volume.ts

import * as pathModule from 'path';
import { PathLike, symlink } from 'fs';
import { Node, Link, File } from './node';
import Stats from './Stats';
import Dirent from './Dirent';
import { Buffer, bufferAllocUnsafe, bufferFrom } from './internal/buffer';
import setImmediate from './setImmediate';
import process from './process';
import setTimeoutUnref, { TSetTimeout } from './setTimeoutUnref';
import { Readable, Writable } from 'stream';
import { constants } from './constants';
import { EventEmitter } from 'events';
import { TEncodingExtended, TDataOut, assertEncoding, strToEncoding, ENCODING_UTF8 } from './encoding';
import * as errors from './internal/errors';
import util = require('util');
import createPromisesApi from './promises';

const resolveCrossPlatform = pathModule.resolve;
const {
  O_RDONLY,
  O_WRONLY,
  O_RDWR,
  O_CREAT,
  O_EXCL,
  O_TRUNC,
  O_APPEND,
  O_SYNC,
  O_DIRECTORY,
  F_OK,
  COPYFILE_EXCL,
  COPYFILE_FICLONE_FORCE,
} = constants;

const { sep, relative, join, dirname } = pathModule.posix ? pathModule.posix : pathModule;

const isWin = process.platform === 'win32';

// ---------------------------------------- Types

// Node-style errors with a `code` property.
export interface IError extends Error {
  code?: string;
}

export type TFileId = PathLike | number; // Number is used as a file descriptor.
export type TData = TDataOut | Uint8Array; // Data formats users can give us.
export type TFlags = string | number;
export type TMode = string | number; // Mode can be a String, although docs say it should be a Number.
export type TTime = number | string | Date;
export type TCallback<TData> = (error?: IError | null, data?: TData) => void;
// type TCallbackWrite = (err?: IError, bytesWritten?: number, source?: Buffer) => void;
// type TCallbackWriteStr = (err?: IError, written?: number, str?: string) => void;

// ---------------------------------------- Constants

// Default modes for opening files.
const enum MODE {
  FILE = 0o666,
  DIR = 0o777,
  DEFAULT = MODE.FILE,
}

const kMinPoolSpace = 128;
// const kMaxLength = require('buffer').kMaxLength;

// ---------------------------------------- Error messages

// TODO: Use `internal/errors.js` in the future.

const ERRSTR = {
  PATH_STR: 'path must be a string or Buffer',
  // FD:             'file descriptor must be a unsigned 32-bit integer',
  FD: 'fd must be a file descriptor',
  MODE_INT: 'mode must be an int',
  CB: 'callback must be a function',
  UID: 'uid must be an unsigned int',
  GID: 'gid must be an unsigned int',
  LEN: 'len must be an integer',
  ATIME: 'atime must be an integer',
  MTIME: 'mtime must be an integer',
  PREFIX: 'filename prefix is required',
  BUFFER: 'buffer must be an instance of Buffer or StaticBuffer',
  OFFSET: 'offset must be an integer',
  LENGTH: 'length must be an integer',
  POSITION: 'position must be an integer',
};
const ERRSTR_OPTS = tipeof => `Expected options to be either an object or a string, but got ${tipeof} instead`;
// const ERRSTR_FLAG = flag => `Unknown file open flag: ${flag}`;

const ENOENT = 'ENOENT';
const EBADF = 'EBADF';
const EINVAL = 'EINVAL';
const EPERM = 'EPERM';
const EPROTO = 'EPROTO';
const EEXIST = 'EEXIST';
const ENOTDIR = 'ENOTDIR';
const EMFILE = 'EMFILE';
const EACCES = 'EACCES';
const EISDIR = 'EISDIR';
const ENOTEMPTY = 'ENOTEMPTY';
const ENOSYS = 'ENOSYS';
const ERR_FS_EISDIR = 'ERR_FS_EISDIR';

function formatError(errorCode: string, func = '', path = '', path2 = '') {
  let pathFormatted = '';
  if (path) pathFormatted = ` '${path}'`;
  if (path2) pathFormatted += ` -> '${path2}'`;

