# Sandbox for Proposed Challenges

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

Sandbox FAQ

## Posting

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

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

## Discussion

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

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

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

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

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

## Other

Search the sandbox / Browse your pending proposals

The sandbox works best if you sort posts by active.

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

Get the Sandbox Viewer to view the sandbox more easily!

# Count smooth numbers

Define $$\\Psi(x,B)\$$ as the number of integers between $$\1\$$ and $$\x\$$, whose prime factors are all $$\B\$$ or less. (These are the $$\B\$$-smooth numbers.)

For example, there are 34 integers between 1 and 100 that have only 2, 3 and 5 as prime factors. These are:

 1  2  3  4  5  6  8  9 10 12
15 16 18 20 24 25 27 30 32 36
40 45 48 50 54 60 64 72 75 80
81 90 96 100


Therefore, $$\\Psi(100,5)=34\$$. The next prime is 7, so $$\\Psi(100,6)\$$ is also 34.

This is a challenge to calculate exact values of $$\\Psi\$$ as quickly as possible.

## Methods

You may not use libraries or built-ins that calculate this function. That being said, I don't know any language that has a built-in for this (not even Mathematica?)

I also can't seem to find any fast algorithms for this problem, except for a basic meet-in-the-middle by Daniel J. Bernstein which might be a good starting point.

Bernstein also describes an approximation method that may or may not be useful.

## Input range

You may assume that $$\1 and $$\B<10^6\$$.

## Scoring

You will be scored on tiers of increasing difficulty.

Each tier has several inputs $$\(x,B)\$$ such that the $$\\Psi(x,B)\$$ have similar values, and the $$\x\$$ values are distributed exponentially. Tiers will start at around $$\\Psi(x,B)\approx 10^9\$$ and go up to $$\\Psi(x,B)\approx 10^{20}\$$ or more, depending on how fast the entries get.

The time limit for each tier is 60 CPU seconds per input on average. In other words, if a tier has $$\n\$$ inputs, you will essentially have $$\n\$$ CPU minutes to obtain the outputs. You are welcome to submit parallel code but each thread will count towards the time limit.

Your score is the highest tier that your program can solve. If there is a tie, the program that is fastest on the highest tier wins.

For practical reasons, your program will be limited to 15GiB of memory.

## Sample tiers

Warm-ups:

3:
Ψ(10^3, 997) = 1000
Ψ(10^6, 7)   = 1273
Ψ(10^9, 5)   = 1530
6:
Ψ(10^6,  999983) = 1000000
Ψ(10^9,  59)     = 1060717
Ψ(10^12, 29)     = 1469549
Ψ(10^15, 17)     =  919814
Ψ(10^36, 7)      =  936046


Tiers:

9:
Ψ(10^9,  999983) =  616220853
Ψ(10^10, 4567)   =  954965955
Ψ(10^12, 337)    = 1180049403
Ψ(10^15, 97)     = 1016358704
Ψ(10^18, 59)     = 1106651678
Ψ(10^24, 31)     =  791377032
Ψ(10^30, 23)     =  812060729
Ψ(10^48, 17)     = 1435897064
10:
Ψ(10^11, 11987) = 10016301575
Ψ(10^12, 1499)  = 10753426440
Ψ(10^15, 199)   = 12766644440
Ψ(10^18, 89)    =  9052115006
Ψ(10^24, 47)    = 11298682134
Ψ(10^30, 37)    = 14838208717
Ψ(10^48, 19)    =  7868307089
11:
Ψ(10^13, 2297) =  84344528150
Ψ(10^15, 443)  =  96272828440
Ψ(10^18, 163)  = 107816435926
Ψ(10^36, 37)   =  94053521936
Ψ(10^24, 67)   =  81421195505
Ψ(10^48, 29)   = 151266342065


etc.

I may choose to do the actual scoring on different inputs, including using $$\x\$$'s that are not powers of 10.

# Squish these Numbers

META: I think my wording is not very clear, if you have a suggestion to make it easier to understand, feel free add a commment or edit it directly in to this post.

Given a finite sequence of real floating point numbers, map it into the Interval $$\[-1,1]\$$, such that the order of these numbers is preserved.

### Details

• You can choose any mapping you like, it doesn't have to be linear.
• The mapping you choose doesn't have to stem from a function, that means some number $$\x\in \mathbb R\$$ doesn't have to get mapped to the same number in $$\[-1,1]\$$, the value it does get mapped to can change depending on the other values in the input list.
• Two equal values in the input list, should remain equal in the output.
• If some value in the input list is strictly smaller than some other value, then the corresponding values in the output should satisfy the same relation, in theory. In practice it might happend that two different numbers will get mapped to the same output due to floating point arithmetic issues, which is fine - as long as it would work with an arbitrary precision.
• You can assume the input sequence contains at least two distinct entries.

### Examples

Following example finds a linear map that maps the least entry to $$\-1\$$ and the greatest entry to $$\1\$$. Note how this map depends on the sequence.

$$(x_1,x_2,\ldots,x_n) \mapsto \left( \frac{x_i - \min_k x_k}{\max_k x_k - \min_k x_k} \right)_{i=1}^n$$

Following example is a fixed function that always behave the same. Because it is monotonic and strictly increasing, it satisfies all conditions. $$(x_i)_{i=1}^n \mapsto \left(\frac{\arctan(x_i)}{\pi} \right)_{i=1}^n$$

Thanks @PeterTaylor for following example. Here $$\\operatorname{sort}(x)\$$ sorts the input sequence in ascending order and $$\\operatorname{indexof}(u,v)\$$ returns the index of the first occurence of $$\v\$$ in the sequence $$\u\$$.

$$(x_i)_{i=1}^n \mapsto \left( \frac{1}{n} \operatorname{indexof}(\operatorname{sort}(x),x_i)\right)_{i=1}^n$$

• How about a function which sorts the array and then maps $x_i \to \frac1n \textrm{indexof}(\textrm{sorted}(x), x_i)$? Dec 18 '18 at 14:59
• Yeah that looks fine (assuming for e.g. $x_1 = x_2$ we get the same index). - If you do not mind I'll add that as an example too. Dec 18 '18 at 17:22
• I think many languages have builtin tanh which trivializes this. Are you sure there are enough languages where this challenge is interesting? Dec 19 '18 at 0:40
• I expect it will be hard for a list-aware function to beat a general mapping like x->1+1/(1+2**-x), or just tanh if that's available, but maybe Peter Taylor's method will be shorter in golfing languages. I think it's worth noting in the spec that it's OK if due to float inaccuracies two very close values are mapped to the same value in practice.
– xnor
Dec 19 '18 at 0:42
• @xnor I'll add the comment about the floating point problems, that is a good point. I'm convinced that there will be a lot of different approach that will be optimal in the different languages. Dec 19 '18 at 9:16

# Shortest JsFuck code for a number

JsFuck is a language using only []()!+ to run and express anything in JavaScript. Below is a simplified model of JavaScript to express numbers:

• Types
1. Number
2. String
3. Boolean (true)
4. Array
• Functions
• IEEEdouble(x):
• Let $$\u\$$ is the number in $$\\{a\cdot 2^{b}|-2^{53}-1075, a,b\in \mathbb Z\}\$$ nearest to x, maximizing $$\b\$$ on tie
• If $$\|u|<2^{1024}\$$, return $$\u\$$
• toNumber(x):
• If x is a Number, return x;
• If x is a Boolean, return 1;
• If x is an Array [], return 0;
• Otherwise, x is a String. In this case,
• If x matches /^([\+\-]?(?:\d*\.?\d+|\d+\.))(?:e([\+\-]?\d+)?$/, let $$\v=\text{＄1}\times 10^\text{＄2}\$$ ($2 is zero if not present)
• Return IEEEdouble($$\v\$$)
• toString(x):
• If x is a String, return x;
• If x is a Boolean, return 'true';
• If x is an Array [], return '';
• Otherwise, x is a Number. In this case,
• Find $$\p, q\in \mathbb Z\$$ such that IEEEdouble($$\p\times 10^q\$$)=x, maximizing $$\q\$$(there may be multiple $$\p\$$ satisfying the restriction, in which case choosing which one is unknown);
• Let $$\y=p\times 10^q\$$;
• If $$\y=0\$$ or $$\10^{-6}\leq|y|<10^{21}\$$, write it normally without scientific notation mapping the RegEx /^([1-9]\d*|0)(\.\d*[1-9])?$/; • Otherwise, write it in scientific notation mapping the RegEx /^[1-9](\.\d*[1-9])?e[\+\-][1-9]\d*$/
• x+y
• If either x or y is a String or an Array, return toString(x) concatted with toString(y);
• Otherwise, return IEEEdouble(the sum of toNumber(x) and toNumber(y))
• +x
• [] and [x]
• Return an Array.
• x[y]
• If x is a String, return the yth character(0-index) in x
• !![] and !+[]
• Return a Boolean

A valid JsFuck code is always parenthesis balanced without two symbols + together. Expression in parenthesis is calculated before the one out. On the same layer operations go from left to right.

You are required to output the shortest JsFuck code that generates a given IEEE double (A possible output of IEEEdouble). Shortest generator wins.

It's fine if your generator runs slow, but beware of potential infinite loop if you eval.

