lib: convert threader/container to c++

Did change the code much, but it's now compiled as c++

lib/mu-threader.hh

@@ -0,0 +1,49 @@
+/*
+** Copyright (C) 2012-2020 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+**
+** This program is free software; you can redistribute it and/or modify it
+** under the terms of the GNU General Public License as published by the
+** Free Software Foundation; either version 3, or (at your option) any
+** later version.
+**
+** This program is distributed in the hope that it will be useful,
+** but WITHOUT ANY WARRANTY; without even the implied warranty of
+** MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+** GNU General Public License for more details.
+**
+** You should have received a copy of the GNU General Public License
+** along with this program; if not, write to the Free Software Foundation,
+** Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+**
+*/
+
+#ifndef MU_THREADER_HH__
+#define MU_THREADER_HH__
+
+#include <glib.h>
+#include <mu-msg-iter.h>
+
+/**
+ * takes an iter and the total number of matches, and from this
+ * generates a hash-table with information about the thread structure
+ * of these matches.
+ *
+ * the algorithm to find this structure is based on JWZ's
+ * message-threading algorithm, as descrbed in:
+ *     http://www.jwz.org/doc/threading.html
+ *
+ * the returned hashtable maps the Xapian docid of iter (msg) to a ptr
+ * to a MuMsgIterThreadInfo structure (see mu-msg-iter.h)
+ *
+ * @param iter an iter; note this function will mu_msgi_iter_reset this iterator
+ * @param matches the number of matches in the set *
+ * @param sortfield the field to sort results by, or
+ * MU_MSG_FIELD_ID_NONE if no sorting should be performed
+ * @param revert if TRUE, if revert the sorting order
+ *
+ * @return a hashtable; free with g_hash_table_destroy when done with it
+ */
+GHashTable *mu_threader_calculate (MuMsgIter *iter, size_t matches,
+				   MuMsgFieldId sortfield, gboolean revert);
+
+#endif /*MU_THREADER_HH__*/