factorygame/util/audio.c

/*
// Created by bruno on 16.2.2025.
*/

#include "audio.h"

AudioData audioData;

uint16_t getAvailableChannel() {
    for (uint16_t i = 0; i < NUM_SYNTH_VOICES; i++) {
        if (audioData.synthVoices[i].volume == 0) {
            return i;
        }
    }
    return -1;
}

// Helper: compute left/right gains from a pan value in [–1..+1]
// pan = –1.0 → full left (L=1, R=0)
// pan = +1.0 → full right (L=0, R=1)
// pan =  0.0 → center (L=R=1/sqrt(2) or just 0.707 to avoid clipping)
static void compute_stereo_gains(float pan, float *outL, float *outR) {
    // Simple linear panning (no constant‐power law).
    // If you prefer constant‐power, you could do:
    //   float angle = (pan + 1.0f) * (M_PI / 4.0f);
    //   *outL = cosf(angle);
    //   *outR = sinf(angle);
    //
    // Here we’ll just do linear:
    pan = fmaxf(-1.0f, fminf(+1.0f, pan));
    if (pan <= 0.0f) {
        *outL = 1.0f;
        *outR = 1.0f + pan; // pan is negative, so R < 1
    } else {
        *outL = 1.0f - pan;  // pan is positive, so L < 1
        *outR = 1.0f;
    }
    // Optionally, scale down both so we never exceed 1.0f / sqrt(2)
    // e.g. *outL *= 0.7071f; *outR *= 0.7071f;
}

// This callback now writes stereo frames: interleaved L/R floats.
void audio_callback(void *userdata, Uint8 *stream, int len) {
    AudioData *audio = (AudioData *) userdata;

    // 'len' is total bytes; each sample‐frame is 2 floats (L+R), i.e. 2 * sizeof(float).
    int frames = len / (2 * sizeof(float));

    // Zero out the entire output buffer (silence)
    // We’ll accumulate into it.
    // Each float is 4 bytes, so total floats = 2 * frames.
    float *outBuf = (float *) stream;
    for (int i = 0; i < 2 * frames; ++i) {
        outBuf[i] = 0.0f;
    }

    // Precompute the listener center
    float listenerCx = audio->playerRect->x + audio->playerRect->w * 0.5f;

    // For each synth voice, mix into the stereo buffer
    for (int v = 0; v < NUM_SYNTH_VOICES; v++) {
        SynthVoice *voice = &audio->synthVoices[v];
        if (voice->volume == 0 || voice->frequency == 0) {
            continue; // skip silent or inactive voices
        }

        // Compute source center X
        float sourceCx = voice->sourceRect.x + voice->sourceRect.w * 0.5f;
        float dx = sourceCx - listenerCx;

        // Normalize for pan. If |dx| >= maxPanDistance → full left or full right.
        float pan = dx / audio->maxPanDistance;
        if (pan < -1.0f) pan = -1.0f;
        if (pan > +1.0f) pan = +1.0f;

        float gainL, gainR;
        compute_stereo_gains(pan, &gainL, &gainR);

        // Optional: You could also attenuate overall volume with distance
        // float dist = fabsf(dx);
        // float distanceAtten = 1.0f - fminf(dist / audio->maxPanDistance, 1.0f);
        // float finalVolume = (voice->volume / 255.0f) * distanceAtten;
        // But for now, we’ll just use voice->volume for amplitude.

        float amp = (voice->volume / 255.0f);

        // Phase increment per sample‐frame:
        //   (freq * 256) / SAMPLE_RATE tells how many phase steps per mono-sample.
        //   Because we’re writing stereo, we still advance phase once per frame.
        uint8_t phaseInc = (uint8_t)((voice->frequency * 256) / SAMPLE_RATE);

        // Mix into each frame
        for (int i = 0; i < frames; i++) {
            float t = (float) voice->phase / 255.0f * 2.0f - 1.0f;
            float sample;
            switch (voice->waveform) {
                default:
                case WAVE_SINE:
                    sample = sinf(voice->phase * 2.0f * M_PI / 256.0f);
                    break;
                case WAVE_SQUARE:
                    sample = (t >= 0.0f) ? 1.0f : -1.0f;
                    break;
                case WAVE_SAWTOOTH:
                    sample = t;
                    break;
                case WAVE_TRIANGLE:
                    sample = (t < 0.0f) ? -t : t;
                    break;
                case WAVE_NOISE:
                    sample = ((float) rand() / RAND_MAX) * 2.0f - 1.0f;
                    break;
            }

            voice->phase += phaseInc;

            // Interleaved index: left = 2*i, right = 2*i + 1
            int idxL = 2 * i;
            int idxR = 2 * i + 1;

            // Accumulate into buffer
            outBuf[idxL] += sample * amp * gainL;
            outBuf[idxR] += sample * amp * gainR;
        }
    }

    // Note: We did not normalize by active voices here, because each voice already
    //   uses its own volume. If you still want an automatic “divide by N active voices”,
    //   you would need to track active voices per‐frame, which is relatively expensive.
    //   In practice, you manage the volume per voice so clipping doesn’t occur.
}