# Samples

1     -> +!![]
2     -> !![]+!![]
10    -> +[+!+[]+[+[]]]
1e10  -> +(+!![]+(!![]+[])[!![]+!![]+!![]]+(+!![])+(+[]))
0.1   -> +((+(+!![]+[+!![]]+(!![]+[])[!![]+!![]+!![]]+(!![]+!![])+(+[]))+[])[+!![]]+(+!![]))
1e-10 -> +((+(+!![]+[+!![]]+(!![]+[])[!![]+!![]+!![]]+(!![]+!![])+(+[]))+[])[+!![]]+(+[])+(+[])+(+[])+(+[])+(+[])+(+[])+(+[])+(+[])+(+[])+(+!![]))
5e-324-> +(!![]+!![]+!![]+(!![]+[])[!![]+!![]+!![]]+(+((+(+!![]+[+!![]]+(!![]+[])[!![]+!![]+!![]]+(!![]+!![])+(+[]))+[])[+!![]]+(+[])+(+[])+(+[])+(+[])+(+[])+(+[])+(+!![]))+[])[!![]+!![]]+(!![]+!![]+!![])+(!![]+!![])+(!![]+!![]+!![]+!![]))
9999999999
-> +(+!![]+(!![]+[])[!![]+!![]+!![]]+(+!![])+(+[]))+(+((+((+(+!![]+[+!![]]+(!![]+[])[!![]+!![]+!![]]+(!![]+!![])+(+[]))+[])[+!![]]+(+[])+(+[])+(+[])+(+[])+(+[])+(+[])+(+!![]))+[])[!![]+!![]]+(+!![])))


SN: Another way to ask is requiring to be testable and shouldn't be longer than a chosen generator

• Challenge seems a bit too long-- maybe restrict the domain to integers? Also I can't see any competitive solution actually finishing for nontrivial test cases because they'd just eval all valid JSFuck strings in parallel. Jan 6 '19 at 3:41
• @lirtosiast Do your "integer" mean safe integer? Also I don't see how it simplifies the problem much
– l4m2
Jan 6 '19 at 7:30

# Is it shifted?

Consider a standard US-International QWERTY keyboard, without a numeric keypad, and Caps Lock mysteriously missing.

< insert image >

The goal is to write two programs or functions that take no input, and each give a distinct output through any default output method: Shift and No shift respectively. The program that outputs No shift has to be written without use of the ⇧ Shift key. The program that outputs Shift has to be written while holding the ⇧ Shift key throughout. Both programs have to be written using the exact same sequence of keypresses. A valid entry would be a1b2 + A!B@, if they output No shift and Shift respectively.

The symbols allowed for the "No-shift" program are as follows:

1234567890-=
qwertyuiop[]\
asdfghjkl;'
zxcvbnm,./
<space> <newline> <tab>


The symbols allowed for the "Shift" program are as follows:

~!@#$%^&*()_+ QWERTYUIOP{}| ASDFGHJKL:" ZXCVBNM<>? <space> <newline>  Note that Tab ↹ is missing, since Shift+Tab ↹ does not produce a \t tab symbol in most editors. ### Sandbox note I think using case-sensitive output makes the challenge more challenging, but it might become too challenging. What are your thoughts? • This already appears to be impossible in most productive languages, so I would allow output in any case. Jan 13 '19 at 8:53 # Touch-typing distance Tags: , Related: Levenshtein distance There are many different string metrics, a simple one is the Levenshtein distance given by $$\\texttt{ld}_{a,b}(|a|,|b|)\$$: $$\texttt{ld}_{a,b}(i,j) = \begin{cases} \max(i,j), & \text{if i = 0 or j = 0} \\ \\ \min \begin{cases} \texttt{ld}_{a,b}(i-1,j) + c_\text{deletion} \\ \texttt{ld}_{a,b}(i,j-1) + c_\text{insertion} \\ \texttt{ld}_{a,b}(i-1,j-1) + w(a_i, b_j) \end{cases}, & \text{otherwise} \end{cases}$$ The avid reader may have noticed that there are missing pieces in the above definition, it makes use of a weight function $$\w\$$ which was never defined and the costs $$\c_\text{deletion}\$$ as well as $$\c_\text{insertion}\$$. The Levenshtein distance uses an indicator function which evaluates to $$\1\$$ if the two characters are not equal and $$\0\$$ otherwise and costs $$\1\$$ for deletion and insertion. This does not take into account that a mistake of typing u instead of w should cost more than mistakenly typing an e. Let's try to fix this! ## Challenge For this challenge we'll assume a QWERTY keyboard and only take lower-case letters into account. We will use the usual letter-to-finger assignment (left-most finger to right-most) ["qaz","wsx","edc","rtfgvb","yuhjnm","ik","ol","p"]: ### Left Hand • pinky: qaz • ring finger: wsx • middle finger: edc • index finger: rtfgvb ### Right Hand • index finger: yuhjnm • middle finger: ik • ring finger: ol • pinky: p Now, to define a new string metric we will use the following definitions in the above generalized Levenshtein distance: Set $$\c_\text{deletion} = c_\text{insertion} = 8\$$ and for $$\w(a_i,b_j)\$$ we will use the distance of the two characters plus $$\1\$$ according to the above assignments (unless they are equal, then we'll use $$\0\$$). Here are a few examples: 'q' 'q' -> 0 'q' 'a' -> 1 'q' 'w' -> 2 'l' 'g' -> 4 'p' 'a' -> 8  ## Rules Input will be two strings $$\a\$$ and $$\b\$$ which • are non-empty • only contain lower-case letters (ie. match ^[a-z]+$)

Output will be the "touch-typing distance" as defined above.

## Test cases

"todo" "todo" → 0


# Shift right by half a bit code-golfmathnumberarithmetic

The challenge is to implement a program or function (subsequently referred to as "program") that takes a nonnegative integer $$\n\$$ as input and returns $$\n\over\sqrt{2}\$$ (the input divided by the square root of two) as output, rounded to a nonnegative integer.

You may take your input and output in any reasonable format; for example stdin/stdout, files, or arguments/return values would all be acceptable.

You are required to use, at minimum, the largest fixed-size integer type offered by your language, and if an unsigned variant of this is available, you must use it. If your language has no built-in integer type (e.g. JavaScript) you are allowed to use its default numerical type (e.g. floating point); for languages with no concept of a number (e.g. regex), input and output can be e.g. the length of a string.

It is not required to reject negative integers; a submission that returns correct answers for negative inputs is allowed, but not required. Undefined behavior with negative inputs is allowed.

You are allowed and encouraged to use arbitrary-precision integer types if you so desire, but the type must either be a built-in, part of a standard library, or implemented from scratch in your program.

Despite what the title might imply, you may use any rounding algorithm you want (floor, ceiling, nearest half up, nearest half even, arbitrary, or even random), as long as the difference between the integer returned value and the theoretical exact (irrational) value is always less than $$\1\$$ for all inputs that fit in your chosen integer type. All inputs up to the maximum representable value must return a correct output.

In a way, the job of this program is to calculate the irrational number $$\\sqrt{2}\$$ to the requested precision, presenting it in the form of an integer. This is why solutions using arbitrary-precision types are encouraged, but not required.

This is a challenge. Standard loopholes are denied. The program with the least number of bytes wins. If there's a tie, the choice of accepted answer will be at my discretion. That said, this challenge is not only about which answer wins; it's also about seeing how concisely the challenge can be solved in each language, and seeing how each language "prefers" to handle rounding. And for those submissions that choose to use arbitrary precision, it's about seeing how concisely this can be done in the language.

# Meta

The primary reason for the question is that I want to post my ECMAScript regex solving it. Currently all I have is an 849 byte (very heavily golfed down from an initial 1159 bytes) ECMAScript + molecular lookahead regex, i.e. not purely ECMAScript-compatible and only works on my regex engine, so this question can stew in the Sandbox for a while until I port the regex to pure ECMAScript at some point, and/or put the regex engine on TIO.

That said, I am genuinely interested in what submissions PPCGers will come up with for this challenge (including in the languages more frequently seen in PPCG posts), and will treat the hosting of it seriously. Also there's the chance that someone will attempt solving it in a more powerful regex flavor, and I'd be fascinated to see if this could be done in significantly less length than ECMA (I actually doubt it can). Or somebody could come up with a crazy solution for it in some other language that has limits imposed which make it hard to do.

And if somebody could think of a way to solve it in ECMA in fewer bytes than I have (or even just golf down my regex), that would be fascinating (or thrilling) as well.

I'd be interested in putting up a bounty for the regex aspects of this question, and would appreciate any suggestions people would have as to how to do this and how much the bounty should be.

• The whole stuff about type widths is confusing and I think it gets self-contradictory. E.g. "You are required to use an integer type of at least the precision of a native int. If your language has a native unsigned integer type, you must use it (if you're using a native type)". Java has signed 8-, 16-, 32- and 64-bit integers, and unsigned 16-bit integers. Which should it use? What does "native int" even mean in the context of languages which aren't C? Why should I get a better score in C if I'm using an underpowered computer which doesn't distinguish between uint and long long uint? Jan 26 '19 at 9:11
• @PeterTaylor What I'm really trying to achieve with that requirement is increasing the likelihood that in at least some languages, simply calculating it using floating point, e.g. convert the int to double, divide by sqrt(2) and convert back to int, would lose precision, making it necessary to implement the sqrt function in integer math. But I wanted to not actually prohibit floating point from being used. I agree with your criticism though. What if I simply require that the largest built-in fixed-size integer type be used at minimum if it exists (or arbitrary-precision if desired)? Jan 26 '19 at 10:13
• Thanks for explaining your goal. I'll think about it and see whether I can think of another way of achieving it. If you haven't already done it, you could pop into chat and ask the people there to think about it too. Jan 26 '19 at 20:00
• Thank you, @PeterTaylor. I actually did talk about it a bit in the chat earlier, which is actually why I required native ints in the original version... Jan 26 '19 at 20:20
• I hadn't noticed your edit. That's probably as good a solution as is possible. One other minor suggestion: how about changing the title to "Shift right by half a bit"? Jan 27 '19 at 8:30
• @PeterTaylor I love your title suggestion. Thank you so much! I kept the text "divide by the square root of two" and moved it to the description, in case of Math Processing Errors and also so it will show up in searches more easily. Jan 27 '19 at 8:53
• Can you add some test cases? Jan 27 '19 at 15:37

# Irregular English Verbs

Given the infinitive of an irregular English verb, output its simple past and its past participle.

