var (
	ErrInvalid    = errors.New("invalid argument")
	ErrPermission = errors.New("permission denied")
	ErrExist      = errors.New("file already exists")
	ErrNotExist   = errors.New("file does not exist")
	ErrClosed     = errors.New("file already closed")
)

type DirEntry interface {
	// Name returns the name of the file (or subdirectory) described by the entry.
	// This name is only the final element of the path (the base name), not the entire path.
	// For example, Name would return "hello.go" not "home/gopher/hello.go".
	Name() string

	// IsDir reports whether the entry describes a directory.
	IsDir() bool

	// Type returns the type bits for the entry.
	// The type bits are a subset of the usual FileMode bits, those returned by the FileMode.Type method.
	Type() FileMode

	// Info returns the FileInfo for the file or subdirectory described by the entry.
	// The returned FileInfo may be from the time of the original directory read
	// or from the time of the call to Info. If the file has been removed or renamed
	// since the directory read, Info may return an error satisfying errors.Is(err, ErrNotExist).
	// If the entry denotes a symbolic link, Info reports the information about the link itself,
	// not the link's target.
	Info() (FileInfo, error)
}

type FS interface {
	// Open opens the named file.
	//
	// When Open returns an error, it should be of type *PathError
	// with the Op field set to "open", the Path field set to name,
	// and the Err field describing the problem.
	//
	// Open should reject attempts to open names that do not satisfy
	// ValidPath(name), returning a *PathError with Err set to
	// ErrInvalid or ErrNotExist.
	Open(name string) (File, error)
}

type File interface {
	Stat() (FileInfo, error)
	Read([]byte) (int, error)
	Close() error
}

type FileInfo interface {
	Name() string       // base name of the file
	Size() int64        // length in bytes for regular files; system-dependent for others
	Mode() FileMode     // file mode bits
	ModTime() time.Time // modification time
	IsDir() bool        // abbreviation for Mode().IsDir()
	Sys() interface{}   // underlying data source (can return nil)
}

type FileMode uint32

const (
	// The single letters are the abbreviations
	// used by the String method's formatting.
	ModeDir        FileMode = 1 << (32 - 1 - iota) // d: is a directory
	ModeAppend                                     // a: append-only
	ModeExclusive                                  // l: exclusive use
	ModeTemporary                                  // T: temporary file; Plan 9 only
	ModeSymlink                                    // L: symbolic link
	ModeDevice                                     // D: device file
	ModeNamedPipe                                  // p: named pipe (FIFO)
	ModeSocket                                     // S: Unix domain socket
	ModeSetuid                                     // u: setuid
	ModeSetgid                                     // g: setgid
	ModeCharDevice                                 // c: Unix character device, when ModeDevice is set
	ModeSticky                                     // t: sticky
	ModeIrregular                                  // ?: non-regular file; nothing else is known about this file

	// Mask for the type bits. For regular files, none will be set.
	ModeType = ModeDir | ModeSymlink | ModeNamedPipe | ModeSocket | ModeDevice | ModeCharDevice | ModeIrregular

	ModePerm FileMode = 0777 // Unix permission bits
)

type PathError struct {
	Op   string
	Path string
	Err  error
}

type ReadDirFS interface {
	FS

	// ReadDir reads the named directory
	// and returns a list of directory entries sorted by filename.
	ReadDir(name string) ([]DirEntry, error)
}

type ReadDirFile interface {
	File

	// ReadDir reads the contents of the directory and returns
	// a slice of up to n DirEntry values in directory order.
	// Subsequent calls on the same file will yield further DirEntry values.
	//
	// If n > 0, ReadDir returns at most n DirEntry structures.
	// In this case, if ReadDir returns an empty slice, it will return
	// a non-nil error explaining why.
	// At the end of a directory, the error is io.EOF.
	//
	// If n <= 0, ReadDir returns all the DirEntry values from the directory
	// in a single slice. In this case, if ReadDir succeeds (reads all the way
	// to the end of the directory), it returns the slice and a nil error.
	// If it encounters an error before the end of the directory,
	// ReadDir returns the DirEntry list read until that point and a non-nil error.
	ReadDir(n int) ([]DirEntry, error)
}

type ReadFileFS interface {
	FS

	// ReadFile reads the named file and returns its contents.
	// A successful call returns a nil error, not io.EOF.
	// (Because ReadFile reads the whole file, the expected EOF
	// from the final Read is not treated as an error to be reported.)
	//
	// The caller is permitted to modify the returned byte slice.
	// This method should return a copy of the underlying data.
	ReadFile(name string) ([]byte, error)
}

type SortedDir []DirEntry

type StatFS interface {
	FS

	// Stat returns a FileInfo describing the file.
	// If there is an error, it should be of type *PathError.
	Stat(name string) (FileInfo, error)
}