const indexMap = [
  'firstCall',
  'secondCall',
  'thirdCall',
]

export function spy() {
  return stub()
}

export function stub(returnFunc = null) {
  if (returnFunc && typeof(returnFunc) !== 'function') {
    throw new Error('stub() was called with non-function argument')
  }
  let manualReturners = new Map()
  let nextManual = null
  let returner = returnFunc ? returnFunc : null
  let calls = []
  let func = function(...args) {
    func.called = true
    calls.push(args)
    func.lastCall = args
    if (func.callCount < indexMap.length) {
      func[indexMap[func.callCount]] = args
    }
    func.callCount++
    if (manualReturners.has(func.callCount -1)) {
      return manualReturners.get(func.callCount -1)(...args)
    }
    if (returner) {
      return returner(...args)
    }
  }
  func.lastCall = null
  func.called = false
  func.callCount = 0

  func.findCall = function(fn) {
    for (let call of calls) {
      if (fn(call)) return call
    }
    return null
  }

  func.getCall = function(i) {
    return calls[i]
  }
  func.getCallN = function(i) {
    return calls[i - 1]
  }
  func.onCall = function(i) {
    if (i !== null && typeof(i) !== 'number') {
      throw new Error('onCall must be called with either null or number')
    }
    nextManual = i
    return func
  }
  func.onCallN = function(i) {
    return func.onCall(i - 1)
  }

  func.reset = function() {
    func.lastCall = null
    func.called = false
    func.callCount = 0
    manualReturners.clear()
    for (let i = 0; i < indexMap.length; i++) {
      func[indexMap[i]] = null
    }
    returner = returnFunc ? returnFunc : null
    calls.splice(0, calls.length)
    return func
  }
  func.returns = function(data) {
    func.returnWith(function() { return data })
    return func
  }
  func.throws = function(data) {
    func.returnWith(function() { throw data })
    return func
  }
  func.resolves = function(data) {
    func.returnWith(function() { return Promise.resolve(data) })
    return func
  }
  func.rejects = function(data) {
    func.returnWith(function() { return Promise.reject(data) })
    return func
  }
  func.returnWith = function(returnFunc) {
    if (typeof(returnFunc) !== 'function') {
      throw new Error('stub() was called with non-function argument')
    }
    if (nextManual !== null) {
      manualReturners.set(nextManual, returnFunc)
      nextManual = null
    } else {
      returner = returnFunc
    }
    return func
  }
  for (let i = 0; i < indexMap.length; i++) {
    func[indexMap[i]] = null
  }
  return func
}