## Rules

• The input and output can be given in any convenient format.
• No need to handle verbs not in the given list.
• Either a full program or a function are acceptable. If a function, you can return the output rather than printing it.
• If possible, please include a link to an online testing environment so other people can try out your code!
• Standard loopholes are forbidden.
• This is so all usual golfing rules apply, and the shortest code (in bytes) wins.

## Example

fall    --> fell; fallen
beat    --> beat; beaten or beat
bereave --> bereaved or bereft; bereaved or bereft
shall   --> should;(no participle)


# List of Irregular Verbs

Infinitive;Simple Past;Past Participle
alight;alighted or alit;alighted or alit
arise;arose;arisen
awake;awoke or awaked;awoken or awaked
be;was or were;been
bear;bore;borne or born
beat;beat;beaten or beat
become;became;become
beget;begot;begotten
begin;began;begun
bend;bent;bent
bereave;bereaved or bereft;bereaved or bereft
beseech;besought or beseeched;besought or beseeched
bet;bet or betted;bet or betted
bind;bound;bound
bite;bit;bitten
bleed;bled;bled
bless;blessed or blest;blessed or blest
blow;blew;blown
break;broke;broken
breed;bred;bred
bring;brought;brought
build;built;built
burn;burnt or burned;burnt or burned
burst;burst;burst
bust;bust or busted;bust or busted
can;could;(no participle)
cast;cast;cast
catch;caught;caught
choose;chose;chosen
cleave;cleft or cleaved or clove;cleft or cleaved or cloven
cling;clung;clung
come;came;come
cost;cost;cost
creep;crept;crept
crow;crowed;crew or crowed
cut;cut;cut
deal;dealt;dealt
dig;dug;dug
do;did;done
draw;drew;drawn
dream;dreamt or dreamed;dreamt or dreamed
drink;drank;drunk
drive;drove;driven
dwell;dwelt or dwelled;dwelt or dwelled
eat;ate;eaten
fall;fell;fallen
feed;fed;fed
feel;felt;felt
fight;fought;fought
find;found;found
flee;fled;fled
fling;flung;flung
fly;flew;flown
forecast;forecast or forecasted;forecast or forecasted
forget;forgot;forgotten
forsake;forsook;forsaken
freeze;froze;frozen
geld;gelded or gelt;gelded or gelt
get;got;got or gotten
gild;gilded or gilt;gilded or gilt
give;gave;given
gnaw;gnawed;gnawed or gnawn
go;went;gone
grind;ground;ground
grip;gripped or gript;gripped or gript
grow;grew;grown
hang;hung;hung
hear;heard;heard
heave;heaved or hove;heaved or hove
hew;hewed;hewed or hewn
hide;hid;hidden or hid
hit;hit;hit
hold;held;held
hurt;hurt;hurt
keep;kept;kept
kneel;knelt or kneeled;knelt or kneeled
knit;knitted or knit;knitted or knit
know;knew;known
lay;laid;laid
lean;leant or leaned;leant or leaned
leap;leapt or leaped;leapt or leaped
learn;learnt or learned;learnt or learned
leave;left;left
lend;lent;lent
let;let;let
lie;lay;lain
light;lit or lighted;lit or lighted
lose;lost;lost
may;might;(no participle)
mean;meant;meant
meet;met;met
melt;melted;molten or melted
mow;mowed;mown or mowed
pay;paid;paid
pen;pent or penned;pent or penned
prove;proved;proven or proved
put;put;put
quit;quit or quitted;quit or quitted
rid;rid or ridded;rid or ridded
ride;rode;ridden
ring;rang;rung
rise;rose;risen
run;ran;run
saw;sawed;sawn or sawed
say;said;said
see;saw;seen
seek;sought;sought
sell;sold;sold
send;sent;sent
set;set;set
sew;sewed;sewn or sewed
shake;shook;shaken
shall;should;(no participle)
shear;sheared;shorn or sheared
shed;shed;shed
shine;shone;shone
shit;shit or shitted or shat;shit or shitted or shat
shoe;shod or shoed;shod or shoed
shoot;shot;shot
show;showed;shown or showed
shred;shred or shredded;shred or shredded
shrink;shrank or shrunk;shrunk
shut;shut;shut
sing;sang;sung
sink;sank;sunk
sit;sat;sat
slay;slew;slain
sleep;slept;slept
slide;slid;slid
sling;slung;slung
slit;slit;slit
smell;smelt or smelled;smelt or smelled
smite;smote;smitten
sow;sowed;sown or sowed
speak;spoke;spoken
speed;sped or speeded;sped or speeded
spell;spelt or spelled;spelt or spelled
spend;spent;spent
spill;spilt or spilled;spilt or spilled
spin;spun;spun
spit;spat;spat
split;split;split
spoil;spoilt or spoiled;spoilt or spoiled
spring;sprang or sprung;sprung
stand;stood;stood
steal;stole;stolen
stick;stuck;stuck
sting;stung;stung
stink;stank or stunk;stunk
stride;strode;stridden
strike;struck;struck
string;strung;strung
strive;strove;striven
swear;swore;sworn
sweat;sweat or sweated;sweat or sweated
sweep;swept;swept
swell;swelled;swollen or swelled
swim;swam;swum
swing;swung;swung
take;took;taken
teach;taught;taught
tear;tore;torn
telecast;telecast or telecasted;telecast or telecasted
tell;told;told
think;thought;thought
throw;threw;thrown
thrust;thrust;thrust
understand;understood;understood
wake;woke or waked;woken or waked
wear;wore;worn
weave;wove;woven
wed;wed or wedded;wed or wedded
weep;wept;wept
wet;wet or wetted;wet or wetted
win;won;won
wind;wound;wound
wring;wrung;wrung
write;wrote;written

• So this is a simple lookup, meaning that the bulk of the challenge is compressing the data?
Jan 24 '19 at 12:48
• Shouldn't the correct output from can be could;was able to rather than could;(kein Participle)?
Jan 24 '19 at 12:49
• @Adám what about simply can -->could; ? Jan 24 '19 at 12:54
• Up to you. Using (kein Participle) makes the challenge more interesting, as it has the only uppercase character, and the only parentheses.
Jan 24 '19 at 14:02
• @Adám, no. It should be could or was able to or were able to;been able to. It certainly shouldn't be (kein Participle): the spec asks for output in English, not German. Jan 24 '19 at 15:04
• @PeterTaylor OP hasn't answered, but to me it looks like lookup KC, so the actual content doesn't actually matter.
Jan 24 '19 at 16:59
• @Adám Yes, I think it's a simple lookup, it can be other than that? Jan 28 '19 at 10:39
• Imho, it would be much more interesting to take a regular verb as input, and output the past tense: Add "ed" or "d" if verb ends with "e", but change final consonant-"y" to "consonant-"i" and insert "k" if verb ends in "c" and double final consonant if verb ends with a single vowel followed by a single consonant.
Jan 28 '19 at 13:04
• Suggested tag: kolmogorov-complexity Jan 28 '19 at 13:19
• @Adám Do u mean adding the rules, or extending the current list with some regular verbs? Jan 28 '19 at 14:00
• I meant replacing the list, so the challenge will only concern completely regular verbs. It is of course your choice. I can also post this related (but not really related, just inspired by) challenge myself. Let me know.
Jan 28 '19 at 14:39
• @Adám Using only regular verbs would make it a very different challenge. I think this one is interesting as-is. Even if these verbs are irregular, they do follow some rules of their own (such as i becomes u). You may want to identify and store the transformation rules that are common to several verbs rather than just do string compression. Feb 5 '19 at 23:21
• @Arnauld can I use your comment to improve the text of the challenge? Feb 6 '19 at 12:00
• Sure. (But maybe you should use a better example than just 'i becomes u' and make it clear that this is not a general rule but just a rule that applies to several verbs.) Feb 6 '19 at 13:53

# Generate a 3D spiral

Inspired by this chat message

The spiral used in The Path Of The Wildebeest is a contiguous mapping of the positive integers to lattice points in 2D. Your task is to generalize this to 3D.

Specifically, create a function $$\f\$$ from $$\\mathbb Z^+\$$ to $$\ \mathbb Z^3\$$ with the following properties:

• $$\f\$$ is a bijection (All points are eventually reached).
• $$\f(1) = (0,0,0)\$$ (The spiral starts at the origin).
• $$\ |f(n+1) - f(n)| = 1\$$ (The spiral is contiguous).
• The Chebyshev distance from the origin $$\ |f(n)|_\infty \$$ is a nondecreasing function ($$\f\$$ fills all points in each concentric cubical shell $$\k\$$ before moving to shell $$\k+1\$$).

One possible $$\f\$$ is given by this Python implementation, but any $$\f\$$ that satisfies the above properties is allowed. Please describe the function your answer generates.

Because all your computer's memory is taken up by the wildebeest simulation you're running, typing is very slow, so your code must be as short as possible.

