 is incompatible with "You can assume the functions will give integer results back". But basically it's a case of guessing and verifying a rational function of some bounded degree? \$\endgroup\$ – Peter Taylor Jan 18 '17 at 11:09
  • \$\begingroup\$ If you want a variety of languages to be able to compete without having to implement arbitrary size integer arithmetic, you could use modular arithmetic. For example, everything is mod 256, or 65536. \$\endgroup\$ – trichoplax Jan 18 '17 at 11:10
  • \$\begingroup\$ @Emigna, good question :) One solution could be: Provide x number of functions on a certain format. After the submission is posted it will be tested against the "real" cases that are of similar difficulty, but different. I think I can have some fairly easy, and some quite hard. But it's a really good question, so I'm not sure... I'm open for ideas :) \$\endgroup\$ – Stewie Griffin Jan 18 '17 at 11:11
  • \$\begingroup\$ @PeterTaylor, I made a blooper when making the examples. Using rational functions with input arguments and 8/10(?) significant figures in the output is probably a good idea. I think 10 is low enough that rounding shouldn't be an issue as long as you have the correct function..? And it can be restricted to 3/4/5..9th degree polynomials. \$\endgroup\$ – Stewie Griffin Jan 18 '17 at 11:15
  • \$\begingroup\$ Peter's comment also makes me wonder what happens for division by zero. Will this be avoided somehow or will we be informed that a particular input causes an error? Will the error be specifically "division by zero" or just "error"? \$\endgroup\$ – trichoplax Jan 18 '17 at 11:16
  • \$\begingroup\$ First comment @trichoplax: You have a point, but it will also make it harder, since it will no longer be a continuous function. Second comment: I think an error message / warning of some sort would suffice. Handling that error should be very simple, since this isn't code golf. It will probably be 1-3 lines of code. Giving inf for divison by zero, and error for 0/0 could also be a solution (that's the default solution in MATLAB). \$\endgroup\$ – Stewie Griffin Jan 18 '17 at 11:21
  • \$\begingroup\$ Note that if you want continuous functions you would need to avoid rational functions, but it sounds like you're happy with piecewise continuous if you don't mind "inf" and "error" in some places? \$\endgroup\$ – trichoplax Jan 18 '17 at 11:34
  • \$\begingroup\$ @trichoplax, What do you think? Is modular arithmetic a good idea? Should I skip division (impossible to avoid division by zero if not)? \$\endgroup\$ – Stewie Griffin Jan 18 '17 at 11:51
  • \$\begingroup\$ I really don't know. I'm just suggesting possibilities. Someone more mathematical would probably be useful for assessing whether modular and rational functions would be interesting to crack. Are you aiming for a situation where most people can write code that cracks all 10, but getting a low score is challenging, or where most people can only crack a proportion of the 10? \$\endgroup\$ – trichoplax Jan 18 '17 at 11:59
  • 1
    \$\begingroup\$ Parentheses should be removed since it makes this challenge much more difficult... and it is already really difficult without them \$\endgroup\$ – Anthony Pham Jan 19 '17 at 0:44

Exiting Vim — Cops & Robbers

In honour of the recent milestone, let's turn escaping Vim into a game!

Rules for Cops

Starting from launching vim with no arguments (i.e. no initial file open), provide a sequence of keys to be typed in to the editor.

Rules for Robbers

Starting from the state described by the cop, provide a sequence of keys to exit Vim.


Cops are scored by the difference between key counts <robber_key_count> - <cop_key_count>, and robbers are scored by the ratio of key counts <cop_key_count> / <robber_key_count>. Higher scores are better.

Keys are counted as one per key-down event (e.g. a sequence of Ctrl+X, Ctrl+Y, Ctrl+Z only need count the Ctrl once, unless it must be released during the sequence). Note that this is not the same as the golf-rules scoring for Vim.

Plugins are not permitted.

