packfile: refactor hash search with fanout table

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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/packfile.c b/packfile.c
index 58bdced3b146c83c977c4ab417a07ce2570ffa0a..59648a1820706cc155c1e75586d220cedf7ae25c 100644 (file)
--- 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;
+       uint32_t result;
 
        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 +1733,8 @@ 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;
-       }
+       if (bsearch_hash(sha1, level1_ofs, index, stride, &result))
+               return nth_packed_object_offset(p, result);
        return 0;
 }
 

diff --git a/sha1-lookup.c b/sha1-lookup.c
index 4cf3ebd9212f6d5c9b9829373e58c34b83f0a548..8d0b1db3e27c99b80faa2867565de237581802f4 100644 (file)
--- a/sha1-lookup.c
+++ b/sha1-lookup.c
@@ -99,3 +99,31 @@ 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 uint32_t *fanout_nbo,
+                const unsigned char *table, size_t stride, uint32_t *result)
+{
+       uint32_t hi, lo;
+
+       hi = ntohl(fanout_nbo[*sha1]);
+       lo = ((*sha1 == 0x0) ? 0 : ntohl(fanout_nbo[*sha1 - 1]));
+
+       while (lo < hi) {
+               unsigned mi = lo + (hi - lo) / 2;
+               int cmp = hashcmp(table + mi * stride, sha1);
+
+               if (!cmp) {
+                       if (result)
+                               *result = mi;
+                       return 1;
+               }
+               if (cmp > 0)
+                       hi = mi;
+               else
+                       lo = mi + 1;
+       }
+
+       if (result)
+               *result = lo;
+       return 0;
+}

diff --git a/sha1-lookup.h b/sha1-lookup.h
index cf5314f402ce78f0d5ab2bd72ee7f334b6394e04..7678b23b36c291c7b1b6656e0c42fc14ea12da15 100644 (file)
--- a/sha1-lookup.h
+++ b/sha1-lookup.h
@@ -7,4 +7,26 @@ 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 1 if found, 0 if not.
+ *
+ * Takes the following parameters:
+ *
+ *  - sha1: the hash to search for
+ *  - fanout_nbo: 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)
+ *  - result: if not NULL, this function stores the element index of the
+ *    position found (if the search is successful) or the index of the least
+ *    element that is greater than sha1 (if the search is not successful)
+ *
+ * This function does not verify the validity of the fanout table.
+ */
+int bsearch_hash(const unsigned char *sha1, const uint32_t *fanout_nbo,
+                const unsigned char *table, size_t stride, uint32_t *result);
 #endif