Easy way to implement karplus strong

easy_karplus.js

var freq1 = Slot(1.0);
var ad1 = AdjustableDelay(1.0, freq1);
var lp1 = LowPass();

function dsp(t) {
  freq1(1/(400 + Math.floor(cos(t*tau)*40) + 100*cos(110*t*tau)));
  var ts = t % 0.4;
  var stroke = arp(ts, 1*noise(), 1, 100) + ad1();
  ad1(lp1(stroke, 10000) * 0.99);
  return 0.5 * stroke;
}

function arp(ts, x, y, z) {
  return Math.sin(x * Math.exp(-ts * y)) * Math.exp(-ts * z);
}

var tau = Math.PI*2;
var cos = Math.cos;
var noise = function() {
  return Math.random()*2 - 1;
}
var clamp = function(x, low, high) {
  return Math.max(low, Math.min(high, x));
}
var sqr = function(x) {
  return (x % (2*Math.PI)) >= Math.PI ? 1 : -1;
}

/*
    butterworth filters, copied from http://basicsynth.com
 */
var sqr2 = 1.414213562;
 
function LowPass() {
  var dlyOut2 = 0, dlyOut1 = 0;
  var dlyIn2  = 0, dlyIn1  = 0;
  return function (x, cutoff_freq) {
    var c  = 1.0 / Math.tan((Math.PI / sampleRate) * cutoff_freq);
    var c2 = c*c;
    var csqr2 = sqr2 * c;
    var d = c2 + csqr2 + 1;
    var In0 = 1 / d;
    var In1 = In0 + In0;
    var In2 = In0;
    var Out1 = (2 * (1 - c2)) / d;
    var Out2 = (c2 - csqr2 + 1) / d;
    
    var out = (In0 * x)
            + (In1 * dlyIn1)
            + (In2 * dlyIn2)
            - (Out1 * dlyOut1)
            - (Out2 * dlyOut2);
    dlyOut2 = dlyOut1; dlyOut1 = out;
    dlyIn2  = dlyIn1;  dlyIn1  = x;
    return out;
  };
}
 
function HighPass() {
  var dlyOut2 = 0, dlyOut1 = 0;
  var dlyIn2  = 0, dlyIn1  = 0;
  return function (x, cutoff_freq) {
    var c  = Math.tan((Math.PI / sampleRate) * cutoff_freq);
    var c2 = c*c;
    var csqr2 = sqr2 * c;
    var d = c2 + csqr2 + 1;
    var In0 = 1 / d;
    var In1 = -(In0 + In0);
    var In2 = In0;
    var Out1 = (2 * (c2 - 1)) / d;
    var Out2 = (1 - csqr2 + c2) / d;
    
    var out = (In0 * x)
            + (In1 * dlyIn1)
            + (In2 * dlyIn2)
            - (Out1 * dlyOut1)
            - (Out2 * dlyOut2);
    dlyOut2 = dlyOut1; dlyOut1 = out;
    dlyIn2  = dlyIn1;  dlyIn1  = x;
    return out;
  };
}
 
function BandPass() {
  var dlyOut2 = 0, dlyOut1 = 0;
  var dlyIn2  = 0, dlyIn1  = 0;
  return function (x, cutoff_freq) {
    var c = 1 / Math.tan((Math.PI / sampleRate) * cutoff_freq);
    var d = 1 + c;
    var In0 = 1 / d;
    var In1 = 0;
    var In2 = -In0;
    var Out1 = (-c*2*Math.cos(2*Math.PI*cutoff_freq/sampleRate)) / d;
    var Out2 = (c - 1) / d;
    
    var out = (In0 * x)
            + (In1 * dlyIn1)
            + (In2 * dlyIn2)
            - (Out1 * dlyOut1)
            - (Out2 * dlyOut2);
    dlyOut2 = dlyOut1; dlyOut1 = out;
    dlyIn2  = dlyIn1;  dlyIn1  = x;
    return out;
  };
}

function Delay(duration) {
  var line = new Float32Array(Math.ceil(duration * sampleRate));
  var i = 0;
  return function (x) {
    if (x === undefined) return line[(i+1)%line.length];
    line[i] = x;
    return line[(i = (i + 1)%line.length)];
  };
}

function AdjustableDelay(maxDuration, fn) {
  var line = new Float32Array(Math.ceil(maxDuration * sampleRate));
  var L = line.length;
  var i = 0;
  return function (x) {
    k = (i + (L-1) - Math.min(L-2, Math.floor(fn() * sampleRate))) % L;
    if (x === undefined) return line[k];
    line[i] = x;
    i = (i + 1) % L;
    return line[k];
  };
}

function Slot(value) {
  return function (x) {
    if (x !== undefined) value = x;
    return value;
  };
}