Programming-Game-AI-by-Example-Go/chapter-2-state-driven-agent-design/westworld/state_machine.go

package westworld

// StateMachine is the Go version of the C++ class template:
//
//	template <class entity_type>
//	class StateMachine { ... };
//
// Same trick as State[T]: [T any] is a blank slot filled in with
// whichever entity type owns this machine, e.g. StateMachine[Miner].
// Instead of an agent managing its own currentState field and
// ChangeState method by hand (like Miner did before), the agent now
// just owns one of these and hands it all the work.
type StateMachine[T any] struct {
	// owner is the agent this state machine belongs to and acts on.
	// Go version of "entity_type* m_pOwner".
	owner *T

	// currentState is whatever mode the agent is in right now.
	currentState State[T]

	// previousState is a record of the last state the agent was in,
	// so RevertToPreviousState can jump back to it.
	previousState State[T]

	// globalState's Execute runs on every single Update, no matter
	// what currentState is - for behavior that should always be
	// checked (the book will show a real use for this later).
	globalState State[T]
}

// NewStateMachine is the Go stand-in for the C++ constructor
// "StateMachine(entity_type* owner)". Go doesn't run constructor
// initializer lists automatically, so this function does that job
// by hand: remember the owner, leave every state slot empty. An
// empty interface value in Go is nil - that's the direct equivalent
// of the book's NULL.
func NewStateMachine[T any](owner *T) *StateMachine[T] {
	return &StateMachine[T]{owner: owner}
}

// SetCurrentState, SetGlobalState and SetPreviousState are for
// initializing the FSM directly - for example, giving an agent its
// very first state without running it through the Enter/Exit dance
// that ChangeState does.
func (sm *StateMachine[T]) SetCurrentState(s State[T]) {
	sm.currentState = s
}

func (sm *StateMachine[T]) SetGlobalState(s State[T]) {
	sm.globalState = s
}

func (sm *StateMachine[T]) SetPreviousState(s State[T]) {
	sm.previousState = s
}

// Update runs every tick. The C++ version marked this method const
// (a promise that it won't change the StateMachine's own fields) -
// Go has no equivalent keyword, and doesn't need one; nothing here
// requires special marking. If a global state is set, its Execute
// always runs; then, separately, whatever the current state is also
// runs. Both can fire in the same tick - that's the point of having
// two slots instead of one.
func (sm *StateMachine[T]) Update() {
	// if a global state exists, call its execute method
	if sm.globalState != nil {
		sm.globalState.Execute(sm.owner)
	}

	// same for the current state
	if sm.currentState != nil {
		sm.currentState.Execute(sm.owner)
	}
}

// ChangeState switches the agent into a new state properly: it
// remembers where it came from, tells the old state it's being
// left, swaps over, then tells the new state it's being entered.
func (sm *StateMachine[T]) ChangeState(newState State[T]) {
	// The C++ code used assert() to make sure the new state isn't a
	// null pointer. Go's equivalent is panic() - nil is Go's version
	// of a null pointer/null value.
	if newState == nil {
		panic("StateMachine.ChangeState: trying to change to a nil state")
	}

	// keep a record of the previous state
	sm.previousState = sm.currentState

	// call the exit method of the existing state
	sm.currentState.Exit(sm.owner)

	// change state to the new state
	sm.currentState = newState

	// call the entry method of the new state
	sm.currentState.Enter(sm.owner)
}

// RevertToPreviousState switches back to whatever state the agent
// was in right before its current one.
func (sm *StateMachine[T]) RevertToPreviousState() {
	sm.ChangeState(sm.previousState)
}

// CurrentState, GlobalState and PreviousState are plain read-only
// accessors for the three state slots.
func (sm *StateMachine[T]) CurrentState() State[T] {
	return sm.currentState
}

func (sm *StateMachine[T]) GlobalState() State[T] {
	return sm.globalState
}

func (sm *StateMachine[T]) PreviousState() State[T] {
	return sm.previousState
}

// IsInState is supposed to report whether the agent's current state
// is the same type as the state passed in. The book only declared
// this method here - its body comes later - so this is a
// placeholder for now.
func (sm *StateMachine[T]) IsInState(st State[T]) bool {
	// TODO: fill in once the book shows this method's body.
	return false
}