sha1-file: provide functions to look up hash algorithms

There are several ways we might refer to a hash algorithm: by name, such
as in the config file; by format ID, such as in a pack; or internally,
by a pointer to the hash_algos array.  Provide functions to look up hash
algorithms based on these various forms and return the internal constant
used for them.  If conversion to another form is necessary, this
internal constant can be used to look up the proper data in the
hash_algos array.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

hash.h

@@ -98,4 +98,17 @@ struct git_hash_algo {
 };
 extern const struct git_hash_algo hash_algos[GIT_HASH_NALGOS];
 
+/*
+ * Return a GIT_HASH_* constant based on the name.  Returns GIT_HASH_UNKNOWN if
+ * the name doesn't match a known algorithm.
+ */
+int hash_algo_by_name(const char *name);
+/* Identical, except based on the format ID. */
+int hash_algo_by_id(uint32_t format_id);
+/* Identical, except for a pointer to struct git_hash_algo. */
+static inline int hash_algo_by_ptr(const struct git_hash_algo *p)
+{
+	return p - hash_algos;
+}
+
 #endif

sha1-file.c

@@ -122,6 +122,27 @@ const char *empty_blob_oid_hex(void)
 	return oid_to_hex_r(buf, the_hash_algo->empty_blob);
 }
 
+int hash_algo_by_name(const char *name)
+{
+	int i;
+	if (!name)
+		return GIT_HASH_UNKNOWN;
+	for (i = 1; i < GIT_HASH_NALGOS; i++)
+		if (!strcmp(name, hash_algos[i].name))
+			return i;
+	return GIT_HASH_UNKNOWN;
+}
+
+int hash_algo_by_id(uint32_t format_id)
+{
+	int i;
+	for (i = 1; i < GIT_HASH_NALGOS; i++)
+		if (format_id == hash_algos[i].format_id)
+			return i;
+	return GIT_HASH_UNKNOWN;
+}
+
+
 /*
  * This is meant to hold a *small* number of objects that you would
  * want read_sha1_file() to be able to return, but yet you do not want