song/Filter: move each class into a separate source

src/song/StringFilter.cxx

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2003-2018 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 "config.h"
+#include "StringFilter.hxx"
+#include "util/StringCompare.hxx"
+
+#include <assert.h>
+
+bool
+StringFilter::Match(const char *s) const noexcept
+{
+#if !CLANG_CHECK_VERSION(3,6)
+	/* disabled on clang due to -Wtautological-pointer-compare */
+	assert(s != nullptr);
+#endif
+
+	if (fold_case) {
+		return fold_case.IsIn(s);
+	} else {
+		return StringIsEqual(s, value.c_str());
+	}
+}