hashmap: introduce hashmap_reserve()

With the current hashmap implementation that uses chaining, placing a
reservation can serve two purposes:
 - To optimize putting of entries if the number of entries to put is
   known. The reservation allocates buckets, so later resizing can be
   avoided.
 - To avoid having very long bucket chains after using
   hashmap_move(_one).

In an alternative hashmap implementation it will serve an additional
purpose:
 - To guarantee a subsequent hashmap_move(_one) will not fail with
   -ENOMEM (this never happens in the current implementation).

1 files changed, 4 insertions, 0 deletions

diff --git a/src/shared/hashmap.h b/src/shared/hashmap.h
index 9789ad77ba..e0ff26c006 100644
--- a/src/shared/hashmap.h
+++ b/src/shared/hashmap.h
@@ -156,6 +156,10 @@ int hashmap_merge(Hashmap *h, Hashmap *other);
 static inline int ordered_hashmap_merge(OrderedHashmap *h, OrderedHashmap *other) {
         return hashmap_merge((Hashmap*) h, (Hashmap*) other);
 }
+int hashmap_reserve(Hashmap *h, unsigned entries_add);
+static inline int ordered_hashmap_reserve(OrderedHashmap *h, unsigned entries_add) {
+        return hashmap_reserve((Hashmap*) h, entries_add);
+}
 void hashmap_move(Hashmap *h, Hashmap *other);
 static inline void ordered_hashmap_move(OrderedHashmap *h, OrderedHashmap *other) {
         hashmap_move((Hashmap*) h, (Hashmap*) other);