package debruijn

func IsViable(e *Expression) (*Abstraction, Expression, bool) {
	if e == nil {
		return nil, nil, false
	} else if app, appOk := (*e).(*Application); !appOk {
		return nil, nil, false
	} else if fn, fnOk := app.abstraction.(*Abstraction); !fnOk {
		return nil, nil, false
	} else {
		return fn, app.argument, true
	}
}

func ReduceAll(e *Expression, step func()) {
	it := NewIterator(e)

	for !it.Done() {
		if fn, arg, ok := IsViable(it.Current()); !ok {
			it.Next()
		} else {
			it.Swap(Substitute(fn.body, arg))
			step()

			if _, _, ok := IsViable(it.Parent()); ok {
				it.Back()
			}
		}
	}
}