package convert

import (
	"git.maximhutz.com/max/lambda/pkg/debruijn"
	"git.maximhutz.com/max/lambda/pkg/lambda"
)

// LambdaToDeBruijn converts a lambda expression to De Bruijn index representation.
func LambdaToDeBruijn(expr lambda.Expression) debruijn.Expression {
	return lambdaToDeBruijn(expr, []string{})
}

func lambdaToDeBruijn(expr lambda.Expression, context []string) debruijn.Expression {
	switch e := expr.(type) {
	case *lambda.Variable:
		index := findIndex(e.Value(), context)
		return debruijn.NewVariable(index, e.Value())

	case *lambda.Abstraction:
		newContext := append([]string{e.Parameter()}, context...)
		body := lambdaToDeBruijn(e.Body(), newContext)
		return debruijn.NewAbstraction(body)

	case *lambda.Application:
		abs := lambdaToDeBruijn(e.Abstraction(), context)
		arg := lambdaToDeBruijn(e.Argument(), context)
		return debruijn.NewApplication(abs, arg)

	default:
		return nil
	}
}

// findIndex returns the De Bruijn index for a variable name in the context.
// Returns the index if found, or -1 if the variable is free.
func findIndex(name string, context []string) int {
	for i, v := range context {
		if v == name {
			return i
		}
	}
	return -1
}