Example 1

Cop (1 key): i

Robber (4 keys): Esc : q Enter

Score for cop = 4 - 1 = 3, score for robber = 1 / 4 = 0.25

Example 2

Cop (2 keys): i i

Robber (5 keys): Esc : q ! Enter

Score for cop = 5 - 2 = 3, score for robber = 2 / 5 = 0.4

Example 3

Cop (3 keys): i Ctrl+V

Robber (6 keys): Return Esc : q ! Enter

Score for cop = 6 - 3 = 3, score for robber = 3 / 6 = 0.5

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  • 1
    \$\begingroup\$ I'm not sure that the scoring here rewards cops enough for ingenuity (all the examples score 3 after all!), so ideas on that would be great. Also I'm considering banning Esc & Ctrl+[ to make this more interesting, but I suspect even with Esc permitted there are ways to get seriously tied-up. \$\endgroup\$ – Dave May 23 '17 at 20:13
  • \$\begingroup\$ I'm worried that there may not be much scope for the cops to improve; it'd be something of a design flaw in the challenge if there's a hard limit to how well a cop can do. Also, what about key sequences that depend on the environment within which vim is running? (For example, Ctrl-Z will suspend vim and require the use of the shell to either exit or restart it, but how you do that depends on which shell is running; or on Linux, Ctrl-Alt-F1 will probably switch to a different virtual terminal altogether, and what assumptions can you make about its state?) \$\endgroup\$ – user62131 May 31 '17 at 21:35
  • \$\begingroup\$ Another potential problem: exit sequences which have side effects. In particular, I'm thinking about Alt-SysRq-K, which is guaranteed to exit vim, in addition to everything else, on Linux systems which have it enabled. That compares favourably to basically all the exit strings you have right now, and there's no way, short of reconfiguring the OS, to block it. \$\endgroup\$ – user62131 May 31 '17 at 21:36
  • \$\begingroup\$ @ais523 I'm thinking it could easily be restricted to "vim-only" commands, so anything which is intercepted by the shell can be forbidden. But I agree that it feels like there's a hard-limit on the cop (though I don't know vim nearly well enough to be sure, and I've seen some hints around the internet that it's possible to get exceedingly stuck, but perhaps only because some modes need obscure non-esc keys, rather than needing more keys). Any ideas for better scoring? \$\endgroup\$ – Dave Jun 5 '17 at 20:36

Me, Me, Me!

Edit: changed success when the input matches the source code to when the input is any permutation of the source.

Your code is clearly superior to all other code. In fact, your code is so great that it prints itself when the opportunity arises (but not when any other, inferior code is around.)


Write a program or function that takes a string as input. If the string is equal to some permutation of the characters in your source code, then output the entire source code. Otherwise, output Gross.

If your language uses a non-ascii encoding, "character" is defined as whatever a character in your source looks like. If it's unreasonable to take input in that format, you can treat the bytes of your source as their respective extended ascii codes.


Takes a string using whatever input mechanism your programming language provides.


Prints either the entire source of your program or the word Gross. No additional output is permitted.


  • You can take input using any reasonable method. (Stdin, function parameter, etc.)
  • A string a is a permutation of a string b iff each character in the alphabet appears the same number of times in a and b.
  • This is Shortest code (in bytes) wins.
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  • \$\begingroup\$ quine? -- \$\endgroup\$ – MD XF Jun 7 '17 at 21:32
  • \$\begingroup\$ @MDXF it will have the quine tag, of course – but it's not strictly a quine unless it gets the right input! I just didn't see how to add tags in the answer... \$\endgroup\$ – vroomfondel Jun 7 '17 at 21:34
  • 1
    \$\begingroup\$ Use [tag:quine] in the header. And yes, we accept the quine tag for quine variants :-) \$\endgroup\$ – MD XF Jun 7 '17 at 21:34
  • 2
    \$\begingroup\$ trivial extension of this challenge \$\endgroup\$ – Destructible Lemon Jun 7 '17 at 23:14
  • \$\begingroup\$ aw shucks. I figured something like this existed already, just didn't know what to search for. \$\endgroup\$ – vroomfondel Jun 8 '17 at 14:36
  • \$\begingroup\$ actually, @DestructibleLemon I could be missing something obvious but I don't think the extension is trivial. Printing its own source code is, I would say, significantly harder than printing "true" as long as I add the usual restriction that it can't read its own source. \$\endgroup\$ – vroomfondel Jun 8 '17 at 14:38
  • \$\begingroup\$ @rogaos If the test on the input passes, printing its source code is as simple as printing the input string. \$\endgroup\$ – NonlinearFruit Jun 8 '17 at 15:33
  • \$\begingroup\$ @NonlinearFruit true of course, but adding the print statement may make the quine harder to construct in the first place. (Except in languages like javascript, which are basically reading their own source anyway.) Maybe the challenge would be improved by transforming the input string in some way, or crashing on "correct" input? \$\endgroup\$ – vroomfondel Jun 8 '17 at 15:38
  • \$\begingroup\$ @rogaos it is the difference between printing true and printing the input \$\endgroup\$ – Destructible Lemon Jun 8 '17 at 23:29
  • \$\begingroup\$ modified to use permutations of the input rather than the input itself \$\endgroup\$ – vroomfondel Jun 19 '17 at 18:21

