Add support for MixRamp tags

Adds mixrampdb and mixrampdelay commands.  Reads MIXRAP_START and
MIXRAMP_END tags from FLAC files and overlaps instead of crossfading.

src/crossfade.c

@@ -26,34 +26,111 @@
 
 #include <assert.h>
 #include <string.h>
+#include <stdlib.h>
+#include <glib.h>
+
+#undef G_LOG_DOMAIN
+#define G_LOG_DOMAIN "crossfade"
+
+static float mixramp_interpolate(char *ramp_list, float required_db)
+{
+	float db, secs, last_db = nan(""), last_secs = 0;
+	char *ramp_str, *save_str = NULL;
+
+	/* ramp_list is a string of pairs of dBs and seconds that describe the
+	 * volume profile. Delimiters are semi-colons between pairs and spaces
+	 * between the dB and seconds of a pair.
+	 * The dB values must be monotonically increasing for this to work. */
+
+	while (1) {
+		/* Parse the dB tokens out of the input string. */
+		ramp_str = strtok_r(ramp_list, " ", &save_str);
+
+		/* Tell strtok to continue next time round. */
+		ramp_list = NULL;
+
+		/* Parse the dB value. */
+		if (NULL == ramp_str) {
+			return nan("");
+		}
+		db = (float)atof(ramp_str);
+
+		/* Parse the time. */
+		ramp_str = strtok_r(NULL, ";", &save_str);
+		if (NULL == ramp_str) {
+			return nan("");
+		}
+		secs = (float)atof(ramp_str);
+
+		/* Check for exact match. */
+		if (db == required_db) {
+			return secs;
+		}
+
+		/* Save if too quiet. */
+		if (db < required_db) {
+			last_db = db;
+			last_secs = secs;
+			continue;
+		}
+
+		/* If required db < any stored value, use the least. */
+		if (isnan(last_db)) {
+			return secs;
+		}
+
+		/* Finally, interpolate linearly. */
+		secs = last_secs + (required_db - last_db) * (secs - last_secs) / (db - last_db);
+		return secs;
+	}
+}
 
 unsigned cross_fade_calc(float duration, float total_time,
+			 float mixramp_db, float mixramp_delay,
+			 char *mixramp_start, char *mixramp_prev_end,
 			 const struct audio_format *af,
 			 const struct audio_format *old_format,
 			 unsigned max_chunks)
 {
-	unsigned int chunks;
+	unsigned int chunks = 0;
+	float chunks_f;
+	float mixramp_overlap;
 
-	if (duration <= 0 || duration >= total_time ||
+	if (duration < 0 || duration >= total_time ||
 	    /* we can't crossfade when the audio formats are different */
 	    !audio_format_equals(af, old_format))
 		return 0;
 
-	assert(duration > 0);
+	assert(duration >= 0);
 	assert(audio_format_valid(af));
 
-	chunks = audio_format_time_to_size(af) / CHUNK_SIZE;
-	chunks = (chunks * duration + 0.5);
+	chunks_f = (float)audio_format_time_to_size(af) / (float)CHUNK_SIZE;
 
-	if (chunks > max_chunks)
+	if (isnan(mixramp_delay) || !(mixramp_start) || !(mixramp_prev_end)) {
+		chunks = (chunks_f * duration + 0.5);
+	} else {
+		/* Calculate mixramp overlap.
+		 * FIXME factor in ReplayGain for both songs. */
+		mixramp_overlap = mixramp_interpolate(mixramp_start, mixramp_db)
+		  + mixramp_interpolate(mixramp_prev_end, mixramp_db);
+		if (!isnan(mixramp_overlap) && (mixramp_delay <= mixramp_overlap)) {
+			chunks = (chunks_f * (mixramp_overlap - mixramp_delay));
+			g_debug("will overlap %d chunks, %fs", chunks,
+				mixramp_overlap - mixramp_delay);
+		}
+	}
+
+	if (chunks > max_chunks) {
 		chunks = max_chunks;
+		g_warning("audio_buffer_size too small for computed MixRamp overlap");
+	}
 
 	return chunks;
 }
 
 void cross_fade_apply(struct music_chunk *a, const struct music_chunk *b,
 		      const struct audio_format *format,
-		      unsigned int current_chunk, unsigned int num_chunks)
+		      float mix_ratio)
 {
 	size_t size;
 
@@ -61,7 +138,6 @@ void cross_fade_apply(struct music_chunk *a, const struct music_chunk *b,
 	assert(b != NULL);
 	assert(a->length == 0 || b->length == 0 ||
 	       audio_format_equals(&a->audio_format, &b->audio_format));
-	assert(current_chunk <= num_chunks);
 
 	if (a->tag == NULL && b->tag != NULL)
 		/* merge the tag into the destination chunk */
@@ -75,7 +151,7 @@ void cross_fade_apply(struct music_chunk *a, const struct music_chunk *b,
 		b->data,
 		size,
 		format,
-		((float)current_chunk) / num_chunks);
+		mix_ratio);
 
 	if (b->length > a->length) {
 		/* the second buffer is larger than the first one: