package lambda

func Substitute(e *Expression, target string, replacement Expression) {
	switch typed := (*e).(type) {
	case *Variable:
		if typed.Value == target {
			*e = replacement.Copy()
		}
	case *Abstraction:
		if typed.Parameter == target {
			return
		}

		if IsFreeVariable(typed.Parameter, replacement) {
			replacementFreeVars := GetFreeVariables(replacement)
			used := GetFreeVariables(typed.Body)
			used.Merge(replacementFreeVars)
			freshVar := GenerateFreshName(used)
			Rename(typed, typed.Parameter, freshVar)
		}

		Substitute(&typed.Body, target, replacement)
	case *Application:
		Substitute(&typed.Abstraction, target, replacement)
		Substitute(&typed.Argument, target, replacement)
	}
}