decoder/opus: move code to new class OggDecoder

Prepare to reuse the same code for a new Vorbis decoder plugin based
on libvorbis instead of libvorbisfile.

src/decoder/plugins/OggDecoder.hxx

@@ -0,0 +1,64 @@
+/*
+ * Copyright 2003-2016 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.
+ */
+
+#ifndef MPD_OGG_DECODER_HXX
+#define MPD_OGG_DECODER_HXX
+
+#include "config.h" /* must be first for large file support */
+#include "lib/xiph/OggVisitor.hxx"
+#include "decoder/Reader.hxx"
+
+class Error;
+
+class OggDecoder : public OggVisitor {
+	ogg_int64_t end_granulepos;
+
+protected:
+	Decoder &decoder;
+	InputStream &input_stream;
+
+public:
+	explicit OggDecoder(DecoderReader &reader)
+		:OggVisitor(reader),
+		 decoder(reader.GetDecoder()),
+		 input_stream(reader.GetInputStream()) {}
+
+	bool Seek(OggSyncState &oy, uint64_t where_frame);
+
+private:
+	/**
+	 * Load the end-of-stream packet and restore the previous file
+	 * position.
+	 */
+	bool LoadEndPacket(ogg_packet &packet) const;
+	ogg_int64_t LoadEndGranulePos() const;
+
+protected:
+	ogg_int64_t UpdateEndGranulePos() {
+		return end_granulepos = LoadEndGranulePos();
+	}
+
+	bool IsSeekable() const {
+		return end_granulepos > 0;
+	}
+
+	bool SeekGranulePos(ogg_int64_t where_granulepos, Error &error);
+};
+
+#endif