sha1_lookup: drop sha1_entry_pos function

This was pulled over from git.git, and is an experiment in
making binary-searching lists of sha1s faster. It was never
compiled by default (nor was it used upstream by default
without a special environment variable).

Unfortunately, it is actually slower in practice, and
upstream is planning to drop it in
git/git@f1068ef (which has
some timing results). It's worth doing the same here for
simplicity.

Author: peff

src/pack.c

@@ -1316,11 +1316,7 @@ static int pack_entry_find_offset(
 		short_oid->id[0], short_oid->id[1], short_oid->id[2], lo, hi, p->num_objects);
 #endif
 
-#ifdef GIT_USE_LOOKUP
-	pos = sha1_entry_pos(index, stride, 0, lo, hi, p->num_objects, short_oid->id);
-#else
 	pos = sha1_position(index, stride, lo, hi, short_oid->id);
-#endif
 
 	if (pos >= 0) {
 		/* An object matching exactly the oid was found */

src/sha1_lookup.c

@@ -11,220 +11,6 @@
 #include "common.h"
 #include "oid.h"
 
-/*
- * Conventional binary search loop looks like this:
- *
- *	unsigned lo, hi;
- *		do {
- *				unsigned mi = (lo + hi) / 2;
- *				int cmp = "entry pointed at by mi" minus "target";
- *				if (!cmp)
- *						return (mi is the wanted one)
- *				if (cmp > 0)
- *						hi = mi; "mi is larger than target"
- *				else
- *						lo = mi+1; "mi is smaller than target"
- *		} while (lo < hi);
- *
- * The invariants are:
- *
- * - When entering the loop, lo points at a slot that is never
- *	above the target (it could be at the target), hi points at a
- *	slot that is guaranteed to be above the target (it can never
- *	be at the target).
- *
- * - We find a point 'mi' between lo and hi (mi could be the same
- *	as lo, but never can be as same as hi), and check if it hits
- *	the target. There are three cases:
- *
- *	- if it is a hit, we are happy.
- *
- *	- if it is strictly higher than the target, we set it to hi,
- *		and repeat the search.
- *
- *	- if it is strictly lower than the target, we update lo to
- *		one slot after it, because we allow lo to be at the target.
- *
- *	If the loop exits, there is no matching entry.
- *
- * When choosing 'mi', we do not have to take the "middle" but
- * anywhere in between lo and hi, as long as lo <= mi < hi is
- * satisfied. When we somehow know that the distance between the
- * target and lo is much shorter than the target and hi, we could
- * pick mi that is much closer to lo than the midway.
- *
- * Now, we can take advantage of the fact that SHA-1 is a good hash
- * function, and as long as there are enough entries in the table, we
- * can expect uniform distribution. An entry that begins with for
- * example "deadbeef..." is much likely to appear much later than in
- * the midway of the table. It can reasonably be expected to be near
- * 87% (222/256) from the top of the table.
- *
- * However, we do not want to pick "mi" too precisely. If the entry at
- * the 87% in the above example turns out to be higher than the target
- * we are looking for, we would end up narrowing the search space down
- * only by 13%, instead of 50% we would get if we did a simple binary
- * search. So we would want to hedge our bets by being less aggressive.
- *
- * The table at "table" holds at least "nr" entries of "elem_size"
- * bytes each. Each entry has the SHA-1 key at "key_offset". The
- * table is sorted by the SHA-1 key of the entries. The caller wants
- * to find the entry with "key", and knows that the entry at "lo" is
- * not higher than the entry it is looking for, and that the entry at
- * "hi" is higher than the entry it is looking for.
- */
-int sha1_entry_pos(const void *table,
-			size_t elem_size,
-			size_t key_offset,
-			unsigned lo, unsigned hi, unsigned nr,
-			const unsigned char *key)
-{
-	const unsigned char *base = (const unsigned char*)table;
-	const unsigned char *hi_key, *lo_key;
-	unsigned ofs_0;
-
-	if (!nr || lo >= hi)
-		return -1;
-
-	if (nr == hi)
-		hi_key = NULL;
-	else
-		hi_key = base + elem_size * hi + key_offset;
-	lo_key = base + elem_size * lo + key_offset;
-
