makestuff.pl

#!perl

use strict;
use warnings;

# key insights:
#
# * things having to happen "simultaneously" is misleading. Because we only add
#   into the middle each time, you can start with each pair, work out what
#   happens there, and then move on to the next one
#
# * we only need to figure out how many of each letter is created, we don't
#   need to build the sequence
#
# * we are traversing a graph. OV -> V actually means OV -> ( OV, VV ) + add one
#   point to the V count
#

my $debug = 1;

my %starting_counts = ( N => 2, C => 1, B => 1 );

# This is our core data structure. We have a pair, and we have the number of
# iterations the pair started at
my @sites = ( [ NN => 0 ], [ NC => 0 ], [ CB => 0 ] );

my %dictionary =    # CH -> B becomes CH -> ( CB, BH, B )
  map {
    my ( $from, $to ) = split( / \-> /, $_ );
    my $left  = substr( $from, 0, 1 ) . $to;
    my $right = $to . substr( $from, 1, 1 );
    $from => [ $left, $right, $to ];
  } split(
    /\n/,
    "CH -> B
HH -> N
CB -> H
NH -> C
HB -> C
HC -> B
HN -> C
NN -> C
BH -> H
NC -> B
NB -> B
BN -> B
BB -> N
BC -> B
CC -> N
CN -> C"
  );

# Take an element from the front of the sites
while ( my $head = shift(@sites) ) {
    my ( $site, $iteration ) = @$head;
    print "Head is [$site] it[$iteration], stack-size[" . @sites . "]\n"
      if $debug;

    # If it's at 40, then we discard it
    if ( $iteration == 40 ) {
        print "\tDiscarding at 40\n" if $debug;
        next;
    }

    # Is it an active site? If not, discard it
    my $reaction = $dictionary{$site};
    unless ($reaction) {
        print "\tSite [$site] is unreactive\n" if $debug;
    }

    my ( $left_site, $right_site, $product ) = @$reaction;
    print "\tSite [$site] makes [$product], so [$left_site],[$right_site]\n"
      if $debug;

    # Count the new thing we made
    $starting_counts{$product}++;

    # Add the newly-created sites to the front of the stack
    unshift( @sites, [ $right_site, $iteration + 1 ] );
    unshift( @sites, [ $left_site,  $iteration + 1 ] );
}

use Data::Printer;
p %starting_counts;