Example for decoding and playing an ADPCM audio buffer on the Playdate in C, only supports mono IMA-ADPCM with no blocks!

playdate_adpcm.c

// adpcm.h
#define CLAMP(n, l, h) \
	if (n < l) n = l;  \
	if (n > h) n = h;

#define min(a,b) (((a) < (b)) ? (a) : (b))

static const int8_t indexTable[8] =
{ -1, -1, -1, -1, 2, 4, 6, 8 };

static const int16_t stepTable[89] =
{
	7, 8, 9, 10, 11, 12, 13, 14, 16, 17,
	19, 21, 23, 25, 28, 31, 34, 37, 41, 45,
	50, 55, 60, 66, 73, 80, 88, 97, 107, 118,
	130, 143, 157, 173, 190, 209, 230, 253, 279, 307,
	337, 371, 408, 449, 494, 544, 598, 658, 724, 796,
	876, 963, 1060, 1166, 1282, 1411, 1552, 1707, 1878, 2066,
	2272, 2499, 2749, 3024, 3327, 3660, 4026, 4428, 4871, 5358,
	5894, 6484, 7132, 7845, 8630, 9493, 10442, 11487, 12635, 13899,
	15289, 16818, 18500, 20350, 22385, 24623, 27086, 29794, 32767
};

static int diffTable[89][16];

static int32_t predictor;
static int8_t  stepIndex;

typedef struct adpcmplay_ctx
{
	int16_t* pcmBuff;
	AudioSample* sample;
	SamplePlayer* player;
} adpcmplay_ctx;

// adpcm.c

// Call once to set up diff table
void adpcmInit()
{
	int16_t step;
	int diff;
	for (u16 stepIndex = 0; stepIndex < 89; stepIndex++)
	{
		step = stepTable[stepIndex];
		for (u16 sample = 0; sample < 16; sample++)
		{
			diff = step >> 3;
			if (sample & 1) diff += step >> 2;
			if (sample & 2) diff += step >> 1;
			if (sample & 4) diff += step;
			if (sample & 8) diff = -diff;
			diffTable[stepIndex][sample] = diff;
		}
	}
}

// Converts a single 4-bit adpcm sample to pcm
int16_t adpcmDecodeSample(uint8_t sample)
{
	predictor += diffTable[stepIndex][sample];
	CLAMP(predictor, -32768, 32767);

	stepIndex += indexTable[sample & 7];
	CLAMP(stepIndex, 0, 88);

	return (int16_t)predictor;
}

// Converts adpcm buffer (in) to pcm buffer (out)
// assumes mono-channel audio buffer
void adpcmDecodeBuffer(const uint8_t* in, int16_t* out, int length)
{
	predictor = 0;
	stepIndex = 0;
	while (length--)
	{
		*out++ = adpcmDecodeSample(*in  & 0xF);
		*out++ = adpcmDecodeSample(*in++ >> 4);
	}
}

// Creates a sample player for an adpcm buffer
adpcmplay_ctx* adpcmPlayerNew(const uint8_t* in, int numSamples, int sampleRate)
{
	adpcmplay_ctx* ctx = pd->system->realloc(NULL, sizeof(adpcmplay_ctx));
	int adpcmSize = numSamples * sizeof(uint8_t) / 2;
	int pcmSize = numSamples * sizeof(int16_t);
	// Allocate a pcm buffer
	ctx->pcmBuff = pd->system->realloc(NULL, pcmSize);
	// Decode ADPCM
	adpcmDecodeBuffer(in, ctx->pcmBuff, adpcmSize);
	// Create an audio sample from the data buffer
	// the data buffer isn't copied, so it *should* be possible to update it on the fly and hear that reflected in the audio
	ctx->sample = pd->sound->sample->newSampleFromData((uint8_t*)ctx->pcmBuff, kSound16bitMono, sampleRate, pcmSize);
	ctx->player = pd->sound->sampleplayer->newPlayer();
	// Create a sample player with the given sample
	pd->sound->sampleplayer->setSample(ctx->player, ctx->sample);
	return ctx;
}

void adpcmPlayerPlay(ctx)
{
	pd->sound->sampleplayer->play(ctx->player, 1, 1.0);
}

void adpcmPlayerStop(ctx)
{
	pd->sound->sampleplayer->stop(ctx->player);
}

// Cleans up the sample player and the pcm buffer
void adpcmPlayerFree(ctx)
{
	pd->sound->sampleplayer->stop(ctx->player);
	pd->sound->sampleplayer->freePlayer(ctx->player);
	pd->sound->sample->freeSample(ctx->sample);
	pd->system->realloc(ctx->pcmBuff, 0);
}