# Sandbox for Proposed Challenges

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

Sandbox FAQ

## Posting

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

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

## Discussion

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

• Parts of the challenge you found unclear
• Problems that could make the challenge uninteresting or unfit for the site

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

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

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

## Other

Search the sandbox / Browse your pending proposals

The sandbox works best if you sort posts by active.

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

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# Fletcher's 16 bit checksum code-golf

## Intro

No story this time :( This is something that I needed for a program and I thought that it would make for a fun little challenge. I'd love to see some unusual languages here!

## Challenge

Implement Fletcher16. The algorithm is very straight forward, as seen here in pseudocode:

    int sum1, sum2, check1, check2;
char[] message;

sum1 = 0;
sum2 = 0;
for i from 1 to message_length do
sum1 = ( sum1 + message[i] ) modulo 255;
sum2 = ( sum2 + sum1       ) modulo 255;
end for


There are two accumulators sum1 and sum2. We iterate over the message and for every byte inside of it we add the byte to the first sum and then modulo the value by 255. We then add the value of the first accumulator to the second one and modulo that by 255 as well. This is Fletcher's algorithm.

    check1 = 255 - (( sum1 + sum2) modulo 255);
check2 = 255 - (( sum1 + check1 ) modulo 255);


We then calculate the checksum by "simulating" the algorithm for the two resulting values. After subtracting the numbers from 255, we get two values check1 and check2. They are then appended to the input in that order.

#### To recap:

• Run the first part to get a "raw" checksum
• Run the second part on that raw checksum to get the Fletcher16 checksum.
• Append the Fletch16 checksum to the input.

Evaluating the checksum

• Run the first part on the string with the appended Fletcher16 checksum
• If 0: String is valid, else String isn't valid.
• Note: If we run the second part as well, the result will be 0xFFFF

## Input

• Binary data in any format, length is at least 1.
• If needed, the length of the data.

## Output

• The Fletcher16 checksum check2 << 8 | check1, since this is what gives the algorithm its name.

## Rules

• This is , shortest answer wins
• No standard loopholes
• No builtins. I doubt that there's a language that has one for this, but if so that that's no fun.
• A submissiom may be a program/function/link/lambda/chain/etc.

## Test cases

abcde -> 0xC846
abcdef -> 0x2088
abcdefgh -> 0x06D2

abcde\x46\xC8 -> 0xFFFF
abcdef\x88\x20 -> 0xFFFF
abcdefgh\xD2\x06 -> 0xFFFF


You can find a reference implementation in C here

## Sandbox

• Are there any mistakes?
• Is something unclear? I'm really not sure about the wording of the input.
• Is the output restriction reasonable?
• The pseudocode is nice but I don't think it is an adequate substitution for an actual explanation of the algorithm.
– Wheat Wizard Mod
Aug 25 '21 at 9:20
• Ok, I'll see what I can do. I'll also add a C reference implementation as soon as I get one working. Aug 25 '21 at 9:24

# A phonetic letter

input
a letter of the alphabet, A-Z (upper or lower or a mix, I don't mind)

output
the phonetic (i.e spelled name of the )letter of the one passed in, along with the appropriate indefinite article a or an (again, in any case), and any amount of leading/trailing whitespace you like (i.e. it's fairly flexible)

full set of test cases (lowercase) - see This english.se Answer

input output
a an a
b a bee
c a cee
d a dee
e an e
f an eff (or "an ef")
g a gee
h an aitch
i an i (or "an eye")
j a jay
k a kay
l an el (or "an ell")
m an em
n an en
o an o (or "an oh")
p a pee
q a cue (or "a queue")
r an ar (or "an arrrrr") with an arbitrary number of rs
s an ess (or "an es")
t a tee
u a u (or "a you")
v a vee
w a double-u (or "a double-you", matching u above)
x an ex
y a wy (or "a wye")
z a zed (or "a zee", if you must)

, usual rules and exceptions, shortest bytes wins.

# Interpret Tarfish

Tarfish is a more tarpit-style version of ><> that I recently created.

It is two-dimensional, and has a stack (array of numbers that can be popped and pushed) and an instruction pointer, which has x and y coordinates and a direction. The instruction pointer starts at 0,0 (top left corner of program), moving right.

It has the following commands (some are not included because they would overcomplicate this challenge):

Command action
> Set the IP's direction to right
v Set the IP's direction to down
< Set the IP's direction to left
^ Set the IP's direction to up
. Pop stack and output as character
, Pop stack and output as number
x Push x position of the IP to stack
y Push y position of the IP to stack
+ Increment the top item of the stack
- Decrement the top item of the stack
= Pop the top two items from the stack, if they're equal, skip the next instruction.
{ Shift the stack right - put the ToS on the bottom of the stack
} Shift the stack left - put the bottom item on top of the stack
_ Pop the stack

Every tick, the command pointed to by the IP is executed (Unless it was skipped), and the instruction pointer moves one place in its direction.

Despite having so few commands, Tarfish is in theory Turing-Complete.