-	ofs_0 = 0;
-	do {
-		int cmp;
-		unsigned ofs, mi, range;
-		unsigned lov, hiv, kyv;
-		const unsigned char *mi_key;
-
-		range = hi - lo;
-		if (hi_key) {
-			for (ofs = ofs_0; ofs < 20; ofs++)
-				if (lo_key[ofs] != hi_key[ofs])
-					break;
-			ofs_0 = ofs;
-			/*
-			 * byte 0 thru (ofs-1) are the same between
-			 * lo and hi; ofs is the first byte that is
-			 * different.
-			 *
-			 * If ofs==20, then no bytes are different,
-			 * meaning we have entries with duplicate
-			 * keys. We know that we are in a solid run
-			 * of this entry (because the entries are
-			 * sorted, and our lo and hi are the same,
-			 * there can be nothing but this single key
-			 * in between). So we can stop the search.
-			 * Either one of these entries is it (and
-			 * we do not care which), or we do not have
-			 * it.
-			 *
-			 * Furthermore, we know that one of our
-			 * endpoints must be the edge of the run of
-			 * duplicates. For example, given this
-			 * sequence:
-			 *
-			 *     idx 0 1 2 3 4 5
-			 *     key A C C C C D
-			 *
-			 * If we are searching for "B", we might
-			 * hit the duplicate run at lo=1, hi=3
-			 * (e.g., by first mi=3, then mi=0). But we
-			 * can never have lo > 1, because B < C.
-			 * That is, if our key is less than the
-			 * run, we know that "lo" is the edge, but
-			 * we can say nothing of "hi". Similarly,
-			 * if our key is greater than the run, we
-			 * know that "hi" is the edge, but we can
-			 * say nothing of "lo".
-			 *
-			 * Therefore if we do not find it, we also
-			 * know where it would go if it did exist:
-			 * just on the far side of the edge that we
-			 * know about.
-			 */
-			if (ofs == 20) {
-				mi = lo;
-				mi_key = base + elem_size * mi + key_offset;
-				cmp = memcmp(mi_key, key, 20);
-				if (!cmp)
-					return mi;
-				if (cmp < 0)
-					return -1 - hi;
-				else
-					return -1 - lo;
-			}
-
-			hiv = hi_key[ofs_0];
-			if (ofs_0 < 19)
-				hiv = (hiv << 8) | hi_key[ofs_0+1];
-		} else {
-			hiv = 256;
-			if (ofs_0 < 19)
-				hiv <<= 8;
-		}
-		lov = lo_key[ofs_0];
-		kyv = key[ofs_0];
-		if (ofs_0 < 19) {
-			lov = (lov << 8) | lo_key[ofs_0+1];
-			kyv = (kyv << 8) | key[ofs_0+1];
-		}
-		assert(lov < hiv);
-
-		if (kyv < lov)
-			return -1 - lo;
-		if (hiv < kyv)
-			return -1 - hi;
-
-		/*
-		 * Even if we know the target is much closer to 'hi'
-		 * than 'lo', if we pick too precisely and overshoot
-		 * (e.g. when we know 'mi' is closer to 'hi' than to
-		 * 'lo', pick 'mi' that is higher than the target), we
-		 * end up narrowing the search space by a smaller
-		 * amount (i.e. the distance between 'mi' and 'hi')
-		 * than what we would have (i.e. about half of 'lo'
-		 * and 'hi'). Hedge our bets to pick 'mi' less
-		 * aggressively, i.e. make 'mi' a bit closer to the
-		 * middle than we would otherwise pick.
-		 */
-		kyv = (kyv * 6 + lov + hiv) / 8;
-		if (lov < hiv - 1) {
-			if (kyv == lov)
-				kyv++;
-			else if (kyv == hiv)
-				kyv--;
-		}
-		mi = (range - 1) * (kyv - lov) / (hiv - lov) + lo;
-
-#ifdef INDEX_DEBUG_LOOKUP
-		printf("lo %u hi %u rg %u mi %u ", lo, hi, range, mi);
-		printf("ofs %u lov %x, hiv %x, kyv %x\n",
-				ofs_0, lov, hiv, kyv);
-#endif
-
-		if (!(lo <= mi && mi < hi)) {
-			giterr_set(GITERR_INVALID, "assertion failure: binary search invariant is false");
-			return -1;
-		}
-
-		mi_key = base + elem_size * mi + key_offset;
-		cmp = memcmp(mi_key + ofs_0, key + ofs_0, 20 - ofs_0);
-		if (!cmp)
-			return mi;
-		if (cmp > 0) {
-			hi = mi;
-			hi_key = mi_key;
-		} else {
-			lo = mi + 1;
-			lo_key = mi_key + elem_size;
-		}
-	} while (lo < hi);
-	return -((int)lo)-1;
-}
-
 int sha1_position(const void *table,
 			size_t stride,
 			unsigned lo, unsigned hi,

src/sha1_lookup.h

@@ -9,12 +9,6 @@
 
 #include <stdlib.h>
 
-int sha1_entry_pos(const void *table,
-			size_t elem_size,
-			size_t key_offset,
-			unsigned lo, unsigned hi, unsigned nr,
-			const unsigned char *key);
-
 int sha1_position(const void *table,
 			size_t stride,
 			unsigned lo, unsigned hi,