// vim: shiftwidth=4 tabstop=4 noexpandtab

package main

import (
	"log"
)

// This struct represents the statemachine.
type Machine struct {
	name string
	initial string
	current string
	states StateMap
	api chan string
	state_listeners []*(chan string)
	hook HookFunction
}

type StateMap map[string]MachineState

type MachineState struct {
	on TransitionMap
}

type TransitionMap map[string]MachineTransition

type MachineTransition struct {
	to string
}

type HookFunction func(string, string, *Machine)

// This will run the statemachine (blocking).
func (m *Machine) Run() {
	var event string
	for {
		event = <-m.api  // read api (blocking)
		m.processEvent(event)
	}
}

// A simple Getter to retrieve the current state.
func (m Machine) GetState() string {
	if m.current == "" {
		return m.initial
	}
	return m.current
}

// This function blocks until the current state differs
// from the 'known' state. It then returns the current
// state.
func (m *Machine) GetStateOnChange(known string) string {
	listener := make(chan string)
	var current string
	m.registerListener(&listener)
	defer m.deregisterListener(&listener)
	for {
		if m.GetState() != known {
			current = m.GetState()
			break
		}
		current = <-listener
		if current != known {
			break
		}
	}
	return current
}

// Use this function to send an event to the statemachine.
func (m *Machine) SendEvent(event string) {
	m.api <- event  // blocks until m.run() goroutine handles this event
}

func (m *Machine) registerListener(listener *(chan string)) {
	m.state_listeners = append(m.state_listeners, listener)
}

func (m *Machine) deregisterListener(listener *(chan string)) {
	wasRemoved := false
	if len(m.state_listeners) == 0 {
		log.Println("Not able to unregister listener from empty state_listener slice")
	}
	for index,item := range(m.state_listeners) {
		if item == listener {
			if index == len(m.state_listeners) - 1 {
				m.state_listeners = m.state_listeners[: len(m.state_listeners) - 1]
			} else {
				m.state_listeners[index] = m.state_listeners[len(m.state_listeners) - 1]
				m.state_listeners = m.state_listeners[: len(m.state_listeners) - 1]
			}
			wasRemoved = true
		}
	}
	if !wasRemoved {
		log.Println("Could not find listener in slice state_listeners")
	}
}

func (m *Machine) processEvent(event string) string {
	current := m.GetState()
	next := m.states[current].on[event].to
	if next != "" {  // check if transition is possible
		log.Printf("State machine '%s' changes from '%s' to '%s'\n", m.name, current, next)

		// inform all state listeners
		for _,listener := range(m.state_listeners) {
			*listener <- next
		}

		m.hook(current, next, m)  // execute registered callback function
		m.current = next  // set new state
		return next
	}
	return current
}