package fileops

import (
	"errors"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"sort"
	"strings"
	"syscall"
)

const (
	promotedDirectoryMode = 0o755
	promotedFileMode      = 0o644
)

// PromoteDirectory copies an existing regular-file tree into a new directory
// and installs the complete copy atomically. It never removes the source or
// replaces an existing destination.
func PromoteDirectory(src, dst string) error {
	return promoteDirectory(src, dst, renameDirectoryNoReplace)
}

func promoteDirectory(src, dst string, install func(string, string) error) error {
	if strings.TrimSpace(src) == "" || strings.TrimSpace(dst) == "" {
		return fmt.Errorf("source and destination directory paths are required")
	}

	sourceInfo, err := os.Lstat(src)
	if err != nil {
		return fmt.Errorf("inspect source directory: %w", err)
	}
	if !sourceInfo.IsDir() {
		return fmt.Errorf("source path %q is not a directory", src)
	}

	if _, err := os.Lstat(dst); err == nil {
		return fmt.Errorf("destination path %q already exists", dst)
	} else if !errors.Is(err, os.ErrNotExist) {
		return fmt.Errorf("inspect destination path: %w", err)
	}

	insideSource, err := pathWithin(src, dst)
	if err != nil {
		return err
	}
	if insideSource {
		return fmt.Errorf("destination path %q must not be inside source directory %q", dst, src)
	}

	destinationParent := filepath.Dir(dst)
	parentInfo, err := os.Lstat(destinationParent)
	if err != nil {
		return fmt.Errorf("inspect destination parent: %w", err)
	}
	if !parentInfo.IsDir() {
		return fmt.Errorf("destination parent %q is not a directory", destinationParent)
	}

	temporary, err := os.MkdirTemp(destinationParent, "."+filepath.Base(dst)+".tmp-*")
	if err != nil {
		return fmt.Errorf("create temporary destination directory: %w", err)
	}
	removeTemporary := true
	defer func() {
		if removeTemporary {
			_ = os.RemoveAll(temporary)
		}
	}()

	if err := copyRegularTree(src, temporary); err != nil {
		return err
	}
	if err := os.Chmod(temporary, promotedDirectoryMode); err != nil {
		return fmt.Errorf("set temporary root permissions: %w", err)
	}
	if err := syncDirectory(temporary); err != nil {
		return fmt.Errorf("sync temporary root: %w", err)
	}
	if err := install(temporary, dst); err != nil {
		return fmt.Errorf("install promoted directory: %w", err)
	}

	removeTemporary = false
	_ = syncDirectory(destinationParent)
	return nil
}

func copyRegularTree(src, dst string) error {
	entries, err := os.ReadDir(src)
	if err != nil {
		return fmt.Errorf("read source directory %q: %w", src, err)
	}
	sort.Slice(entries, func(i, j int) bool {
		return entries[i].Name() < entries[j].Name()
	})

	for _, entry := range entries {
		sourcePath := filepath.Join(src, entry.Name())
		destinationPath := filepath.Join(dst, entry.Name())
		info, err := os.Lstat(sourcePath)
		if err != nil {
			return fmt.Errorf("inspect source entry %q: %w", sourcePath, err)
		}

		switch {
		case info.Mode().IsRegular():
			if err := copyRegularFile(sourcePath, destinationPath, info); err != nil {
				return err
			}
		case info.IsDir():
			if err := os.Mkdir(destinationPath, promotedDirectoryMode); err != nil {
				return fmt.Errorf("create destination directory %q: %w", destinationPath, err)
			}
			if err := copyRegularTree(sourcePath, destinationPath); err != nil {
				return err
			}
			if err := os.Chmod(destinationPath, promotedDirectoryMode); err != nil {
				return fmt.Errorf("set destination directory permissions %q: %w", destinationPath, err)
			}
			if err := syncDirectory(destinationPath); err != nil {
				return fmt.Errorf("sync destination directory %q: %w", destinationPath, err)
			}
		default:
			return fmt.Errorf("source entry %q has unsupported file type %s", sourcePath, info.Mode().Type())
		}
	}

	return nil
}

func copyRegularFile(src, dst string, inspected os.FileInfo) error {
	in, err := os.Open(src)
	if err != nil {
		return fmt.Errorf("open source file %q: %w", src, err)
	}
	defer func() { _ = in.Close() }()

	openedInfo, err := in.Stat()
	if err != nil {
		return fmt.Errorf("inspect opened source file %q: %w", src, err)
	}
	if !openedInfo.Mode().IsRegular() || !os.SameFile(inspected, openedInfo) {
		return fmt.Errorf("source file %q changed while being copied", src)
	}

	out, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_EXCL, promotedFileMode)
	if err != nil {
		return fmt.Errorf("create destination file %q: %w", dst, err)
	}
	closed := false
	defer func() {
		if !closed {
			_ = out.Close()
		}
	}()

	if _, err := io.Copy(out, in); err != nil {
		return fmt.Errorf("copy source file %q: %w", src, err)
	}
	if err := out.Chmod(promotedFileMode); err != nil {
		return fmt.Errorf("set destination file permissions %q: %w", dst, err)
	}
	if err := out.Sync(); err != nil {
		return fmt.Errorf("sync destination file %q: %w", dst, err)
	}
	if err := out.Close(); err != nil {
		return fmt.Errorf("close destination file %q: %w", dst, err)
	}
	closed = true
	return nil
}

func syncDirectory(path string) error {
	directory, err := os.Open(path)
	if err != nil {
		return err
	}
	defer func() { _ = directory.Close() }()

	err = directory.Sync()
	if errors.Is(err, syscall.EINVAL) || errors.Is(err, syscall.ENOTSUP) {
		return nil
	}
	return err
}

func pathWithin(parent, candidate string) (bool, error) {
	absoluteParent, err := filepath.Abs(parent)
	if err != nil {
		return false, fmt.Errorf("resolve source directory: %w", err)
	}
	absoluteCandidate, err := filepath.Abs(candidate)
	if err != nil {
		return false, fmt.Errorf("resolve destination directory: %w", err)
	}
	relative, err := filepath.Rel(absoluteParent, absoluteCandidate)
	if err != nil {
		return false, fmt.Errorf("compare source and destination directories: %w", err)
	}
	return relative != "." && relative != ".." && !strings.HasPrefix(relative, ".."+string(filepath.Separator)), nil
}