### I/O

As is standard with questions, either 1-indexing or 0-indexing is allowed. Any of these I/O formats are acceptable:

• Receive $$\n\$$ as input and output the $$\n\$$th point in the sequence
• Receive $$\n\$$ as input and output the first $$\n\$$ points
• Take no input and output the sequence infinitely.
• Editing this from "is an increasing function" to "is a nondecreasing function" doesn't fix the problem... the function will sometimes have to decrease, because it has to hop over its own path and come back down. If you're already taking this into account, could you please be clearer about what you mean? Jan 30 '19 at 1:40
• @Deadcode I'm not sure what you mean by hopping over its own path. This challenge is just to generate the spiral, not the path of any chess piece. Jan 30 '19 at 1:45
• Okay, imagine an actual ball of yarn. But to simplify things, let's imagine you start with a sphere. Start wrapping a string around it. After you make 1 revolution, you must adjust the angle of the string so that it crosses over its previous path. This will make a small bump in the path the string takes on its 2nd revolution. And on its 3rd revolution, there will be more bumps. Now imagine the analogy with a cube, quantizing the path to a cubical grid. The 2nd revolution will have to cross over the path the 1st revolution took, making a bump, 1 up, 1 down. Jan 30 '19 at 1:56
• I think in the cubical case, the path of the yarn can be arranged such that it never goes over bumps. Jan 30 '19 at 1:58
• Okay, but then it will bear much less resemblance to a ball of yarn. Jan 30 '19 at 2:04
• @Deadcode the key point here is that it's the Chebyshev distance that's non-decreasing. So the 3d lattice is divided into cubic shells centered on the 0 point, and the function has to map to all of the points in shell n before moving on to shell n+1 Jan 30 '19 at 21:37
• (in that sense it's analogous to the 2d case) Jan 30 '19 at 21:41
• You can remove the backticks before and after the list then select it and press Ctrl + K, it will show as a single block of code then Apr 19 '18 at 13:59
• @user202729 This isn't a KC because an answer to the challenge should accept inputs. Nov 19 '18 at 8:05

# Least efficient field order

## Background:

In C and other languages, a struct is a data type composed of fields of other data types. These fields may be different sizes (in bytes) and may have different alignment requirements (e.g. an int field may need to be on a 4-byte boundary).

Padding is added to a struct to keep all of its fields aligned, and to keep its size a multiple of each field's alignment.

For example:

struct test {
char  a; // 1-byte, 1-byte aligned
short b; // 2-byte, 2-byte aligned
int   c; // 4-byte, 4-byte aligned
}

In this struct, there will be 1 byte of padding after a (to keep b on a 2-byte boundary), no padding after b (as c is already on a 4-byte boundary) and no padding after c (as the size is already a multiple of 2 and 4 bytes), so the total size of the struct is 8 bytes (1 + 2 + 4 + 2). However, if we rearrange the order of the fields, the struct size can be bigger.

struct test_2 {
char  d; // 1-byte, 1-byte aligned
int   e; // 4-byte, 4-byte aligned
short f; // 2-byte, 2-byte aligned
}

In this struct, there will be 3 bytes of padding after d (to keep e on a 4-byte boundary) and there will be 2 bytes of padding after f (to keep the size a multiple of 2 and 4 bytes), so the total size of this struct is 12 bytes (1 + 4 + 2 + 3 + 2).

## Challenge:

Given a list of pairs of positive integers (each pair representing the size and alignment of a field), return the same pairs in an order such that a struct with fields in that order would require the most padding, i.e. be the least space-efficient.

Givens:

• A field's size will always be greater than or equal to its alignment. (e.g. (4, 8) is not a valid input pair)

• A field's size will always be a multiple of its alignment. (i.e. (12, 5) is not a valid input pair)

Test cases: (other outputs that give the same total size are valid as well)

[(1, 1), (1, 1), (4, 4)] -> [(1, 1), (4, 4), (1, 1)] # size 12
[(12, 4), (1, 1), (2, 2), (8, 8)] -> [(12, 4), (8, 8), (1, 1), (2, 2)] # size 32
[(7, 7), (5, 1), (2, 2)] -> [(7, 7), (2, 2), (5, 1)] # size 28
[(6, 6), (6, 3), (4, 2), (2, 2)] -> [(6, 3), (4, 2), (6, 6), (2, 2)] # size 24
[(1, 1)] -> [(1, 1)] # size 1


Test case checker online!

• I think in your explanation (1 + 2 + 4 + 2) should be (1 + 2 + 4 + 1). The only thing I see about this challenge that is questionable is whether having separate alignments is interesting or tedious. Would this lose much by assuming each field's alignment is equal to its size? Feb 25 '19 at 21:25

# The Minigame Challenge

The idea of this CnR is fairly simple: The Cops must create a simple minigame (explained in more detail below), with a definite goal. That goal may or may not be achievable. The Robbers must then either complete the minigame, or prove its goal is not achievable.

## Rules

### The Minigame

The Cops will create a suitably simple minigame with a definite goal.

Acceptable minigame examples are functions that take some input and return some output, with the goal being to either achieve a specific output, or make the program terminate or error.

Minigames considered complex (and thus unnacceptable) would be programs/functions that will never terminate in up to 60s (using TIO as a benchmark here), or that simulate complex games (such as blackjack, chess, etc).

### Cops:

The Cops must provide one of the following:

• A solution for your minigame that achieves the goal, or
• A proof that the goal for your minigame is not achievable.

at least 7 days (168h) after the answer was posted for their answer to be considered safe.

If your minigame's goal is achievable, it must always be achievable, and it must not contain "insider information", such as a fixed seed for a pRNG, or pregenerated primes as factors of a number to be factored.

### Robbers:

To crack an (unsafe) answer, the robber must provide either a solution to the minigame that achieves the stated goal, or a proof that the goal is not achievable.

### Scoring:

Cops will be scored based on standard rules, with the fewest number of bytes in an answer being better. Cracked answers will always have a score of $$\\infty\$$.

Robbers are scored according to the number of answers they've cracked.

Standard loopholes are, as usual, forbidden.

# Challenge is still under construction

This is the first CnR I've ever come up with. Suggestions and observations are always appreciated.

• This seems a lot like PCG inside PCG. Mar 15 '19 at 19:16

# Challenge

Create a function takes in two 2-dimensional arrays of Characters (or Strings if the programming language does not have characters as a datatype) as inputs: a and b.

Your task is to determine if b contains a. If this is so, return true. Otherwise, return false.

## Sample Test Cases

a:

123
456
789


b:

123
456
789


should return true.

a:

code
golf


b:

thisis
code!!
golf!!
ohyeah


should return true.

a:

abcd
efgh
ijkl


b:

abcdef
ghijkl
mnopqr


should return false.

Least bytes wins.

• does the block need to be as-is? No lines inbetween? No characters? i.e. does code\golf appear in code!!\trucking!!\golf!!?
– Ven
Mar 25 '19 at 11:04

(No title yet)

I'm traveling away from home, and while visiting a fair a dart-throwing game catches my eye.

To play, I can buy $$\d\$$ darts at a time. There are no other options for purchase.

There are $$\t\$$ identical targets, but since I'm not a great shot, I can't consistently aim at just one. Instead, I can hit one (at random) with nonzero probability $$\p\$$.

Of course, just to make it harder for me management also decided it would be too easy if players won a prize immediately upon hitting a target. Rather, I need to hit the same target $$\n\$$ times in order to get the prize. As long as I keep playing, darts that have already landed on targets will remain there, but if I leave, they will be reset.

I'd rather not be scammed though, so I would like to know how many times I should expect to purchase darts in order to win at least one prize.

Given $$\d\$$, $$\t\$$, $$\p\$$, and $$\n\$$, find the expected cost for me to get at least one prize.

### Test cases

(just some basic ones, for now)

  d,  t,  p,  n  -> E

__, __, __,  1  -> 1/(1-(1-p)^d)
__, __,  1, __  -> t*n


Any suggestions to improve wording or formatting? Is anything unclear or confusing?

Would it be more interesting to ask for the expected cost to get at least $$\x\$$ prizes, instead? In that case, a target would award a prize for every $$\n\$$ darts landed, i.e. at $$\n,2n,3n,\ldots\$$ darts.

What level of accuracy should be required?

Ideas for test cases?

Not entirely sure what other tag(s) I'd like to work with here. Golf should be interesting... I'd also like to see faster solutions though, and I would expect golfy solutions to be very inefficient.

# Finding Points in Convex Hulls

Convex hulls are notoriously hard to deal with, so today's challenge will deal with a relatively simple premise: given a convex hull defined by a set of points, and an additional point, find whether the additional point lies in the hull.

Now, for some definitions

1. The convex hull of a finite set X, a subset of R^n, is the set of convex combinations of points of $X$.
2. A convex combination of points x1,x2,...,xn is a point of the form a1x1+a2x2+...+anxn such that all ai>=0 and the sum of all of the ai is 1.

# Input/Output

The input is rather flexible, as long as it contains the appropriate information to express the convex hull and additional point.

An example of a valid input format to express the convex hull represented by (0,1,2),(4,3,2),(8,8,8) and the point (5,5,5) is

([(0,1,2),(4,3,2),(8,8,8)],(5,5,5))


The output is a truthy/falsy value, depending on whether the point is contained in the hull.

# Remarks

There are a lot of packages and libraries that deal with convex hulls, such as scipy.spatial. Such libraries/packages are explicitly banned. Libraries that facilitate matrix computations, such as numpy, are permitted.

The winning criterion is .

There is no limit on the number of dimensions the points can lie in, as long as they all lie in the same number of dimensions.

# Test Cases

I will be using the following tio link for testing the validity of solutions.

