Re: Horrible re-packing?

From: Linus Torvalds <torvalds@osdl.org>
Date: Jun 5, 2006, 19:03 UTC
Message-ID: <Pine.LNX.4.64.0606051155000.5498@g5.osdl.org>
In-Reply-To: <Pine.LNX.4.64.0606051140530.5498@g5.osdl.org>

On this same thread..

This trivial patch not only simplifies the name hashing, it actually improves packing for both git and the kernel.

The git archive pack shrinks from 6824090->6622627 bytes (a 3% improvement), and the kernel pack shrinks from 108756213 to 108219021 (a mere 0.5% improvement, but still, it's an improvement from making the hashing much simpler!)

I think the hash function with its comment is self-explanatory:

        /*
         * 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);
        }
        return hash;

ie we just create a 32-bit hash, where we "age" previous characters by two bits, so the last characters in a filename count most. So when we then compare the hashes in the sort routine, filenames that end the same way sort the same way.

It takes the subdirectory into account (unless the filename is > 16 characters), but files with the same name within the same subdirectory will obviously sort closer than files in different subdirectories.

And, incidentally (which is why I tried the hash change in the first place, of course) builtin-rev-list.c will sort fairly close to rev-list.c.

And no, it's not a "good hash" in the sense of being secure or unique, but that's not what we're looking for. The whole "hash" thing is misnamed here. It's not so much a hash as a "sorting number".

Comments?

		Linus

----
 pack-objects.c |   41 +++++++++++------------------------------
 1 files changed, 11 insertions(+), 30 deletions(-)

diff --git a/pack-objects.c b/pack-objects.c
index 3590cd5..ae49fba 100644
--- a/pack-objects.c
+++ b/pack-objects.c
@@ -473,38 +473,19 @@ #define DIRBITS 12
 
 static unsigned name_hash(struct name_path *path, const char *name)
 {
-	struct name_path *p = path;
-	const char *n = name + strlen(name);
-	unsigned hash = 0, name_hash = 0, name_done = 0;
-
-	if (n != name && n[-1] == '\n')
-		n--;
-	while (name <= --n) {
-		unsigned char c = *n;
-		if (c == '/' && !name_done) {
-			name_hash = hash;
-			name_done = 1;
-			hash = 0;
-		}
-		hash = hash * 11 + c;
-	}
-	if (!name_done) {
-		name_hash = hash;
-		hash = 0;
-	}
-	for (p = path; p; p = p->up) {
-		hash = hash * 11 + '/';
-		n = p->elem + p->len;
-		while (p->elem <= --n) {
-			unsigned char c = *n;
-			hash = hash * 11 + c;
-		}
-	}
+	unsigned char c;
+	unsigned hash = 0;
+
 	/*
-	 * Make sure "Makefile" and "t/Makefile" are hashed separately
-	 * but close enough.
+	 * 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.
 	 */
-	hash = (name_hash<<DIRBITS) | (hash & ((1U<<DIRBITS )-1));
+	while ((c = *name++) != 0) {
+		if (isspace(c))
+			continue;
+		hash = (hash >> 2) + (c << 24);
+	}
 	return hash;
 }