// Karplus-Strong pluck, tuned: the loop length is the period minus the
// averaging filter's half sample, the fraction is a first-order allpass,
// the excitation is a noise burst minus itself delayed by the pluck
// position (the comb a real pluck makes), and 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)

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 c = 0.0
var ap_x = 0.0
var ap_y = 0.0
var avg = 0.0
var loss = 0.99
var burst = 0.0
var amp = 1.0

fn voice() {
    if trigger {
        let p = sample_rate / freq - 0.5
        let n = floor(p)
        let d = p - n
        len = int(n)
        // Keep the allpass delay in [0.1, 1.1) where it is well behaved.
        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
    }
    // Excitation: filtered noise for one period, minus its comb copy.
    let noise = if t < len { rand() } else { 0.0 }
    burst = burst + (noise - burst) * (0.08 + 0.92 * bright)
    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)
    // The string loop.
    let x0 = buf[w - len]
    let y = c * (x0 - ap_y) + ap_x
    ap_x = x0
    ap_y = y
    let s = (y + avg) * 0.5
    avg = y
    let out = s * loss + input
    buf[w] = out
    w = w + 1
    if gate == 0.0 {
        amp = amp * 0.9995
        if amp < 0.0001 {
            stop()
        }
    }
    out * amp * velocity * 0.6
}