From eb4d0e3f4515e5508fa9c0a695f7a45812a76296 Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Tue, 2 Oct 2007 19:28:19 -0700
Subject: [PATCH] optimize diffcore-delta by sorting hash entries.

Here's a test-patch. I don't guarantee anything, except that when I did
the timings I also did a "wc" on the result, and they matched..

Before:
	[torvalds@woody linux]$ time git diff -l0 --stat -C v2.6.22.. | wc
	   7104   28574  438020

	real    0m10.526s
	user    0m10.401s
	sys     0m0.136s

After:
	[torvalds@woody linux]$ time ~/git/git diff -l0 --stat -C v2.6.22.. | wc
	   7104   28574  438020

	real    0m8.876s
	user    0m8.761s
	sys     0m0.128s

but the diff is fairly simple, so if somebody will go over it and say
whether it's likely to be *correct* too, that 15% may well be worth it.

[ Side note, without rename detection, that diff takes just under three
  seconds for me, so in that sense the improvement to the rename detection
  itself is larger than the overall 15% - it brings the cost of just
  rename detection from 7.5s to 5.9s, which would be on the order of just
  over a 20% performance improvement. ]

Hmm. The patch depends on half-way subtle issues like the fact that the
hashtables are guaranteed to not be full => we're guaranteed to have zero
counts at the end => we don't need to do any steenking iterator count in
the loop. A few comments might in order.

		Linus
---
 diffcore-delta.c | 54 +++++++++++++++++++++++++++---------------------
 1 file changed, 30 insertions(+), 24 deletions(-)

diff --git a/diffcore-delta.c b/diffcore-delta.c
index d9729e5ec..e670f8512 100644
--- a/diffcore-delta.c
+++ b/diffcore-delta.c
@@ -46,22 +46,6 @@ struct spanhash_top {
 	struct spanhash data[FLEX_ARRAY];
 };
 
-static struct spanhash *spanhash_find(struct spanhash_top *top,
-				      unsigned int hashval)
-{
-	int sz = 1 << top->alloc_log2;
-	int bucket = hashval & (sz - 1);
-	while (1) {
-		struct spanhash *h = &(top->data[bucket++]);
-		if (!h->cnt)
-			return NULL;
-		if (h->hashval == hashval)
-			return h;
-		if (sz <= bucket)
-			bucket = 0;
-	}
-}
-
 static struct spanhash_top *spanhash_rehash(struct spanhash_top *orig)
 {
 	struct spanhash_top *new;
@@ -122,6 +106,20 @@ static struct spanhash_top *add_spanhash(struct spanhash_top *top,
 	}
 }
 
+static int spanhash_cmp(const void *a_, const void *b_)
+{
+	const struct spanhash *a = a_;
+	const struct spanhash *b = b_;
+
+	/* A count of zero compares at the end.. */
+	if (!a->cnt)
+		return !b->cnt ? 0 : 1;
+	if (!b->cnt)
+		return -1;
+	return a->hashval < b->hashval ? -1 :
+		a->hashval > b->hashval ? 1 : 0;
+}
+
 static struct spanhash_top *hash_chars(struct diff_filespec *one)
 {
 	int i, n;
@@ -158,6 +156,10 @@ static struct spanhash_top *hash_chars(struct diff_filespec *one)
 		n = 0;
 		accum1 = accum2 = 0;
 	}
+	qsort(hash->data,
+		1ul << hash->alloc_log2,
+		sizeof(hash->data[0]),
+		spanhash_cmp);
 	return hash;
 }
 
@@ -169,7 +171,7 @@ int diffcore_count_changes(struct diff_filespec *src,
 			   unsigned long *src_copied,
 			   unsigned long *literal_added)
 {
-	int i, ssz;
+	struct spanhash *s, *d;
 	struct spanhash_top *src_count, *dst_count;
 	unsigned long sc, la;
 
@@ -190,22 +192,26 @@ int diffcore_count_changes(struct diff_filespec *src,
 	}
 	sc = la = 0;
 
-	ssz = 1 << src_count->alloc_log2;
-	for (i = 0; i < ssz; i++) {
-		struct spanhash *s = &(src_count->data[i]);
-		struct spanhash *d;
+	s = src_count->data;
+	d = dst_count->data;
+	for (;;) {
 		unsigned dst_cnt, src_cnt;
 		if (!s->cnt)
-			continue;
+			break; /* we checked all in src */
+		while (d->cnt) {
+			if (d->hashval >= s->hashval)
+				break;
+			d++;
+		}
 		src_cnt = s->cnt;
-		d = spanhash_find(dst_count, s->hashval);
-		dst_cnt = d ? d->cnt : 0;
+		dst_cnt = d->hashval == s->hashval ? d->cnt : 0;
 		if (src_cnt < dst_cnt) {
 			la += dst_cnt - src_cnt;
 			sc += src_cnt;
 		}
 		else
 			sc += dst_cnt;
+		s++;
 	}
 
 	if (!src_count_p)
-- 
2.26.2