const tweenFns = {
  linear: (from, to, t, d) => from + (to - from) * (t / d)
}

/**
 * only support "linear" timing-function
 * duration unit is "ms"
 * @param {HTMLElement} el
 * @param {({prop: String, value: String, duration: Number})[]} list
 */
function transitionTo(el, list) {
  let startTime
  let oldStyle = new Map()
  let newStyle = new Map()
  for (let prop of list) {
    oldStyle.set(prop.name, window.getComputedStyle(el)[prop.name])
  }
  for (let prop of list) {
    el.style[prop.name] = prop.value
  }
  for (let prop of list) {
    newStyle.set(prop.name, window.getComputedStyle(el)[prop.name])
  }
  for (let prop of list) {
    el.style[prop.name] = oldStyle.get(prop.name)
  }


  requestAnimationFrame(run)

  function run(time) {
    if (startTime == null) startTime = time
    let t = time - startTime
    let done = true
    for (let prop of list) {
      if (t >= prop.duration) {
        el.style[prop.name] = newStyle.get(prop.name)
        continue
      }
      done = false
      let oldPropValue = oldStyle.get(prop.name)
      let newPropValue = newStyle.get(prop.name)
      if (prop.name === 'transform') {
        if (oldPropValue === 'none') oldPropValue = 'matrix(1, 0, 0, 1, 0, 0)'
        if (newPropValue === 'none') newPropValue = 'matrix(1, 0, 0, 1, 0, 0)'
      }
      el.style[prop.name] = generateNewStyle(oldPropValue, newPropValue, t, prop.duration, tweenFns.linear)
    }
    if (!done) requestAnimationFrame(run)
  }
}

function generateNewStyle(from, to, t, duration, tweenFn) {
  let fromExp = /[\d.-]+/g
  let toExp = /[\d.-]+/g
  let fromMatch
  let toMatch
  let result = ''
  let lastIndex = 0
  while (fromMatch = fromExp.exec(from)) {
    result += from.slice(lastIndex, fromMatch.index)
    toMatch = toExp.exec(to)
    result += tweenFn(+fromMatch[0], +toMatch[0], t, duration)
    lastIndex = fromExp.lastIndex
  }
  result += from.slice(lastIndex)
  return result
}