The Chroma Key to Success

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  • \$\begingroup\$ So essentially, for every pixel in the second image, if it's #00FF00, replace it with the corresponding pixel in the first image; otherwise, don't modify that pixel? \$\endgroup\$ – HyperNeutrino Jul 16 '17 at 17:33
  • \$\begingroup\$ @HyperNeutrino Yes. \$\endgroup\$ – ckjbgames Jul 16 '17 at 17:34
  • \$\begingroup\$ I don't know image formats very well, but are we guaranteed that the image will be in 3-byte-RGB format? As in, is there transparency? \$\endgroup\$ – HyperNeutrino Jul 16 '17 at 17:36
  • \$\begingroup\$ @HyperNeutrino Assume the image is full opacity. \$\endgroup\$ – ckjbgames Jul 16 '17 at 17:39
  • 3
    \$\begingroup\$ Please delete this, now that it is posted. \$\endgroup\$ – programmer5000 Jul 17 '17 at 13:05

How high can you count in English?


In 500 bytes (or fewer) write a program that outputs a list of the English word forms of as many consecutive integers greater than zero as you can.

For example, score 6:

one two three four five six

Example submission (hopefully you can do better than this):

Python 3, score 43 (488 bytes)

print(["one","two","three","four","five","six","seven","eight","nine","ten","eleven","twelve","thirteen","fourteen","fifteen", "sixteen","seventeen","eighteen","nineteen","twenty","twenty-one","twenty-two","twenty-three","twenty-four","twenty-five","twenty-six","twenty-seven","twenty-eight","twenty-nine","thirty","thirty-one","thirty-two","thirty-three","thirty-four","thirty-five","thirty-six","thirty-seven","thirty-eight","thirty-nine","forty","forty-one","forty-two","forty-three"])

Try it online!

Scoring and rules

For each language, the person whose code counts the highest wins. In case of a tie, the person who submitted first wins.

  • No modules, libraries, builtins that convert from numeric to word form are allowed.
  • You must output all integers from 1 (one) to n (your score) without missing any. If you want to output 0 (zero) as well, that's fine.
  • You are allowed up to 500 bytes of code. Your code may be a full program or function.
  • Number format: for consistency, all numbers must match the output of this site.
  • Standard loopholes apply (of course)
  • Standard output rules apply
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  • 2
    \$\begingroup\$ Come on, you could at least make your example better by just using a single space-separated string ;) but nice challenge! :) \$\endgroup\$ – HyperNeutrino Jul 18 '17 at 18:00
  • 1
    \$\begingroup\$ Alternate title: How high can you count in English? \$\endgroup\$ – Stephen Jul 18 '17 at 18:06
  • \$\begingroup\$ You may, however, omit "and" if you'd like. do we have to omit and or not? Personally, I'd say you wouldn't as it 'sounds more correct'. \$\endgroup\$ – Okx Jul 18 '17 at 18:11
  • \$\begingroup\$ @StepHen changed, thanks. \$\endgroup\$ – wrymug Jul 18 '17 at 18:15
  • \$\begingroup\$ Your edit doesn't answer my question. Do we have to omit and or not? \$\endgroup\$ – Okx Jul 18 '17 at 18:15
  • \$\begingroup\$ @Okx I take your point. Removed. \$\endgroup\$ – wrymug Jul 18 '17 at 18:28
  • 1
    \$\begingroup\$ I'm not sure this adds to existing number-to-english challenges like this or this. Past getting the same basic pattern of digits down, the question seems to be how many prefixes for powers of 1000 one can compress into the remaining bytes. \$\endgroup\$ – xnor Jul 18 '17 at 18:33
  • 1
    \$\begingroup\$ Off the jokes, what if I can count to infinity? \$\endgroup\$ – V. Courtois Jul 20 '17 at 13:40
  • \$\begingroup\$ @courtois I don't see how that would be possible, but I guess you'd win \$\endgroup\$ – wrymug Jul 20 '17 at 13:42
  • \$\begingroup\$ @rosslh yeah you're right, I just saw the sentence Number format: for consistency, all numbers must match the output of [this](https://lingojam.com/NumbersToWords) site. And for my method, it would have been with million of billion of billion of ... though the site stops at one hundred novenonagintanongentillion, something like 3002 zeros. \$\endgroup\$ – V. Courtois Jul 20 '17 at 13:43

ASCII addition


Given two strings, your job is to:

  1. Convert each character to their respective ASCII decimal value
  2. Concatenate the numbers into one large number
  3. Add these values together
  4. Get the ASCII characters represented of each pair of numbers starting from the right (or if there are not enough numbers, take a number alone)
  5. Leave unprintables (ie not in the range 32 - 126), and output the rest

An example for HELLO and WORLD

"HELLO"    + "WORLD"
H E L L O  + W O R L D
7269767679 + 8779827668
1 60 49 59 53 47    (separated to show ASCII conversion easily)
   <  1  ;  5  /    convert to ASCII by converting pairs to their respective characters (note: you start from the last pair)
<1;5/               output (note there is no 0x01) 


  • Each string will be a maximum of 6 characters long
  • Input will always contain readable ASCII characters
  • You have to take pairs of numbers from the end of the sum and convert each one of them to ASCII
  • You must not print unreadable characters if their values appear and instead skip them


ABC + XYZ  //input
656667 + 888990
1 54 56 57
   6  8  9       //again 0x01 is left out
689      //output


  • Your submission can be either a program or a function

This is so the program with the shortest bytecount wins!

