const headBox = document.getElementById('head')
const boxes = document.getElementsByClassName('box')
const mouseDown$ = Rx.Observable.fromEvent(headBox, 'mousedown')
const mouseMove$ = Rx.Observable.fromEvent(document, 'mousemove')
const mouseUp$ = Rx.Observable.fromEvent(document, 'mouseup')
const delayBoxes$ = Rx.Observable.from([].slice.call(boxes, 0))
  .zip(Rx.Observable.interval(100).startWith(0), (box) => box)

mouseDown$.map((e) => {
  const pos = getTranslate(headBox)
  return {
    pos,
    event: e,
  }
})
.switchMap((initialState) => {
  const initialPos = initialState.pos
  const { clientX, clientY } = initialState.event
  return mouseMove$.map((moveEvent) => ({
    x: moveEvent.clientX - clientX + initialPos.x,
    y: moveEvent.clientY - clientY + initialPos.y,
  }))
  .takeUntil(mouseUp$)
})
.mergeMap((pos) => {
  return delayBoxes$.do((box) => {
    setTranslate(box, pos)
  })
})
.subscribe()

function getTranslate (element) {
  const style = getComputedStyle(element)
  const regExp = /matrix\((\d+,\s){4}(\d+),\s(\d+)/i
  const result = style.transform.match(regExp)
  if (result) {
    return {
      x: parseInt(result[2], 10),
      y: parseInt(result[3], 10)
    }
  } else {
    return {
      x: 0,
      y: 0
    }
  }
}

function setTranslate (element, pos) {
  element.style.transform = `translate(${pos.x}px, ${pos.y}px)`
}