feat: use iterative approach for reduce once

pkg/fifo/fifo.go

@@ -0,0 +1,35 @@
+package fifo
+
+import "fmt"
+
+type FIFO[T any] []T
+
+func New[T any](items ...T) *FIFO[T] {
+	f := FIFO[T](items)
+	return &f
+}
+
+func (f *FIFO[T]) Push(item T) {
+	*f = append(*f, item)
+}
+
+func (f *FIFO[T]) Empty() bool {
+	return len(*f) == 0
+}
+
+func (f *FIFO[T]) MustPop() T {
+	var item T
+
+	*f, item = (*f)[:len(*f)-1], (*f)[len(*f)-1]
+	return item
+}
+
+func (f *FIFO[T]) Pop() (T, error) {
+	var item T
+
+	if f.Empty() {
+		return item, fmt.Errorf("stack is exhausted")
+	}
+
+	return f.MustPop(), nil
+}

pkg/lambda/reduce.go

@@ -1,22 +1,27 @@
 package lambda
 
+import "git.maximhutz.com/max/lambda/pkg/fifo"
+
 func ReduceOnce(e *Expression) bool {
-	switch typed := (*e).(type) {
-	case *Abstraction:
-		return ReduceOnce(&typed.Body)
-	case *Application:
-		if fn, fnOk := typed.Abstraction.(*Abstraction); fnOk {
-			Substitute(&fn.Body, fn.Parameter, typed.Argument)
-			*e = fn.Body
-			return true
-		}
+	stack := fifo.New(e)
 
-		if ReduceOnce(&typed.Abstraction) {
-			return true
-		}
+	for !stack.Empty() {
+		top := stack.MustPop()
 
-		return ReduceOnce(&typed.Argument)
-	default:
-		return false
+		switch typed := (*top).(type) {
+		case *Abstraction:
+			stack.Push(&typed.Body)
+		case *Application:
+			if fn, fnOk := typed.Abstraction.(*Abstraction); fnOk {
+				Substitute(&fn.Body, fn.Parameter, typed.Argument)
+				*top = fn.Body
+				return true
+			}
+
+			stack.Push(&typed.Argument)
+			stack.Push(&typed.Abstraction)
+		}
 	}
+
+	return false
 }