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/set.c b/src/shared/set.c
index ed16067bdc..1a3465ca8b 100644
--- a/src/shared/set.c
+++ b/src/shared/set.c
@@ -137,6 +137,10 @@ int set_merge(Set *s, Set *other) {
         return hashmap_merge(MAKE_HASHMAP(s), MAKE_HASHMAP(other));
 }
 
+int set_reserve(Set *s, unsigned entries_add) {
+        return hashmap_reserve(MAKE_HASHMAP(s), entries_add);
+}
+
 void set_move(Set *s, Set *other) {
         return hashmap_move(MAKE_HASHMAP(s), MAKE_HASHMAP(other));
 }