Sandbox Questions

  • Are the specs clear enough?
  • Will this question give me hats?
  • Any better title suggestions?
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  • \$\begingroup\$ What range of ASCII characters do we need to be able to handle in the input? Only printable characters or any? \$\endgroup\$ – Robert Hickman Dec 20 '16 at 20:07
  • 1
    \$\begingroup\$ I swear we've had this challenge, but I can't seem to find it ... \$\endgroup\$ – AdmBorkBork Dec 20 '16 at 20:21
  • \$\begingroup\$ Edited that into the specs \$\endgroup\$ – user41805 Dec 20 '16 at 20:22
  • \$\begingroup\$ @TimmyD How's this now? \$\endgroup\$ – user41805 Dec 21 '16 at 7:28
  • \$\begingroup\$ what if a character is unprintable? do we still need to output it then? \$\endgroup\$ – Destructible Lemon Dec 21 '16 at 7:46
  • \$\begingroup\$ @DestructibleWatermelon Yes \$\endgroup\$ – user41805 Dec 21 '16 at 7:56
  • \$\begingroup\$ Better wording. Given that a bunch of control codes are possible, some test cases that demonstrate expected behavior when (e.g.) 11 or 13 occur in the output would be of good value. \$\endgroup\$ – AdmBorkBork Dec 21 '16 at 13:49
  • \$\begingroup\$ @TimmyD Do you think it would be a good idea if I restrict the output to displaying only characters if their values are between 32 and 126 because outputting other values might be difficult in some languages? \$\endgroup\$ – user41805 Dec 21 '16 at 13:54
  • \$\begingroup\$ That may be a better way to go, but you'll need to be very careful with the wording. For example, suppose that the output number is ...226... and the 26 is slated to be the pair of digits that get converted to ASCII. Obviously, that's outside the printable range, so let's look at the next digit, but now 226 is also outside. Does that mean just the 6 is skipped? The 26 is skipped? The 226 is skipped? \$\endgroup\$ – AdmBorkBork Dec 21 '16 at 13:58
  • \$\begingroup\$ @Cowsquack You don't say precisely (but we see it in the test case) how many digits we need to parse at a time, before outputting. \$\endgroup\$ – V. Courtois Jul 18 '17 at 14:06
  • \$\begingroup\$ @V.Courtois Is this clearer now? \$\endgroup\$ – user41805 Jul 19 '17 at 12:48
  • \$\begingroup\$ @Cowsquack This is! In fact you could see, in the comments some were wondering too if we had to print things like 125 -> }. \$\endgroup\$ – V. Courtois Jul 19 '17 at 12:55

Let's play Othello (Reversi)!

Open for takeover

I don't have the time to devote to this right new, and not for the foreseeable future. Anyone who wants this idea, the post, and any code I have in the repo should comment below.

Side note on the code, I have a gorilla repl file in the repo that I can use to run test games on, and get screenshots of during any point of the game. If you want I can make up screenshots for the rules section since I have everything setup on my end for that.

This would be a king of the hill about reversi. Yep. I'm going to write up the post and controller later The controller is being written here: https://github.com/JJ-Atkinson/reversi-koth-ppcg , but I'm putting this here so show the idea is taken ;)

Post start


(you can probbably skip this if you've played Reversi before)

enter image description here

(images from http://www.coolmath-games.com/0-reversi)

Reversi is a two player game with a simple goal - own the most pieces on the board.