Some smaller test cases:

([(1,1)],(1,1)) - True
([(1,1)],(1,2)) - False
([(1,1,1),(3,3,1)],(2,3,1)) - False
([(1,1,1),(3,3,1)],(2,2,1)) - True
([(0,0),(3,3),(0,6)],(2,4)) - True
([(0,0),(3,3),(0,6)],(1,1)) - True
([(0,0),(3,3),(0,5)],(2,4)) - False


# Questions for Sandbox

Is code golf the best criteria for this? That might encourage brute force searches on every combination of points... any suggestions?

• You mention R^n but all the examples use Z^n (which is much easier to work with computationally). Please clarify the expected input. Also, what about libraries for linear programming? Apr 1 '19 at 10:03
• @PeterTaylor I was thinking of making it R^n only, since one can bash out the answer if it is in Z^n. And, linear programming libraries probably also should be banned. Apr 1 '19 at 13:08
• I'm not sure how easy it is to bash out the answer. The weights could be rational, and it's not obvious to me that the denominators can be bounded because the entire system can be translated. Apr 1 '19 at 20:35
• @PeterTaylor Hmm, I'll think about how to revise this. Apr 1 '19 at 22:05

This is a post to pre-test for a duplicate question before I spend the time to finish the full post and add test cases.

So, have we ever had a question for a "snake rotation" of a matrix:

    +--------------+
1  2  3  4  5|
+------------  |
|10  9  8  7  6|
|  +-----------+
|11 12 13 14 15
+--------------+

+--------------+
-->  13 14 15  1  2|
+------------  |
| 7  6  5  4  3|
|  +-----------+
| 8  9 10 11 12  -->
+--------------+


The ascii walls are there only for clarity. Actual input/output would be normal matrices

I've decided to leave this very open ended, so that I can write it based on community input. There are a lot of questions in the bottom. All feedback is appreciated, even minor suggestions/comments. :)

Did you know WordArt is still a thing in MS Word? Let's parse it!

(I'll remove the Swag format)

The challenge will contain a test battery with X jpg.files each with a word or sentence shown in WordArt. Your task is to parse as many as you can. The winner will be the submission that correctly parses the most images, with date stamp being the tie breaker.

### Sandboxing:

• Good idea or not?
• Should there be many different styles, or should I stick to one? Colors? Shade/mirroring?
• How many test cases?
• Is jpg the best format (it was used in the Upgoat/Downgoat challenge)
• Should I have a more refined scoring mechanism? It's hard to mix two quantities into one score.
• Should I stick to just letters / alphanumeric / all printable ASCII / multiple lines? Long sentences or single words?
• Orientation? Waves? Circular?
• Resolution?
• Builtin functions?
• Should I require a complete match of images, or character by character?
• Nice challenge! 1 WordArt is a completely objective (good) definition. 2 Some of the questions was mainly for the problem of the challenge being "too hard", so if you want any answer at all you should simplify those. (somehow most PPCG users are lazy). 3 Format is not a very important problem (conversion is easy), but just to be sure, use a lossless format. Nov 7 '17 at 16:48
• There is another problem of the font used. Nov 7 '17 at 16:49
• The challenge is supposed to be hard. Scoring based on the success rate would be useless if not. But it's of course possible to change the scoring, and make the test cases easier. Nov 7 '17 at 17:00
• 1. optical-char-recognition. 2. Upgoat/Downgoat used JPG because it used photos. IMO it's preferable in general to allow any raster image format, because otherwise you pretty much force people to use languages with library support for image decoding. 3. The "Swag" example is ambiguous: is it "Swag Swag"? 4. If you want to refine the scoring mechanism, I would do it as the sum of Levenshtein distances from the correct results. But the big problem with scoring test-battery is the tension between keeping the tests secret (to prevent overfitting) and public (for objective scoring). Nov 8 '17 at 7:25
• 3. I can avoid the Swag-type format. Are the other formats unambiguous? 4. I can either show all the possible formats, and say that I will use words that are say 4-15 characters, or maximum 15 characters per example. I can then pre-select the input list, and provide a hash that ensures I won't change the input list after answers are posted. People can then try their code on some random strings of their own choosing on various formats and approximate their results, while the final score is made public later...? May 22 '19 at 6:43
• Thanks for the link to the other challenges, they are useful as references when writing this challenge. Do you think this is different enough by the way? May 22 '19 at 6:58

# Challenge

You will be given an input represented by x, which is a string containing at least 3 characters. It will consist only of the standard numeric characters, 0 through 9. Your job is to find and output how many right triangles can be formed with the given numbers.

# Rules

• Numbers must be kept in the order they were given in. No mixing them up!

• The numbers for each right triangle must be consecutive.

• The order of numbers has to be a first, b second, and c third, and must satisfy the formula a² + b² = c². a can be greater than or less than b, as long as it satisfies the formula.

• Decimal points may be added between any numbers.

• Decimals require one or more numbers to be placed before them, e.g. .5 cannot be used as a number but 0.5 and 12.5 can.

• Decimals with at least 4 digits after the decimal point truncated to the third digit, e.g. 1.2345 would truncated to 1.234 and 1.9999 would be truncated to 1.999.

• Numbers can be used more than once in 2 or more different triangles, but cannot be used multiple times in the same triangle.

• Multiple representations of the same value can count multiple times.

• Repeating zeros are allowed, e.g. 000.5 counts as a number.

• All possible combinations must be taken into account for your program to be valid.

# Example Inputs and Outputs

Input: 345
Output: 1
This can be split into 3, 4, and 5, which, of course, form a right triangle.

Input: 534
Output: 0
While this does include the necessary numbers to form a right triangle, they are not in the correct order. It has to follow the formula a² + b² = c², but in this case it follows c² = a² + b². The order of numbers cannot be changed from the original input, so in this case no right triangles can be formed.

Input: 3415
Output: 0
This does contain a 3, 4, and a 5, which can form a right triangle, but they are not consecutive; there is a 1 splitting the 5 from the 3 and 4.

Input: 5567507
Output: 1
Because decimals can be added anywhere, it can be changed to 55.67.507, which allows splitting it into 5, 5.6, and 7.507 to form a right triangle. Remember that decimals are truncated to the third digit after the decimal point, which is how we get 7.507.

Input: 345567507
Output: 2
The first right triangle is formed by 3, 4, and 5. The second one is formed by 5567507 (read the previous example for explanation). Numbers can be used more than once, so the first 5 was used in the first and second triangles.

Input: 51125
Output: 0
Because of rule 5, you cannot use .5, 1, and 1.25. An integer is required before .5 for it to work.

Input: 051125
Output: 0
Unlike the previous example, there is a number before the first 5, so it is now legal to use 0.5, 1, and 1.25.

Input: 121418439
Output: 2
The numbers 12 and 14 would form a right triangle where side c has a length of approximately 18.43908891458577462000. Because long decimals are truncated to the third digit after the decimal point, we would be left with 18.439. This fits in with the original input, 121418439. Additionally, 1.2, 1.4, and 1.843 counts as a separate combination, thus giving us our second right triangle.

Input: 10011005
Output: 8
Numbers count separately if they're represented in different ways, so this allows for (1, 00, 1), (1.0, 0, 1), (1, 0, 01), (1, 0.01, 1), (1, 0.01, 1.0), (1, 0.01, 1.00), (1.0, 0.1, 1.005), and (1, 00.1, 1.005).

This is code golf, so shortest answer in bytes wins. Good luck!

• So if I understand correctly we do the following things: get all substrings of the input; get all 3-part partitions of each substring; insert a comma at each possible position in each possible partition; check any combination of three in order is approximately truthy for $a^2+b^2=c^2$ (approximately, because the decimals have been truncated, so aren't exactly a right triangle in some cases). Do I understand this correctly? May 8 '19 at 7:07
• @KevinCruijssen almost correct, except you must find all combinations both with and without decimals at any given location. So it's not necessarily just adding a decimal point at every possible place; it's checking the results that would occur both by adding and not adding a decimal. May 8 '19 at 14:19
• "Numbers can be used more than once." - Could you provide some examples of this or otherwise clarify what you mean? May 10 '19 at 19:52
• @Beefster see the fifth example: one of the 5's is used to form two different right triangles. May 10 '19 at 19:57
• Perhaps a better way to explain it would be something like "Right triangles that only use a subsequence of the digits can overlap with other triangles made from the same sequence of digits" or "You do not need to use all the digits for a single triangle and you may re-use digits that were used in previous triangles". What confused me about the way it's currently written is that it could be taken to mean that you can re-use a digit for the same triangle, so 125059 could be split into 12, 22, 25.059. May 13 '19 at 22:45

# $$\n\$$-Chess KotH king-of-the-hillgridchessboard-game

## Introduction

Inspired by the challenge N-movers: How much of the infinite board can I reach?, I came up with an idea of applying the $$\n\$$-mover rules in chess.

$$\n\$$-Chess™ is a board game played on a 8x8 grid like the original chess. However, the pieces are very different from chess - they are named $$\n\$$-movers ($$\n\$$ can be any natural number) and can only move by the rules set on them (see Rules: Pieces).

