hash: add a function to look up hash algo structs

In C, it's easy for us to look up a hash algorithm structure by its
offset by simply indexing the hash_algos array.  However, in Rust, we
sometimes need a pointer to pass to a C function, but we have our own
hash algorithm abstraction.

To get one from the other, let's provide a simple function that looks up
the C structure from the offset and expose it in Rust.

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

1 files changed, 7 insertions, 0 deletions

diff --git a/hash.c b/hash.c
index 81b4f87027..97fd473607 100644
--- a/hash.c
+++ b/hash.c
@@ -241,6 +241,13 @@ const char *empty_tree_oid_hex(const struct git_hash_algo *algop)
 	return oid_to_hex_r(buf, algop->empty_tree);
 }
 
+const struct git_hash_algo *hash_algo_ptr_by_number(uint32_t algo)
+{
+	if (algo >= GIT_HASH_NALGOS)
+		return NULL;
+	return &hash_algos[algo];
+}
+
 uint32_t hash_algo_by_name(const char *name)
 {
 	if (!name)