Add: Mixer feature for streaming sampled music

src/mixer.cpp

@@ -15,6 +15,7 @@
 #include "framerate_type.h"
 
 #include "safeguards.h"
+#include "mixer.h"
 
 struct MixerChannel {
 	bool active;
@@ -38,6 +39,7 @@ struct MixerChannel {
 static MixerChannel _channels[8];
 static uint32 _play_rate = 11025;
 static uint32 _max_size = UINT_MAX;
+static MxStreamCallback _music_stream = NULL;
 
 /**
  * The theoretical maximum volume for a single sound sample. Multiple sound
@@ -151,6 +153,9 @@ void MxMixSamples(void *buffer, uint samples)
 	/* Clear the buffer */
 	memset(buffer, 0, sizeof(int16) * 2 * samples);
 
+	/* Fetch music if a sampled stream is available */
+	if (_music_stream) _music_stream((int16*)buffer, samples);
+
 	/* Mix each channel */
 	for (mc = _channels; mc != endof(_channels); mc++) {
 		if (mc->active) {
@@ -215,6 +220,17 @@ void MxSetChannelVolume(MixerChannel *mc, uint volume, float pan)
 void MxActivateChannel(MixerChannel *mc)
 {
 	mc->active = true;
+}
+
+/**
+ * Set source of PCM music
+ * @param music_callback Function that will be called to fill sample buffers with music data.
+ * @return Sample rate of mixer, which the buffers supplied to the callback must be rendered at.
+ */
+uint32 MxSetMusicSource(MxStreamCallback music_callback)
+{
+	_music_stream = music_callback;
+	return _play_rate;
 }
 
 
@@ -222,5 +238,6 @@ bool MxInitialize(uint rate)
 {
 	_play_rate = rate;
 	_max_size  = UINT_MAX / _play_rate;
+	_music_stream = NULL; /* rate may have changed, any music source is now invalid */
 	return true;
 }

src/mixer.h

@@ -14,6 +14,14 @@
 
 struct MixerChannel;
 
+/**
+ * Type of callback functions for supplying PCM music.
+ * A music decoder/renderer implements this function and installs it with MxSetMusicSource, which also returns the sample rate used.
+ * @param buffer Pointer to interleaved 2-channel signed 16 bit PCM data buffer, guaranteed to be 0-initialized.
+ * @param samples number of samples that must be filled into \c buffer.
+ */
+typedef void(*MxStreamCallback)(int16 *buffer, size_t samples);
+
 bool MxInitialize(uint rate);
 void MxMixSamples(void *buffer, uint samples);
 
@@ -22,4 +30,6 @@ void MxSetChannelRawSrc(MixerChannel *mc, int8 *mem, size_t size, uint rate, boo
 void MxSetChannelVolume(MixerChannel *mc, uint volume, float pan);
 void MxActivateChannel(MixerChannel*);
 
+uint32 MxSetMusicSource(MxStreamCallback music_callback);
+
 #endif /* MIXER_H */