decoder/flac: calculate time stamp from current frame

Don't update a float timestamp, this will make imprecisions add up
after a while.  We already have the number of the current frame, let's
just calculate the float timestamp from that for every decoder_data()
command.  For this, we need to add the attribute "first_frame", for
CUE sheet songs.

src/decoder/_flac_common.c

@@ -36,9 +36,9 @@ flac_data_init(struct flac_data *data, struct decoder * decoder,
 	pcm_buffer_init(&data->buffer);
 
 	data->have_stream_info = false;
+	data->first_frame = 0;
 	data->next_frame = 0;
 
-	data->time = 0;
 	data->position = 0;
 	data->decoder = decoder;
 	data->input_stream = input_stream;
@@ -116,6 +116,7 @@ flac_common_write(struct flac_data *data, const FLAC__Frame * frame,
 	size_t buffer_size = frame->header.blocksize *
 		audio_format_frame_size(&data->audio_format);
 	void *buffer;
+	float position;
 	unsigned bit_rate;
 
 	buffer = pcm_buffer_get(&data->buffer, buffer_size);
@@ -124,6 +125,12 @@ flac_common_write(struct flac_data *data, const FLAC__Frame * frame,
 		     data->audio_format.bits, buf,
 		     0, frame->header.blocksize);
 
+	if (data->next_frame >= data->first_frame)
+		position = (float)(data->next_frame - data->first_frame) /
+			data->audio_format.sample_rate;
+	else
+		position = 0;
+
 	if (nbytes > 0)
 		bit_rate = nbytes * 8 * frame->header.sample_rate /
 			(1000 * frame->header.blocksize);
@@ -132,7 +139,7 @@ flac_common_write(struct flac_data *data, const FLAC__Frame * frame,
 
 	cmd = decoder_data(data->decoder, data->input_stream,
 			   buffer, buffer_size,
-			   data->time, bit_rate,
+			   position, bit_rate,
 			   data->replay_gain_info);
 	data->next_frame += frame->header.blocksize;
 	switch (cmd) {

src/decoder/_flac_common.h

@@ -51,12 +51,17 @@ struct flac_data {
 	 */
 	FLAC__StreamMetadata_StreamInfo stream_info;
 
+	/**
+	 * The number of the first frame in this song.  This is only
+	 * non-zero if playing sub songs from a CUE sheet.
+	 */
+	FLAC__uint64 first_frame;
+
 	/**
 	 * The number of the next frame which is going to be decoded.
 	 */
 	FLAC__uint64 next_frame;
 
-	float time;
 	struct audio_format audio_format;
 	FLAC__uint64 position;
 	struct decoder *decoder;

src/decoder/flac_plugin.c

@@ -191,14 +191,9 @@ static FLAC__StreamDecoderWriteStatus
 flac_write_cb(const FLAC__StreamDecoder *dec, const FLAC__Frame *frame,
 	      const FLAC__int32 *const buf[], void *vdata)
 {
-	FLAC__uint32 samples = frame->header.blocksize;
 	struct flac_data *data = (struct flac_data *) vdata;
-	float timeChange;
 	FLAC__uint64 nbytes = 0;
 
-	timeChange = ((float)samples) / frame->header.sample_rate;
-	data->time += timeChange;
-
 	if (FLAC__stream_decoder_get_decode_position(dec, &nbytes)) {
 		if (data->position > 0 && nbytes > data->position) {
 			nbytes -= data->position;
@@ -431,6 +426,8 @@ flac_decoder_loop(struct flac_data *data, FLAC__StreamDecoder *flac_dec,
 	struct decoder *decoder = data->decoder;
 	enum decoder_command cmd;
 
+	data->first_frame = t_start;
+
 	while (true) {
 		if (data->tag != NULL && !tag_is_empty(data->tag)) {
 			cmd = decoder_tag(data->decoder, data->input_stream,
@@ -448,8 +445,6 @@ flac_decoder_loop(struct flac_data *data, FLAC__StreamDecoder *flac_dec,
 			    (t_end == 0 || seek_sample <= t_end) &&
 			    FLAC__stream_decoder_seek_absolute(flac_dec, seek_sample)) {
 				data->next_frame = seek_sample;
-				data->time = (float)(seek_sample - t_start) /
-					data->audio_format.sample_rate;
 				data->position = 0;
 				decoder_command_finished(decoder);
 			} else

src/decoder/oggflac_plugin.c

@@ -155,11 +155,6 @@ oggflac_write_cb(G_GNUC_UNUSED const OggFLAC__SeekableStreamDecoder *decoder,
 		 void *vdata)
 {
 	struct flac_data *data = (struct flac_data *) vdata;
-	FLAC__uint32 samples = frame->header.blocksize;
-	float time_change;
-
-	time_change = ((float)samples) / frame->header.sample_rate;
-	data->time += time_change;
 
 	return flac_common_write(data, frame, buf, 0);
 }
@@ -338,8 +333,6 @@ oggflac_decode(struct decoder * mpd_decoder, struct input_stream *input_stream)
 			if (OggFLAC__seekable_stream_decoder_seek_absolute
 			    (decoder, seek_sample)) {
 				data.next_frame = seek_sample;
-				data.time = ((float)seek_sample) /
-				    data.audio_format.sample_rate;
 				data.position = 0;
 				decoder_command_finished(mpd_decoder);
 			} else