package pathsafe

import (
	"fmt"
	"path/filepath"
	"strings"
)

// JoinSlashRelativeUnderRoot validates a slash-style relative path and resolves
// it under root. The returned path uses the host filepath separator.
func JoinSlashRelativeUnderRoot(root, relative string) (string, error) {
	rootClean := filepath.Clean(strings.TrimSpace(root))
	if rootClean == "." || rootClean == "" {
		return "", fmt.Errorf("root path is required")
	}

	normalized, err := NormalizeRelativeDestination(relative)
	if err != nil {
		return "", err
	}

	joined := filepath.Clean(filepath.Join(rootClean, filepath.FromSlash(normalized)))
	rel, err := filepath.Rel(rootClean, joined)
	if err != nil {
		return "", fmt.Errorf("resolve relative path under root: %w", err)
	}
	if rel == "." || rel == ".." || strings.HasPrefix(rel, ".."+string(filepath.Separator)) {
		return "", ErrRelativePathEscape
	}
	return joined, nil
}

// SlashRelativeFromRoot derives a slash-style relative path for target under
// root. Target may be absolute or relative to root.
func SlashRelativeFromRoot(root, target string) (string, error) {
	rootClean := filepath.Clean(strings.TrimSpace(root))
	if rootClean == "." || rootClean == "" {
		return "", fmt.Errorf("root path is required")
	}

	targetClean := filepath.Clean(strings.TrimSpace(target))
	if targetClean == "." || targetClean == "" {
		return "", ErrRelativePathRequired
	}
	if !filepath.IsAbs(targetClean) {
		targetClean = filepath.Clean(filepath.Join(rootClean, targetClean))
	}

	rel, err := filepath.Rel(rootClean, targetClean)
	if err != nil {
		return "", fmt.Errorf("derive path relative to root: %w", err)
	}
	normalized, err := NormalizeRelativeDestination(filepath.ToSlash(rel))
	if err != nil {
		return "", err
	}
	return normalized, nil
}