SDL_AudioStream latency control

latency.c

#define SDL_PROP_AUDIO_STREAM_LATENCY_CONTROL "SDL.audio.stream.latency_control"

#define SDL_AUDIO_LATENCY_CONTROL_SAMPLES 16

typedef struct SDL_AudioLatencyControl
{
    float target_latency;
    float min_latency;
    float max_latency;

    int gets[SDL_AUDIO_LATENCY_CONTROL_SAMPLES];
    int avails[SDL_AUDIO_LATENCY_CONTROL_SAMPLES];
    int samples;

    float avg_latency;

    float ratio;
} SDL_AudioLatencyControl;

static void SDLCALL SDL_AudioLatencyControlGetCallback(void *userdata, SDL_AudioStream *stream, int additional_amount, int total_amount)
{
    SDL_AudioLatencyControl *self = userdata;
    int i;

    self->gets[self->samples] = (int)(total_amount);
    self->avails[self->samples] = SDL_GetAudioStreamQueued(stream);
    self->samples = (self->samples + 1) % SDL_AUDIO_LATENCY_CONTROL_SAMPLES;

    int max_request = self->gets[0];
    int min_queued = self->avails[0];

    for (i = 1; i < 8; ++i) {
        max_request = SDL_max(max_request, self->gets[i]);
        min_queued = SDL_min(min_queued, self->avails[i]);
    }

    self->avg_latency += (((float)min_queued / (float)max_request) - self->avg_latency) * 0.5f;

    float ratio = self->ratio;

    // Check if we are above or below the target latency
    if (self->avg_latency > self->target_latency) {
        if (self->avg_latency > self->max_latency * 2.0f) {
            // If we are way above the max latency, just clear the stream.
            SDL_ClearAudioStream(stream);
            ratio = 1.0f;
        } else if (ratio < 1.0f) {
            // We've been trying to increase the latency, and now we've reached our target.
            ratio = 1.0f;
        } else if (self->avg_latency > self->max_latency) {
            // Slowly decrease the latency.
            ratio = 1.001f;
        }
    } else {
        if (ratio > 1.0f) {
            // We've been trying to decrease the latency, and now we've reached our target.
            ratio = 1.0f;
        } else if (self->avg_latency < self->min_latency) {
            // Slowly increase the latency.
            ratio = 0.999f;
        }
    }

    SDL_SetAudioStreamFrequencyRatio(stream, ratio);

    self->ratio = ratio;
}

static void SDLCALL SDL_CleanupLatencyControl(void *userdata, void *value)
{
    SDL_free(value);
}

int SDL_SetAudioStreamLatencyControl(SDL_AudioStream *stream, float min_latency, float max_latency)
{
    // TODO: Check errors

    SDL_AudioLatencyControl *self = SDL_calloc(1, sizeof(*self));
    self->min_latency = min_latency;
    self->max_latency = max_latency;
    self->target_latency = (min_latency + max_latency) * 0.5f;
    self->ratio = 1.0f;

    SDL_SetPropertyWithCleanup(SDL_GetAudioStreamProperties(stream), SDL_PROP_AUDIO_STREAM_LATENCY_CONTROL, self, SDL_CleanupLatencyControl, NULL);
    SDL_SetAudioStreamGetCallback(stream, SDL_AudioLatencyControlGetCallback, self);

    return 0;
}