[PATCH v2 1/8] pack-objects: create new name-hash function version

From: Jonathan Tan <jonathantanmy@google.com>

As we will explore in later changes, the default name-hash function used in 'git pack-objects' has a tendency to cause collisions and cause poor delta selection. This change creates an alternative that avoids some collisions while preserving some amount of hash locality.

The pack_name_hash() method has not been materially changed since it was introduced in ce0bd64 (pack-objects: improve path grouping heuristics., 2006-06-05). The intention here is to group objects by path name, but also attempt to group similar file types together by making the most-significant digits of the hash be focused on the final characters.

Here's the crux of the implementation:

	/*
	 * This effectively just creates a sortable number from the
	 * last sixteen non-whitespace characters. Last characters
	 * count "most", so things that end in ".c" sort together.
	 */
	while ((c = *name++) != 0) {
		if (isspace(c))
			continue;
		hash = (hash >> 2) + (c << 24);
	}

As the comment mentions, this only cares about the last sixteen non-whitespace characters. This cause some filenames to collide more than others. This collision is somewhat by design in order to promote hash locality for files that have similar types (.c, .h, .json) or could be the same file across a directory rename (a/foo.txt to b/foo.txt). This leads to decent cross-path deltas in cases like shallow clones or packing a repository with very few historical versions of files that share common data with other similarly-named files.

However, when the name-hash instead leads to a large number of name-hash collisions for otherwise unrelated files, this can lead to confusing the delta calculation to prefer cross-path deltas over previous versions of the same file.

The new pack_name_hash_v2() function attempts to fix this issue by taking more of the directory path into account through its hash function. Its naming implies that we will later wire up details for choosing a name-hash function by version.

The first change is to be more careful about paths using non-ASCII characters. With these characters in mind, reverse the bits in the byte as the least-significant bits have the highest entropy and we want to maximize their influence. This is done with some bit manipulation that swaps the two halves, then the quarters within those halves, and then the bits within those quarters.

The second change is to perform hash composition operations at every level of the path. This is done by storing a 'base' hash value that contains the hash of the parent directory. When reaching a directory boundary, we XOR the current level's name-hash value with a downshift of the previous level's hash. This perturbation intends to create low-bit distinctions for paths with the same final 16 bytes but distinct parent directory structures.

The collision rate and effectiveness of this hash function will be explored in later changes as the function is integrated with 'git pack-objects' and 'git repack'.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Derrick Stolee <stolee@gmail.com>
---
 pack-objects.h | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)