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  • \$\begingroup\$ You will need a complete set of rules, and some high-level description of the controller (as well as a detailed spec when it's written), and you'll need to work out some victory criteria. \$\endgroup\$ – VisualMelon Jun 29 '17 at 6:50
  • \$\begingroup\$ Yep, all that is in the works. I'll add a questions section when it is ready for review. \$\endgroup\$ – J Atkin Jun 29 '17 at 18:46
  • \$\begingroup\$ As for a longer title, you could simply do "Let's play Reversi" \$\endgroup\$ – Shelvacu Jul 3 '17 at 22:46
  • 1
    \$\begingroup\$ This is not Reversi. This is Othello. Source. In Reversi as it was originally played, the starting configuration was not predetermined, like it is in Othello. Players would take turns setting down the first four pieces in the center of the board \$\endgroup\$ – Okx Jul 4 '17 at 11:23
  • \$\begingroup\$ Thanks for the heads up! I'll keep reversi in the title, since I've always seen this game called reversi. \$\endgroup\$ – J Atkin Jul 4 '17 at 15:54
  • \$\begingroup\$ @JAtkin But it is not called Reversi. I quote: Unfortunately, because the games look similar and have similar rules, and “Othello” is a trademark and “Reversi” is not, many board game sellers, websites, software makers, etc., wanted to piggy back on the popularity of Othello by calling their game “Reversi”. Quite aggravatingly, they never use the rules of real Reversi (as far as we've ever seen). I insist that you remove all instances of Reversi from your post. \$\endgroup\$ – Okx Jul 5 '17 at 17:22
  • \$\begingroup\$ I understand fully that Reversi is not the correct name of this game. However, many people (myself included) have not heard of Othello, and only know about Reversi, even if our understanding was incorrect. I plan to keep both names. I think I'll include the article you reference, but I won't remove Reversi. This is not because I dislike being correct, it is because I believe that it brings the tangible benefit of wider recognition and perhaps higher viewership of the post. I will have a strongly defined rules section so any concern about confusion about rules can be dismissed. \$\endgroup\$ – J Atkin Jul 7 '17 at 1:11

Prime encode integers!

In this challenge, you must convert inputted natural numbers into a prime encoding.

The sequence of the primes, and 1, is a complete sequence (We're going to consider 1 an honourary prime for this challenge). What this means is that it's possible to express any positive integer as a sum of the terms of the sequence (without reusing terms). For example, the powers of two are a complete sequence, and you can encode numbers in them (this is binary).

As with binary, you use a sequence of 1s and 0s to represent which terms are used. 1011 will represent 5 + 1 + 2, or 8. 8 could also be represented as 10001, or 7+1. the place values represent primes:

... 13 11 7 5 3 2 1

continuing with all of the primes to the left

In this challenge, you must output a string of 1s and 0s, such that the place value primes sum to inputted strictly positive integer

Test cases

8 -> 10001 or 1011
2 -> 10
11 -> 1111 or 10101 or 100000 (I might have missed one of the possibilities?)
13 -> 1000000 or any other possibility

you may use any valid representation of the in

input note:

input is 1 strictly positive integer

you do not need to consider the value 0, even though it is possible to represent in this system, by just outputting zero

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  • \$\begingroup\$ I'm assuming input is n>0? IO in all normal means? Is this code-golf? I'm also assuming we can return any valid combination? \$\endgroup\$ – TheLethalCoder Aug 4 '17 at 10:48
  • \$\begingroup\$ didn't it specifically say n>0? and also specifically say you can return any valid representation? \$\endgroup\$ – Destructible Lemon Aug 4 '17 at 22:49
  • \$\begingroup\$ It does specifically say "strictly positive", but a long way after it says "inputted integers". It's best to state restrictions like that as early as possible to avoid misleading or confusing readers. \$\endgroup\$ – Peter Taylor Aug 6 '17 at 19:00

Find the longest Factor-Multiple sequence

Inspired by this riddle.

A Factor-Multiple sequence is any sequence where A[n+1] is either a multiple or factor of A[n].


Create a full program or function that, given a list (or any other accepted input) of positive integers, returns (one of the) the longest possible Factor-Multiple sequence containing those numbers. Each number can only be used once and each number in the input will be unique.


As mentioned above, input is a list of integers. If your language only supports strings, or if you like doing so, you may take input as strings instead.


You can output your sequence in any way you like as defined on meta.


Test cases

More need to be added.

Input                                                Output                 
1 2 3 5 7 11                                         2 1 3 or 3 1 2 or 11 1 7 etc.
1 2 3 4 5 6 7 8 9 10                                 5 10 1 4 8 2 6 3 9  

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20   11  1  7 14  2  8 16  4 12  6 18  9  3 15  5 10 20 


  • Need to add more test-cases

  • Can't think of any relevant rules, did I miss any?

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  • \$\begingroup\$ Related (same problem, different graph). \$\endgroup\$ – Peter Taylor Aug 15 '17 at 22:28
  • \$\begingroup\$ @PeterTaylor Thanks for the link. From what I can see, that challenge has a one-directional graph, while in this challenge if a is connected to b => b connects to a. I am not sure if that changes the algorithms involved, but it would be interesting to see them golfed. \$\endgroup\$ – JAD Aug 16 '17 at 7:20
  • \$\begingroup\$ Actually the biggest difference is that the other one asks for approximate answers and this one asks for exact answers. \$\endgroup\$ – Peter Taylor Aug 16 '17 at 7:25

Case Matching Find Replace

Take three inputs, a string of text, T; a string of characters to replace, F; and a string of characters to replace them with, R. For each substring of T with the same (case insensitive) characters as F, replace them with the characters in R. However, keep the same case as the original text.

