DatabasePlugin: add method VisitUniqueTags()

Optimize the ProxyDatabase by invoking "list" on the peer, instead of
visiting all songs.

src/DatabaseHelpers.cxx

@@ -0,0 +1,78 @@
+/*
+ * Copyright (C) 2003-2012 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * 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 2 of the License, 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.
+ */
+
+#include "DatabaseHelpers.hxx"
+#include "DatabasePlugin.hxx"
+#include "song.h"
+#include "tag.h"
+
+#include <functional>
+#include <set>
+
+#include <string.h>
+
+struct StringLess {
+	gcc_pure
+	bool operator()(const char *a, const char *b) const {
+		return strcmp(a, b) < 0;
+	}
+};
+
+typedef std::set<const char *, StringLess> StringSet;
+
+static bool
+CollectTags(StringSet &set, enum tag_type tag_type, song &song)
+{
+	struct tag *tag = song.tag;
+	if (tag == nullptr)
+		return true;
+
+	bool found = false;
+	for (unsigned i = 0; i < tag->num_items; ++i) {
+		if (tag->items[i]->type == tag_type) {
+			set.insert(tag->items[i]->value);
+			found = true;
+		}
+	}
+
+	if (!found)
+		set.insert("");
+
+	return true;
+}
+
+bool
+VisitUniqueTags(const Database &db, const DatabaseSelection &selection,
+		enum tag_type tag_type,
+		VisitString visit_string,
+		GError **error_r)
+{
+	StringSet set;
+
+	using namespace std::placeholders;
+	const auto f = std::bind(CollectTags, std::ref(set), tag_type, _1);
+	if (!db.Visit(selection, f, error_r))
+		return false;
+
+	for (auto value : set)
+		if (!visit_string(value, error_r))
+			return false;
+
+	return true;
+}