[PATCH 2/2] packfile: refactor hash search with fanout table

[Jonathan Tan <jonathantanmy@google.com>]

Subsequent patches will introduce file formats that make use of a fanout
array and a sorted table containing hashes, just like packfiles.
Refactor the hash search in packfile.c into its own function, so that
those patches can make use of it as well.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
---
 packfile.c    | 19 +++++--------------
 sha1-lookup.c | 24 ++++++++++++++++++++++++
 sha1-lookup.h | 21 +++++++++++++++++++++
 3 files changed, 50 insertions(+), 14 deletions(-)

diff --git a/packfile.c b/packfile.c
index 58bdced3b..29f5dc239 100644
--- a/packfile.c
+++ b/packfile.c
@@ -1712,7 +1712,8 @@ off_t find_pack_entry_one(const unsigned char *sha1,
 {
 	const uint32_t *level1_ofs = p->index_data;
 	const unsigned char *index = p->index_data;
-	unsigned hi, lo, stride;
+	unsigned stride;
+	int ret;
 
 	if (!index) {
 		if (open_pack_index(p))
@@ -1725,8 +1726,6 @@ off_t find_pack_entry_one(const unsigned char *sha1,
 		index += 8;
 	}
 	index += 4 * 256;
-	hi = ntohl(level1_ofs[*sha1]);
-	lo = ((*sha1 == 0x0) ? 0 : ntohl(level1_ofs[*sha1 - 1]));
 	if (p->index_version > 1) {
 		stride = 20;
 	} else {
@@ -1734,17 +1733,9 @@ off_t find_pack_entry_one(const unsigned char *sha1,
 		index += 4;
 	}
 
-	while (lo < hi) {
-		unsigned mi = lo + (hi - lo) / 2;
-		int cmp = hashcmp(index + mi * stride, sha1);
-
-		if (!cmp)
-			return nth_packed_object_offset(p, mi);
-		if (cmp > 0)
-			hi = mi;
-		else
-			lo = mi+1;
-	}
+	ret = bsearch_hash(sha1, level1_ofs, index, stride);
+	if (ret >= 0)
+		return nth_packed_object_offset(p, ret);
 	return 0;
 }
 
diff --git a/sha1-lookup.c b/sha1-lookup.c
index 4cf3ebd92..d11c5e526 100644
--- a/sha1-lookup.c
+++ b/sha1-lookup.c
@@ -99,3 +99,27 @@ int sha1_pos(const unsigned char *sha1, void *table, size_t nr,
 	} while (lo < hi);
 	return -lo-1;
 }
+
+int bsearch_hash(const unsigned char *sha1, const void *fanout_,
+		 const void *table_, size_t stride)
+{
+	const uint32_t *fanout = fanout_;
+	const unsigned char *table = table_;
+	int hi, lo;
+
+	hi = ntohl(fanout[*sha1]);
+	lo = ((*sha1 == 0x0) ? 0 : ntohl(fanout[*sha1 - 1]));
+
+	while (lo < hi) {
+		unsigned mi = lo + (hi - lo) / 2;
+		int cmp = hashcmp(table + mi * stride, sha1);
+
+		if (!cmp)
+			return mi;
+		if (cmp > 0)
+			hi = mi;
+		else
+			lo = mi + 1;
+	}
+	return -lo - 1;
+}
diff --git a/sha1-lookup.h b/sha1-lookup.h
index cf5314f40..3c59e9cb1 100644
--- a/sha1-lookup.h
+++ b/sha1-lookup.h
@@ -7,4 +7,25 @@ extern int sha1_pos(const unsigned char *sha1,
 		    void *table,
 		    size_t nr,
 		    sha1_access_fn fn);
+
+/*
+ * Searches for sha1 in table, using the given fanout table to determine the
+ * interval to search, then using binary search. Returns the element index of
+ * the position found if successful, -i-1 if not (where i is the index of the
+ * least element that is greater than sha1).
+ *
+ * Takes the following parameters:
+ *
+ *  - sha1: the hash to search for
+ *  - fanout: a 256-element array of NETWORK-order 32-bit integers; the integer
+ *    at position i represents the number of elements in table whose first byte
+ *    is less than or equal to i
+ *  - table: a sorted list of hashes with optional extra information in between
+ *  - stride: distance between two consecutive elements in table (should be
+ *    GIT_MAX_RAWSZ or greater)
+ *
+ * This function does not verify the validity of the fanout table.
+ */
+extern int bsearch_hash(const unsigned char *sha1, const void *fanout,
+			const void *table, size_t stride);
 #endif
-- 
2.16.0.rc1.238.g530d649a79-goog