Synthesizer playing the Tetris music

tetris.formula.c

// Generates a note for a given frequency
float synth(float freq) {
    
    float output = freq*fmod(TIME, 1/freq) / 2;
    output += freq*fmod(TIME, 1/(freq*1.005)) / 2;
    output += freq*fmod(TIME, 1/(freq*0.995)) / 2;
    output += 0.5*pow(sin(2*M_PI*freq / 4 * TIME), 5);
    
    return output;
}

// Linear interpolation
float lerp(float x, float x1, float x2, float y1, float y2) {
    return y1 + (x-x1) * (y2-y1) / (x2-x1) ;
}

// Manages attak, decay, sustain and release
float adsr(float sample, float t, float duration) {
    float attackTime = 0.01;
    float decayTime = 0.1;
    float sustainGain = 0.5;
    float releaseTime = 0.1;
    
    if (t < attackTime) {
        sample *= lerp(t, 0, attackTime, 0, 1);
    }    
    else if (t < attackTime + decayTime) {
        sample *= lerp(t, attackTime, attackTime + decayTime, 1, sustainGain);
    }
    else if (t < duration - releaseTime) {
        sample *= sustainGain;
    }
    else {
        sample *= lerp(t, duration - releaseTime, duration, sustainGain, 0);
    }
    return sample;
}

// Tetris notes and durations
const float notes[41][2] = {
    {659.25, 0.5}, {493.88, 0.25}, {523.25, 0.25}, {587.33, 0.5},  {523.25, 0.25}, {493.88, 0.25},
    {440.00, 0.5}, {440.00, 0.25}, {523.25, 0.25}, {659.25, 0.5},  {587.33, 0.25}, {523.25, 0.25},
    {493.88, 0.5}, {493.88, 0.25}, {523.25, 0.25}, {587.33, 0.5},  {659.25, 0.5},
    {523.25, 0.5}, {440.00, 0.5},  {440.00, 0.5},  {0, 0.5},
    {0, 0.25},     {587.33, 0.5},  {698.46, 0.25}, {880.00, 0.5},  {783.99, 0.25}, {698.46, 0.25},
    {659.25, 0.75},{523.25, 0.25}, {659.25, 0.5},  {587.33, 0.25}, {523.25, 0.25},
    {493.88, 0.5}, {493.88, 0.25}, {523.25, 0.25}, {587.33, 0.5},  {659.25, 0.5},
    {523.25, 0.5}, {440.00, 0.5},  {440.00, 0.5},  {0, 0.5},
};
const float totalDuration = 16;

formula_main {
    
    float output = 0;
    
    float musicTime = fmod(TIME, totalDuration);
    
    float noteLength, noteFreq;
    for (int i = 0; i < 41; i++) {
        noteFreq = notes[i][0];
        noteLength = notes[i][1];
        if (musicTime < noteLength) {
            break;
        }
        else {
            musicTime -= noteLength;
        }
    }
    
    output = synth(noteFreq);
    output = adsr(output, musicTime, noteLength);
    
    return output;
}