hashmap: add disallow_rehash setting

Teach hashmap to allow rehashes to be suppressed.
This is useful when hashmaps are accessed by multiple
threads.  It still requires the caller to properly
manage their locking.  This just prevents unexpected
rehashing during inserts and deletes.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/hashmap.c b/hashmap.c
index 93793cb954e4bde0df9c57131a782c32fc15dc36..7d1044eb5de0faf69a151ca16c1a21500d9d8a10 100644 (file)
--- a/hashmap.c
+++ b/hashmap.c
@@ -104,11 +104,19 @@ static inline unsigned int bucket(const struct hashmap *map,
        return key->hash & (map->tablesize - 1);
 }
 
+int hashmap_bucket(const struct hashmap *map, unsigned int hash)
+{
+       return 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;
 
+       if (map->disallow_rehash)
+               return;
+
        alloc_table(map, newsize);
        for (i = 0; i < oldsize; i++) {
                struct hashmap_entry *e = oldtable[i];
@@ -141,7 +149,9 @@ void hashmap_init(struct hashmap *map, hashmap_cmp_fn equals_function,
                size_t initial_size)
 {
        unsigned int size = HASHMAP_INITIAL_SIZE;
-       map->size = 0;
+
+       memset(map, 0, sizeof(*map));
+
        map->cmpfn = equals_function ? equals_function : always_equal;
 
        /* calculate initial table size and allocate the table */

diff --git a/hashmap.h b/hashmap.h
index 45eda693bdbc7efc53f6b0ce29a172bcb574cfdf..de6022a3a916605d8e3330b23259110dd3522c7d 100644 (file)
--- a/hashmap.h
+++ b/hashmap.h
@@ -39,6 +39,7 @@ struct hashmap {
        struct hashmap_entry **table;
        hashmap_cmp_fn cmpfn;
        unsigned int size, tablesize, grow_at, shrink_at;
+       unsigned disallow_rehash : 1;
 };
 
 struct hashmap_iter {
@@ -77,6 +78,29 @@ static inline void *hashmap_get_from_hash(const struct hashmap *map,
        return hashmap_get(map, &key, keydata);
 }
 
+int hashmap_bucket(const struct hashmap *map, unsigned int hash);
+
+/*
+ * Disallow/allow rehashing of the hashmap.
+ * This is useful if the caller knows that the hashmap
+ * needs multi-threaded access.  The caller is still
+ * required to guard/lock searches and inserts in a
+ * manner appropriate to their usage.  This simply
+ * prevents the table from being unexpectedly re-mapped.
+ *
+ * If is up to the caller to ensure that the hashmap is
+ * initialized to a reasonable size to prevent poor
+ * performance.
+ *
+ * When value=1, prevent future rehashes on adds and deleted.
+ * When value=0, allow future rehahses.  This DOES NOT force
+ * a rehash now.
+ */
+static inline void hashmap_disallow_rehash(struct hashmap *map, unsigned value)
+{
+       map->disallow_rehash = value;
+}
+
 /* hashmap_iter functions */
 
 extern void hashmap_iter_init(struct hashmap *map, struct hashmap_iter *iter);