var Scroller = (function(){
  var Scroller = {}
  
  var prev, next
  var webkitRatio = 1
  
  Scroller.init = function(opt){
    previous = opt.previous
    next = opt.next
    if (is_mobile) {
//       document.body.addEventListener("touchstart", touchstart)
//       document.body.addEventListener("touchmove", touchmove)
//       document.body.addEventListener("touchend", touchend)
    }
    else {
      document.body.addEventListener("wheel", wheelDelta)
      document.body.addEventListener("keydown", keydown)
    }
  }
  
  var touching = false, touchStartTime, touchStartY, touchEndY, touchDist, touchScrollTop
  function touchstart (e) {
    touching = true
    touchStartTime = +(new Date)
    touchStartY = touchEndY = e.touches ? e.touches[0].pageY : e.pageY
  }
  function touchmove (e) {
    if (! touching) return
    e.preventDefault()
    var timestamp = +(new Date)
    var duration = timestamp - touchStartTime
    touchEndY = e.touches ? e.touches[0].pageY : e.pageY
    touchDist = touchStartY - touchEndY
  }
  function touchend (e) {
    if (! touching) return
    e.preventDefault()
    touching = false
    var timestamp = +(new Date)
    var duration = timestamp - touchStartTime
    touchDist = touchStartY - touchEndY
		if (Math.abs(touchDist) < 10 || duration < 100) {
			step(1)
		}
		else {
			touchDist > 0 ? step(1) : step(-1)
		}
  }
  
  function wheelDelta (e){
    e.preventDefault()
		var deltaY = 0
		// WebKit
		if ( e.deltaY ) {
			deltaY += e.deltaY * webkitRatio
		}
		else if ( e.wheelDeltaY ) {
			deltaY += e.wheelDeltaY * webkitRatio
		}
		// Opera / Explorer 9
		else if ( e.wheelDelta ) {
			deltaY += e.wheelDelta * webkitRatio
		}
		// Firefox
		else if ( e.detail ) {
			deltaY -= e.detail * 2
		}
    wheel(deltaY)
  }
  
  var lastScrollEvent = +new Date(), lastDY = 0, lastSign = 0
  function wheel (dy) {
    if (isNaN(dy)) return
    var now = +new Date(), newSign = sign(dy)

    if ((now > lastScrollEvent + 300) ||
        (lastSign !== newSign && Math.abs(dy) > Math.abs(lastDY*2)) ||
        (lastSign === newSign && Math.log(Math.abs(dy)) > 1+Math.log(Math.abs(lastDY)))) {
      step(sign(dy))
    }
    lastDY = dy
    lastSign = newSign
    lastScrollEvent = now
  }
  function step (n) {
    var now = +new Date()
    if (now < lastScrollEvent + 300) {
      return
    }
    lastScrollEvent = now
    if (n < 0) {
      previous()
    }
    else {
      next()
    }
  }
  function keydown (e) {
    // console.log(e.keyCode)
    switch (e.keyCode) {
      case 32: // space
        e.preventDefault()
        step(1)
        break
      case 36: // home
        e.preventDefault()
        break
      case 35: // end
        e.preventDefault()
        break
      case 33: // page up
      case 38: // up
        e.preventDefault()
        step(-1)
        break
      case 34: // page down
      case 40: // down
        e.preventDefault()
        step(1)
        break
    }
  }
  
  return Scroller
})()