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.cxx

@@ -0,0 +1,87 @@
+/*
+ * 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.
+ */
+
+#include "config.h" /* must be first for large file support */
+#include "OggDecoder.hxx"
+#include "lib/xiph/OggFind.hxx"
+#include "input/InputStream.hxx"
+#include "util/Error.hxx"
+
+/**
+ * Load the end-of-stream packet and restore the previous file
+ * position.
+ */
+bool
+OggDecoder::LoadEndPacket(ogg_packet &packet) const
+{
+	if (!input_stream.CheapSeeking())
+		/* we do this for local files only, because seeking
+		   around remote files is expensive and not worth the
+		   trouble */
+		return false;
+
+	const auto old_offset = input_stream.GetOffset();
+
+	/* create temporary Ogg objects for seeking and parsing the
+	   EOS packet */
+
+	bool result;
+
+	{
+		DecoderReader reader(decoder, input_stream);
+		OggSyncState sync2(reader);
+		OggStreamState stream2(GetSerialNo());
+		result = OggSeekFindEOS(sync2, stream2, packet,
+					input_stream);
+	}
+
+	/* restore the previous file position */
+	input_stream.LockSeek(old_offset, IgnoreError());
+
+	return result;
+}
+
+ogg_int64_t
+OggDecoder::LoadEndGranulePos() const
+{
+	ogg_packet packet;
+	if (!LoadEndPacket(packet))
+		return -1;
+
+	return packet.granulepos;
+}
+
+bool
+OggDecoder::SeekGranulePos(ogg_int64_t where_granulepos, Error &error)
+{
+	assert(IsSeekable());
+
+	/* interpolate the file offset where we expect to find the
+	   given granule position */
+	/* TODO: implement binary search */
+	offset_type offset(where_granulepos * input_stream.GetSize()
+			   / end_granulepos);
+
+	if (!input_stream.LockSeek(offset, error))
+		return false;
+
+	PostSeek();
+	return true;
+}
+