notmuch-index-message: Start generating correct thread_id values.

Currently we're looking up all parents (based on In-reply-to and
References header) and using the list of all thread_id values
from those as our thread_id value. We're missing one step which
sup does which is to also look up any children in the database
that have reference our message ID. So we'll need to do that next.

notmuch-index-message.cc

@@ -33,6 +33,9 @@ using namespace std;
 
 #define ARRAY_SIZE(arr) (sizeof (arr) / sizeof (arr[0]))
 
+/* Xapian complains if we provide a term longer than this. */
+#define NOTMUCH_MAX_TERM 245
+
 /* These prefix values are specifically chosen to be compatible
  * with sup, (http://sup.rubyforge.org), written by
  * William Morgan <wmorgan-sup@masanjin.net>, and released
@@ -109,6 +112,7 @@ add_term (Xapian::Document doc,
 
     term = g_strdup_printf ("%s%s", prefix, value);
 
+    if (strlen (term) <= NOTMUCH_MAX_TERM)
 	doc.add_term (term);
 
     g_free (term);
@@ -204,6 +208,58 @@ skip_re_in_subject (const char *subject)
     return s;
 }
 
+Xapian::Document
+find_message (Xapian::Database db, const char *message_id)
+{
+    Xapian::PostingIterator i;
+    char *term;
+
+    term = g_strdup_printf ("%s%s", find_prefix ("msgid"), message_id);
+    i = db.postlist_begin (term);
+    if (i != db.postlist_end (term))
+	return db.get_document (*i);
+    else
+	return Xapian::Document ();
+}
+
+static char *
+find_thread_id (Xapian::Database db, GPtrArray *parents)
+{
+    Xapian::Document doc;
+    GHashTable *thread_ids;
+    GList *keys, *l;
+    GString *result = NULL;
+    unsigned int i;
+    string value_string;
+    const char *value;
+
+    thread_ids = g_hash_table_new (g_str_hash, g_str_equal);
+
+    for (i = 0; i < parents->len; i++) {
+	doc = find_message (db, (char *) g_ptr_array_index (parents, i));
+	value_string = doc.get_value (NOTMUCH_VALUE_THREAD);
+	value = value_string.c_str();
+	if (strlen (value))
+	    g_hash_table_insert (thread_ids, strdup (value), NULL);
+    }
+
+    keys = g_hash_table_get_keys (thread_ids);
+    for (l = keys; l; l = l->next) {
+	char *id = (char *) l->data;
+	if (result == NULL) {
+	    result = g_string_new (id);
+	} else {
+	    g_string_append_printf (result, ",%s", id);
+	}
+	free (id);
+    }
+
+    if (result)
+	return g_string_free (result, FALSE);
+    else
+	return NULL;
+}
+
 /* Add a term for each message-id in the References header of the
  * message. */
 static void
@@ -334,7 +390,7 @@ index_file (Xapian::WritableDatabase db,
 
     FILE *file;
 
-    const char *value, *from;
+    const char *value, *from, *thread_id;
 
     time_t time;
     struct tm gm_time_tm;
@@ -386,7 +442,9 @@ index_file (Xapian::WritableDatabase db,
     parse_references (parents, value);
 
     for (i = 0; i < parents->len; i++)
-	add_term (doc, "ref", (const char *) g_ptr_array_index (parents, i));
+	add_term (doc, "ref", (char *) g_ptr_array_index (parents, i));
+
+    thread_id = find_thread_id (db, parents);
 
     for (i = 0; i < parents->len; i++)
 	g_free (g_ptr_array_index (parents, i));
@@ -420,10 +478,17 @@ index_file (Xapian::WritableDatabase db,
 
     value = g_mime_message_get_message_id (message);
     add_term (doc, "msgid", value);
-    add_term (doc, "thread", value);
-
     doc.add_value (NOTMUCH_VALUE_MESSAGE_ID, value);
+
+    if (thread_id) {
+	add_term (doc, "thread", thread_id);
+	doc.add_value (NOTMUCH_VALUE_THREAD, thread_id);
+	free ((void *) thread_id);
+    } else {
+	/* If not referenced thread, use the message ID */
+	add_term (doc, "thread", value);
 	doc.add_value (NOTMUCH_VALUE_THREAD, value);
+    }
 
     doc.add_value (NOTMUCH_VALUE_DATE, Xapian::sortable_serialise (time));