function Handler(f, once, priority) {
  this.f = f
  this.once = once
  this.priority = priority
}

function Subscription() {
  this.handlers = []
}
exports.Subscription = Subscription

function insert(s, handler) {
  var pos = 0
  for (; pos < s.handlers.length; pos++)
    if (s.handlers[pos].priority < handler.priority) break
  s.handlers = s.handlers.slice(0, pos).concat(handler).concat(s.handlers.slice(pos))
}

Subscription.prototype.handlersForDispatch = function() {
  var handlers = this.handlers, updated = null
  for (var i = handlers.length - 1; i >= 0; i--) if (handlers[i].once) {
    if (!updated) updated = handlers.slice()
    updated.splice(i, 1)
  }
  if (updated) this.handlers = updated
  return handlers
}

Subscription.prototype.add = function(f, priority) {
  insert(this, new Handler(f, false, priority || 0))
}

Subscription.prototype.addOnce = function(f, priority) {
  insert(this, new Handler(f, true, priority || 0))
}

Subscription.prototype.remove = function(f) {
  for (var i = 0; i < this.handlers.length; i++) if (this.handlers[i].f == f) {
    this.handlers = this.handlers.slice(0, i).concat(this.handlers.slice(i + 1))
    return
  }
}

Subscription.prototype.hasHandler = function() {
  return this.handlers.length > 0
}

Subscription.prototype.dispatch = function() {
  var handlers = this.handlersForDispatch()
  for (var i = 0; i < handlers.length; i++)
    handlers[i].f.apply(null, arguments)
}

function PipelineSubscription() {
  Subscription.call(this)
}
exports.PipelineSubscription = PipelineSubscription

PipelineSubscription.prototype = new Subscription

PipelineSubscription.prototype.dispatch = function(value) {
  var handlers = this.handlersForDispatch()
  for (var i = 0; i < handlers.length; i++)
    value = handlers[i].f(value)
  return value
}

function StoppableSubscription() {
  Subscription.call(this)
}
exports.StoppableSubscription = StoppableSubscription

StoppableSubscription.prototype = new Subscription

StoppableSubscription.prototype.dispatch = function() {
  var handlers = this.handlersForDispatch()
  for (var i = 0; i < handlers.length; i++) {
    var result = handlers[i].f.apply(null, arguments)
    if (result) return result
  }
}

function DOMSubscription() {
  Subscription.call(this)
}
exports.DOMSubscription = DOMSubscription

DOMSubscription.prototype = new Subscription

DOMSubscription.prototype.dispatch = function(event) {
  var handlers = this.handlersForDispatch()
  for (var i = 0; i < handlers.length; i++)
    if (handlers[i].f(event) || event.defaultPrevented) return true
  return false
}