properties: add notmuch_message_count_properties

The user can already do this manually, of course, but (a) it's nice to
have a convenience function, and (b) exposing this interface means
that someone more clever with a _notmuch_string_map_t than i am can
write a more efficient version if they like, and it will just
accelerate the users of the convenience function.

lib/message-property.cc

@@ -36,6 +36,31 @@ notmuch_message_get_property (notmuch_message_t *message, const char *key, const
     return NOTMUCH_STATUS_SUCCESS;
 }
 
+notmuch_status_t
+notmuch_message_count_properties (notmuch_message_t *message, const char *key, unsigned int *count)
+{
+    if (! count || ! key || ! message)
+	return NOTMUCH_STATUS_NULL_POINTER;
+
+    notmuch_string_map_t *map;
+    map = _notmuch_message_property_map (message);
+    if (! map)
+	return NOTMUCH_STATUS_NULL_POINTER;
+
+    notmuch_string_map_iterator_t *matcher = _notmuch_string_map_iterator_create (map, key, true);
+    if (! matcher)
+	return NOTMUCH_STATUS_OUT_OF_MEMORY;
+
+    *count = 0;
+    while (_notmuch_string_map_iterator_valid (matcher)) {
+	(*count)++;
+	_notmuch_string_map_iterator_move_to_next (matcher);
+    }
+
+    _notmuch_string_map_iterator_destroy (matcher);
+    return NOTMUCH_STATUS_SUCCESS;
+}
+
 static notmuch_status_t
 _notmuch_message_modify_property (notmuch_message_t *message, const char *key, const char *value,
 				  bool delete_it)

lib/notmuch.h

@@ -1890,6 +1890,22 @@ typedef struct _notmuch_string_map_iterator notmuch_message_properties_t;
 notmuch_message_properties_t *
 notmuch_message_get_properties (notmuch_message_t *message, const char *key, notmuch_bool_t exact);
 
+/**
+ * Return the number of properties named "key" belonging to the specific message.
+ *
+ * @param[in] message  The message to examine
+ * @param[in] key      key to count
+ * @param[out] count   The number of matching properties associated with this message.
+ *
+ * @returns
+ *
+ * NOTMUCH_STATUS_SUCCESS: successful count, possibly some other error.
+ *
+ * @since libnotmuch 5.2 (notmuch 0.27)
+ */
+notmuch_status_t
+notmuch_message_count_properties (notmuch_message_t *message, const char *key, unsigned int *count);
+
 /**
  * Is the given *properties* iterator pointing at a valid (key,value)
  * pair.