Linear time list operations

[haampie]

Avoids string concatenation overhead in extend and preextend

"tricks" used:

• Build a local temporary string instead of extending a big string one item at a time (quadratic complexity)
• When prefix not set: use set -- and "$*" with different IFS for fast concatenation, presumably linear complexity.
• When prefix is set: shift the first element, followed by a tight loop (still quadratic complexity).
• Eliminate the array reversal in preextend

In the included benchmark, runtime goes from 100ms to 21ms on my macbook.

[sethrj]

Very nice! Good testing and clever scripting.

[sethrj]

Is there a nontrivial performance penalty for merging these and the similar lines at 135 into a single function, e.g. one that uses printf and is called with _ext_str="$(merge "$_dst" "$sep" "$_prefix" "$@")"? Hiding the symbol-filled code (or at least making clear that it's an exact duplicate) woudl be good.

[haampie]

Yeah, this is a bit dense and repetitive... So far we've tried to avoid printf, or rather subshells "$(printf ...)". I'm not sure if it can be improved without subshells.

[kwryankrattiger]

Looks good, the expected/result are consistent for all of the tests look correct.

I ran all of the test and the benchmark before and after these changes and found one bug in the old preextend command that was double prepending the PREFIX. The speed up for the benchmark as about 4x which is pretty good.