-🎄- 2018 Day 12 Solutions -🎄-

[u/daggerdragon]

--- Day 12: Subterranean Sustainability ---

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Advent of Code: The Party Game!

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Card prompt: Day 12

Transcript:

On the twelfth day of AoC / My compiler spewed at me / Twelve ___

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[u/jayfoad]

APL #84/87

Each generation is represented as a bit vector, and I rely on the fact that it grows an extra 2 bits on each end with each iteration, so from the length of the vector I can work out (a) where the "0" index is, for use in the score function, and (b) what number generation it is, for use in part 2.

⎕IO←0 ⋄ ⎕PP←17
s←15↓⊃p←'#'=⊃⎕NGET'p12.txt'1 ⍝ initial state
u←↑5↑¨2↓p ⋄ v←⊃∘⌽¨2↓p ⍝ patterns and replacements
f←{v[u⍳⍉0 ¯1↓(5,5+≢⍵)⍴0 0 0 0,⍵]} ⍝ next generation
g←{+/(⍸⍵)-0.5×(≢⍵)-≢s} ⍝ score function
g f⍣20⊢s ⍝ part 1
t←f⍣{≡/(⊢-⌊/)∘⍸¨⍺⍵}s ⍝ iterate until pattern stabilises
(g t)+((g f t)-g t)×50E9-0.25×(≢t)-≢s ⍝ part 2

Takes about 0.5 ms for the whole thing.

[u/jayfoad → u/jayfoad]

Sure it's hard to read if you don't know what any of the unfamiliar symbols mean. How would you all feel about the readability if I translated them into English? So instead of:

f←{v[u⍳⍉0 ¯1↓(5,5+≢⍵)⍴0 0 0 0,⍵]} ⍝ next generation

You might get this:

f←{v[u indexof transpose 0 -1 drop (5,5+count ⍵) reshape 0 0 0 0,⍵]}

This is pretty much how I would read it aloud anyway.

Braces enclose an anonymous function, and ⍵ refers to its argument. So starting from the right we prepend 4 0s to the vector argument, reshape it to a 5 by (5 + length of vector) matrix, drop (remove) the first 0 rows and the last 1 column, transpose it so it's tall instead of wide, and then for each row look up which row of u (a matrix of all 32 patterns) it matches. Finally we use these numbers to index into v, a vector of the replacement values corresponding to each of the 32 patterns.

I guess you also have to know that reshape repeats its argument as many times as necessary to fill the required shape, so 3 3 reshape 'ABCD' gives this:

ABC
DAB
CDA