Your challenge is to interpret this language.

You do not have to implement wrapping - If the instruction poiinter leaves the grid, that is undefined behaviour. If the program tries to pop from an empty stack, that is undefined behaviour.

# Scoring

This is , shortest wins!

# Testcases

x++++++++++++++++++++++++++++++++:+:+++++++++++:++++++++++++++++++++++++++++:.+++++++++++++++:++++++++++++++:.+++++++:::..+++:.{{{.{.{{.}}.:++++++.:.--------..; => Hello, World!

x+, => 1

v > v > x,
> ^ > ^
=> 8

x:++++++++++v
>+}     :y-=v{,;
^+++++++++{-<
=> 100

• this may be closed as a subset of the existing ><> challenge. Aug 27 '21 at 5:40
• @Razetime About half the commands are different, and this is much simpler. Aug 27 '21 at 8:16
• What's the stack item's range? And what should happen if output as a character is chosen when the top of the stack isn't valid ascii (like x-.)? Sep 8 '21 at 18:23
• Is the input guaranteed to be valid? And what's up with the colons in the example? Sep 8 '21 at 18:26

# Counting and so on

• Re: scoring. What if somebody achieves infinity? I suppose it would become impossible if you required different languages for each step. But either require or don't allow at all — merely allowing different languages would create two different challenges in one post. Prescribing a specific shape for each digit would make things less subjective, but then it'll be hard to find something that works both for Jelly and Java :P "And also individual winners for each program too" I suppose you meant language, but anyway, that's not a distinction we usually mention. Aug 18 '21 at 0:22
• the acceptance criteria for the numbers needs to be a lot more specific. Aug 19 '21 at 3:59
• @Razetime What about being able to stretch bits of the number in any orthogonal direction? Or just that it has to be the same shape, but at different scales? Aug 20 '21 at 23:50
• I'd define "X resembles Y" as "Y can be obtained from X by repeatedly removing one out of two consecutive identical rows or columns". Aug 25 '21 at 0:58

# Implement the binary operators*

*of INTERCAL

## Intro

As we all know, there are 5 different binary operations. Two combine two values, the rest modifies the value given.

## Challenge

Your task will be to implement some if not all of the operators. They are defined as followed

• $ (mingle) Given two values, returns a value that consists of the bits of both values alternating. Right argument has the first bit. • ~ (select) Given two values, discard all bits of the right value that aren't 1 in the same place in the second value. Then, pack them to the right. If this explanation is too confusing, refer to this diagram. • &, V and ? (AND, OR and XOR) Given a value, AND/OR/XOR neighbouring the bits. ## Rules • I/O in any format that isn't a binary representation of the number(s). • Each answer can contain up to five submissions, each handling a different operator. You don't need to use one language for all programs. • A valid submission can be a program/function/link/lambda/chain/etc. • This is . Shortest answer for each operator wins. ## Test cases  44$     4 = 2224
2 \$     9 =   73

255 ~   341 =   15
4214 ~ 47818 =   96

&  1999 =  967
&    71 =    3

V     4 =    6
V   100 =  118

?    42 =   63
?   428 =  387


## Sandbox things

• I'm unsure about what to submit. Is this "up-to-five-programs-one-per-op" fine or should it be changed?
• Anything else unclear?
• Is this question different enough from Implement INTERCAL's Binary Operators ?
• "If this explanation is too confusing, refer to this diagram" I have no idea what's going on in this diagram Aug 19 '21 at 19:05
• "As we all know, there are 5 different binary operations." It'd be good to specify that these are the operations of INTERCAL.
– user
Aug 19 '21 at 19:06
• What does the "(lists of)" provision mean--in what case is a list received as input, and what do you do with it? I'd also note that restricting input formats is generally frowned upon here, but if you're doing it just to block INTERCAL programs from using their native numeric I/O do note that the input is already base 10 ;) Aug 19 '21 at 21:49
• @Dudecoinheringaahing me neither but that's what the manpage shows :) I think that the procedere should be clear if you look at the first test case. Aug 20 '21 at 6:25
• @user The footnote at the end is easy to overlook, I'll change it. Aug 20 '21 at 6:26
• @UnrelatedString I agree, "(lists of)" should be removed to avoid confusion. Bad wording on my part. The intent of the second part was to block I/O from being binary strings. I shoud have worded it like that or drop that entirely Aug 20 '21 at 6:28
• Impractical if your time zone offset from Greenwich is not a whole number multiple of an hour (see e.g. wolframalpha.com/input/?i=Australia+time+zones). Sep 5 '21 at 22:20
• @JonathanFrech Not really. I frequently do LPs with people in Iran which is one of these places. If you live there you know it. But this is a simple code golf challenge and I don't think it serves it to complicate this with that sort of stuff.
– Wheat Wizard Mod
Sep 6 '21 at 8:15
• Yes, I meant your proposed scheme would be impractical if one were to live there; just as a comment. As a string-manipulating golfing challenge I think it has some merit. Sep 7 '21 at 1:24
• @jonathanFrech And I'm saying that while the scheme works best for integer offsets, from actual experience it is perfectly practical for people living in Iran which is UTC+3.5 or UTC+4.5 because if you live there you are aware of the offset and you can easily account for it.
– Wheat Wizard Mod
Sep 7 '21 at 7:12
• No, the Jimmies are on the backs of more Jimmies, and it's Jimmies all the way dow. Sep 10 '21 at 10:19
• What's the center of mass of -- or ----, and does it suffice to have a Jimmy's arm on one side of the central two? Sep 10 '21 at 21:15
• @emanresuA In the first it is at 1 and in the second it's at 2. (relative to the start of the platform) In either case the arm cannot be a place a Jimmy is directly touching so it must be in between two.
– Wheat Wizard Mod
Sep 10 '21 at 21:37

