soundThing/include/synth.h

#ifndef SYNTH_H
#define SYNTH_H

#include <stdint.h>

#define VOICE_COUNT 8
#define OSC_COUNT 2
#define LFO_COUNT 2
#define OSC_MAX_GAIN 4.0f

typedef enum {
    WAVE_SINE,
    WAVE_TRIANGLE,
    WAVE_SAW,
    WAVE_RAMP,
    WAVE_PULSE,
    WAVE_NOISE,
    WAVE_COUNT  // handy for the modulo wrap on waveform switching
} Waveform;

// Envelope structures:
typedef enum {
    ENV_IDLE,
    ENV_ATTACK,
    ENV_DECAY,
    ENV_SUSTAIN,
    ENV_RELEASE
} EnvStage;

typedef enum {
    MOD_SOURCE_NONE    = 0,
    MOD_SOURCE_AMP_ENV = 1,
    MOD_SOURCE_MOD_ENV = 2,
    MOD_SOURCE_LFO     = 3,
    MOD_SOURCE_LFO2    = 4
} ModSource;

typedef struct {
    EnvStage stage;
    float    value;      // current output value, 0.0 to 1.0
    float    attackSec;
    float    decaySec;
    float    sustainLevel;
    float    releaseSec;
} Envelope;

typedef struct {
    float    phase;
    float    rate;        // Hz
    Waveform waveform;
    int      active;
    float    noiseHeld;
    float    noisePhase;
} LFO;

typedef enum {
    FILTER_LOWPASS,
    FILTER_HIGHPASS,
    FILTER_BANDPASS,
    FILTER_COUNT
} FilterType;

typedef struct {
    float      cutoff;      // Hz
    float      resonance;   // 0.0 (flat) to 0.99 (near self-oscillation)
    FilterType type;
    int        active;
    float      low, band;   // TPT integrator states s1, s2
    int        modRouting;
    float      modDepth;    // Hz
    int        resModRouting;
    float      resModDepth; // resonance units (-0.99..0.99)
} Filter;

typedef struct {
    float    phase;
    float    dutyCycle;
    Waveform waveform;
    float    detune;      // cents, 0 = no detune
    float    noiseHeld;   // last drawn random value for clocked noise
    float    noisePhase;  // tracks when to draw a new noise value
    float    gain;
    int      active;          // whether this oscillator contributes to output
    int      octave;          // transposition in octaves, -2 to +2
    // Modulation routing: one source per destination by design.
    // Each parameter (pwm, detune, gain) has exactly one mod source and one depth.
    int      modRouting[3];   // 0=off, 1=env0, 2=env1, 3=lfo0, 4=lfo1
    float    modDepth[3];     // Depth of modulation parameter
} Oscillator;

typedef struct {
    Oscillator oscillators[OSC_COUNT];
    float      freqHz;
    int        active;
    int        midiNote;
    Envelope   ampEnv;
    Envelope   modEnv;
    Filter     filter;
} Voice;

typedef struct {
    float    sampleRate;
    float    pitchBend;
    float    pitchBendRange;
    int      lastStolenVoice;
    Voice    voices[VOICE_COUNT];
    float    volume;  // 0.0 to 1.0, master output level
    LFO      lfos[LFO_COUNT];
} Synth;

// Envelope functions:
void  envelopeInit(Envelope *e, float attackSec, float decaySec, float sustainLevel, float releaseSec);
void  envelopeNoteOn(Envelope *e);
void  envelopeNoteOff(Envelope *e);
float envelopeTick(Envelope *e, float sampleRate);

// Oscillator functions:
void oscillatorInit(Oscillator *o, Waveform waveform, float dutyCycle, float detune, float gain);
float oscillatorTick(Oscillator *o, float freqHz, float bendMultiplier, float sampleRate,
                     float dutyCycle, float detune, float gain);

// Synth functions:
void  synthInit(Synth *s, float sampleRate);
void  synthResetPatch(Synth *s);
void  synthNoteOn(Synth *s, int midiNote);
void  synthNoteOff(Synth *s, int midiNote);
void  synthFillBuffer(Synth *s, int16_t *out, int frames);
void  synthSyncVoices(Synth *s);

// LFO functions:
float lfoTick(LFO *l, float sampleRate);

// Filter functions:
float filterTick(Filter *f, float input, float cutoff, float resonance, float sampleRate);
const char *filterTypeName(FilterType t);

// Waveform functions:
float waveformSample(Waveform w, float phase, float dutyCycle, float noiseHeld);
const char *waveformName(Waveform w);

#endif