package normalorder

import "git.maximhutz.com/max/lambda/pkg/lambda"

func ReduceOnce(e lambda.Expression) (lambda.Expression, bool) {
	switch e := e.(type) {
	case lambda.Abstraction:
		body, reduced := ReduceOnce(e.Body())
		if reduced {
			return lambda.NewAbstraction(e.Parameter(), body), true
		}
		return e, false

	case lambda.Application:
		if fn, fnOk := e.Abstraction().(lambda.Abstraction); fnOk {
			return fn.Body().Substitute(fn.Parameter(), e.Argument()), true
		}

		abs, reduced := ReduceOnce(e.Abstraction())
		if reduced {
			return lambda.NewApplication(abs, e.Argument()), true
		}

		arg, reduced := ReduceOnce(e.Argument())
		if reduced {
			return lambda.NewApplication(e.Abstraction(), arg), true
		}

		return e, false

	default:
		return e, false
	}
}