package downloader

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

// TaskPaths are fixed local names derived only from RequestID.
type TaskPaths struct {
	Root      string
	TasksDir  string
	FilesDir  string
	Metadata  string
	Backup    string
	Part      string
	Completed string
}

// DeriveTaskPaths derives all local paths without accepting remote filenames
// or Content-Disposition values.
func DeriveTaskPaths(downloadsRoot, requestID string) (TaskPaths, error) {
	if downloadsRoot == "" {
		return TaskPaths{}, fmt.Errorf("%w: empty downloads root", ErrInvalidTask)
	}
	if !ValidRequestID(requestID) {
		return TaskPaths{}, fmt.Errorf("%w: invalid request_id", ErrInvalidTask)
	}
	absoluteRoot, err := filepath.Abs(downloadsRoot)
	if err != nil {
		return TaskPaths{}, fmt.Errorf("%w: resolve downloads root", ErrInvalidTask)
	}
	tasksDir := filepath.Join(absoluteRoot, "tasks")
	filesDir := filepath.Join(absoluteRoot, "files")
	paths := TaskPaths{
		Root:      absoluteRoot,
		TasksDir:  tasksDir,
		FilesDir:  filesDir,
		Metadata:  filepath.Join(tasksDir, requestID+".json"),
		Backup:    filepath.Join(tasksDir, requestID+".json.backup"),
		Part:      filepath.Join(filesDir, requestID+".part"),
		Completed: filepath.Join(filesDir, requestID+".download"),
	}
	for _, candidate := range []string{
		paths.TasksDir,
		paths.FilesDir,
		paths.Metadata,
		paths.Backup,
		paths.Part,
		paths.Completed,
	} {
		relative, relativeErr := filepath.Rel(absoluteRoot, candidate)
		if relativeErr != nil ||
			relative == ".." ||
			strings.HasPrefix(relative, ".."+string(filepath.Separator)) {
			return TaskPaths{}, fmt.Errorf("%w: derived path escapes root", ErrInvalidTask)
		}
	}
	return paths, nil
}