package ucontainer

// Memo holds a cached value.
type Memo[T any] struct {
	cache  T
	valid  bool
	update func () T
}

// NewMemo creates a new Memo which will take its value from the specified
// update callback.
func NewMemo[T any] (update func () T) Memo[T] {
	return Memo[T] {
		update: update,
	}
}

// Value returns the Memo's value, updating it if the current cached value is
// invalid.
func (this *Memo[T]) Value () T {
	if !this.valid {
		this.cache = this.update()
	}
	return this.cache
}

// Invalidate marks the Memo's value as invalid, which will cause it to be
// updated the next time Value is called.
func (this *Memo[T]) Invalidate () {
	this.valid = false
}

// InvalidateTo invalidates the Memo and sets its value. The new value will be
// entirely inaccessible. This is only intended to be used for setting a
// reference to nil
func (this *Memo[T]) InvalidateTo (value T) {
	this.Invalidate()
	this.cache = value
}