Add function notmuch_query_count_threads() to get the number of threads
matching a search. This is done by performing a search and figuring out the
number of unique thread IDs in the matching messages, a significantly
heavier operation than notmuch_query_count_messages().

Signed-off-by: Jani Nikula <jani@nikula.org>
---
 lib/notmuch.h |   14 ++++++++++++++
 lib/query.cc  |   40 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 54 insertions(+), 0 deletions(-)

diff --git a/lib/notmuch.h b/lib/notmuch.h
index c4330e4..662d594 100644
--- a/lib/notmuch.h
+++ b/lib/notmuch.h
@@ -609,6 +609,20 @@ notmuch_threads_destroy (notmuch_threads_t *threads);
 unsigned
 notmuch_query_count_messages (notmuch_query_t *query);
  
+/* Return the number of threads matching a search.
+ *
+ * This function performs a search and returns the number of unique thread IDs
+ * in the matching messages. This is the same as number of threads matching a
+ * search.
+ *
+ * Note that this is a fairly heavy operation, much more so than
+ * notmuch_query_count_messages().
+ *
+ * If an error occurs, this function may return 0.
+ */
+unsigned
+notmuch_query_count_threads (notmuch_query_t *query);
+
 /* Get the thread ID of 'thread'.
  *
  * The returned string belongs to 'thread' and as such, should not be
diff --git a/lib/query.cc b/lib/query.cc
index 6f02b04..2c0610a 100644
--- a/lib/query.cc
+++ b/lib/query.cc
@@ -457,3 +457,43 @@ notmuch_query_count_messages (notmuch_query_t *query)
 
     return count;
 }
+
+unsigned
+notmuch_query_count_threads (notmuch_query_t *query)
+{
+    notmuch_messages_t *messages;
+    GHashTable *hash;
+    unsigned int count;
+
+    messages = notmuch_query_search_messages (query);
+    if (messages == NULL)
+	return 0;
+
+    hash = g_hash_table_new_full (g_str_hash, g_str_equal, NULL, NULL);
+    if (hash == NULL) {
+	talloc_free (messages);
+	return 0;
+    }
+
+    while (notmuch_messages_valid (messages)) {
+	notmuch_message_t *message = notmuch_messages_get (messages);
+	const char *thread_id = notmuch_message_get_thread_id (message);
+	char *thread_id_copy = talloc_strdup(messages, thread_id);
+	if (unlikely (thread_id_copy == NULL)) {
+	    notmuch_message_destroy (message);
+	    count = 0;
+	    goto DONE;
+	}
+	g_hash_table_insert (hash, thread_id_copy, NULL);
+	notmuch_message_destroy (message);
+	notmuch_messages_move_to_next (messages);
+    }
+
+    count = g_hash_table_size(hash);
+
+  DONE:
+    g_hash_table_unref (hash);
+    talloc_free (messages);
+
+    return count;
+}
-- 
1.7.5.4