This question has been posted on the main site

• You might want to link to the question from here next time, although it doesn't really matter much.
– user
Sep 26 '21 at 19:08
• Why not allow giving the first n numbers in the sequence?
Sep 19 '21 at 13:29
• @Adám I don't really see any reason why one would want to do that.
– Wheat Wizard Mod
Sep 19 '21 at 13:44

## Pythagoras' Golfing Grid

• Does it matter which solution is output? At the very least e and h can be any pair of factors for $f - T$, and you could swap dg with jk, etc. You say "integer" but your example only uses positive integers. Do you mean positive integers? If you do, there might be some T which do not have a solution. Sep 17 '21 at 16:06

# Let's go for a rollercodester ride

• also considering using absolute change for scoring instead of curvature Sep 10 '21 at 20:53
• Seems interesting, but I think a program that just switches between something like unary and denser languages like Jelly would be optimal and probably not too difficult. (Although it does seem like it could be a good challenge, making something like that is probably non trivial) Sep 10 '21 at 20:53
• Would it be improved by only allowing each language once? I was also thinking a zip bomb type answer could dominate, but maybe that's not a bad thing. I could divide the score of each step by the sum of the code lengths that went into it, so it would be normalized and always less than 2 (theoretically 0,100,0 becomes +100,-100 becomes -200, from code lengths 0+100+0, leaving 200/100=2. That's the best I can think of). I like that because longer sequences would be the only way to reach higher score values. I guess the minimum sequence would need 3 languages to be scorable. Sep 11 '21 at 2:09
• I'd assumed that each language was only allowed once anyway, so that's probably the best thing to do. A zip bomb type answer probably wouldn't win, because you need more than just a massive increase in size, you need to be able to control what the output is so you can make it do something useful. I think the number of languages should definitely be factored into the score though, since a single massive increase or decrease in size could be the easiest way to win at the moment. Sep 11 '21 at 15:23

# Count occurences in Pascal's Triangle

• The challenge itself looks good to post. That said, I don't think there are any interesting strategies beyond generating up to the nth row (starting from the zeroth), flattening and counting Sep 18 '21 at 3:03
• Also, it appears this sequence isn't on OEIS, and that the closest sequence is A006987 Sep 18 '21 at 3:07
• @cairdcoinheringaahing oeis.org/A003016
– tsh
Sep 18 '21 at 8:26

# Minimizing flights of stairs climbed

• I think the bonus is the most interesting part and should be the main challenge. For now it's just a simple formula. Also, why the room numbers, not just floor numbers (which are room//100)? Sep 23 '21 at 12:03
• @pajonk I'll make the bonus be the main challenge then. They are room numbers to add a bit of extra challenge for converting into the floor number. Sep 23 '21 at 16:05

# Multiplication for geometric algebra

• If you're looking for feedback here in the sandbox, I think it would be nice to remove the challenge from main site for now. Sep 23 '21 at 11:56
• very interesting challenge. Sep 27 '21 at 2:15
• I've edited this down to a stub now that it's been posted to save space Sep 30 '21 at 1:02

# Newton Polynomial

A newton polynomial is a interpolation polynomial where the coefficients are found using the Newton's divided differences method. The relevant Wiki is here.

1. Given a set of inputs and outputs to find and display the polynomial that describes the series. The input can be as two lists or a dictionary like structure. e.g. $$[x_1:y_2,\;x_2:y_2,\;...\;, x_{n-1}:y_{n-1},\; x_n:y_n]$$
2. Given any real number, return the interpolated value.

## Examples

Presentation of the polynomial must be in a format that is recognisable.

As ever this is code golf: so shortest answer wins.

