Re: [PATCH v2 10/10] diffcore-rename: use directory rename guided basename comparisons

[Derrick Stolee <stolee@gmail.com>]

On 2/23/2021 6:44 PM, Elijah Newren via GitGitGadget wrote:
> For the testcases mentioned in commit 557ac0350d ("merge-ort: begin
> performance work; instrument with trace2_region_* calls", 2020-10-28),
> this change improves the performance as follows:
> 
>                             Before                  After
>     no-renames:       12.775 s ±  0.062 s    12.596 s ±  0.061 s
>     mega-renames:    188.754 s ±  0.284 s   130.465 s ±  0.259 s
>     just-one-mega:     5.599 s ±  0.019 s     3.958 s ±  0.010 s

Hooray!

> +	for (i = 0; i < rename_src_nr; ++i) {
> +		char *filename = rename_src[i].p->one->path;
> +		const char *base = NULL;
> +		intptr_t src_index;
>  		intptr_t dst_index;
>
> +		/* Is this basename unique among remaining sources? */

This comment sent me down a confusing direction. Perhaps, we can
instead say:

	/*
	 * If the basename is unique among remaining sources, then
	 * src_index will equal 'i' and we can attempt to match it
	 * to a unique basename in the destinations. Otherwise, use
	 * directory rename heuristics, if possible.
	 */

> +		base = get_basename(filename);
> +		src_index = strintmap_get(&sources, base);
> +		assert(src_index == -1 || src_index == i);
> +
> +		if (strintmap_contains(&dests, base)) {
>  			struct diff_filespec *one, *two;
>  			int score;
>  
> +			/* Find a matching destination, if possible */
> +			dst_index = strintmap_get(&dests, base);
> +			if (src_index == -1 || dst_index == -1) {
> +				src_index = i;
> +				dst_index = idx_possible_rename(filename, info);
> +			}

It is important that 'src_index == i' from this point on, no
matter whether it was unique or not.

> +			if (dst_index == -1)
> +				continue;
> +
> +			/* Ignore this dest if already used in a rename */
> +			if (rename_dst[dst_index].is_rename)
> +				continue; /* already used previously */
> +

This seems to match all of the complicated special cases.

Thanks,
-Stolee