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 5 SOLUTIONS - 🖨️ (programming.dev)

submitted 3 weeks ago by [email protected] to c/[email protected]

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Day 5: Print Queue

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[–] [email protected] 3 points 2 weeks ago

J

This is a problem where J's biases lead one to a very different solution from most of the others. The natural representation of a directed graph in J is an adjacency matrix, and sorting is specified in terms of a permutation to apply rather than in terms of a comparator: x /: y (respectively x \: y) determines the permutation that would put y in ascending (descending) order, then applies that permutation to x.

data_file_name =: '5.data'
lines =: cutopen fread data_file_name
NB. manuals start with the first line where the index of a comma is &lt; 5
start_of_manuals =: 1 i.~ 5 > ',' i.~"1 > lines
NB. ". can't parse the | so replace it with a space
edges =: ". (' ' &amp; (2}))"1 > start_of_manuals {. lines
NB. don't unbox and parse yet because they aren't all the same length
manuals =: start_of_manuals }. lines
max_page =: >./ , edges
NB. adjacency matrix of the page partial ordering; e.i. makes identity matrix
adjacency =: 1 (&lt; edges)} e. i. >: max_page
NB. ordered line is true if line is ordered according to the adjacency matrix
ordered =: monad define
   pages =. ". > y
   NB. index pairs 0 &lt;: i &lt; j &lt; n; box and raze to avoid array fill
   page_pairs =. ; (&lt; @: (,~"0 i.)"0) i. # pages
   */ adjacency {~ &lt;"1 pages {~ page_pairs
)
midpoint =: ({~ (&lt;. @: -: @: #)) @: ". @: >
result1 =: +/ (ordered"0 * midpoint"0) manuals

NB. toposort line yields the pages of line topologically sorted by adjacency
NB. this is *not* a general topological sort but works for our restricted case:
NB. we know that each individual manual will be totally ordered
toposort =: monad define
   pages =. ". > y
   NB. for each page, count the pages which come after it, then sort descending
   pages \: +/"1 adjacency {~ &lt;"1 pages ,"0/ pages
)
NB. midpoint2 doesn't parse, but does remove trailing zeroes
midpoint2 =: ({~ (&lt;. @: -: @: #)) @: ({.~ (i. &amp; 0))
result2 =: +/ (1 - ordered"0 manuals) * midpoint2"1 toposort"0 manuals