• A summary of the method would be welcome (in addition to the Wikipedia link). From a super quick skim I think there might be errors in the final example output: shouldn't $(n-1)(n-2)$ be $(n-1)(n-3)$, and similarly $(n-1)(n-2)(n-3)$ be $(n-1)(n-3)(n-7)$? Oct 12 '21 at 4:49
• computers don't use real numbers. but still an interesting challenge. do you care what format the output polynomial is in? Oct 18 '21 at 3:14
• @Dingus thanks, yes working on a summary of the method, wanting to keep it consice though. no every term is included so '(n-1)(n-2)(n-3) etc. Oct 18 '21 at 20:44
• @donbright true, but I haven't had time to look into using complex numbers. I'm currently not interested in using them for this. Id want the polynomial to be readable, what did you have in mind? Oct 18 '21 at 20:47
• no i mean, "given any real number" should be something like "given a 32 bit floating point number in your languages format". as far as the polynomial format i was wondering if you would accept it all expanded out to $a_nx^n + a_{n-1}x^{n-1}...a_0$ form, to the point where it can be represented as an array of numbers, the numbers in the array are coefficients and the position in the array is the exponent. thanks. Oct 19 '21 at 2:09
• @donbright ah I see what you're saying, yes I will amend that then. I'm not sure I quite understand how you want to layout it out. Maybe expecting a particular format is too hard and I should just ask for an array of coefficient, were you thinking of expanding (n-1)(n-2)(n-3)... for example? Oct 19 '21 at 23:06
• yes the final result should be a polynomial, would it be ok to return a string saying 23.4n^4 + 3920n^3 ... Oct 20 '21 at 2:32

# Wave square

Given an integer $$\n\$$ with the constraints $$\0 < n \leq 9\$$, output the corresponding square, that for each row shifts the sequence from $$\1\to n\$$ by one, overflowing when needed.

## I/O

Input is an integer $$\n\$$, $$\ 0 < n \leq 9\$$. Output can be as a returned String, matrix, array or to STDOUT. You can write a full program or a function.

## Examples

5:

12345
23451
34512
45123
51234


2:

12
21


1:

1


9:

123456789
234567891
345678912
456789123
567891234
678912345
789123456
891234567
912345678


## meta

Thoughts on extending the challenge to allow for inputs bigger than 9, which would then just be taken mod 10?

• One idea - using letters for numbers greater than 9 (i.e a for 10, b for 11, etc.) would allow for inputs of up to 35 without disrupting the format. Oct 18 '21 at 2:33
• Thanks! I don't know if that'd be enough to make it distinct from another challenge though (someone posted it was a dupe in the nineteenth byte) Oct 18 '21 at 8:22

# Largest turing incomplete subset of ASCII

We have challenges about trying to find a short, turing complete number of characters in a language. In this challenge, you'll do the opposite, trying to find the largest subset of characters which is not turing complete.

Rules:

• Like , competition is purely within a language, not between different ones
• Languages with SBCSs or other custom code pages may use those instead of ASCII
• SBCS submissions will not compete against ASCII submissions in the same language
• In this challenge, "ASCII" is not short for "printable ASCII". All of 0x00 to 0x7f is allowed
• So, the highest score possible would be 127 (for non-SBCS submissions), if there was a single character which is always needed for a TC program
• The language you use must be turing complete (and must be TC with full ASCII or its SBCS, whichever you choose)

Possible rule:

You must be able to write a valid program using only (and all of) the chosen subset of ASCII/the SBCS. It doesn't need to do anything useful, it just can't error.

This prevents padding the subset with characters that would never be allowed in a valid program in your language, or not including just one critical character needed in every program. However, strings and comments could be a problem.

• If a language is using SBCS, that would allow for a score higher than 94, wouldn't it? Oct 20 '21 at 15:03
• @AaroneousMiller It would, yes (this is fine since it's a competition within the language) Oct 20 '21 at 15:10
• An empty program is valid in some languages.
– xnor
Oct 20 '21 at 18:09
• @xnor Ah, I meant using all of the characters in the chosen subset. Will fix. Oct 20 '21 at 18:18
• Maybe just allow all 256 bytes 0x00 to 0xff so it's a fair playing field? Oct 20 '21 at 19:00
• I don't think the "possible rule" is needed. The equivalent challenge is to find a small set of chars/bytes such that some of them are absolutely needed for the lang to be TC, and take the set complement. IMO the first part is totally fair across languages (assuming full byte range), and the second part only depends on how you define the full set. So for maximum fairness I'd suggest "every char/byte is on the table, even the ones that cannot appear in any valid program". Oct 20 '21 at 23:30
• @Bubbler To be fair, it's not meant to be fair to all languages. It'll be like code golf where scoring is per language. But I guess I agree the possible rule probably isn't needed. Oct 20 '21 at 23:45

# Find all paths

• More test cases would be needed. Is this oeis.org/A121788? Jun 27 '21 at 15:24
• I dont think this is that problem in the link Jun 27 '21 at 15:34
• Anyway, please include more test cases Jun 27 '21 at 16:24
• Is this oeis.org/A001184?
– tsh
Jun 28 '21 at 8:22
• yes it is @tsh updated! Jun 28 '21 at 8:37
• === Looks good! === Jun 28 '21 at 8:43

# What's the valency?

## The challenge.

Given an element, for simplicity lets assume the element is entered as a string of the symbol of the element (any consistent casing of your choice) and the only elements are the first 20 elements of the periodic table.

