hashmap: implement api to iterate a hashmap backwards

src/pulsecore/hashmap.c

@@ -237,6 +237,39 @@ at_end:
     return NULL;
 }
 
+void *pa_hashmap_iterate_backwards(pa_hashmap *h, void **state, const void **key) {
+    struct hashmap_entry *e;
+
+    pa_assert(h);
+    pa_assert(state);
+
+    if (*state == (void*) -1)
+        goto at_beginning;
+
+    if (!*state && !h->iterate_list_tail)
+        goto at_beginning;
+
+    e = *state ? *state : h->iterate_list_tail;
+
+    if (e->iterate_previous)
+        *state = e->iterate_previous;
+    else
+        *state = (void*) -1;
+
+    if (key)
+        *key = e->key;
+
+    return e->value;
+
+at_beginning:
+    *state = (void *) -1;
+
+    if (key)
+        *key = NULL;
+
+    return NULL;
+}
+
 void* pa_hashmap_first(pa_hashmap *h) {
     pa_assert(h);
 

src/pulsecore/hashmap.h

@@ -26,7 +26,8 @@
 
 /* Simple Implementation of a hash table. Memory management is the
  * user's job. It's a good idea to have the key pointer point to a
- * string in the value data. */
+ * string in the value data. The insertion order is preserved when
+ * iterating. */
 
 typedef struct pa_hashmap pa_hashmap;
 
@@ -59,6 +60,9 @@ pa_bool_t pa_hashmap_isempty(pa_hashmap *h);
    returned. */
 void *pa_hashmap_iterate(pa_hashmap *h, void **state, const void**key);
 
+/* Same as pa_hashmap_iterate() but goes backwards */
+void *pa_hashmap_iterate_backwards(pa_hashmap *h, void **state, const void**key);
+
 /* Remove the oldest entry in the hashmap and return it */
 void *pa_hashmap_steal_first(pa_hashmap *h);
 
@@ -69,4 +73,8 @@ void* pa_hashmap_first(pa_hashmap *h);
 #define PA_HASHMAP_FOREACH(e, h, state) \
     for ((state) = NULL, (e) = pa_hashmap_iterate((h), &(state), NULL); (e); (e) = pa_hashmap_iterate((h), &(state), NULL))
 
+/* A macro to ease iteration through all entries, backwards */
+#define PA_HASHMAP_FOREACH_BACKWARDS(e, h, state) \
+    for ((state) = NULL, (e) = pa_hashmap_iterate_backwards((h), &(state), NULL); (e); (e) = pa_hashmap_iterate_backwards((h), &(state), NULL))
+
 #endif