package convert

import (
	"fmt"

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

// DeBruijnToLambda converts a De Bruijn expression back to named lambda calculus.
func DeBruijnToLambda(expr debruijn.Expression) lambda.Expression {
	return deBruijnToLambda(expr, []string{})
}

func deBruijnToLambda(expr debruijn.Expression, context []string) lambda.Expression {
	switch e := expr.(type) {
	case *debruijn.Variable:
		if e.Index() >= 0 && e.Index() < len(context) {
			return lambda.NewVariable(context[e.Index()])
		}
		if e.Label() != "" {
			return lambda.NewVariable(e.Label())
		}
		return lambda.NewVariable(fmt.Sprintf("free_%d", e.Index()))

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

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

	default:
		return nil
	}
}

// generateParamName generates a fresh parameter name that doesn't conflict with context.
func generateParamName(context []string) string {
	base := 'a'
	for i := 0; ; i++ {
		name := string(rune(base + rune(i%26)))
		if i >= 26 {
			name = fmt.Sprintf("%s%d", name, i/26)
		}

		conflict := false
		for _, existing := range context {
			if existing == name {
				conflict = true
				break
			}
		}

		if !conflict {
			return name
		}
	}
}