Advent Of Code

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Advent of Code is an annual Advent calendar of small programming puzzles for a variety of skill sets and skill levels that can be solved in any programming language you like.

AoC 2024

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🪐 - 2024 DAY 11 SOLUTIONS -🪐 (programming.dev)

submitted 2 weeks ago by CameronDev@programming.dev to c/advent_of_code@programming.dev

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Day 11: Plutonian Pebbles

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[–] ystael@beehaw.org 2 points 2 weeks ago

J

If one line of code needs five lines of comment, I'm not sure how much of an improvement the "expressive power" is! But I learned how to use J's group-by operator (/. or /..) and a trick with evoke gerund (`:0"1) to transform columns of a matrix separately. It might have been simpler to transpose and apply to rows.

data_file_name =: '11.data'
data =: ". > cutopen fread data_file_name
NB. split splits an even digit positive integer into left digits and right digits
split =: ; @: ((10 &amp; #.) &amp;.>) @: (({.~ ; }.~) (-: @: #)) @: (10 &amp; #.^:_1)
NB. step consumes a single number and yields the boxed count-matrix of acting on that number
step =: monad define
   if. y = 0 do. &lt; 1 1
   elseif. 2 | &lt;. 10 ^. y do. &lt; (split y) ,. 1 1
   else. &lt; (y * 2024), 1 end.
)
NB. reduce_count_matrix consumes an unboxed count-matrix of shape n 2, left column being
NB. the item and right being the count of that item, and reduces it so that each item
NB. appears once and the counts are summed; it does not sort the items. Result is unboxed.
NB. Read the vocabulary page for /.. to understand the grouped matrix ;/.. builds; the
NB. gerund evoke `:0"1 then sums under boxing in the right coordinate of each row.
reduce_count_matrix =: > @: (({. ` ((+/&amp;.>) @: {:)) `:0"1) @: ({. ;/.. {:) @: |:
initial_count_matrix =: reduce_count_matrix data ,. (# data) $ 1
NB. iterate consumes a count matrix and yields the result of stepping once across that
NB. count matrix. There's a lot going on here. On rows (item, count) of the incoming count
NB. matrix, (step @: {.) yields the (boxed count matrix) result of step item;
NB. (&lt; @: (1&amp;,) @: {:) yields &lt;(1, count); then *"1&amp;.> multiplies those at rank 1 under
NB. boxing. Finally raze and reduce.
iterate =: reduce_count_matrix @: ; @: (((step @: {.) (*"1&amp;.>) (&lt; @: (1&amp;,) @: {:))"1)
count_pebbles =: +/ @: ({:"1)
result1 =: count_pebbles iterate^:25 initial_count_matrix
result2 =: count_pebbles iterate^:75 initial_count_matrix