## Rules

### Pieces

In $$\n\$$-chess, a piece is called an $$\n\$$-mover, where $$\n\$$ denotes its possible movements. Specifically:

1. The $$\n\$$-mover can only move one step of Euclidean distance $$\\sqrt n\$$ each time, i.e. for a move with $$\x\$$ units horizontal and $$\y\$$ units vertical, $$\x\$$ and $$\y\$$ must fulfill the equation $$\x^2+y^2=n\$$ in order to be valid.
• For example, a 5-mover can only move 1 unit in one direction and 2 units in the another at the same time; while a 25-mover can move 5 units in either direction, or 3 units in one direction and 4 units in the another at the same time.
2. A piece can have at most 2 rules set at the same time. For a piece that can act as either an $$\m\$$-mover or an $$\n\$$-mover, we can call it an $$\(m,n)\$$-mover. An $$\(m,n)\$$-mover is allowed to move one step of Euclidean distance of either $$\\sqrt m\$$ or $$\\sqrt n\$$ or $$\\sqrt{m+n}\$$.
• It is allowed to have $$\m=n\$$.
• Only pieces that has captured other pieces can have multiple rules (See Rules: Capture).
3. If more than 2 rules are set on a piece, only 2 rules can be reserved. You can choose which 2 rules to reserve.

### Board

The chess is played on a 8x8 board. The initial setup is as follows:

### Capture

1. If a piece lands on a grid that an opponent piece has occupied (capture), that opponent piece shall be moved away from the board.
2. After that, the piece will be granted rules from the captured opponent piece. As stated in Piece, if there are more than 2 rules in total, only 2 shall be reserved.

### Playing

1. In a turn, White and Black play a move in turn. White always moves first.
2. Only one piece can be move per turn.
3. You cannot move a piece in the way that is not allowed by the rules set on it, beyond the boundaries of the board, or onto a grid already occupied by your other pieces.
4. You cannot move your last piece in the way that after moving it it can be captured instantly.

### Victory Condition

The game ends with either player winning if:

1. The last piece of the opponent is being checkmated, i.e. no possible moves that can save this last piece from being captured; or
2. The opponent resigns and loses. (In this challenge, no resignation will be allowed)

The game ends with a draw if :

1. No checkmate is possible for both sides;
2. Either player has no more possible moves, but is not in checkmate;
3. There is no capture in 50 moves; or
4. The identical board arrangement has appeared 3 times with the same player to move.

## Implementation

A player bot is a class that implements a function nextMove(). This function receives 3 arguments and output an array of 6 integers:

function nextMove(
color: int,                    // Your color: 0 = white, 1 = black
board: int[8][8][3],           // Current board:
//  First index: x-coordinate
//  Second index: y-coordinate
//  The third layer will be as follows -
//   [0]: the color of the grid
//       (-1: Not occupied; 0: white; 1: black)
//   [1]: rule 1 (0 if not exists)
//   [2]: rule 2 (0 if not exists)
moves: int[][10]               // Previous moves:
//  First index: move number (0-indexed)
//  The second layer will be as follows -
//   [0]: the color of the player
//   [1]: is this a capture (0: no, 1: yes)
//   [2]: source x-coordinate
//   [3]: source y-coordinate
//   [4]: source rule 1
//   [5]: source rule 2 (0 if not exists)
//   [6]: target x-coordinate
//   [7]: target y-coordinate
//   [8]: target rule 1
//   [9]: target rule 2 (0 if not exists)
): int[6]                          // Return value: as follows:
//  [0]: source x-coordinate
//  [1]: source y-coordinate
//  [2]: target x-coordinate
//  [3]: target y-coordinate
//  [4]: target rule 1
//  [5]: target rule 2 (0 if not exists)

• Rule 1 and Rule 2 denotes the 2 rules a piece has.
• Source rule entries are used to denote which rules a piece has, while target rule entries are used to denote which rules are decide to keep.

You may use JavaScript, pseudo-codes, or any languages similar to JavaScript, but using JavaScript is strongly encouraged, because all submissions will be translated to JavaScript before using for competition, and the test drive only supports JavaScript.

## Restriction

1. Standard loopholes are forbidden by default.
2. You cannot by any means read or modify other bots. You can only use data within the class and inputs passed to nextMove() function.
3. You may define other variables inside your class if needed.
4. You may use a random number generator.
5. Your bot, specifically the nextMove() function, must return value in 10 second per move. Timeout leads to disqualification.

## Procedure

Round-robin will be used to determine the opponents, i.e. all bots will face each of the other bots twice, one as black and one as white. For each game, a win gets 2 points, a lose gets 0 points, and a draw gets 1 points.

The bot will the highest points wins, and if there are any ties, they will be broken by:

1. Number of victories;
2. Average time to checkmate opponent (shorter is better); and
3. Drawing lots. (Hopefully this step is not necessary)

## TODO

• KISS: since I assume that this is actually a game you've made up for the question, why not ditch checkmate and make the win condition that you capture all of the opposing pieces? I'm not sure about n-Chess™ for a name, either: it feels more like checkers than chess to me. May 29 '19 at 16:34
• @PeterTaylor I'd say the question I had my inspiration from had its inspiration from chess, so I chose chess, and tbh I don't even know what checkers is until I've googled it right now. Well, it does look like checkers in the way capturing occurs, but I think I need to borrow some rules from chess, especially the ending rules, because I need to draw a line if the game never ends. Anyway thanks for your useful feedback! May 30 '19 at 2:41

# Introduction

The story below is fictional, don't worry.

Times table hard. More curriculum. Class advance. Son behind. Horror. Help.

ahem Pardon me.

So... I've got a big problem. The times table is huge, and all the mnemonic rules we've taught our son haven't make him fully memorize it yet. However, his class must advance to the next chapter, long multiplication. As a result, he is prone to making many mistakes while multiplying numbers.

See, they're already making the students multiply a multi-digit number by a single-digit number. My son often forgets to carry digits over and, of course, his times table. The good thing is that he never misses digits!

The situation is awful. Help me before they start with two multi-digit numbers!

# Challenge

Your challenge is to write a program that takes a multiplier, a multiplicand and a product, and analyzes the product for any of the following mistakes. Here's how:

• If a digit is the correct ones digit of the product of the multiplicand times the digit of the multiplier he's currently multiplying, but a non-zero carryover was supposed to be added to it, then this digit has a carryover error.
• If a digit is otherwise wrong, there's a times table error around it (we can't be sure if the ones digit was wrong or a carryover digit was wrong), so it has a times table error.
• There will never be another kind of mistake in the product, my son doesn't skip digits.

For each wrong digit, return an array of two or three elements:

• Position of the digit from the left or right, 0- or 1-indexed (your choice must be consistent). Keep in mind that, if the leftmost digit is absent due to a carryover error, then it's still the leftmost digit and the place has the relevant index.
• Correct carryover, if the mistake was a carryover error, or omitted otherwise. You may also put a 0 instead of omitting the element (you can't blame my son for forgetting to add a zero carryover!), but be consistent.
• Correct digit.

# Rules

• The numbers will all be positive integers.
• The multiplicand will be an integer from 1 to 9, inclusive.
• You can get the three numbers via any reasonable way you want.
• The output can be in any order, as can the elements of each array, as long as the latter is consistent.
• You may not make use of any standard loophole.
• This is a challenge, so the shortest answer, measured in bytes, wins.

# Test cases

Multiplier, Multiplicand, Product -> Mistakes (0-indexed from the left, possibly absent carryover)

5, 3, 15 -> [] (15)
3, 5, 5 -> [[0, 1, 1]] (05 -> 15)
7, 3, 31 -> [[0, 2]] (31 -> 21)
1551, 9, 4959 -> [[0, 1, 1], [1, 4, 3]] (04959 -> 14959 -> 13959)
2121, 7, 14847 -> [] (14847)
2121, 7, 4727 -> [[0, 1, 1], [2, 1, 8], [3, 4]] (04727 -> 14727 -> 14827 -> 14847)
33333, 9, 11111 -> [[0, 2, 2], [1, 9], [2, 9], [3, 9], [4, 9], [5, 7]] (011111 -> 211111 -> 291111 -> 299111 -> 299911 -> 299991 -> 299997)


# Sandbox

• Is the challenge unclear?
• Is the output format weird or too strict?
• I think it's a pretty interesting challenge! My only nitpick is in the introduction, when you're bringing up 'My son makes these mistakes:': make this section a bit more formal/a bit less story-level detail. The rest of the introduction is funny and gives some good context, but while reading the challenge I was kind of thrown off by the unnecessary detail in that section, e.g 'Ah, the crux of my worries.' Jul 9 '19 at 21:25
• @GezaKerecsenyi That might be because those were part of the challenge text before I decided to move it all to the Challenge section. I'll shorten them to one sentence. EDIT: I've removed that part. Jul 9 '19 at 22:01
• Also, I think that making the carryover optional is quite confusing, and it also means that you have to check if it was a carryover error just to know whether to output it or not. I would suggest to make them always give the correct carryover or 0, even if it wasn't a carryover error. Jul 10 '19 at 7:46
• @GezaKerecsenyi I'm giving two options for times table errors, either exclude the carryover or make it 0. The answerer can choose one of the two consistently. Do you think there's an issue with the first one? Jul 10 '19 at 17:48

## Sandbox Notes

• Any other/better tags?
• Is my math right?

# Knot Function

TL;DR: Define a closed curve in 3D space that forms a knot.

A function $$\f : [0,1] \to \mathbb R^3\$$ is considered a loop function if the following conditions hold:

• It is continuous.
• $$\f(0) = f(1)\$$.
• It is injective everywhere else.

Intuitively, this means that the function traces out a curve in 3D space that does not intersect itself.

Let the set of loop functions be denoted $$\\text{Loop}\$$. One such function is $$\\text{circle}(t) = (0, \sin 2\pi t, \cos 2\pi t)\$$, which traces out a circle in 3D space.

A function $$\f : \text{Loop}\$$ is considered a knot function if there exists no continuous function $$\d : [0,1] \to \text {Loop}\$$ such that $$\d(0) = f\$$ and $$\d(1) = \text{circle}\$$. Intuitively, this means that the loop that the function traces out cannot be continuously deformed into a circle without intersecting itself.

