add a hashtable implementation that supports O(1) removal

The existing hashtable implementation (in hash.[ch]) uses open addressing
(i.e. resolve hash collisions by distributing entries across the table).
Thus, removal is difficult to implement with less than O(n) complexity.
Resolving collisions of entries with identical hashes (e.g. via chaining)
is left to the client code.

Add a hashtable implementation that supports O(1) removal and is slightly
easier to use due to builtin entry chaining.

Supports all basic operations init, free, get, add, remove and iteration.

Also includes ready-to-use hash functions based on the public domain FNV-1
algorithm (http://www.isthe.com/chongo/tech/comp/fnv).

The per-entry data structure (hashmap_entry) is piggybacked in front of
the client's data structure to save memory. See test-hashmap.c for usage
examples.

The hashtable is resized by a factor of four when 80% full. With these
settings, average memory consumption is about 2/3 of hash.[ch], and
insertion is about twice as fast due to less frequent resizing.

Lookups are also slightly faster, because entries are strictly confined to
their bucket (i.e. no data of other buckets needs to be traversed).

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

1 files changed, 228 insertions, 0 deletions

diff --git a/hashmap.c b/hashmap.c
new file mode 100644
index 000000000..d1b8056d8
--- /dev/null
+++ b/hashmap.c
@@ -0,0 +1,228 @@
+/*
+ * Generic implementation of hash-based key value mappings.
+ */
+#include "cache.h"
+#include "hashmap.h"
+
+#define FNV32_BASE ((unsigned int) 0x811c9dc5)
+#define FNV32_PRIME ((unsigned int) 0x01000193)
+
+unsigned int strhash(const char *str)
+{
+	unsigned int c, hash = FNV32_BASE;
+	while ((c = (unsigned char) *str++))
+		hash = (hash * FNV32_PRIME) ^ c;
+	return hash;
+}
+
+unsigned int strihash(const char *str)
+{
+	unsigned int c, hash = FNV32_BASE;
+	while ((c = (unsigned char) *str++)) {
+		if (c >= 'a' && c <= 'z')
+			c -= 'a' - 'A';
+		hash = (hash * FNV32_PRIME) ^ c;
+	}
+	return hash;
+}
+
+unsigned int memhash(const void *buf, size_t len)
+{
+	unsigned int hash = FNV32_BASE;
+	unsigned char *ucbuf = (unsigned char *) buf;
+	while (len--) {
+		unsigned int c = *ucbuf++;
+		hash = (hash * FNV32_PRIME) ^ c;
+	}
+	return hash;
+}
+
+unsigned int memihash(const void *buf, size_t len)
+{
+	unsigned int hash = FNV32_BASE;
+	unsigned char *ucbuf = (unsigned char *) buf;
+	while (len--) {
+		unsigned int c = *ucbuf++;
+		if (c >= 'a' && c <= 'z')
+			c -= 'a' - 'A';
+		hash = (hash * FNV32_PRIME) ^ c;
+	}
+	return hash;
+}
+
+#define HASHMAP_INITIAL_SIZE 64
+/* grow / shrink by 2^2 */
+#define HASHMAP_RESIZE_BITS 2
+/* load factor in percent */
+#define HASHMAP_LOAD_FACTOR 80
+
+static void alloc_table(struct hashmap *map, unsigned int size)
+{
+	map->tablesize = size;
+	map->table = xcalloc(size, sizeof(struct hashmap_entry *));
+
+	/* calculate resize thresholds for new size */
+	map->grow_at = (unsigned int) ((uint64_t) size * HASHMAP_LOAD_FACTOR / 100);
+	if (size <= HASHMAP_INITIAL_SIZE)
+		map->shrink_at = 0;
+	else
+		/*
+		 * The shrink-threshold must be slightly smaller than
+		 * (grow-threshold / resize-factor) to prevent erratic resizing,
+		 * thus we divide by (resize-factor + 1).
+		 */
+		map->shrink_at = map->grow_at / ((1 << HASHMAP_RESIZE_BITS) + 1);
+}
+
+static inline int entry_equals(const struct hashmap *map,
+		const struct hashmap_entry *e1, const struct hashmap_entry *e2,
+		const void *keydata)
+{
+	return (e1 == e2) || (e1->hash == e2->hash && !map->cmpfn(e1, e2, keydata));
+}
+