  switch (errorCode) {
    case ENOENT:
      return `ENOENT: no such file or directory, ${func}${pathFormatted}`;
    case EBADF:
      return `EBADF: bad file descriptor, ${func}${pathFormatted}`;
    case EINVAL:
      return `EINVAL: invalid argument, ${func}${pathFormatted}`;
    case EPERM:
      return `EPERM: operation not permitted, ${func}${pathFormatted}`;
    case EPROTO:
      return `EPROTO: protocol error, ${func}${pathFormatted}`;
    case EEXIST:
      return `EEXIST: file already exists, ${func}${pathFormatted}`;
    case ENOTDIR:
      return `ENOTDIR: not a directory, ${func}${pathFormatted}`;
    case EISDIR:
      return `EISDIR: illegal operation on a directory, ${func}${pathFormatted}`;
    case EACCES:
      return `EACCES: permission denied, ${func}${pathFormatted}`;
    case ENOTEMPTY:
      return `ENOTEMPTY: directory not empty, ${func}${pathFormatted}`;
    case EMFILE:
      return `EMFILE: too many open files, ${func}${pathFormatted}`;
    case ENOSYS:
      return `ENOSYS: function not implemented, ${func}${pathFormatted}`;
    case ERR_FS_EISDIR:
      return `[ERR_FS_EISDIR]: Path is a directory: ${func} returned EISDIR (is a directory) ${path}`
    default:
      return `${errorCode}: error occurred, ${func}${pathFormatted}`;
  }
}

function createError(errorCode: string, func = '', path = '', path2 = '', Constructor = Error) {
  const error = new Constructor(formatError(errorCode, func, path, path2));
  (error as any).code = errorCode;
  return error;
}

// ---------------------------------------- Flags

// List of file `flags` as defined by Node.
export enum FLAGS {
  // Open file for reading. An exception occurs if the file does not exist.
  r = O_RDONLY,
  // Open file for reading and writing. An exception occurs if the file does not exist.
  'r+' = O_RDWR,
  // Open file for reading in synchronous mode. Instructs the operating system to bypass the local file system cache.
  rs = O_RDONLY | O_SYNC,
  sr = FLAGS.rs,
  // Open file for reading and writing, telling the OS to open it synchronously. See notes for 'rs' about using this with caution.
  'rs+' = O_RDWR | O_SYNC,
  'sr+' = FLAGS['rs+'],
  // Open file for writing. The file is created (if it does not exist) or truncated (if it exists).
  w = O_WRONLY | O_CREAT | O_TRUNC,
  // Like 'w' but fails if path exists.
  wx = O_WRONLY | O_CREAT | O_TRUNC | O_EXCL,
  xw = FLAGS.wx,
  // Open file for reading and writing. The file is created (if it does not exist) or truncated (if it exists).
  'w+' = O_RDWR | O_CREAT | O_TRUNC,
  // Like 'w+' but fails if path exists.
  'wx+' = O_RDWR | O_CREAT | O_TRUNC | O_EXCL,
  'xw+' = FLAGS['wx+'],
  // Open file for appending. The file is created if it does not exist.
  a = O_WRONLY | O_APPEND | O_CREAT,
  // Like 'a' but fails if path exists.
  ax = O_WRONLY | O_APPEND | O_CREAT | O_EXCL,
  xa = FLAGS.ax,
  // Open file for reading and appending. The file is created if it does not exist.
  'a+' = O_RDWR | O_APPEND | O_CREAT,
  // Like 'a+' but fails if path exists.
  'ax+' = O_RDWR | O_APPEND | O_CREAT | O_EXCL,
  'xa+' = FLAGS['ax+'],
}

export type TFlagsCopy =
  | typeof constants.COPYFILE_EXCL
  | typeof constants.COPYFILE_FICLONE
  | typeof constants.COPYFILE_FICLONE_FORCE;

export function flagsToNumber(flags: TFlags | undefined): number {
  if (typeof flags === 'number') return flags;

  if (typeof flags === 'string') {
    const flagsNum = FLAGS[flags];
    if (typeof flagsNum !== 'undefined') return flagsNum;
  }

  // throw new TypeError(formatError(ERRSTR_FLAG(flags)));
  throw new errors.TypeError('ERR_INVALID_OPT_VALUE', 'flags', flags);
}

// ---------------------------------------- Options

function getOptions<T extends IOptions>(defaults: T, options?: T | string): T {
  let opts: T;
  if (!options) return defaults;
  else {
    const tipeof = typeof options;
    switch (tipeof) {
      case 'string':
        opts = Object.assign({}, defaults, { encoding: options as string });
        break;
      case 'object':
        opts = Object.assign({}, defaults, options);
        break;
      default:
        throw TypeError(ERRSTR_OPTS(tipeof));
    }
  }

  if (opts.encoding !== 'buffer') assertEncoding(opts.encoding);

  return opts;
}

function optsGenerator<TOpts>(defaults: TOpts): (opts) => TOpts {
  return options => getOptions(defaults, options);
}

function validateCallback(callback) {
  if (typeof callback !== 'function') throw TypeError(ERRSTR.CB);
  return callback;
}

function optsAndCbGenerator<TOpts, TResult>(getOpts): (options, callback?) => [TOpts, TCallback<TResult>] {
  return (options, callback?) =>
    typeof options === 'function' ? [getOpts(), options] : [getOpts(options), validateCallback(callback)];
}