Write a program or function that implements a knot function, where real numbers are replaced with (to best approximation) floating point numbers.

This is , so the shortest valid answer (measured in bytes) wins.

• This is going to need a formal definition of continuity, in particular of $d$. E.g. if using epsilon-delta, what's the distance metric between loops? Jul 5 '19 at 11:06
• @PeterTaylor I think it's pointwise fixing each input value of t that's given to d.
– xnor
Jul 6 '19 at 1:54
• You need to be careful about input because it is not possible for a computable function to take an arbitrary real number between $[0,1]$ as input, so for this challenge $[0,1]$ is going to have to mean something else likely $\mathbb{Q}\cap[0,1]$. You might also want to replace $\mathbb{R}^3$ with $\mathbb{Q}^3$, but I don't think this is technically necessary.
– Wheat Wizard Mod
Jul 6 '19 at 2:59
• Also I am going to recommend the topological definition of continuity over $\varepsilon-\delta$ for this purpose because it meshes neatly with topology. (and IMO is easier to understand anyway)
– Wheat Wizard Mod
Jul 6 '19 at 3:02
• I think it would help to give a short informal statement of the challenge before going into the definition, so as not to needlessly scare away anyone intimidated by the math terms.
– xnor
Jul 10 '19 at 4:23
• The TL;DR needs "closed", because open polynomial curves which are knots are not valid answers. Jul 11 '19 at 22:12
• @PeterTaylor Fixed. Jul 11 '19 at 22:13

# Print the sum of the previous answer's ASCII values

## Challenge

Output the sum of the previous answer's characters. For example, if the previous answer was print(25), the next answer would have to output the next answer would have to output 741.

The initial answer has to output the number 1000

## Rules

• Your code can only use ascii characters (sorry Jelly), no extended-ASCII or Unicode characters.
• Your output can be in Decimal, Unary or Base 256 (represented by ASCII characters)
• You cannot use a language that has been used before. (Note: Different versions of the same language are allowed, like Python 2 and Python 3)
• To prevent overly long languages (such as Unary) ruining the chain, your answer cannot be longer then 1000000 bytes.
• The same person cannot post twice in a row
• If two users post "at the same time", the earlier one will stand
• If a solution breaks one of these rules, it and all answers that chain off of it will be deleted
• Don't edit your code after you've posted it, otherwise it could invalidate code that chains off of it

## Winning

The winner of the challenge is whoever has posted the most recent answer, since it will get harder and harder the more languages that are used.

• Can you clarify base 256? If extended ASCII is excluded, is there a specific way to represent 256 distinct digits using 127 codepoints? Jul 13 '19 at 20:12
• IMO this is too trivial to make a good answer chaining question. Finding an unused language in which you can figure out how to write (essentially) Hello, World is not going to get difficult until a couple of thousand answers in. Jul 14 '19 at 8:24
• Wouldn't every answer just be variants of print(n) where $n$ is the integer literal of the sum of the last answer?
– Jo King Mod
Jul 15 '19 at 22:52
• Yeah, I’m gonna scrap this challenge because I didn’t think it through properly Jul 16 '19 at 6:22

## I'm sorry, Code Golf

This challenge is inspired by a series of Garfield-inspired artwork in which Garfield is depicted as a horror creature, stalking his owner, Jon. Commonly referred to as "Creepy Garfield", the line "I'm sorry, Jon" can be found in one of the earliest Creepy Garfield images, created by DubbleBaby.

After being used over and over again by the Code Golf community, our beloved Jimmy has transformed - no longer bound to his weak, mortal body. He has become the embodiment of spaghetti code, the ascii manifestation of hours-long debugging sessions.

Take 3 ordered numerical inputs and output Jimmy's body

Inputs

Input is received in the form x y z where:

x is the number of left limbs (integer between 1 & 10)
y is the number of right limbs (integer between 1 & 10)
z is the number of heads (integer between 1 & 10)

----Sample Input----
2 8 1


Meta Note: I don't think increasing the possible amount of body parts will influence the solutions but please let me know if you think they should be higher.

Body Parts

Head =          <o>
Left Limb =     /
Right Limb =    \
Filler Limb =   |
Body Piece =    0


Body Generation Rules

   1. Left limbs must be attached to the left side of a body piece
(excl rule 3)
- Correct:  /0
- Incorrect: /<o>
- Incorrect: 0/
- Incorrect: /0\0\

2. Right limbs must be attached to the right side of a body piece
(excl rule 3)
- Correct: 0\
- Incorrect: <o>\
- Incorrect: \0

3. Limbs must not touch
- Correct: /00\
- Incorrect: /0\/0\

4. An external body piece without a limb must have a | in its place
- Correct: /0|
- Correct: |0|
- Correct: /00\
- Incorrect: /0
- Incorrect: 0\

5. Heads must be between body pieces (horizontally not vertically)
- Correct: 0<o>0
- Incorrect: 0<o><o>0
- Incorrect: 0<o>

6. All lines must be aligned centre
- Correct:      /0\
/0<o>0|
/0\

- Incorrect:    /0\
/0<o>0|
/0\

7. The bottom line of Jimmy's body must always contain a left and right limb
- Correct:      /0\
/0<o>0|
/0\

- Incorrect:    /0\
/0<o>0\
/0|

8. Jimmy's body must be whole and cannot be separated into sections
- Correct:    /0\
/0<o>0|

- Incorrect:    /0\   /0\   |0<o>0|

- Incorrect:      /0\

/0<o>0|


Examples

Input:
2 8 2

Sample Output:

/0<o>0<o>0\
|0\
|0\
|0\
|0\
|0\
|0\
/0\

OR

/0\
|0\
|0<o>0\
|0\
|0\
|0<o>0\
|0\
/0\


## Rules & Win Conditions

1. You may receive the stated input via any method into a program or function.
2. You may display the output in any clear and discernible way.
3. Your output must obey all Body Generation Rules.
4. The previous 3 rules must be followed for all 1000 combinations of inputs.
5. Standard loophole rules apply.

As this is , least amount of code in bytes wins.

• I don't like bonuses in code golf in general, and this one doesn't even affect the score so it's just an invitation to distract from the challenge Jul 16 '19 at 20:36
• (1) Whenever you see a bandwagon, run the other way. It's better to ask an interesting question than the nth variant on a theme. (2) That aside, there's no clear explanation of what the input is. Where are the values of that enum? Jul 17 '19 at 10:53
• @PeterTaylor (1) This challenge doesn't exist to hop on the bandwagon. I thought that generating an eldritch-like stick figure would be neat. If you don't think this is an interesting question, I'd love to hear your feedback on that. (2) I've amended the input section to be a bit clearer but I'm not sure what I was missing in the first place.
– BDM
Jul 17 '19 at 21:13
• What's missing is an explanation of the interpretation of the numbers. The input section defines the range of legal inputs but not the meaning of the inputs. Jul 17 '19 at 21:33
• @PeterTaylor Ah I understand now. Hopefully my recent edit solves this.
– BDM
Jul 17 '19 at 21:44
• Somewhat related ... I think?! Jul 21 '19 at 21:23

# String attractors

Let $$\S\$$ be a string of length $$\n\$$. An attractor of $$\S\$$ is a subset of indices $$\A \subset \{0, \ldots, n-1\}\$$ such that every nonempty contiguous substring $$\S[i \ldots j]\$$ of $$\S\$$ has an occurrence $$\S[i' \ldots j']\$$ whose index set $$\[i' \ldots j']\$$ intersects $$\A\$$. A minimum attractor is one with minimal size. Minimum attractors can be used to implement very efficient string compression algorithms. In this challenge, your task is to find a minimum attractor for a given string.

# Example

Consider the string acabaccabc. Using 0-based indexing, one of its attractors is $$\A = \{2,3,5,8\}\$$. This graphic shows the positions of $$\A\$$:

acabaccabc
^^ ^  ^


For example, the occurrence of the substring ac in the beginning doesn't intersect $$\A\$$, but there is another occurrence after the first b that does. It turns out that no attractor of size 3 exists for this string, so $$\A\$$ is a minimum attractor.

# Specifications

Your input is a nonempty string of lowercase ASCII characters. Your output is a minimum attractor for this string, in any reasonable format. You can use either 0-based or 1-based indexing.

There may be several choices for the minimum attractor. In that case you may return any one of them, but only one.

The lowest byte count wins.

