Added git.git sha1 lookup method to replace simple binary search in pack backend.

Implemented find_unique_short_oid for pack backend, based on git sha1 lookup method;
finding an object given its full oid is just a particular case of searching
the unique object matching an oid prefix (short oid).

Added git_odb_read_unique_short_oid, which iterates over all the backends to
find and read the unique object matching the given oid prefix.

Added a git_object_lookup_short_oid method to find the unique object in
the repository matching a given oid prefix : it generalizes git_object_lookup
which now does nothing but calls git_object_lookup_short_oid.

1 files changed, 196 insertions, 0 deletions

diff --git a/src/sha1_lookup.c b/src/sha1_lookup.c
new file mode 100644
index 000000000..f4a3c42cc
--- /dev/null
+++ b/src/sha1_lookup.c
@@ -0,0 +1,196 @@
+/*
+ * This file is basically taken from git code.
+ * This file is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License, version 2,
+ * as published by the Free Software Foundation.
+ *
+ * In addition to the permissions in the GNU General Public License,
+ * the authors give you unlimited permission to link the compiled
+ * version of this file into combinations with other programs,
+ * and to distribute those combinations without any restriction
+ * coming from the use of this file.  (The General Public License
+ * restrictions do apply in other respects; for example, they cover
+ * modification of the file, and distribution when not linked into
+ * a combined executable.)
+ *
+ * This file is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; see the file COPYING.  If not, write to
+ * the Free Software Foundation, 51 Franklin Street, Fifth Floor,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#include <stdio.h>
+
+#include "sha1_lookup.h"
+#include "common.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 = 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.
+			 */
+			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)) {
+			return git__throw(GIT_ERROR, "Assertion failure. Binary search invariant is false");
+		}
+
+		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 -lo-1;
+}