Merge branch 'kb/hashmap-updates'

* kb/hashmap-updates:
  hashmap: add string interning API
  hashmap: add simplified hashmap_get_from_hash() API
  hashmap: improve struct hashmap member documentation
  hashmap: factor out getting a hash code from a SHA1

Author: gitster

Documentation/technical/api-hashmap.txt

@@ -8,11 +8,19 @@ Data Structures
 
 `struct hashmap`::
 
-	The hash table structure.
+	The hash table structure. Members can be used as follows, but should
+	not be modified directly:
 +
-The `size` member keeps track of the total number of entries. The `cmpfn`
-member is a function used to compare two entries for equality. The `table` and
-`tablesize` members store the hash table and its size, respectively.
+The `size` member keeps track of the total number of entries (0 means the
+hashmap is empty).
++
+`tablesize` is the allocated size of the hash table. A non-0 value indicates
+that the hashmap is initialized. It may also be useful for statistical purposes
+(i.e. `size / tablesize` is the current load factor).
++
+`cmpfn` stores the comparison function specified in `hashmap_init()`. In
+advanced scenarios, it may be useful to change this, e.g. to switch between
+case-sensitive and case-insensitive lookup.
 
 `struct hashmap_entry`::
 
@@ -58,6 +66,15 @@ Functions
 +
 `strihash` and `memihash` are case insensitive versions.
 
+`unsigned int sha1hash(const unsigned char *sha1)`::
+
+	Converts a cryptographic hash (e.g. SHA-1) into an int-sized hash code
+	for use in hash tables. Cryptographic hashes are supposed to have
+	uniform distribution, so in contrast to `memhash()`, this just copies
+	the first `sizeof(int)` bytes without shuffling any bits. Note that
+	the results will be different on big-endian and little-endian
+	platforms, so they should not be stored or transferred over the net.
+
 `void hashmap_init(struct hashmap *map, hashmap_cmp_fn equals_function, size_t initial_size)`::
 
 	Initializes a hashmap structure.
@@ -101,6 +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
@@ -162,6 +193,21 @@ more entries.
 `hashmap_iter_first` is a combination of both (i.e. initializes the iterator
 and returns the first entry, if any).
 
+`const char *strintern(const char *string)`::
+`const void *memintern(const void *data, size_t len)`::
+
+	Returns the unique, interned version of the specified string or data,
+	similar to the `String.intern` API in Java and .NET, respectively.
+	Interned strings remain valid for the entire lifetime of the process.
++
+Can be used as `[x]strdup()` or `xmemdupz` replacement, except that interned
+strings / data must not be modified or freed.
++
+Interned strings are best used for short strings with high probability of
+duplicates.
++
+Uses a hashmap to store the pool of interned strings.
+
 Usage example
 -------------
 

builtin/describe.c

@@ -56,18 +56,9 @@ static int commit_name_cmp(const struct commit_name *cn1,
 	return hashcmp(cn1->peeled, peeled ? peeled : cn2->peeled);
 }
 
-static inline unsigned int hash_sha1(const unsigned char *sha1)
-{
-	unsigned int hash;
-	memcpy(&hash, sha1, sizeof(hash));
-	return hash;
-}
-
 static inline struct commit_name *find_commit_name(const unsigned char *peeled)
 {
-	struct commit_name key;
-	hashmap_entry_init(&key, hash_sha1(peeled));
-	return hashmap_get(&names, &key, peeled);
+	return hashmap_get_from_hash(&names, sha1hash(peeled), peeled);
 }
 
 static int replace_name(struct commit_name *e,
@@ -114,7 +105,7 @@ static void add_to_known_names(const char *path,
 		if (!e) {
 			e = xmalloc(sizeof(struct commit_name));
 			hashcpy(e->peeled, peeled);
-			hashmap_entry_init(e, hash_sha1(peeled));
+			hashmap_entry_init(e, sha1hash(peeled));
 			hashmap_add(&names, e);
 			e->path = NULL;
 		}

decorate.c

@@ -8,10 +8,7 @@
 
 static unsigned int hash_obj(const struct object *obj, unsigned int n)
 {
-	unsigned int hash;
-
-	memcpy(&hash, obj->sha1, sizeof(unsigned int));
-	return hash % n;
+	return sha1hash(obj->sha1) % n;
 }
 
 static void *insert_decoration(struct decoration *n, const struct object *base, void *decoration)

diffcore-rename.c

@@ -242,14 +242,12 @@ struct file_similarity {
 
 static unsigned int hash_filespec(struct diff_filespec *filespec)
 {
-	unsigned int hash;
 	if (!filespec->sha1_valid) {
 		if (diff_populate_filespec(filespec, 0))
 			return 0;
 		hash_sha1_file(filespec->data, filespec->size, "blob", filespec->sha1);
 	}
-	memcpy(&hash, filespec->sha1, sizeof(hash));
-	return hash;
+	return sha1hash(filespec->sha1);
 }
 
 static int find_identical_files(struct hashmap *srcs,
@@ -259,15 +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.c

