NoiceSynth/synth_engine.cpp

#include "synth_engine.h"
#include <math.h>

// A simple sine lookup table for the sine oscillator
const int SINE_TABLE_SIZE = 256;
static int16_t sine_table[SINE_TABLE_SIZE];
static bool sine_table_filled = false;

/**
 * @brief Fills the global sine table. Called once on startup.
 */
void fill_sine_table() {
    if (sine_table_filled) return;
    for (int i = 0; i < SINE_TABLE_SIZE; ++i) {
        // M_PI is not standard C++, but it's common. If it fails, use 3.1415926535...
        sine_table[i] = static_cast<int16_t>(sin(2.0 * M_PI * i / SINE_TABLE_SIZE) * 32767.0);
    }
    sine_table_filled = true;
}

SynthEngine::SynthEngine(uint32_t sampleRate)
    : _sampleRate(sampleRate),
      _phase(0),
      _increment(0),
      _volume(0.5f),
      _waveform(SAWTOOTH),
      _isGateOpen(false),
      _envState(ENV_IDLE),
      _envLevel(0.0f),
      _attackInc(0.0f),
      _decayDec(0.0f),
      _sustainLevel(1.0f),
      _releaseDec(0.0f),
      _lpAlpha(1.0f), _hpAlpha(0.0f),
      _lpVal(0.0f), _hpVal(0.0f)
{
    fill_sine_table();
    // Initialize with a default frequency
    setFrequency(440.0f);
    setADSR(0.05f, 0.1f, 0.7f, 0.2f); // Default envelope
}

void SynthEngine::setFrequency(float freq) {
    // Calculate the phase increment for a given frequency.
    // The phase accumulator is a 32-bit unsigned integer (0 to 2^32-1).
    // One full cycle of the accumulator represents one cycle of the waveform.
    // increment = (frequency * 2^32) / sampleRate
    // The original calculation was incorrect for float frequencies.
    _increment = static_cast<uint32_t>((double)freq * (4294967296.0 / (double)_sampleRate));
}

void SynthEngine::setVolume(float vol) {
    if (vol < 0.0f) vol = 0.0f;
    if (vol > 1.0f) vol = 1.0f;
    _volume = vol;
}

void SynthEngine::setWaveform(Waveform form) {
    _waveform = form;
}

void SynthEngine::setGate(bool isOpen) {
    _isGateOpen = isOpen;
    if (isOpen) {
        _envState = ENV_ATTACK;
    } else {
        _envState = ENV_RELEASE;
    }
}

void SynthEngine::setADSR(float attack, float decay, float sustain, float release) {
    // Calculate increments per sample based on time in seconds
    // Avoid division by zero
    _attackInc = (attack > 0.001f) ? (1.0f / (attack * _sampleRate)) : 1.0f;
    _decayDec  = (decay > 0.001f)  ? (1.0f / (decay * _sampleRate))  : 1.0f;
    _sustainLevel = sustain;
    _releaseDec = (release > 0.001f) ? (1.0f / (release * _sampleRate)) : 1.0f;
}

void SynthEngine::setFilter(float lpCutoff, float hpCutoff) {
    // Simple one-pole filter coefficient calculation: alpha = 2*PI*fc/fs
    _lpAlpha = 2.0f * M_PI * lpCutoff / _sampleRate;
    if (_lpAlpha > 1.0f) _lpAlpha = 1.0f;
    if (_lpAlpha < 0.0f) _lpAlpha = 0.0f;

    _hpAlpha = 2.0f * M_PI * hpCutoff / _sampleRate;
    if (_hpAlpha > 1.0f) _hpAlpha = 1.0f;
    if (_hpAlpha < 0.0f) _hpAlpha = 0.0f;
}

float SynthEngine::getFrequency() const {
    return (float)((double)_increment * (double)_sampleRate / 4294967296.0);
}

void SynthEngine::process(int16_t* buffer, uint32_t numFrames) {
    for (uint32_t i = 0; i < numFrames; ++i) {
        _phase += _increment;

        int16_t sample = 0;
        
        // Oscillator Generation
        switch (_waveform) {
            case SAWTOOTH:
                sample = static_cast<int16_t>(_phase >> 16);
                break;
            case SQUARE:
                sample = (_phase < 0x80000000) ? 32767 : -32768;
                break;
            case SINE:
                sample = sine_table[(_phase >> 24) & 0xFF];
                break;
        }

        float sampleF = static_cast<float>(sample);

        // Apply Filters (One-pole)
        // Low Pass
        _lpVal += _lpAlpha * (sampleF - _lpVal);
        sampleF = _lpVal;
        
        // High Pass (implemented as Input - LowPass(hp_cutoff))
        _hpVal += _hpAlpha * (sampleF - _hpVal);
        sampleF = sampleF - _hpVal;

        // Apply ADSR Envelope
        switch (_envState) {
            case ENV_ATTACK:
                _envLevel += _attackInc;
                if (_envLevel >= 1.0f) { _envLevel = 1.0f; _envState = ENV_DECAY; }
                break;
            case ENV_DECAY:
                _envLevel -= _decayDec;
                if (_envLevel <= _sustainLevel) { _envLevel = _sustainLevel; _envState = ENV_SUSTAIN; }
                break;
            case ENV_SUSTAIN:
                _envLevel = _sustainLevel;
                break;
            case ENV_RELEASE:
                _envLevel -= _releaseDec;
                if (_envLevel <= 0.0f) { _envLevel = 0.0f; _envState = ENV_IDLE; }
                break;
            case ENV_IDLE:
                _envLevel = 0.0f;
                break;
        }
        
        sampleF *= _envLevel;

        // Apply Master Volume and write to buffer
        buffer[i] = static_cast<int16_t>(sampleF * _volume);
    }
}