This is very interesting. Your version rates at the same speed as moritz's and avar's once the bug is fixed - it's 0<, not -1<.

One thing that is odd is that the specific problem I'm working has all of the chr(0)'s in groups of 3. So, I figured that I could use that and change the primary line to:

vec( $$s1, $pos, 24 ) ||= vec( $$s2, $pos, 24 )
  while 0 < ( $pos = index $$s1, chr(0)x3, $pos );
[download]

Except, that slows it down by 20%. If I change it so that the 24's stay, but it goes back to being chr(0) without the x3, it's back to being the same speed. I wonder why that is. I also wonder why the knowledge of being able to work 3 bytes at a time doesn't speed things up at all.

As for why this wasn't in the problem statement - I wanted to solve the general problem and was willing to pay a meter of beer to see the various solutions. That there's an additional constraint in what I'm actually using the solutions for doesn't change what I was willing to pay a meter of beer for. :-)

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