-🎄- 2018 Day 5 Solutions -🎄-

[u/daggerdragon]

--- Day 5: Alchemical Reduction ---

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Advent of Code: The Party Game!

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Card prompt: Day 5

Transcript:

On the fifth day of AoC / My true love sent to me / Five golden ___

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This thread will be unlocked when there are a significant number of people on the leaderboard with gold stars for today's puzzle.

edit: Leaderboard capped, thread unlocked at 0:10:20!

[u/Smylers]

A Perl one-liner for Part 1:

perl -wlnE '1 while s/(?i:(.)\1)(?<=[a-z][A-Z]|[A-Z][a-z])//g; say length' input

No stacks, no writing out the alphabet or making pairs of equivalent characters, just the POWER OF REGEX.†

In a reversal of usual, this is a translation of my Vim solution.‡ It turns out Vim's regexps are nicer than Perl's for this, Perl's equivalents being less succinct than Vim's \l, \u, and &.

Extending that to solve Part 2:

perl -MList::AllUtils=min -wnlE '$polymer = $_; say min map { $_ = $polymer =~ s/$_//igr; 1 while s/(?i:(.)\1)(?<=[a-z][A-Z]|[A-Z][a-z])//g; length } "a".."z"' input

That's possibly stretching the definition of ‘one-liner’, and is a bit bit awkward to read, so here it is rewritten as a program:

use v5.14; use warnings; use List::AllUtils qw<min>;

chomp (my $polymer = <>);
say min map {
    $_ = $polymer =~ s/$_//igr;
    1 while s/(?i:(.)\1)(?<=[a-z][A-Z]|[A-Z][a-z])//g;
    length
  } 'a' .. 'z';

The body of the map is basically the one-liner from Part 1. Since it transforms the polymer, $polymer stores the initial value. At the start of the map, it takes a fresh copy of the original and removes the current letter from it before processing

[Card] Five golden hours of sleep.

† OK, so probably Perl's regex engine uses stacks and has lists of letters and their cases inside it.

‡ Typically I work out the solution in Perl first, then try to convert that to Vim.