You have to output the valency of the element. The valency of the element has to be with the sign and as per the below cases.

## All cases

h  ->  1
he ->  0
li ->  1
be ->  2
b  ->  3
c  ->  4
n  -> -3
o  -> -2
f  -> -1
ne ->  0
na ->  1
mg ->  2
al ->  3
si ->  4
p  -> -3
s  -> -2
cl -> -1
ar ->  0
k  ->  1
ca ->  2


This is code-golf so the shortest answer in bytes wins.

• Since we require that challenges be self contained, you would need to provide the valency for each of the elements that you want to include. Sep 20 '21 at 18:25
• Adding to FryAmTheEggman's comment, several elements have more than one valency. For each element you need either to specify which valency/valencies to output or allow answers to choose. Oct 19 '21 at 4:49
• @FryAmTheEggman edited the question. It now includes all 20 cases. Oct 19 '21 at 6:11
• This looks much better! Now I would recommend allowing the inputs to be in whatever consistent casing (upper, lower, title) the answerer prefers. Otherwise, I think it is good to go. Oct 19 '21 at 14:08
• @FryAmTheEggman edited. Oct 19 '21 at 15:18
• @PyGamer0 The wording "(any case)" isn't clear enough – it could also mean that the program has to be able to handle any case.
– m90
Oct 19 '21 at 15:21
• Perhaps "any consistent casing of your choice"? Oct 19 '21 at 15:38
• How many more "build an optimal lookup table" questions do we need? Oct 19 '21 at 16:18

Input two positive integers m, n; Output a grid looks like below:

┌──────────────┐
├──┬──┬──┬──┬──┤
│  │  │  │  │  │
├──┴──┴──┴──┴──┤
├──┬──┬──┬──┬──┤
│  │  │  │  │  │
├──┴──┴──┴──┴──┤
├──┬──┬──┬──┬──┤
│  │  │  │  │  │
├──┴──┴──┴──┴──┤
├──┬──┬──┬──┬──┤
│  │  │  │  │  │
├──┴──┴──┴──┴──┤
└──────────────┘


Here is an example for 5×4 grid.

Characters used in the grid is shown as this table:

Char Code Point
Space U+0020
─ U+2500
│ U+2502
┌ U+250C
┐ U+2510
└ U+2514
┘ U+2518
├ U+251C
┤ U+2524
┬ U+252C
┴ U+2534

You may alternative use a CJK space 　 (U+3000) instead of two ASCII spaces   (U+0020 U+0020). If you chose this option, you should use a single horizontal grid line ─ (U+2500) instead of two. You may verify this behavior in the code snippet at bottom.

You may output:

• a single string
• an array of characters, an array of integers (whose value is the code point of each character);
• an array of strings, as the single string split by new lines;
• or an array of array of characters, an array of array of integers;

Use ASCII Space:

<pre lang="en">┌──────────────┐
├──┬──┬──┬──┬──┤
│  │  │  │  │  │
├──┴──┴──┴──┴──┤
├──┬──┬──┬──┬──┤
│  │  │  │  │  │
├──┴──┴──┴──┴──┤
├──┬──┬──┬──┬──┤
│  │  │  │  │  │
├──┴──┴──┴──┴──┤
├──┬──┬──┬──┬──┤
│  │  │  │  │  │
├──┴──┴──┴──┴──┤
└──────────────┘</pre>

Use CJK Space:

<pre lang="zh">┌─────────┐
├─┬─┬─┬─┬─┤
│　│　│　│　│　│
├─┴─┴─┴─┴─┤
├─┬─┬─┬─┬─┤
│　│　│　│　│　│
├─┴─┴─┴─┴─┤
├─┬─┬─┬─┬─┤
│　│　│　│　│　│
├─┴─┴─┴─┴─┤
├─┬─┬─┬─┬─┤
│　│　│　│　│　│
├─┴─┴─┴─┴─┤
└─────────┘</pre>

• Nov 4 '21 at 2:13

# Calculate Smith normal form of an integer matrix

Given an $$\m \times n\$$ matrix of integers A, there exist a $$\m \times m\$$ matrix P, an $$\m \times n\$$ matrix D, and an $$\n \times n\$$ matrix Q such that:

• P and Q are unimodular matrices (i.e. matrices which are invertible and whose inverses are also integer matrices);
• D is diagonal;
• each diagonal entry $$\d_{ii}\$$ of D is nonnegative; and
• $$\d_{11} \mid d_{22} \mid \cdots \mid d_{nn} \$$.

Furthermore, the matrix D is unique in this representation.

One common way to calculate D is via an algorithm that looks like a combination of the Euclidean algorithm for calculating gcd and Gaussian elimination -- applying elementary row and column operations until the matrix is in the desired format for D. Another way to calculate D is to use the fact that for each i, $$\d_{11} d_{22} \cdots d_{ii}\$$ is equal to the gcd of all determinants of $$\i\times i\$$ submatrices (including non-contiguous submatrices) of A.