// General options with optional `encoding` property that most commands accept.
export interface IOptions {
  encoding?: BufferEncoding | TEncodingExtended;
}

export interface IFileOptions extends IOptions {
  mode?: TMode;
  flag?: TFlags;
}

const optsDefaults: IOptions = {
  encoding: 'utf8',
};
const getDefaultOpts = optsGenerator<IOptions>(optsDefaults);
const getDefaultOptsAndCb = optsAndCbGenerator<IOptions, any>(getDefaultOpts);

// Options for `fs.readFile` and `fs.readFileSync`.
export interface IReadFileOptions extends IOptions {
  flag?: string;
}
const readFileOptsDefaults: IReadFileOptions = {
  flag: 'r',
};
const getReadFileOptions = optsGenerator<IReadFileOptions>(readFileOptsDefaults);

// Options for `fs.writeFile` and `fs.writeFileSync`
export interface IWriteFileOptions extends IFileOptions {}
const writeFileDefaults: IWriteFileOptions = {
  encoding: 'utf8',
  mode: MODE.DEFAULT,
  flag: FLAGS[FLAGS.w],
};
const getWriteFileOptions = optsGenerator<IWriteFileOptions>(writeFileDefaults);

// Options for `fs.appendFile` and `fs.appendFileSync`
export interface IAppendFileOptions extends IFileOptions {}
const appendFileDefaults: IAppendFileOptions = {
  encoding: 'utf8',
  mode: MODE.DEFAULT,
  flag: FLAGS[FLAGS.a],
};
const getAppendFileOpts = optsGenerator<IAppendFileOptions>(appendFileDefaults);
const getAppendFileOptsAndCb = optsAndCbGenerator<IAppendFileOptions, void>(getAppendFileOpts);

// Options for `fs.realpath` and `fs.realpathSync`
export interface IRealpathOptions {
  encoding?: TEncodingExtended;
}
const realpathDefaults: IReadFileOptions = optsDefaults;
const getRealpathOptions = optsGenerator<IRealpathOptions>(realpathDefaults);
const getRealpathOptsAndCb = optsAndCbGenerator<IRealpathOptions, TDataOut>(getRealpathOptions);

// Options for `fs.watchFile`
export interface IWatchFileOptions {
  persistent?: boolean;
  interval?: number;
}

// Options for `fs.createReadStream`
export interface IReadStreamOptions {
  flags?: TFlags;
  encoding?: BufferEncoding;
  fd?: number;
  mode?: TMode;
  autoClose?: boolean;
  start?: number;
  end?: number;
}

// Options for `fs.createWriteStream`
export interface IWriteStreamOptions {
  flags?: TFlags;
  defaultEncoding?: BufferEncoding;
  fd?: number;
  mode?: TMode;
  autoClose?: boolean;
  start?: number;
}

// Options for `fs.watch`
export interface IWatchOptions extends IOptions {
  persistent?: boolean;
  recursive?: boolean;
}

// Options for `fs.mkdir` and `fs.mkdirSync`
export interface IMkdirOptions {
  mode?: TMode;
  recursive?: boolean;
}
const mkdirDefaults: IMkdirOptions = {
  mode: MODE.DIR,
  recursive: false,
};
const getMkdirOptions = (options): IMkdirOptions => {
  if (typeof options === 'number') return Object.assign({}, mkdirDefaults, { mode: options });
  return Object.assign({}, mkdirDefaults, options);
};

// Options for `fs.rmdir` and `fs.rmdirSync`
export interface IRmdirOptions {
  recursive?: boolean;
}
const rmdirDefaults: IRmdirOptions = {
  recursive: false,
};
const getRmdirOptions = (options): IRmdirOptions => {
  return Object.assign({}, rmdirDefaults, options);
};

export interface IRmOptions {
  force?: boolean;
  maxRetries?: number;
  recursive?: boolean;
  retryDelay?: number;
}
const getRmOpts = optsGenerator<IOptions>(optsDefaults);
const getRmOptsAndCb = optsAndCbGenerator<IRmOptions, any>(getRmOpts);

