#! perl -slw
use strict;
use Time::HiRes qw[ time ];

my $start = time;

my @res;
for ( 1 .. 1e6 ) {
    my $c = 0;
    my $n = $_;
    while( $n > 1 ) {
        ++$c;
        $n = $n&1 ? 3*$n+1 : $n/2;
        if( defined $res[ $n ] ) {
            $c += $res[ $n ];
            last;
        }
    }
    $res[ $_ ] = $c;
}

printf "Took %.6f seconds\n", time() - $start;

my( $n, $i ) = 0;
$n < $res[ $_ ] and ( $n, $i ) = ( $res[ $_ ], $_ )
    for 1 .. $#res;

print "longest sequence is $n steps starting with $i";

__END__
c:\test>collatz.pl
Took 2.000000 seconds
longest sequence is 524 steps starting with 837799