"use strict"

var guid = 0, HALT = {}
function createStream() {
  function stream() {
    if (arguments.length > 0 && arguments[0] !== HALT) updateStream(stream, arguments[0])
    return stream._state.value
  }
  initStream(stream)

  if (arguments.length > 0 && arguments[0] !== HALT) updateStream(stream, arguments[0])

  return stream
}
function initStream(stream) {
  stream.constructor = createStream
  stream._state = {id: guid++, value: undefined, state: 0, derive: undefined, recover: undefined, deps: {}, parents: [], endStream: undefined}
  stream.map = stream["fantasy-land/map"] = map, stream["fantasy-land/ap"] = ap, stream["fantasy-land/of"] = createStream
  stream.valueOf = valueOf, stream.toJSON = toJSON, stream.toString = valueOf

  Object.defineProperties(stream, {
    end: {get: function() {
      if (!stream._state.endStream) {
        var endStream = createStream()
        endStream.map(function(value) {
          if (value === true) unregisterStream(stream), unregisterStream(endStream)
          return value
        })
        stream._state.endStream = endStream
      }
      return stream._state.endStream
    }}
  })
}
function updateStream(stream, value) {
  updateState(stream, value)
  for (var id in stream._state.deps) updateDependency(stream._state.deps[id], false)
  finalize(stream)
}
function updateState(stream, value) {
  stream._state.value = value
  stream._state.changed = true
  if (stream._state.state !== 2) stream._state.state = 1
}
function updateDependency(stream, mustSync) {
  var state = stream._state, parents = state.parents
  if (parents.length > 0 && parents.every(active) && (mustSync || parents.some(changed))) {
    var value = stream._state.derive()
    if (value === HALT) return false
    updateState(stream, value)
  }
}
function finalize(stream) {
  stream._state.changed = false
  for (var id in stream._state.deps) stream._state.deps[id]._state.changed = false
}

function combine(fn, streams) {
  if (!streams.every(valid)) throw new Error("Ensure that each item passed to stream.combine/stream.merge is a stream")
  return initDependency(createStream(), streams, function() {
    return fn.apply(this, streams.concat([streams.filter(changed)]))
  })
}

function initDependency(dep, streams, derive) {
  var state = dep._state
  state.derive = derive
  state.parents = streams.filter(notEnded)

  registerDependency(dep, state.parents)
  updateDependency(dep, true)

  return dep
}
function registerDependency(stream, parents) {
  for (var i = 0; i < parents.length; i++) {
    parents[i]._state.deps[stream._state.id] = stream
    registerDependency(stream, parents[i]._state.parents)
  }
}
function unregisterStream(stream) {
  for (var i = 0; i < stream._state.parents.length; i++) {
    var parent = stream._state.parents[i]
    delete parent._state.deps[stream._state.id]
  }
  for (var id in stream._state.deps) {
    var dependent = stream._state.deps[id]
    var index = dependent._state.parents.indexOf(stream)
    if (index > -1) dependent._state.parents.splice(index, 1)
  }
  stream._state.state = 2 //ended
  stream._state.deps = {}
}

function map(fn) {return combine(function(stream) {return fn(stream())}, [this])}
function ap(stream) {return combine(function(s1, s2) {return s1()(s2())}, [stream, this])}
function valueOf() {return this._state.value}
function toJSON() {return this._state.value != null && typeof this._state.value.toJSON === "function" ? this._state.value.toJSON() : this._state.value}

function valid(stream) {return stream._state }
function active(stream) {return stream._state.state === 1}
function changed(stream) {return stream._state.changed}
function notEnded(stream) {return stream._state.state !== 2}

function merge(streams) {
  return combine(function() {
    return streams.map(function(s) {return s()})
  }, streams)
}
createStream["fantasy-land/of"] = createStream
createStream.merge = merge
createStream.combine = combine
createStream.HALT = HALT

if (typeof module !== "undefined") module["exports"] = createStream
else window.stream = createStream