// Plucked string (Karplus-Strong), in tune at every pitch: the loop is one
// period minus the averaging filter's half sample, with the fraction in a
// first-order allpass; the excitation is a filtered noise burst minus its
// copy at the pluck position (the comb a real pluck makes); the loop loss
// gives the same T60 at every pitch.
let MAX = 4096
let bright = param(0.6, 0, 1)
let position = param(0.22, 0.05, 0.5)
let t60 = param(3.0, 0.2, 12)
let damp = param(0.15, 0, 0.9)
let gain = param(0.6, 0, 1)

var buf = [0.0; MAX]
var ex = [0.0; MAX]
var w = int(0)
var len = int(1)
var comb = int(1)
var t = int(0)
var ap = Allpass1{}
var c = 0.0
var avg = 0.0
var lp = 0.0
var loss = 0.99
var burst = 0.0
var amp = 1.0

fn voice() {
    if trigger {
        // One period, minus the averaging filter's half sample and the
        // damping low-pass's phase delay at the fundamental.
        let w0 = TAU * freq / sample_rate
        let lp_delay = atan2(damp * sin(w0), 1.0 - damp * cos(w0)) / w0
        let p = sample_rate / freq - 0.5 - lp_delay
        let d = p - floor(p)
        len = int(floor(p))
        if d < 0.1 {
            len = len - 1
            d = d + 1.0
        }
        c = (1.0 - d) / (1.0 + d)
        comb = max(int(position * p), 1)
        loss = exp(-3.0 * log(10.0) / (freq * t60))
        t = 0
        amp = 1.0
    }
    let noise = if t < len { rand() } else { 0.0 }
    burst = burst + (noise - burst) * (0.08 + 0.92 * bright * velocity)
    let b = if t < len { burst } else { 0.0 }
    ex[t] = b
    let input = if t < len + comb { b - ex[t - comb] } else { 0.0 }
    t = min(t + 1, MAX)
    let y = allpass1(ap, buf[w - len], c)
    let s = (y + avg) * 0.5
    avg = y
    // Extra damping darkens the tail (a one-pole in the loop).
    lp = lp + (s - lp) * (1.0 - damp)
    let out = lp * loss + input
    buf[w] = out
    w = w + 1
    if gate == 0.0 {
        amp = amp * exp(-1.0 / (0.15 * sample_rate))
        if amp < 0.0001 {
            stop()
        }
    }
    out * amp * gain * (0.4 + 0.6 * velocity)
}