// Package tempdir allocates disposable storage outside a source worktree.
package tempdir

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

// Outside uses system temporary storage and falls back to the repository's
// parent when TMPDIR resolves inside the worktree. Cleanup belongs to the caller.
func Outside(repositoryDir, pattern string) (string, error) {
	repository, err := filepath.Abs(repositoryDir)
	if err != nil {
		return "", err
	}
	repository, err = filepath.EvalSymlinks(repository)
	if err != nil {
		return "", err
	}
	directory, err := os.MkdirTemp("", pattern)
	if err != nil {
		return "", err
	}
	created := directory
	directory, err = filepath.Abs(created)
	if err != nil {
		os.RemoveAll(created)
		return "", err
	}
	directory, err = filepath.EvalSymlinks(directory)
	if err != nil {
		os.RemoveAll(directory)
		return "", err
	}
	outside, err := isOutside(repository, directory)
	if err != nil {
		os.RemoveAll(directory)
		return "", err
	}
	if outside {
		return directory, nil
	}
	if err := os.RemoveAll(directory); err != nil {
		return "", fmt.Errorf("remove temporary storage inside repository %q: %w", repository, err)
	}
	directory, err = os.MkdirTemp(filepath.Dir(repository), pattern)
	if err != nil {
		return "", fmt.Errorf("create temporary storage outside repository %q: %w", repository, err)
	}
	created = directory
	directory, err = filepath.Abs(created)
	if err != nil {
		os.RemoveAll(created)
		return "", err
	}
	directory, err = filepath.EvalSymlinks(directory)
	if err != nil {
		os.RemoveAll(directory)
		return "", err
	}
	outside, err = isOutside(repository, directory)
	if err != nil {
		os.RemoveAll(directory)
		return "", err
	}
	if !outside {
		os.RemoveAll(directory)
		return "", fmt.Errorf("temporary storage %q is inside repository %q", directory, repository)
	}
	return directory, nil
}

func isOutside(repository, directory string) (bool, error) {
	if !strings.EqualFold(filepath.VolumeName(repository), filepath.VolumeName(directory)) {
		return true, nil
	}
	relative, err := filepath.Rel(repository, directory)
	if err != nil {
		return false, err
	}
	return relative == ".." || strings.HasPrefix(relative, ".."+string(filepath.Separator)), nil
}