# Test cases

These are 0-indexed and show one possible output.

a -> [0]
aaa -> [0]
ababab -> [0,1]
qgqqgq -> [1,2]
abcbacaa -> [1,3,5,6]
abcdefgh -> [0,1,2,3,4,5,6,7]
jejjeeje -> [2,3,5]
sttststt -> [1,5]
ykkkkvyk -> [0,2,5]
assassins -> [0,2,6,7]
acabaccabc -> [2,4,6,8]
ssssssjjjsjj -> [5,8]
hrhrhhrhhhrr -> [3,7,10]
fubuaabubuau -> [0,1,4,6,10]
jjaahajjhahaaahjjh -> [2,6,8,11,14]

• It took me a few tries to understand the first paragraph. I think it is explained well, but I may just have been unfamiliar with the language. Adding an explanation of "occurrence" may help? Jul 18 '19 at 20:34
• The formal definition in the first paragraph is clear if you are used to such an academic style, though it might unnecessarily hard to grasp for users with no such background. It might be worthwhile to start with an informal explanation before giving the concise formal specification. E.g. something like this: Consider the following game: You are given a string like acabaccabc and need to defend it against attacks by marking some of its characters. An attack is a continuous substring of the given string, e.g. ca, and it is defended if one of the occurrences of the substring has (1/2) Jul 28 '19 at 18:43
• a character that was marked by you. In the given example, ca occurs two times and covers the characters at positions 1,2 and 6,7 (with positions starting with 0). Thus, at least one of the positions 1,2,6 or 7 would need to be marked to defend the attack. Your goal is to find a marking such that the string is defended against any attack. Furthermore, you want to mark as few characters as possible. (2/2) Jul 28 '19 at 18:43

Posted here

# Check If A Binary Search Tree Is Balanced

Now posted on CodeGolf.StackExchange.com:

Write The Shortest Program To Check If A Binary Tree Is Balanced

• Looks like a nice challenge! I think you should specify what representations of a binary tree are accepted. There is certainly the representation as a list (where the children of the entry at index r are at 2*r and 2*r+1), but there might also be other native representations. Aug 5 '19 at 6:44
• Now that you've posted this, please edit it to be only a link to the main challenge and then delete your post to help keep the Sandbox tidy. Thanks! Aug 7 '19 at 14:28

Posted to PPCG. Thx for the votes!

# Find the Lowest Common Ancestor of Two Nodes In A Binary Tree

Any two separate nodes in a binary tree have a common ancestor, which is the root of a binary tree. The lowest common ancestor(LCA) is thus defined as the node that is furthest from the root and that is the ancestor of the two nodes.

The following are binary trees and the lowest common ancestors of the some of their nodes.

The LCA of 13 and 15 is 14.

The LCA of 47 and 49 is 48.

The LCA of 4 and 45 is 40.

The LCA of 13 and 14 is 14.

Challenge

Write the shortest program possible that accepts as input the root of a binary tree and references to any two nodes in the binary tree. The program returns a reference to the LCA node of the two inputted nodes.

Restriction

The binary tree does not have nodes with parent field references in any form. You may not use parent field references in your program.

Input

The root of a binary tree and references to any two nodes in the binary tree. This may be in the form of a reference to the root object or even a list that is a valid representation of a binary tree.

Output

Returns a reference to the node that is the LCA of the two inputted nodes.

Definition of a Binary Tree

A tree is an object that contains a value and either two other trees or pointers to them.

The structure of the binary tree looks something like the following:

typedef struct T
{
struct T *l;
struct T *r;
int v;
}T;


If using a list representation for a binary tree, it may look something like the following:

[root_value, left_node, right_node]

• I mostly like your binary tree questions, but it would be nice if your definition was more inclusive of languages without the concept of pointers. Something like 'input will be a tree where a tree is either something containing a number and two other trees, or else something representing nothing. A tree must not be ambiguous'. This would include strings like [5[][7[6[][]][8[][]]], where a tree is [number tree tree] or [] as well as the usual standard binary tree structures and lists
– Jo King Mod
Aug 9 '19 at 6:45

# A Picture is Worth a Thousand Words

The average word is four characters/bytes long. So a picture, according to the old saying, is worth 4 kilobytes. But how complicated can that picture get?

Using any language or length of code you want, generate an image exactly 4,000 bytes in size with the highest possible entropy. You may use any standard image format. Your score will be the compression ratio when your image is compressed as a .zip file. For example, an image that compresses to 2,000 bytes will score 2, but if it expands to 4,100 bytes it will score 0.9756. Lowest score wins.

• What image formats are allowed? Aug 13 '19 at 2:41
• @wastl All of them. Aug 13 '19 at 3:19
• should the program output to a file or can it print a list of bytes? Aug 13 '19 at 12:20
• Interesting challenge. .zip is a file format, not a compression method, though. You'll need to specify what compression method is used -- DEFLATE, LZMA, etc. -- because that could drastically affect the scoring. Aug 13 '19 at 12:51
• @SuperStormer It shouldn't matter as long as you upload the image file. Aug 13 '19 at 15:03
• @AdmBorkBork How about gzip? I would specify version 1.6 because that's what I have. Aug 13 '19 at 15:04
• If you say something like gzip 1.6 using DEFLATE with default settings that should probably suffice. Aug 13 '19 at 15:58
• I didn't test this but I feel that any uncommon image format of raw data will have the optimal score (assume the compressor is smart enough to store without compression) Aug 16 '19 at 4:42

## Black Hole Evaporation

Background: In empty space, occurring all the time, there are pairs of particles that appear and then, usually soon after, cancel each other out. At a certain distance from a Black Hole, it's possible that a pair of particles might spontaneously appear and, if the distance is just right, one of the particles will fall into the Event Horizon while the other escapes off into space, never getting a chance to cancel each other out. The exit of those particles is Hawking Radiation. Eventually, this process causes the entire Black Hole to evaporate away.

Challenge

Given a string of numbers, emit the negative of the single digit number closest to, in front of, the infinity symbol ∞ (the event horizon), and continue emiting it until there are no digits left in front of the infinity symbol. Then print the rest of the string masking each character uniquely and different from the masked character (burst of energy that might take place at the final instant), excluding any infinity symbols.

Test cases

Input:                      Output:
111∞4567                    -1 -1 -1 jkl;
3∞                          -3
3-145∞962                   -5 -4 1 -3 ™©®
4-567-9∞1-23                 9 -7 -6 5 -4 😐😈😎😻
2-1∞                        1 -2
34∞67∞89                    -4 -3 9876
-123∞456                    -3 -2 1 ♫♪#
6-6∞6                       6 -6 ä
48120∞                      0 (or -0) -2 -1 -8 -4
---5∞555                    5 オゼヰ


Challenge rules

• Anything after the first infinity symbol can be ignored as it has fallen into the Black Hole and we know nothing more about it anyway until after evaporation, where the information is a bit scrambled.
• The code should only expect numbers, negations, and infinity symbols. No decimal points or other symbols. The input can be an array, list, etc..., but each character must be separate, including the negation and infinity characters.
• A negation only applies to the single digit immediately after it. See the test cases above.
• You may mask the remaining characters after evaporation with any character set of your choice, as long as the resulting character is different from the masked character, and each character in the resulting output is unique. For languages limited to smaller character sets (or type constraints), characters may repeat after each possible unique character has already been utilized in the output.
• You may use the '_' symbol instead of the '∞' symbol.

General rules

• This is , so shortest code in bytes in its respective language wins.

• Standard rules apply for your answer with default I/O rules, so you are allowed to use STDIN (with the specification above)/STDOUT, functions/method with the proper parameters and return-type, full programs.

• Default Loopholes are forbidden.
• I'm thinking of expanding this to all alpha numeric characters. Aug 7 '19 at 19:04
• What does it do if there is a single number after the infinity symbol? Aug 8 '19 at 6:53
• I'll update the rules and the test cases. Curiously, I already sort of demonstrated that case by having nothing after the infinity symbol. Nice catch. Aug 8 '19 at 6:55
• If 121 remains after evaporation, is reversing that string an acceptable scrambling?
Aug 8 '19 at 7:21
• Any particular reason to use ∞? Adding Unicode into the mix is an unnecessary complication for many languages. Maybe allow substituting _?
Aug 8 '19 at 7:22
• @Adam, "If 121 remains after evaporation, is reversing that string an acceptable scrambling" Absolutey! Our understanding of what happens to the information after Black Hole evaporation is still under much debate. It is entirely possible that the result of scrambling might result in the same information. We just don't know. Aug 8 '19 at 7:46
• @Adam, "Any particular reason to use ∞? Adding Unicode into the mix is an unnecessary complication for many languages." Yeah I thought about that. It can even be excluding factor, which I don't like. I really want that infinity symbol though.... I'll have a think about it, but any suggestions like yours are welcome. Aug 8 '19 at 7:49
• @Adam, "If 121 remains after evaporation, is reversing that string an acceptable scrambling?". After re-reading through the Information Paradox, I think the answer is 'no'. '121' should definitely end up different. I'll have to find some way to re-word the challenge, and I can be talked out of this. This requirement seemed rather innocent, but as I'm trying to code this up in my head, it can get complicated. Aug 8 '19 at 8:01
• The connection between the background and the task is not very clear. The comment about information suggests that you have a much clearer idea which you haven't included in the question. There's no need to overload the question with back story, but a link to a more detailed explanation might make it feel less arbitrary. Aug 8 '19 at 8:23
• @PeterTaylor, I've been mulling over your comment and I'm not sure what to include. Links to the Information Paradox or Hawking Radiation, or both? Aug 8 '19 at 8:41
• Probably both. AIUI, the infinity symbol represents the event horizon, the symbols to the left represent virtual pairs which become Hawking radiation, the symbols to the right represent the captured information, the emission of the inverted symbols to the left corresponds to the emission of Hawking radiation, and the shuffling of the symbols to the right corresponds to the information paradox. (I'm not sure without checking some reference why there's anything left to the right after evaporation). It would be good to sketch the correspondences and have some "Further reading". Aug 8 '19 at 8:46
• @PeterTaylor, Ok thanks. The final stage of evaporation might release a detectable packet of energy (which has yet to be seen or is even theoretically certain), so that's kind of what that represents. There might be something left over afterwords, or maybe not. When I get the chance, I'll edit some links in and parenthesize some metaphors in there as well. Aug 8 '19 at 8:55
• @ouflak Regarding ∞ you can use it but allow substitution. Btw, _ is the infinity symbol in the J language.
• @ouflak What if your language's infinity is inf? That's three symbols, so it complicates matters a lot. "use it efficiently" is unclear? Does that include causing higher byte count? How about just: You may use the _ symbol instead of ∞`.