speedometer

speed.js

var tick = 1
var maxTick = 65535
var resolution = 4
var inc = function() {
  tick = (tick + 1) & maxTick // increments cycling from 1 to 65535
}

// every 1/4 of second
var timer = setInterval(inc, (1000 / resolution) | 0)
if (timer.unref) timer.unref()

var speed = function(seconds) {
  var size = resolution * (seconds || 5) // 4 * 5 = 20
  var buffer = [0]
  var pointer = 1
  var last = (tick-1) & maxTick // 0 (previous tick when the speedometer started). the number of 1000 / resolution milliseconds  since the clock started to tick

  return function(delta) {
    var dist = (tick - last) & maxTick // how many ticks since the speedometer started
    if (dist > size) dist = size // to a maximum of size
    last = tick // last tick

    while (dist--) { // for every tick
      if (pointer === size) pointer = 0
      buffer[pointer] = buffer[pointer === 0 ? size-1 : pointer-1]
      pointer++
    }

    if (delta) buffer[pointer-1] += delta

    var top = buffer[pointer-1]
    var btm = buffer.length < size ? 0 : buffer[pointer === size ? 0 : pointer]

    return buffer.length < resolution ? top : (top - btm) * resolution / buffer.length
  }
}