hashmap: add simplified hashmap_get_from_hash() API

Hashmap entries are typically looked up by just a key. The hashmap_get()
API expects an initialized entry structure instead, to support compound
keys. This flexibility is currently only needed by find_dir_entry() in
name-hash.c (and compat/win32/fscache.c in the msysgit fork). All other
(currently five) call sites of hashmap_get() have to set up a near emtpy
entry structure, resulting in duplicate code like this:

  struct hashmap_entry keyentry;
  hashmap_entry_init(&keyentry, hash(key));
  return hashmap_get(map, &keyentry, key);

Add a hashmap_get_from_hash() API that allows hashmap lookups by just
specifying the key and its hash code, i.e.:

  return hashmap_get_from_hash(map, hash(key), key);

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-hashmap.txt

@@ -118,6 +118,20 @@ hashmap_entry) that has at least been initialized with the proper hash code
 If an entry with matching hash code is found, `key` and `keydata` are passed
 to `hashmap_cmp_fn` to decide whether the entry matches the key.
 
+`void *hashmap_get_from_hash(const struct hashmap *map, unsigned int hash, const void *keydata)`::
+
+	Returns the hashmap entry for the specified hash code and key data,
+	or NULL if not found.
++
+`map` is the hashmap structure.
++
+`hash` is the hash code of the entry to look up.
++
+If an entry with matching hash code is found, `keydata` is passed to
+`hashmap_cmp_fn` to decide whether the entry matches the key. The
+`entry_or_key` parameter points to a bogus hashmap_entry structure that
+should not be used in the comparison.
+
 `void *hashmap_get_next(const struct hashmap *map, const void *entry)`::
 
 	Returns the next equal hashmap entry, or NULL if not found. This can be

builtin/describe.c

@@ -58,9 +58,7 @@ static int commit_name_cmp(const struct commit_name *cn1,
 
 static inline struct commit_name *find_commit_name(const unsigned char *peeled)
 {
-	struct commit_name key;
-	hashmap_entry_init(&key, sha1hash(peeled));
-	return hashmap_get(&names, &key, peeled);
+	return hashmap_get_from_hash(&names, sha1hash(peeled), peeled);
 }
 
 static int replace_name(struct commit_name *e,

diffcore-rename.c

@@ -257,15 +257,14 @@ static int find_identical_files(struct hashmap *srcs,
 	int renames = 0;
 
 	struct diff_filespec *target = rename_dst[dst_index].two;
-	struct file_similarity *p, *best, dst;
+	struct file_similarity *p, *best = NULL;
 	int i = 100, best_score = -1;
 
 	/*
 	 * Find the best source match for specified destination.
 	 */
-	best = NULL;
-	hashmap_entry_init(&dst, hash_filespec(target));
-	for (p = hashmap_get(srcs, &dst, NULL); p; p = hashmap_get_next(srcs, p)) {
+	p = hashmap_get_from_hash(srcs, hash_filespec(target), NULL);
+	for (; p; p = hashmap_get_next(srcs, p)) {
 		int score;
 		struct diff_filespec *source = p->filespec;
 

hashmap.h

@@ -68,6 +68,14 @@ extern void *hashmap_put(struct hashmap *map, void *entry);
 extern void *hashmap_remove(struct hashmap *map, const void *key,
 		const void *keydata);
 
+static inline void *hashmap_get_from_hash(const struct hashmap *map,
+		unsigned int hash, const void *keydata)
+{
+	struct hashmap_entry key;
+	hashmap_entry_init(&key, hash);
+	return hashmap_get(map, &key, keydata);
+}
+
 /* hashmap_iter functions */
 
 extern void hashmap_iter_init(struct hashmap *map, struct hashmap_iter *iter);

name-hash.c

@@ -213,12 +213,11 @@ struct cache_entry *index_dir_exists(struct index_state *istate, const char *nam
 struct cache_entry *index_file_exists(struct index_state *istate, const char *name, int namelen, int icase)
 {
 	struct cache_entry *ce;
-	struct hashmap_entry key;
 
 	lazy_init_name_hash(istate);
 
-	hashmap_entry_init(&key, memihash(name, namelen));
-	ce = hashmap_get(&istate->name_hash, &key, NULL);
+	ce = hashmap_get_from_hash(&istate->name_hash,
+				   memihash(name, namelen), NULL);
 	while (ce) {
 		if (same_name(ce, name, namelen, icase))
 			return ce;

test-hashmap.c

@@ -115,9 +115,8 @@ static void perf_hashmap(unsigned int method, unsigned int rounds)
 
 		for (j = 0; j < rounds; j++) {
 			for (i = 0; i < TEST_SIZE; i++) {
-				struct hashmap_entry key;
-				hashmap_entry_init(&key, hashes[i]);
-				hashmap_get(&map, &key, entries[i]->key);
+				hashmap_get_from_hash(&map, hashes[i],
+						      entries[i]->key);
 			}
 		}
 
@@ -199,12 +198,8 @@ int main(int argc, char *argv[])
 
 		} else if (!strcmp("get", cmd) && l1) {
 
-			/* setup static key */
-			struct hashmap_entry key;
-			hashmap_entry_init(&key, hash);
-
 			/* lookup entry in hashmap */
-			entry = hashmap_get(&map, &key, p1);
+			entry = hashmap_get_from_hash(&map, hash, p1);
 
 			/* print result */
 			if (!entry)