## The challenge

You are to write a function or program that calculates the Smith normal form of an input matrix. The output may either be in the form of the full matrix D, or in the form of a list of the diagonal entries of D. In an imperative programming language, it is also acceptable to write a function that takes a matrix by reference and mutates the matrix into its Smith normal form.

## Rules

• This is code-golf: shortest code wins.
• Standard loophole prohibitions apply.
• You do not need to worry about integer overflow, if that applies in your language.

## Examples

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

6  10       1 0
10 15  ->   0 10

6 0  0        1 0  0
0 10 0   ->   0 30 0
0 0  15       0 0  30

2 2       2 0
2 -2  ->  0 4

2 2  4 6       2 0 0 0
2 -2 0 -2  ->  0 4 0 0


Note: Mathematica already has a built-in to calculate Smith normal form. As such, you can use this to play around with test cases: Try it online!

• Three new matrices are introduced at the top of the post and it isn't immediately obvious which one is the Smith normal form. Suggest stating that it's D after the first set of bullet points. Nov 9 '21 at 23:34

# Climbing up slippery stairs code-golfmathcombinatorics

## Challenge

You're standing in front of a stairway with $$\n\$$ stairs in total. (If you label each staircase $$\1,\cdots,n\$$, the starting point counts as the 0th staircase.) You can climb up $$\1,2,\cdots,k\$$ stairs at a time. But some stairs are slippery; if you step on it, it will cause you to slip backwards until you stand on a non-slippery one (or the starting point). In how many ways can you get to the $$\n\$$th staircase with exactly $$\s\$$ steps?

Task: Given the list of staircases of length $$\n\$$ marked with either "slippery" or "non-slippery" (you can choose any two consistent values) and the values of $$\k\$$ and $$\s\$$, answer the above question.

The $$\n\$$th staircase is guaranteed to be non-slippery, and you cannot move further than the $$\n\$$th. This implies that reaching the destination before you spend $$\s\$$ steps doesn't count.

For example, if the stairway is [non-slippery, slippery, non-slippery] and $$\k=2\$$, two possible first steps (climbing one or two stairs) will result in the same position (the first staircase), but count as two distinct possible moves. And if you try to climb one stair as the second step, you will end up not moving at all, but it still counts as a step.

You may assume $$\1 \le k \le n\$$ and $$\s \ge 1\$$. Some inputs may have no way to reach the destination at all; in that case, your code should output the value of zero for any value of $$\s\$$.

## Example and test cases

N means non-slippery, and S means slippery. 1-1-3 notation means that you can reach the top in three steps, trying to advance 1, 1, and 3 stairs at once.

N = [N, S, N, N]
k = 3
s = 1: answer = 0 (no way to climb 4 stairs at once)
s = 2: answer = 3 (1-3, 2-3, 3-1)
s = 3: answer = 4 (1-1-3, 1-2-1, 2-1-3, 2-2-1)

N = [S, S, N, S, S, N]
k = 4
s = 1: answer = 0
s = 2: answer = 2 (3-3, 4-3)
s = 3: answer = 8 (1-3-3, 1-4-3, 2-3-3, 2-4-3, 3-1-3, 3-2-3, 4-1-3, 4-2-3)


More test cases coming soon.

# ⧵begin{alignat} ... ⧵end{alignat} code-golfstring

Note: The title intentionally uses &#10741; (reverse solidus operator) instead of plain backslash, because otherwise MathJax would happily translate the entire title to... uh... a MathJax error box.

## Background

\begin{alignat}{n} ... \end{alignat} is a lesser known LaTeX/MathJax block that allows aligning multiple parts of multi-line equations.

\begin{alignat}{5}
A&=&B&\\
& &B&=&C&\\
& & & &C&=&D&\\
& & & & & &D&=&E
\end{alignat}


\begin{alignat}{5} A&=&B\\ & &B&=&C&\\ & & & &C&=&D&\\ & & & & & &D&=&E \end{alignat}

\begin{alignat}{5}
x&=&-3&a&-&2&b\\
y&=& 2&a&+& &b
\end{alignat}


\begin{alignat}{5} x&=& 3&a&-&2&b\\ y&=&-2&a&+& &b \end{alignat}

## Challenge

Implement "Poor Man's ASCII-Only Alignat", which tries to replicate the behavior of MathJax but in plain text. The spec of PMAOA is as follows:

• Let's define a "word" to be each part of a line delimited by the & character. A word can be empty, and the parts before the first & and after the last & also count as words.
• Given a multi-line string input, identify the 1st, 2nd, 3rd, ... word of each line.
• Right-align the 1st words of all lines, right-align the 2nd words of all lines, ..., and concatenate the aligned blocks horizontally. Padding is done with minimal amount of space characters. Trailing whitespace at the end of each line is optional.

Standard rules apply. The shortest code in bytes wins.

## Test cases

Coming soon.

