[minor_nitwit]

that seems overly complicated for ruby if a is a file.

   c = []; a.lines{c << a.pos}

As far as $, you may know it from Regex as the new line indicator.

[geocar]

a is a mapped string. You could do:

    c=[];n=0;a.lines{|x|c<<(n+=x.size)-1}

but $"\n" wasn't special, and this allocates tons of memory. tinco's implementation is much closer to what k is actually doing.