module SassListen::FSM

def self.included(klass)
  klass.send :extend, ClassMethods
end

def current_state
  states[@state]
end

def current_state_name
  current_state && current_state.name || ''
end

def default_state
  self.class.default_state
end

def initialize
  @state = self.class.default_state
end

def states
  self.class.states
end

def transition(state_name)
  new_state = validate_and_sanitize_new_state(state_name)
  return unless new_state
  transition_with_callbacks!(new_state)
end

def transition!(state_name)
  @state = state_name
end

def transition_with_callbacks!(state_name)
  transition! state_name.name
  state_name.call(self)
end

def validate_and_sanitize_new_state(state_name)
  state_name = state_name.to_sym
  return if current_state_name == state_name
  if current_state && !current_state.valid_transition?(state_name)
    valid = current_state.transitions.map(&:to_s).join(', ')
    msg = "#{self.class} can't change state from '#{@state}'"\
      " to '#{state_name}', only to: #{valid}"
    fail ArgumentError, msg
  end
  new_state = states[state_name]
  unless new_state
    return if state_name == default_state
    fail ArgumentError, "invalid state for #{self.class}: #{state_name}"
  end
  new_state
end