• By saying align, is extra spaces allowed? For example, "a&b\ncd&e" -> "..a..b\n.cd..e"
– tsh
Nov 9 '21 at 10:02
• @tsh No, the padding should be minimal. Nov 9 '21 at 10:14

# What is the maximum value generated by interleaving?

INTERCAL has an interleave operator which does the following operation. Let left operand be asdf and right one qwer in binary, respectively. The operation produces a binary value aqswdefr.

INTERCAL internally treats data as unsigned integers, so the value of the eight-bit value represents 0 to 255 in decimal, inclusively.

If one operand has fewer bits than the other, the fewer one gets padded with zero before operation. So, asd interleaving with qwer is equal to 0asd interleaving with qwer, which is 0qawsedr: it represents 0 to 127 in decimal.

Also, INTERCAL has an extension that handles any bases. Let's assume if 3-base numbers are handled. If each operand has 1 and 3 digits respectively, the maximum value for each operand is represented as 2 and 222 in 3-base number, respectively. Interleaving them results in 020222, which is 188 in decimal.

Given an input of three unsigned integers, output the largest possible value generated by interleaving. The three integers are: number of digits for left operand, number of digits for right operand, and in what base those operands are described with.

## Restrictions

• Base shall be 2 or greater.
• Each operand has at least 1 digit.

## Rules

• In either function or a program.
• Standard I/O rules apply, as long as every input and output value is represented as same base or same representation.
• So varying output base is not allowed.
• If input is represented as a list-like format, any orders of arguments or input values are fine.
• No standard loopholes.
• Shortest code wins

TODO.

# meta

• Am I missing any appropriate tags yet?
• The I/O rules seem fine. I'd suggest changing the title to something like 'What is the maximum value generated by interleaving?' Also, 'output what the possibly largest value interleaving generates' -> 'output the largest possible value generated by interleaving'. Nov 12 '21 at 4:14

# Constructing Solar Panels from Squares part 2 code-golfnumberoptimization

Thanks to all your generous contributions of code, my horde of minions can now precisely calculate how to construct solar panels of any size, but there's a problem.

The logistics department used these results to calculate how many square panels they would need for every size up to 1,000,000, which is way too many panels. As it turns out, larger panels are more expensive than cheaper ones, so I've decided we need to try to use the largest square panels we can wherever possible.

To make matters worse, my team of scientists tell me that we should avoid using multiple panels of the same size in our configuration, except for our tiny 1x1 panels, to make sure the panels don't fall apart.

## The Challenge

Given a positive integer n, output a list of square numbers that sums to n containing the largest squares possible such that no square number other than 1 appears multiple times.

A square number is any integer that is the result of multiplying an integer by itself. For example 16 is a square number because 4 x 4 = 16 This is A000290

For example: For n = 12, you could achieve the desired size with 4 panels of sizes [9, 1, 1, 1]. As 9 is the largest square possible in this configuration, this is the best answer. For n = 13, you can achieve the desired size with only 2 panels: [9, 4]

If n is a square number, the output should be [n].

## Input

A positive integer n representing the total desired surface area of the solar panels. Note that 0 is not positive.

## Output

A list of square numbers that sums to n containing the largest squares possible.

### Testcases

1 -> [1]
2 -> [1,1]
3 -> [1,1,1]
4 -> [4]
7 -> [4,1,1,1]
8 -> [4,4]
9 -> [9]
12 -> [9,1,1,1]
13 -> [9,4]
18 -> [16,1,1]
30 -> [25,4,1]
50 -> [49,1]
60 -> [49,9,1,1]
70 -> [64,4,1,1]
95 -> [81,9,4,1]
300 -> [289,9,1,1]
1246 -> [1225,16,4,1]
12460 -> [12321,121,16,1,1]
172593 -> [172225,361,4,1,1,1]


# Sandbox

I don't think this challenge is similar enough to part 1 to be considered a dupe, as while some answers from part 1 could be trivially modified to work for part 2, they would likely be out-golfed by better approaches.

That said I'm not sure how well-worded the output requirement is. a list of square numbers that sums to n containing the largest squares possible such that no square number other than 1 appears multiple times. feels poorly worded, but I'm not sure how I could word it better.

• The challenge is unclear. I think you copied part 1 but forgot to edit testcase and example of challenge
– okie
Nov 17 '21 at 1:04
• such that no square number other than 1 appears multiple times - test cases don't agree. Nov 17 '21 at 5:24
• I did generate new testcases when I wrote this, but I guess I messed up when putting them in. I'll re-run and update the testcases now Nov 17 '21 at 22:15
• 8->4,4 having 2 4
– okie
Nov 18 '21 at 0:43
• Am not sure if this question may be solved by modify the previous one trivially. But there would be many answer be very similar though. I'm not sure if it would be a duplicate to that one in such case.
– tsh
Nov 18 '21 at 3:59
• @okie yeah, the problem is with the wording of the challenge, not the testcases. Not sure how best to word it... Nov 18 '21 at 14:07
• So only the largest and 1 can appear multiple time?
– okie
Nov 18 '21 at 23:27

# Implement Unix Timestamp to Daytime

Given an unsigned integer that represents a timestamp since 1970/01/01 00:00:00 (which is Unix epoch time), output one of these to represent the daytime in GMT timezone:

• An array that stores year, month, date, hour, minute, second.
• A string in format YYYYMMDDHHmmss.
• Or whatever similar, as long as it complies with standard i/o rules.

