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#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };
sub collatz {
my ($start) = @_;
my @seq = $start;
push @seq, ($seq[-1] / 2, 3 * $seq[-1] + 1)[$seq[-1] % 2]
while $seq[-1] != 1;
return @seq
}
my @sizes;
push @sizes, [$_, scalar collatz($_)] for 1 .. 1e6;
say "@$_" for (sort { $b->[1] <=> $a->[1] } @sizes)[0 .. 19];
__END__
use Test::More;
is_deeply [collatz(23)],
[23, 70, 35, 106, 53, 160, 80, 40, 20, 10, 5, 16, 8, 4, 2, 1];
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