class Promise

# File lib/promise.rb, line 7
def initialize(&block)
  @state = :pending
  @value = nil
  @reason = nil
  @callbacks = []
  @errbacks = []
  instance_eval(&block) if block_given?
end

# File lib/promise.rb, line 20
def then(on_fulfilled = nil, on_rejected = nil)
  result = Promise.new

  call_and_fulfill = ->(value) {
    begin
      if function?(on_fulfilled)
        new_value = on_fulfilled.call(value)
        if Promise === new_value
          new_value.then(->(v) { result.fulfill(v) }, ->(r) { result.reject(r) })
        else
          result.fulfill(new_value)
        end
      else
        result.fulfill(value)
      end
    rescue Exception => e
      result.reject(e)
    end
  }

  call_and_reject = ->(reason) {
    begin
      if function?(on_rejected)
        new_value = on_rejected.call(reason)
        if Promise === new_value
          new_value.then(->(v) { result.fulfill(v) }, ->(r) { result.reject(r) })
        else
          result.fulfill(new_value)
        end
      else
        result.reject(reason)
      end
    rescue Exception => e
      result.reject(e)
    end
  }

  case @state
  when :pending
    @callbacks << call_and_fulfill
    @errbacks << call_and_reject
  when :fulfilled
    call_and_fulfill.call(@value)
  when :rejected
    call_and_reject.call(@reason)
  end

  result
end

# File lib/promise.rb, line 72
def fulfill(value)
  return unless @state == :pending
  @state = :fulfilled
  @value = value
  while callback = @callbacks.shift
    callback.call(value) rescue nil
  end
end

# File lib/promise.rb, line 81
def reject(reason)
  return unless @state == :pending
  @state = :rejected
  @reason = reason
  while errback = @errbacks.shift
    errback.call(reason) rescue nil
  end
end

# File lib/promise.rb, line 92
def function?(obj)
  obj.respond_to? :call
end