You are right, I didn't do the math on paper originally, and due to false laziness, lack of an existing encode() or motivation to write one made me not verify my code with different data, even though I knew that might come back and bite me in the tender parts.

I fixed the code; this time, no comments, since I don't know how to explain it briefly. I guess it warrants a longer comment in front of the subroutine containing the math.

sub decode3 {
    my ($diff, $delta, $next);
    $diff = $_[0];
    $delta = -$_[0];
    $diff -= $delta += $_ for @_;
    $_[0] = $diff/@_;
    return map { $next += $_ } @_;
}

[download]

Your tests verify this one, and the benchmark on my 5.6.1 says it is consistently about 42% faster than the for version and 35% faster than the map one. The main win was removing the intermediary @results. If I put that step back in, it only beats the vanilla versions by 12% and 8%, respectively, which means you can speed up your own versions by 25% if you omit the temporary array.

The rest of the speed difference is mainly because I avoid a couple off-by-one corrections by not shifting off the total. It is debatable whether the loss of clarity (or the requirement for comments..) is worth the speed gains. Some profiling would be in order to determine the impact of decode()'s speed on overall program perforamce.

Makeshifts last the longest.

In reply to Re^3: Perl Drag Racing - sum/delta list by Aristotle
in thread Perl Drag Racing - sum/delta list by John M. Dlugosz

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