raspi-synth/include/synth/dsp/adsr.h

#ifndef __ADSR_H__
#define __ADSR_H__

#include <cmath>

#include "util.h"

class ADSR {
public:
    typedef struct {
        float attackStep;
        float decayStep;
        float sustain;
        float releaseStep;
    } Envelope;

    const Envelope* env;

    ADSR() = delete;
    ADSR(const Envelope *env);
    void noteOn();
    void noteOff();

    inline float tick() {
        float out = curve(gain);

        switch(state) {
            case IDLE:
                break;

            case ATTACK:
                if(gain < 1) {
                    gain += env->attackStep;
                } else {
                    state = DECAY;
                    gain = 1;
                }
                break;

            case DECAY:
                if(gain > env->sustain) {
                    gain -= env->decayStep;
                } else {
                    state = SUSTAIN;
                    gain = env->sustain;
                }
                break;

            case SUSTAIN:
                break;

            case RELEASE:
                if(gain > 0) {
                    gain -= env->releaseStep;
                } else {
                    state = IDLE;
                    gain = 0;
                }
                break;
        }

        return out;
    }

private:
    enum { IDLE, ATTACK, DECAY, SUSTAIN, RELEASE } state;
    float gain;

    float curve(float gain) {
        return 0.5 - 0.5 * cos(clamp(gain * M_PI, 0, M_PI));
    }
};

#endif