If there are more characters in R than F, the extra characters should be the same case as they are in R. If there are numbers or symbols in F, then the corresponding characters in R should keep the case they have in R. F will not necessarily appear in T.

You can assume all text will be in the printable ASCII range.


"Text input", "text", "test" -> "Test input"

"tHiS Is a PiEcE oF tExT", "is", "abcde" -> "tHaBcde Abcde a PiEcE oF tExT"

"The birch canoe slid on the smooth planks", "o", " OH MY " -> "The birch can OH MY e slid  OH MY n the sm OH MY  OH MY th planks"

"The score was 10 to 5", "10", "tEn" -> "The score was tEn to 5"

"I wrote my code in Brain$#@!", "$#@!", "Friend" -> "I wrote my code in BrainFriend"

"This challenge was created by Andrew Piliser", "Andrew Piliser", "Martin Ender" -> "This challenge was created by Martin Ender"

// Has a match, but does not match case 
"John does not know", "John Doe", "Jane Doe" -> "Jane does not know"

// No match
"Glue the sheet to the dark blue background", "Glue the sheet to the dark-blue background", "foo" -> "Glue the sheet to the dark blue background"

// Only take full matches
"aaa", "aa", "b" -> "ba"

// Apply matching once across the string as a whole, do not iterate on replaced text
"aaaa", "aa", "a" -> "aa"
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  • \$\begingroup\$ Example(s) where the replacement string narrowly does not appear in the text might help. \$\endgroup\$ – isaacg Aug 17 '17 at 19:55
  • \$\begingroup\$ @isaacg Added one, let me know if it's not what you were thinking \$\endgroup\$ – Andrew Aug 17 '17 at 21:11
  • \$\begingroup\$ What is the expected output for "aaa","aa","b" and "aaaa","aa","a"? \$\endgroup\$ – fireflame241 Aug 17 '17 at 21:33
  • \$\begingroup\$ Will we guaranteed that F will appear at least once in T? \$\endgroup\$ – Shaggy Aug 17 '17 at 21:58
  • \$\begingroup\$ @Shaggy F will not necessarily appear in T. \$\endgroup\$ – Andrew Aug 17 '17 at 22:04
  • \$\begingroup\$ @fireflame241 Added your two examples, good catch. \$\endgroup\$ – Andrew Aug 17 '17 at 22:07
  • \$\begingroup\$ I would personally appreciate some kind of note on test case 3. It took me quite a while to realize that John Doe actually appears in full in the input string, rather than requiring we be able to replace all instances of John with Jane in that situation. \$\endgroup\$ – Kamil Drakari Aug 18 '17 at 17:45
  • 1
    \$\begingroup\$ @KamilDrakari Added explanation for that case and a few others. Thanks for the feedback! \$\endgroup\$ – Andrew Aug 18 '17 at 19:03

I'm symmetric, not palindromic!


Inspired by I'm a palindrome. Are you?, where it is presented the shocking fact that “()() is not a palindrome, but ())(”, I asked myself what instead is ()() and the answer is simply: it is a string with a vertical symmetry axis!

The task

Write a program or function that takes a string S (or the appropriate equivalent in your language) as input, checks for symmetry along the vertical axis, and returns a truthy or falsy value accordingly. You can use any reasonable means to take the input and provide the output.

Reflectional symmetry

Reflectional symmetry around a vertical axis (or left-right symmetry) means that if you put a mirror vertically at the exact center of the string, the reflected image of the first half of the string is identical to the second half of the string.

For example, the following strings are reflectional symmetric around a vertical axis:

[A + A]

while the following are not:

WOW ! wow

Rules of the contest

• Your program or function will receive only printable ASCII characters. You can include or not the empty string, (which is symmetric, of course!) as legal input, which is better for you.

• The ASCII characters that can be considered symmetric with respect to the vertical axes are the following (note the initial space, and the difference between uppercase and lowercase letters):


The ASCII characters that can be considered “mirrored” and their corresponding characters are:


Note that, since they are mirrored, you can have both () as well as )(, /\ and \/, etc.

All the other ASCII printable characters must be considered asymmetric and without a mirrored corresponding character.

• This is a challenge: the shorter your program is, measured in bytes, the better, in any programming language.

• Kudos to people that will produce a symmetric program!

