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#!/usr/bin/env perl
use strict;
use warnings;
use feature qw<say switch>;
no warnings qw<experimental recursion>;
my $max=$ARGV[0]//23; #sane default without cmd line args
my @seqs=( ([]) x 20); #Initalise the largest 20 sequences found
for ( 1..$max) {
my $s=collaz([int($_)]);
for my $i (0..@seqs-1) {
if (@$s > @{$seqs[$i]}) {
pop @seqs unless @seqs < 20;
splice @seqs,$i,0,$s;
last;
}
}
};
say "Top 20 Collaz Sequence lengths for starting numbers 1..$max";
for(grep { @$_ != 0} @seqs) {
printf "Starting Number: %10d Sequence Length: %d\n", $_->[0],scalar @$_;
}
sub collaz {
my ($seq)=@_;
given ($seq->[-1]) {
when ($_%2 == 0) {
push @$seq, $_/2;
}
default {
push @$seq, 3*$_+1;
}
}
&collaz unless $seq->[-1]==1;
$seq;
}
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