package convert

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

// LambdaToDeBruijn converts a lambda calculus expression to De Bruijn indexed form.
// The context parameter tracks bound variables from outer abstractions.
func LambdaToDeBruijn(expr lambda.Expression) debruijn.Expression {
	return lambdaToDeBruijnWithContext(expr, []string{})
}

func lambdaToDeBruijnWithContext(expr lambda.Expression, context []string) debruijn.Expression {
	switch e := expr.(type) {
	case *lambda.Variable:
		name := e.Value()
		// Search for the variable in the context (innermost to outermost).
		for i := len(context) - 1; i >= 0; i-- {
			if context[i] == name {
				index := len(context) - 1 - i
				return debruijn.NewVariable(index, name)
			}
		}
		// Free variable: use a negative index to mark it.
		// We encode free variables with index = len(context) + position.
		// For simplicity, we use a large index that won't conflict.
		return debruijn.NewVariable(len(context), name)

	case *lambda.Abstraction:
		// Add the parameter to the context.
		newContext := append(context, e.Parameter())
		body := lambdaToDeBruijnWithContext(e.Body(), newContext)
		return debruijn.NewAbstraction(body)

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

	default:
		panic("unknown expression type")
	}
}