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  • \$\begingroup\$ You seem to be missing two pairs of mirror-able characters: qpdb. Also, I'm not sure about the mixed-win criteria: "this is code-golf ...but I will mark as accepted the shortest symmetric program, if there will be at least one!", I'd go for "imaginary brownies" or just "kudos" (let the upvotes or bounties reward). \$\endgroup\$ – Jonathan Allan Sep 6 '17 at 9:54
  • \$\begingroup\$ Thanks a lot, @JonathanAllan, I will correct the text about mirrored characters, and the criteria for winning. \$\endgroup\$ – Renzo Sep 6 '17 at 9:57
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    \$\begingroup\$ This question is very confusing, because the string it gives as an example of vertical symmetry is actually vertically symmetric, but the definition it gives of vertical symmetry is actually the definition of horizontal symmetry. \$\endgroup\$ – Peter Taylor Sep 6 '17 at 11:10
  • \$\begingroup\$ @PeterTaylor, my definition is in accord with wikipedia: “If the letter T is reflected along a vertical axis, it appears the same. This is sometimes called vertical symmetry.” I said that one should put “a mirror vertically”. If you think it is ambiguous, however, I will change “vertically symmetric” with “has a vertical symmetry axis” or “has a left-right symmetry”. Or I could add a picture. \$\endgroup\$ – Renzo Sep 6 '17 at 11:51
  • \$\begingroup\$ The very next sentence in the Wikipedia article you quote says that this is an ambiguous phrasing and best avoided. I must say that I didn't know it was ambiguous: this is the first time I've seen anyone say that an object with a vertical axis of symmetry has vertical symmetry, and I have understood since I was a small child that an object with a vertical axis of symmetry has horizontal symmetry (and so, presumably, has the person who upvoted my comment). Since questions on PPCG should be unambiguous, I can only advise a rewrite which explicitly uses the word "axis" everywhere. \$\endgroup\$ – Peter Taylor Sep 6 '17 at 12:00
  • \$\begingroup\$ @PeterTaylor, ok, thank you very much for your comment, I will change the question. Do you think I should change also the title? \$\endgroup\$ – Renzo Sep 6 '17 at 12:14
  • \$\begingroup\$ Yes, but in the title it would suffice to say "symmetric". Which symmetry is a detail which can be left for the body. \$\endgroup\$ – Peter Taylor Sep 6 '17 at 12:36
  • \$\begingroup\$ @geokavel, thanks, I corrected it. \$\endgroup\$ – Renzo Sep 6 '17 at 15:06

Zoom box drawing characters

Here are some sample box drawing characters:

╶┤ ├╴

How do we zoom them? Well, we need to triple their size. The result looks like this:

   ┃     ┃
   ┃     ┃
╺━━┫     ┣━━╸
   ┃     ┃
   ┃     ┃

As you can see, what happens is this:

  • Each box drawing character is replaced by its heavy version (for extra thickness)
  • The box drawing characters are extended using the heavy horizontal and heavy vertical characters, resulting in a separation of three between the original characters

You can use any reasonable character I/O format. You will only need to support spaces and the 15 basic box drawing characters, plus newlines if you need them as line separators. You can only require rectangular input, but your output may contain arbitrary whitespace padding, except on the left, so that the characters in the zoomed image are aligned.

This is , so the shortest solution in bytes that violates no standard loopholes wins, but if you're using UTF-8 encoding then you can score all box drawing characters as 1 each.

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  • 3
    \$\begingroup\$ RIP Charcoal, it doesn't use UTF-8 :P \$\endgroup\$ – ASCII-only Oct 6 '17 at 11:58

Does it have a square? (simple version)

Given a matrix of 0s and 1s, determine if there are 4 points that are 1 and are the corners of a square.

Here's an example to clarify, with a possible square (in bold):

0 0 0 0 0 0 0 1
0 0 0 1 0 1 0 0
0 0 0 0 1 1 0 0
0 0 0 1 0 1 0 0
1 0 0 0 0 0 0 1
0 1 1 0 0 1 1 1
1 1 1 1 1 0 0 1
0 1 0 0 1 0 1 1
1 1 1 1 0 0 1 0
1 0 0 1 0 1 0 1
0 0 1 0 1 0 1 1
1 0 0 1 1 0 1 1
Another possible square is the following:

0 0 0 0 0 0 0 1
0 0 0 1 0 1 0 0
0 0 0 0 1 1 0 0
0 0 0 1 0 1 0 0
1 0 0 0 0 0 0 1
0 1 1 0 0 1 1 1
1 1 1 1 1 0 0 1
0 1 0 0 1 0 1 1
1 1 1 1 0 0 1 0
1 0 0 1 0 1 0 1
0 0 1 0 1 0 1 1
1 0 0 1 1 0 1 1

There are many squares in this matrix, but the point isn't to count the squares, just to determine if there's a square in it. Since there is a square in it, your solution, given this matrix, must return a truthy value.

