=begin pod

Algorithm::DawkinsWeasel is a simple model illustrating the idea of cumulative
selection in evolution.

The original form of it looked like this:

  1. Start with a random string of 28 characters.
  2. Make 100 copies of this string, with a 5% chance per character of that
     character being replaced with a random character.
  3. Compare each new string with "METHINKS IT IS LIKE A WEASEL", and give
     each a score (the number of letters in the string that are correct and
     in the correct position).
  4. If any of the new strings has a perfect score (== 28), halt.
  5. Otherwise, take the highest scoring string, and go to step 2

This module parametrizes the target string, mutation threshold, and number of
copies per round.

The goal is to be able to use it something like this:

  my $weasel = Algorithm::DawkinsWeasel.new;
  say $weasel.current-phrase while $weasel;

=end pod
unit class Algorithm::DawkinsWeasel:ver<0.0.2> does Iterator;

has Str @.target-phrase;
has Rat $.mutation-threshold;
has Int $.copies;

has Str @!charset;
has Int $!count;
has Str @!current-phrase;
has Int $!hi-score;
submethod BUILD(Str :$target-phrase = 'METHINKS IT IS LIKE A WEASEL',
Rat :$mutation-threshold = 0.05, Int :$copies = 100) {
    @!target-phrase = $target-phrase.comb;
    $!mutation-threshold = $mutation-threshold;
    $!copies = $copies;
}

submethod TWEAK {
    @!charset  = | ['A' .. 'Z'] , ' ';
    @!current-phrase = map { @!charset.pick }, 0 .. @!target-phrase.end;
    $!hi-score = 0;
    $!count = 0;
}

method evolve {
    for (1 .. $!copies) {
        my @trial = map {
            1.rand < $!mutation-threshold ?? @!charset.pick !! $_;
        }, @!current-phrase;
        
        my Int $score = 0;
        for 0 .. @!target-phrase.end -> $i {
            if @trial[$i] eq @!target-phrase[$i] {
                $score++;
            }
        }

        if $score > $!hi-score {
            $!hi-score = $score;
            @!current-phrase = @trial;
        }
    }
}

method pull-one {
    $!count++;

    self.evolve;
    return $!hi-score == @!target-phrase.elems ?? IterationEnd !! self;
}