package pathsafe

import (
	"errors"
	"path"
	"regexp"
	"strings"
)

var (
	ErrRelativePathRequired = errors.New("relative path is required")
	ErrRelativePathAbsolute = errors.New("relative path must not be absolute")
	ErrRelativePathEscape   = errors.New("relative path escapes root")
)

var windowsAbsPathRE = regexp.MustCompile(`^[a-zA-Z]:[\\/]`)

// NormalizeRelativeDestination validates and normalizes a relative destination
// path to slash-separated form.
func NormalizeRelativeDestination(relative string) (string, error) {
	trimmed := strings.TrimSpace(relative)
	if trimmed == "" {
		return "", ErrRelativePathRequired
	}
	if strings.HasPrefix(trimmed, "/") || strings.HasPrefix(trimmed, `\`) || windowsAbsPathRE.MatchString(trimmed) {
		return "", ErrRelativePathAbsolute
	}

	normalized := strings.ReplaceAll(trimmed, `\`, "/")
	cleaned := path.Clean(normalized)
	if cleaned == "." || cleaned == "" {
		return "", ErrRelativePathRequired
	}
	if cleaned == ".." || strings.HasPrefix(cleaned, "../") {
		return "", ErrRelativePathEscape
	}
	return cleaned, nil
}