+static inline unsigned int bucket(const struct hashmap *map,
+		const struct hashmap_entry *key)
+{
+	return key->hash & (map->tablesize - 1);
+}
+
+static void rehash(struct hashmap *map, unsigned int newsize)
+{
+	unsigned int i, oldsize = map->tablesize;
+	struct hashmap_entry **oldtable = map->table;
+
+	alloc_table(map, newsize);
+	for (i = 0; i < oldsize; i++) {
+		struct hashmap_entry *e = oldtable[i];
+		while (e) {
+			struct hashmap_entry *next = e->next;
+			unsigned int b = bucket(map, e);
+			e->next = map->table[b];
+			map->table[b] = e;
+			e = next;
+		}
+	}
+	free(oldtable);
+}
+
+static inline struct hashmap_entry **find_entry_ptr(const struct hashmap *map,
+		const struct hashmap_entry *key, const void *keydata)
+{
+	struct hashmap_entry **e = &map->table[bucket(map, key)];
+	while (*e && !entry_equals(map, *e, key, keydata))
+		e = &(*e)->next;
+	return e;
+}
+
+static int always_equal(const void *unused1, const void *unused2, const void *unused3)
+{
+	return 0;
+}
+
+void hashmap_init(struct hashmap *map, hashmap_cmp_fn equals_function,
+		size_t initial_size)
+{
+	unsigned int size = HASHMAP_INITIAL_SIZE;
+	map->size = 0;
+	map->cmpfn = equals_function ? equals_function : always_equal;
+
+	/* calculate initial table size and allocate the table */
+	initial_size = (unsigned int) ((uint64_t) initial_size * 100
+			/ HASHMAP_LOAD_FACTOR);
+	while (initial_size > size)
+		size <<= HASHMAP_RESIZE_BITS;
+	alloc_table(map, size);
+}
+
+void hashmap_free(struct hashmap *map, int free_entries)
+{
+	if (!map || !map->table)
+		return;
+	if (free_entries) {
+		struct hashmap_iter iter;
+		struct hashmap_entry *e;
+		hashmap_iter_init(map, &iter);
+		while ((e = hashmap_iter_next(&iter)))
+			free(e);
+	}
+	free(map->table);
+	memset(map, 0, sizeof(*map));
+}
+
+void *hashmap_get(const struct hashmap *map, const void *key, const void *keydata)
+{
+	return *find_entry_ptr(map, key, keydata);
+}
+
+void *hashmap_get_next(const struct hashmap *map, const void *entry)
+{
+	struct hashmap_entry *e = ((struct hashmap_entry *) entry)->next;
+	for (; e; e = e->next)
+		if (entry_equals(map, entry, e, NULL))
+			return e;
+	return NULL;
+}
+
+void hashmap_add(struct hashmap *map, void *entry)
+{
+	unsigned int b = bucket(map, entry);
+
+	/* add entry */
+	((struct hashmap_entry *) entry)->next = map->table[b];
+	map->table[b] = entry;
+
+	/* fix size and rehash if appropriate */
+	map->size++;
+	if (map->size > map->grow_at)
+		rehash(map, map->tablesize << HASHMAP_RESIZE_BITS);
+}
+
+void *hashmap_remove(struct hashmap *map, const void *key, const void *keydata)
+{
+	struct hashmap_entry *old;
+	struct hashmap_entry **e = find_entry_ptr(map, key, keydata);
+	if (!*e)
+		return NULL;
+
+	/* remove existing entry */
+	old = *e;
+	*e = old->next;
+	old->next = NULL;
+
+	/* fix size and rehash if appropriate */
+	map->size--;
+	if (map->size < map->shrink_at)
+		rehash(map, map->tablesize >> HASHMAP_RESIZE_BITS);
+	return old;
+}
+
+void *hashmap_put(struct hashmap *map, void *entry)
+{
+	struct hashmap_entry *old = hashmap_remove(map, entry, NULL);
+	hashmap_add(map, entry);
+	return old;
+}
+
+void hashmap_iter_init(struct hashmap *map, struct hashmap_iter *iter)
+{
+	iter->map = map;
+	iter->tablepos = 0;
+	iter->next = NULL;
+}
+
+void *hashmap_iter_next(struct hashmap_iter *iter)
+{
+	struct hashmap_entry *current = iter->next;
+	for (;;) {
+		if (current) {
+			iter->next = current->next;
+			return current;
+		}
+
+		if (iter->tablepos >= iter->map->tablesize)
+			return NULL;
+
+		current = iter->map->table[iter->tablepos++];
+	}
+}