Add a pack index 'virtual function' to fetch an index entry

Given an index entry number, the idx_get() function returns an
(version agnostic) index_entry structure containing all of the
information required to unpack the corresponding object from
the '.pack' file.

Since the v1 and v2 file formats differ in the layout of the
object records, we provide two implementations of the get
function and initialise the function pointer appropriately.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>

src/odb.c

@@ -31,6 +31,14 @@
 #include "odb.h"
 
 #define GIT_PACK_NAME_MAX (5 + 40 + 1)
+
+typedef struct {
+	uint32_t      n;
+	unsigned char *oid;
+	off_t         offset;
+	off_t         size;
+} index_entry;
+
 struct git_pack {
 	git_odb *db;
 	git_lck lock;
@@ -44,6 +52,10 @@ struct git_pack {
 		uint32_t *,
 		struct git_pack *,
 		off_t);
+	int (*idx_get)(
+		index_entry *,
+		struct git_pack *,
+		uint32_t n);
 
 	/** The .idx file, mapped into memory. */
 	git_file idx_fd;
@@ -764,6 +776,27 @@ static int idxv1_search_offset(uint32_t *out, git_pack *p, off_t offset)
 	return GIT_ENOTFOUND;
 }
 
+static int idxv1_get(index_entry *e, git_pack *p, uint32_t n)
+{
+	unsigned char *data = p->im_oid;
+	uint32_t *next = p->im_off_next;
+
+	if (n < p->obj_cnt) {
+		uint32_t pos = n * (GIT_OID_RAWSZ + 4);
+		off_t next_off = p->pack_size - GIT_OID_RAWSZ;
+		e->n = n;
+		e->oid = data + pos + 4;
+		e->offset = decode32(data + pos);
+		if (next[n] < p->obj_cnt) {
+			pos = next[n] * (GIT_OID_RAWSZ + 4);
+			next_off = decode32(data + pos);
+		}
+		e->size = next_off - e->offset;
+		return GIT_SUCCESS;
+	}
+	return GIT_ENOTFOUND;
+}
+
 static int pack_openidx_v1(git_pack *p)
 {
 	uint32_t *src_fanout = p->idx_map.data;
@@ -794,6 +827,7 @@ static int pack_openidx_v1(git_pack *p)
 
 	p->idx_search = idxv1_search;
 	p->idx_search_offset = idxv1_search_offset;
+	p->idx_get = idxv1_get;
 	p->im_fanout = im_fanout;
 	p->im_oid = (unsigned char *)(src_fanout + 256);
 
@@ -869,6 +903,35 @@ static int idxv2_search_offset(uint32_t *out, git_pack *p, off_t offset)
 	return GIT_ENOTFOUND;
 }
 
+static int idxv2_get(index_entry *e, git_pack *p, uint32_t n)
+{
+	unsigned char *data = p->im_oid;
+	uint32_t *next = p->im_off_next;
+
+	if (n < p->obj_cnt) {
+		uint32_t o32 = decode32(p->im_offset32 + n);
+		off_t next_off = p->pack_size - GIT_OID_RAWSZ;
+		e->n = n;
+		e->oid = data + n * GIT_OID_RAWSZ;
+		e->offset = o32;
+		if (o32 & 0x80000000) {
+			uint32_t o64_idx = (o32 & ~0x80000000);
+			e->offset = decode64(p->im_offset64 + 2*o64_idx);
+		}
+		if (next[n] < p->obj_cnt) {
+			o32 = decode32(p->im_offset32 + next[n]);
+			next_off = o32;
+			if (o32 & 0x80000000) {
+				uint32_t o64_idx = (o32 & ~0x80000000);
+				next_off = decode64(p->im_offset64 + 2*o64_idx);
+			}
+		}
+		e->size = next_off - e->offset;
+		return GIT_SUCCESS;
+	}
+	return GIT_ENOTFOUND;
+}
+
 static int pack_openidx_v2(git_pack *p)
 {
 	unsigned char *data = p->idx_map.data;
@@ -900,6 +963,7 @@ static int pack_openidx_v2(git_pack *p)
 
 	p->idx_search = idxv2_search;
 	p->idx_search_offset = idxv2_search_offset;
+	p->idx_get = idxv2_get;
 	p->im_fanout = im_fanout;
 	p->im_oid = (unsigned char *)(src_fanout + 256);
 	p->im_crc = (uint32_t *)(p->im_oid + 20 * p->obj_cnt);