Suppose you have a list of numbers, presumably closely grouped. Instead of listing the numbers, you can list the total followed by the delta between each item and the previous (no delta listed for the first item).

It's a slight bit of algebra to figure out how to get the original list back, and I illustrated it with a Perl function. I used a couple foreach loops rather than map, because I wanted to make the math clear to non-Perl programmers.

Then I benchmarked it with a map instead, and found about 8% speedup.

So, how else might this function be improved, in terms of speed? I don't think it's possible without making two passes through the data. But, just like someone who doesn't know how to map is missing something, I wonder if there are any other techniques I can employ here.

sub decode2
 {
 my $total= shift;
 my $delta= 0;
 my $diff= 0;
 foreach (@_) {
    $delta += $_;
    $diff += $delta;
    }
 my $original= ($total-$diff)/(1+scalar @_);
 my $value= $original;
 my @results= map {
    $value += $_;
    } @_;
 return ($original, @results);
 }

[download]
Test vector: input: 5028 1 -21 44 -1
output: 1000 1001 980 1024 1023

In reply to Perl Drag Racing - sum/delta list by John M. Dlugosz

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