# Rules

• Implement the function or the full program from vanilla.
• So no built-ins nor libraries that has the fuctionality (see next section).
• Implement leap year, too; but don't leap second (although leap second is not supported).
• You just need to support up to 2038/01/19 (so input range shall be 0 to 2147483647 (inclusive)).
• Standard loopholes apply.
• Standard I/O rules apply.
• Input and output format should be consistent and not ambiguous.
• So, for example, if output is a string like 1970121000, it's unqualified, as it can be recognized as 1970/01/21 00:00:00, 1970/12/01 00:00:00, or 1970/01/02 10:00:00.
• Shortest code wins.

# Test cases

0
-> 1970/01/01 00:00:00
999999999
-> 2001/09/09 01:46:39
1145141919
-> 2006/04/15 22:58:39
1330500000
-> 2012/02/29 07:20:00
1633773293
-> 2021/10/09 09:54:53
2147483647
-> 2038/01/19 03:14:07


# Hint

Here are implementations:

# Meta

• I once posted here but it's closed.
• TODO. define what builtins are.
• The most problem is that that KSH answer has a function that directly converts to the objective string; should I prohibit it?
• Maybe I wanted to say no date objects allowed.
• Is what is so-called date object ambiguos?
• Unix Timestamp does not support leap seconds. So there is nothing to do to require someone implement leap second support for an Unix Timestamp.
– tsh
Oct 12 '21 at 6:28
• Am I required to support year above 2038? As, you know, support only 1970~2038 would avoid some leap year issues for 2100 or 2200.
– tsh
Oct 12 '21 at 6:31
• @tsh 1. Thank you for pointing out leap seconds. 2. I clarified to support until 2038.
– user100411
Oct 12 '21 at 12:27

## Permutation of all the number which separated by ':' and ','

• Welcome to Code Golf and thanks for using the Sandbox! This is a great challenge, but as it stands it's a little unclear from the instructions what's being asked. For example it's clear from the example that 1-2,11,44:110-113 means output every permutation of 1,2,11,44 on the left column and 110,111,112,113 on the right column, but that's not explained in the instructions. Nov 12 '21 at 20:52
• @Mayube Thanks for your comment, I've updated the instructions. Nov 12 '21 at 22:02

# Print random characters indefinitely

Continuously print a random character (a-z, A-Z, 0-9) not separated by a newline (\n).

### Expected output

b7gFDRtgFc67h90h8H76f5dD55f7GJ6GRT86hG7TH6T7302f2f4 ...
`

Note: output should be randomised.

## Requirements/Rules

1. Output must be continuous (i.e. never ending),
2. Output may not comprise of newlines,
3. Output must be random,
4. Output must be only composed of characters: a-z, A-Z, 0-9.
5. No loopholes, regarding programming language version(s). Use only the tools available with that programming language, no third-party packages, modules, or other addons.

## Points

• Smallest file size (in bytes)

### Notes

Concerns: None at the moment...

• I think this would count as a duplicate of this. It's a language specific version, and indefinite, but making something loop forever isn't really the hard part of the challenge I think. Nov 29 '21 at 0:51
• @Rɪᴋᴇʀ no, not because different. Nov 29 '21 at 0:51
• Is "creativity" part of the scoring? If so, there's no way to objectively determine how creative something is, so you can't use that as a scoring criterion. You should probably clarify Python 3.8 is the only allowed language, or just drop that entirely (which I'd strongly recommend). Nov 29 '21 at 0:52
• @RedwolfPrograms and Riker thank you for the tips, i'll make the suggested edits to the criterias ASAP. Nov 29 '21 at 0:57
• Not limited to python, this might be different enough. I consider something a duplicate if the more specific challenge doesn't really encourage any different approaches than whatever worked for the previous ones, but limiting to alphanumeric and forcing an infinite stream is probably good. You should probably check if there's any other ones tagged [random] that look too similar though. Nov 29 '21 at 0:59
• @Rɪᴋᴇʀ Understood. Thanks again! Nov 29 '21 at 1:01
• No problem! It's an interesting challenge, and if it really hasn't been done before then I think it would be quite fun to do. Nov 29 '21 at 1:13
• Ok! Will post this in ~22 hours... Nov 29 '21 at 1:16
• We've had similar versions of this challenge before, and they've all been closed as a dupe of this and/or this. Personally, I would hammer this closed as a dupe of those two, as I don't think this adds anything that they don't - it's just a trivial combination of the two Nov 29 '21 at 1:20

# Largest SKI Reduction in under 100 characters

## The challenge

Create a program in SKI Combinator Calculus in under 100 characters that produces the largest result after terminating.

## Scoring

The score will be the number of characters in the final reduction. The highest is the winner.