Move audio resampling out of SDL code

src/gba/gba-audio.c

@@ -6,6 +6,7 @@
 #include "gba-thread.h"
 
 #include <limits.h>
+#include <math.h>
 
 const unsigned GBA_AUDIO_SAMPLES = 512;
 const unsigned GBA_AUDIO_FIFO_SIZE = 8 * sizeof(int32_t);
@@ -419,6 +420,41 @@ unsigned GBAAudioCopy(struct GBAAudio* audio, void* left, void* right, unsigned
 	return read;
 }
 
+void GBAAudioResampleNN(struct GBAAudio* audio, float ratio, float* drift, struct GBAStereoSample* output, unsigned nSamples) {
+	int32_t left[GBA_AUDIO_SAMPLES];
+	int32_t right[GBA_AUDIO_SAMPLES];
+
+	// toRead is in GBA samples
+	// TODO: Do this with fixed-point math
+	unsigned toRead = ceilf(nSamples / ratio);
+	while (nSamples) {
+		unsigned currentRead = GBA_AUDIO_SAMPLES;
+		if (currentRead > toRead) {
+			currentRead = toRead;
+		}
+		unsigned read = GBAAudioCopy(audio, left, right, currentRead);
+		toRead -= read;
+		unsigned i;
+		for (i = 0; i < read; ++i) {
+			*drift += ratio;
+			while (*drift >= 1.f) {
+				output->left = left[i];
+				output->right = right[i];
+				++output;
+				--nSamples;
+				*drift -= 1.f;
+				if (!nSamples) {
+					return;
+				}
+			}
+		}
+		if (read < currentRead) {
+			memset(output, 0, nSamples * sizeof(struct GBAStereoSample));
+			return;
+		}
+	}
+}
+
 static int32_t _updateSquareChannel(struct GBAAudioSquareControl* control, int duty) {
 	control->hi = !control->hi;
 	int period = 16 * (2048 - control->frequency);

src/gba/gba-audio.h

@@ -7,7 +7,7 @@
 
 struct GBADMA;
 
-const unsigned GBA_AUDIO_SAMPLES;
+extern const unsigned GBA_AUDIO_SAMPLES;
 
 struct GBAAudioEnvelope {
 	union {
@@ -204,6 +204,11 @@ struct GBAAudio {
 	int32_t sampleInterval;
 };
 
+struct GBAStereoSample {
+	int16_t left;
+	int16_t right;
+};
+
 void GBAAudioInit(struct GBAAudio* audio);
 void GBAAudioDeinit(struct GBAAudio* audio);
 
@@ -229,6 +234,7 @@ void GBAAudioWriteFIFO(struct GBAAudio* audio, int address, uint32_t value);
 void GBAAudioSampleFIFO(struct GBAAudio* audio, int fifoId, int32_t cycles);
 
 unsigned GBAAudioCopy(struct GBAAudio* audio, void* left, void* right, unsigned nSamples);
+void GBAAudioResampleNN(struct GBAAudio*, float ratio, float* drift, struct GBAStereoSample* output, unsigned nSamples);
 
 struct GBASerializedState;
 void GBAAudioSerialize(const struct GBAAudio* audio, struct GBASerializedState* state);

src/gba/gba.h

@@ -7,7 +7,7 @@
 #include "gba-video.h"
 #include "gba-audio.h"
 
-const uint32_t GBA_ARM7TDMI_FREQUENCY;
+extern const uint32_t GBA_ARM7TDMI_FREQUENCY;
 
 enum GBAIRQ {
 	IRQ_VBLANK = 0x0,