hash: add a function to finalize object IDs

To avoid the penalty of having to branch in hash comparison functions,
we'll want to always compare the full hash member in a struct object_id,
which will require that SHA-1 object IDs be zero-padded.  To do so, add
a function which finalizes a hash context and writes it into an object
ID that performs this padding.

Move the definition of struct object_id and the constant definitions
higher up so we they are available for us to use.

Signed-off-by: brian m. carlson <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: bk2204

hash.h

@@ -95,6 +95,29 @@ static inline void git_SHA256_Clone(git_SHA256_CTX *dst, const git_SHA256_CTX *s
 /* Number of algorithms supported (including unknown). */
 #define GIT_HASH_NALGOS (GIT_HASH_SHA256 + 1)
 
+/* The length in bytes and in hex digits of an object name (SHA-1 value). */
+#define GIT_SHA1_RAWSZ 20
+#define GIT_SHA1_HEXSZ (2 * GIT_SHA1_RAWSZ)
+/* The block size of SHA-1. */
+#define GIT_SHA1_BLKSZ 64
+
+/* The length in bytes and in hex digits of an object name (SHA-256 value). */
+#define GIT_SHA256_RAWSZ 32
+#define GIT_SHA256_HEXSZ (2 * GIT_SHA256_RAWSZ)
+/* The block size of SHA-256. */
+#define GIT_SHA256_BLKSZ 64
+
+/* The length in byte and in hex digits of the largest possible hash value. */
+#define GIT_MAX_RAWSZ GIT_SHA256_RAWSZ
+#define GIT_MAX_HEXSZ GIT_SHA256_HEXSZ
+/* The largest possible block size for any supported hash. */
+#define GIT_MAX_BLKSZ GIT_SHA256_BLKSZ
+
+struct object_id {
+	unsigned char hash[GIT_MAX_RAWSZ];
+	int algo;
+};
+
 /* A suitably aligned type for stack allocations of hash contexts. */
 union git_hash_ctx {
 	git_SHA_CTX sha1;
@@ -106,6 +129,7 @@ typedef void (*git_hash_init_fn)(git_hash_ctx *ctx);
 typedef void (*git_hash_clone_fn)(git_hash_ctx *dst, const git_hash_ctx *src);
 typedef void (*git_hash_update_fn)(git_hash_ctx *ctx, const void *in, size_t len);
 typedef void (*git_hash_final_fn)(unsigned char *hash, git_hash_ctx *ctx);
+typedef void (*git_hash_final_oid_fn)(struct object_id *oid, git_hash_ctx *ctx);
 
 struct git_hash_algo {
 	/*
@@ -138,6 +162,9 @@ struct git_hash_algo {
 	/* The hash finalization function. */
 	git_hash_final_fn final_fn;
 
+	/* The hash finalization function for object IDs. */
+	git_hash_final_oid_fn final_oid_fn;
+
 	/* The OID of the empty tree. */
 	const struct object_id *empty_tree;
 
@@ -161,29 +188,6 @@ static inline int hash_algo_by_ptr(const struct git_hash_algo *p)
 	return p - hash_algos;
 }
 
-/* The length in bytes and in hex digits of an object name (SHA-1 value). */
-#define GIT_SHA1_RAWSZ 20
-#define GIT_SHA1_HEXSZ (2 * GIT_SHA1_RAWSZ)
-/* The block size of SHA-1. */
-#define GIT_SHA1_BLKSZ 64
-
-/* The length in bytes and in hex digits of an object name (SHA-256 value). */
-#define GIT_SHA256_RAWSZ 32
-#define GIT_SHA256_HEXSZ (2 * GIT_SHA256_RAWSZ)
-/* The block size of SHA-256. */
-#define GIT_SHA256_BLKSZ 64
-
-/* The length in byte and in hex digits of the largest possible hash value. */
-#define GIT_MAX_RAWSZ GIT_SHA256_RAWSZ
-#define GIT_MAX_HEXSZ GIT_SHA256_HEXSZ
-/* The largest possible block size for any supported hash. */
-#define GIT_MAX_BLKSZ GIT_SHA256_BLKSZ
-
-struct object_id {
-	unsigned char hash[GIT_MAX_RAWSZ];
-	int algo;
-};
-
 #define the_hash_algo the_repository->hash_algo
 
 extern const struct object_id null_oid;

object-file.c

@@ -89,6 +89,12 @@ static void git_hash_sha1_final(unsigned char *hash, git_hash_ctx *ctx)
 	git_SHA1_Final(hash, &ctx->sha1);
 }
 
+static void git_hash_sha1_final_oid(struct object_id *oid, git_hash_ctx *ctx)
+{
+	git_SHA1_Final(oid->hash, &ctx->sha1);
+	memset(oid->hash + GIT_SHA1_RAWSZ, 0, GIT_MAX_RAWSZ - GIT_SHA1_RAWSZ);
+}
+
 
 static void git_hash_sha256_init(git_hash_ctx *ctx)
 {
@@ -110,6 +116,16 @@ static void git_hash_sha256_final(unsigned char *hash, git_hash_ctx *ctx)
 	git_SHA256_Final(hash, &ctx->sha256);
 }
 
+static void git_hash_sha256_final_oid(struct object_id *oid, git_hash_ctx *ctx)
+{
+	git_SHA256_Final(oid->hash, &ctx->sha256);
+	/*
+	 * This currently does nothing, so the compiler should optimize it out,
+	 * but keep it in case we extend the hash size again.
+	 */
+	memset(oid->hash + GIT_SHA256_RAWSZ, 0, GIT_MAX_RAWSZ - GIT_SHA256_RAWSZ);
+}
+
 static void git_hash_unknown_init(git_hash_ctx *ctx)
 {
 	BUG("trying to init unknown hash");
@@ -130,6 +146,12 @@ static void git_hash_unknown_final(unsigned char *hash, git_hash_ctx *ctx)
 	BUG("trying to finalize unknown hash");
 }
 
+static void git_hash_unknown_final_oid(struct object_id *oid, git_hash_ctx *ctx)
+{
+	BUG("trying to finalize unknown hash");
+}
+
+
 const struct git_hash_algo hash_algos[GIT_HASH_NALGOS] = {
 	{
 		NULL,
@@ -141,6 +163,7 @@ const struct git_hash_algo hash_algos[GIT_HASH_NALGOS] = {
 		git_hash_unknown_clone,
 		git_hash_unknown_update,
 		git_hash_unknown_final,
+		git_hash_unknown_final_oid,
 		NULL,
 		NULL,
 	},
@@ -155,6 +178,7 @@ const struct git_hash_algo hash_algos[GIT_HASH_NALGOS] = {
 		git_hash_sha1_clone,
 		git_hash_sha1_update,
 		git_hash_sha1_final,
+		git_hash_sha1_final_oid,
 		&empty_tree_oid,
 		&empty_blob_oid,
 	},
@@ -169,6 +193,7 @@ const struct git_hash_algo hash_algos[GIT_HASH_NALGOS] = {
 		git_hash_sha256_clone,
 		git_hash_sha256_update,
 		git_hash_sha256_final,
+		git_hash_sha256_final_oid,
 		&empty_tree_oid_sha256,
 		&empty_blob_oid_sha256,
 	}