Pushdown Automata Godot 4.0

PDA.gd

extends RefCounted
class_name StateMachine

class PDA:
	var stack = []

	func push(state):
		if stack.size() > 0:
			stack[-1].exit(state)
		stack.append(state)
		state.enter(stack.size() > 1 if stack[stack.size() - 2] != null else null)
		Debug.info("Entering state: " + state.get_script().to_string())

	func pop():
		var prev_state = stack.pop_back()
		Debug.info("Leaving state: " + prev_state.get_script().to_string())
		prev_state.exit(stack.size() > 0 if stack.back() != null else null)
		if stack.size() > 0:
			stack.back().enter(prev_state)
		return prev_state

	func pop_and_push(state):
		pop()
		push(state)

States.gd

extends Node
class_name States # Give it a class_name to refer to the states more easily without having to make this a singleton


class State:
	var _character # Reference to character
	
	var Gravity = ProjectSettings.get("physics/2d/default_gravity")

	func _init(character):
		_character = character

	func enter(previous_state):
		pass

	func exit(next_state):
		pass

	func physics_process(delta):
		pass



# Example 1:

class SideIdle extends State:
	func enter(previous_state):
		_character.state_machine.stack = []
		_character.state_machine.stack.append(self)
		_character.play_animation("Idle")
		_character.velocity.x = 0.0

	func physics_process(delta):
		if _character.is_moving():
			_character.state_machine.push(SideMove.new(_character))
			return
		if _character.is_jumping():
			_character.state_machine.push(SideJump.new(_character))


# Example 2:

class SideFalling extends State:
	func enter(previous_state):
		_character.play_animation("Fall")
		if not _character.state_machine.stack.any(func(state): state is SideJump): # Actually use PDA to check for jump in previous states to prevent coyote jumping if you've jumped already
			_character.coyote_timer = _character.coyote_time
      ... # There's a lot more here, this was just to show the use of the PDA's stack