Given this matrix, your solution must return a falsy value:

0 0 0 0 0 0
0 1 1 0 0 1
0 1 0 0 1 1
0 1 1 0 1 0
You can see there's no square in there.


  • Since the input is a matrix, it will always be rectangular.
  • Probably needless to say, the sides of a square must be equal.
  • The corners of a square must all be 1.
  • A square must have at least side length 2 to be considered a square, otherwise this challenge would be extremely trivial.
  • Standard Loopholes, as usual, are forbidden.

Test cases

This section is under construction.

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  • \$\begingroup\$ Related. \$\endgroup\$ – Jonathan Frech Oct 27 '17 at 22:52
  • \$\begingroup\$ Is a 2x2 square a square? \$\endgroup\$ – Magic Octopus Urn Oct 27 '17 at 23:22
  • \$\begingroup\$ @MagicOctopusUrn A square must have at least side length 2 to be considered a square \$\endgroup\$ – Erik the Outgolfer Oct 28 '17 at 9:34
  • \$\begingroup\$ Can one's solution require that the matrix' dimensions are given? If so, can the input be a single list instead of a nested one? \$\endgroup\$ – Jonathan Frech Oct 28 '17 at 20:42
  • \$\begingroup\$ @Jonathan Relevant but not necessarily answering your question. \$\endgroup\$ – ბიმო Oct 28 '17 at 22:06
  • \$\begingroup\$ @JonathanFrech Does your language really not support nested arrays (like e.g. Neim)? Otherwise I don't see why... \$\endgroup\$ – Erik the Outgolfer Oct 28 '17 at 22:08
  • \$\begingroup\$ @EriktheOutgolfer I was thinking of languages like C, where I find it simply nicer to only have to deal with one list, even though the language can handle nested lists. I just wanted to ask about, not necessarily influence, the validity of a solution taking a flat list. \$\endgroup\$ – Jonathan Frech Oct 28 '17 at 22:27

LaTeX truth tables

Write a program or a function that accepts the list of outputs from a logic function and outputs the LaTeX code for its truth table.

The inputs should be labeled as lowercase letters a-z, and the output should be labelled as F. The length of list of inputs will always be shorter than 2^25, which means that number of inputs will always be less than 25, so you can use letters from lowercase alphabet for input names.


A number n of inputs and list of length 2^n of binary numbers which represents the outputs of a logical function.


LaTeX code that produces the truth table for that function. Input and output values should be centered in rows. There must be a line between table header and its values and between inputs and output, so the code should be similar to that below.

\begin{tabular}{c * <NUMBER OF INPUTS>|c}



[0, 0, 0, 1]


a & b & F \\
0 & 0 & 0 \\
0 & 1 & 0 \\
1 & 0 & 0 \\
1 & 1 & 1 \\

Which when displayed in LaTeX shows the following truth table

Truth table

General rules

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  • \$\begingroup\$ What happens if there are >25 outputs? Do we label them abcdef...uvwxz? Also, just to be clear, can we assume that there will be at most 25 variables present? Also, what is the winning criterion? (I recommend code-golf for this challenge). \$\endgroup\$ – HyperNeutrino Oct 31 '17 at 20:09
  • \$\begingroup\$ There are not going to be > 25 input variables. I was thinking of shortest code so yeah, code-golf :) \$\endgroup\$ – drobilc Oct 31 '17 at 20:11
  • \$\begingroup\$ Okay. For the >25 variables part you might want to specify it, and you should add a note saying what the winning criterion is, and you might also want to consider what tags to use (probably just code-golf and maybe math, really). Once you add that ping me and I'd be happy to check it over :) \$\endgroup\$ – HyperNeutrino Oct 31 '17 at 20:16
  • 1
    \$\begingroup\$ "The inputs should be labeled as lowercase letters a-z, and the output should be labelled as y." It isn't really clear from this whether 'y' can also be part of the inputs or not (I guess not). Another thing that came to my mind is that you could give the possibility of taking 'n' as an input in addition to the list of '2^n' values, since that could remove some annoying boilerplate from some answers and this challenge seems more about generating a structured output than computing the base-2 logarithm of a number. \$\endgroup\$ – Leo Nov 1 '17 at 0:17
  • \$\begingroup\$ I agree, should I label the output name to something else? Like uppercase F or something like that? I'll add number of inputs as an input. \$\endgroup\$ – drobilc Nov 1 '17 at 8:03
4 5
7 8

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