@@ -226,3 +226,41 @@ void *hashmap_iter_next(struct hashmap_iter *iter)
 		current = iter->map->table[iter->tablepos++];
 	}
 }
+
+struct pool_entry {
+	struct hashmap_entry ent;
+	size_t len;
+	unsigned char data[FLEX_ARRAY];
+};
+
+static int pool_entry_cmp(const struct pool_entry *e1,
+			  const struct pool_entry *e2,
+			  const unsigned char *keydata)
+{
+	return e1->data != keydata &&
+	       (e1->len != e2->len || memcmp(e1->data, keydata, e1->len));
+}
+
+const void *memintern(const void *data, size_t len)
+{
+	static struct hashmap map;
+	struct pool_entry key, *e;
+
+	/* initialize string pool hashmap */
+	if (!map.tablesize)
+		hashmap_init(&map, (hashmap_cmp_fn) pool_entry_cmp, 0);
+
+	/* lookup interned string in pool */
+	hashmap_entry_init(&key, memhash(data, len));
+	key.len = len;
+	e = hashmap_get(&map, &key, data);
+	if (!e) {
+		/* not found: create it */
+		e = xmallocz(sizeof(struct pool_entry) + len);
+		hashmap_entry_init(e, key.ent.hash);
+		e->len = len;
+		memcpy(e->data, data, len);
+		hashmap_add(&map, e);
+	}
+	return e->data;
+}

hashmap.h

@@ -13,6 +13,17 @@ extern unsigned int strihash(const char *buf);
 extern unsigned int memhash(const void *buf, size_t len);
 extern unsigned int memihash(const void *buf, size_t len);
 
+static inline unsigned int sha1hash(const unsigned char *sha1)
+{
+	/*
+	 * Equivalent to 'return *(unsigned int *)sha1;', but safe on
+	 * platforms that don't support unaligned reads.
+	 */
+	unsigned int hash;
+	memcpy(&hash, sha1, sizeof(hash));
+	return hash;
+}
+
 /* data structures */
 
 struct hashmap_entry {
@@ -57,6 +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);
@@ -68,4 +87,12 @@ static inline void *hashmap_iter_first(struct hashmap *map,
 	return hashmap_iter_next(iter);
 }
 
+/* string interning */
+
+extern const void *memintern(const void *data, size_t len);
+static inline const char *strintern(const char *string)
+{
+	return memintern(string, strlen(string));
+}
+
 #endif

khash.h

@@ -320,19 +320,12 @@ static const double __ac_HASH_UPPER = 0.77;
 		code;												\
 	} }
 
-static inline khint_t __kh_oid_hash(const unsigned char *oid)
-{
-	khint_t hash;
-	memcpy(&hash, oid, sizeof(hash));
-	return hash;
-}
-
 #define __kh_oid_cmp(a, b) (hashcmp(a, b) == 0)
 
-KHASH_INIT(sha1, const unsigned char *, void *, 1, __kh_oid_hash, __kh_oid_cmp)
+KHASH_INIT(sha1, const unsigned char *, void *, 1, sha1hash, __kh_oid_cmp)
 typedef kh_sha1_t khash_sha1;
 
-KHASH_INIT(sha1_pos, const unsigned char *, int, 1, __kh_oid_hash, __kh_oid_cmp)
+KHASH_INIT(sha1_pos, const unsigned char *, int, 1, sha1hash, __kh_oid_cmp)
 typedef kh_sha1_pos_t khash_sha1_pos;
 
 #endif /* __AC_KHASH_H */

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;

object.c

@@ -50,18 +50,7 @@ int type_from_string(const char *str)
  */
 static unsigned int hash_obj(const unsigned char *sha1, unsigned int n)
 {
-	unsigned int hash;
-
-	/*
-	 * Since the sha1 is essentially random, we just take the
-	 * required number of bits directly from the first
-	 * sizeof(unsigned int) bytes of sha1.  First we have to copy
-	 * the bytes into a properly aligned integer.  If we cared
-	 * about getting consistent results across architectures, we
-	 * would have to call ntohl() here, too.
-	 */
-	memcpy(&hash, sha1, sizeof(unsigned int));
-	return hash & (n - 1);
+	return sha1hash(sha1) & (n - 1);
 }
 
 /*

pack-objects.c

@@ -7,10 +7,9 @@ static uint32_t locate_object_entry_hash(struct packing_data *pdata,
 					 const unsigned char *sha1,
 					 int *found)
 {
-	uint32_t i, hash, mask = (pdata->index_size - 1);
+	uint32_t i, mask = (pdata->index_size - 1);
 
-	memcpy(&hash, sha1, sizeof(uint32_t));
-	i = hash & mask;
+	i = sha1hash(sha1) & mask;
 
 	while (pdata->index[i] > 0) {
 		uint32_t pos = pdata->index[i] - 1;