// Options for `fs.readdir` and `fs.readdirSync`
export interface IReaddirOptions extends IOptions {
  withFileTypes?: boolean;
}
const readdirDefaults: IReaddirOptions = {
  encoding: 'utf8',
  withFileTypes: false,
};
const getReaddirOptions = optsGenerator<IReaddirOptions>(readdirDefaults);
const getReaddirOptsAndCb = optsAndCbGenerator<IReaddirOptions, TDataOut[] | Dirent[]>(getReaddirOptions);

// Options for `fs.lstat`, `fs.lstatSync`, `fs.stat`, and `fs.statSync`
export interface IStatOptions {
  bigint?: boolean;
  throwIfNoEntry?: boolean;
}

// Options for `fs.fstat`, fs.fstatSync
export interface IFStatOptions {
  bigint?: boolean;
}

const statDefaults: IStatOptions = {
  bigint: false,
};
const getStatOptions: (options?: any) => IStatOptions = (options = {}) => Object.assign({}, statDefaults, options);
const getStatOptsAndCb: (options: any, callback?: TCallback<Stats>) => [IStatOptions, TCallback<Stats>] = (
  options,
  callback?,
) =>
  typeof options === 'function' ? [getStatOptions(), options] : [getStatOptions(options), validateCallback(callback)];

// ---------------------------------------- Utility functions

function getPathFromURLPosix(url): string {
  if (url.hostname !== '') {
    throw new errors.TypeError('ERR_INVALID_FILE_URL_HOST', process.platform);
  }
  const pathname = url.pathname;
  for (let n = 0; n < pathname.length; n++) {
    if (pathname[n] === '%') {
      const third = pathname.codePointAt(n + 2) | 0x20;
      if (pathname[n + 1] === '2' && third === 102) {
        throw new errors.TypeError('ERR_INVALID_FILE_URL_PATH', 'must not include encoded / characters');
      }
    }
  }
  return decodeURIComponent(pathname);
}

export function pathToFilename(path: PathLike): string {
  if (typeof path !== 'string' && !Buffer.isBuffer(path)) {
    try {
      if (!(path instanceof require('url').URL)) throw new TypeError(ERRSTR.PATH_STR);
    } catch (err) {
      throw new TypeError(ERRSTR.PATH_STR);
    }

    path = getPathFromURLPosix(path);
  }

  const pathString = String(path);
  nullCheck(pathString);
  // return slash(pathString);
  return pathString;
}

type TResolve = (filename: string, base?: string) => string;
let resolve: TResolve = (filename, base = process.cwd()) => resolveCrossPlatform(base, filename);
if (isWin) {
  const _resolve = resolve;
  const { unixify } = require('fs-monkey/lib/correctPath');
  resolve = (filename, base) => unixify(_resolve(filename, base));
}

export function filenameToSteps(filename: string, base?: string): string[] {
  const fullPath = resolve(filename, base);
  const fullPathSansSlash = fullPath.substr(1);
  if (!fullPathSansSlash) return [];
  return fullPathSansSlash.split(sep);
}

export function pathToSteps(path: PathLike): string[] {
  return filenameToSteps(pathToFilename(path));
}

export function dataToStr(data: TData, encoding: string = ENCODING_UTF8): string {
  if (Buffer.isBuffer(data)) return data.toString(encoding);
  else if (data instanceof Uint8Array) return bufferFrom(data).toString(encoding);
  else return String(data);
}

export function dataToBuffer(data: TData, encoding: string = ENCODING_UTF8): Buffer {
  if (Buffer.isBuffer(data)) return data;
  else if (data instanceof Uint8Array) return bufferFrom(data);
  else return bufferFrom(String(data), encoding);
}

export function bufferToEncoding(buffer: Buffer, encoding?: TEncodingExtended): TDataOut {
  if (!encoding || encoding === 'buffer') return buffer;
  else return buffer.toString(encoding);
}

function nullCheck(path, callback?) {
  if (('' + path).indexOf('\u0000') !== -1) {
    const er = new Error('Path must be a string without null bytes');
    (er as any).code = ENOENT;
    if (typeof callback !== 'function') throw er;
    process.nextTick(callback, er);
    return false;
  }
  return true;
}

function _modeToNumber(mode: TMode | undefined, def?): number | undefined {
  if (typeof mode === 'number') return mode;
  if (typeof mode === 'string') return parseInt(mode, 8);
  if (def) return modeToNumber(def);
  return undefined;
}

function modeToNumber(mode: TMode | undefined, def?): number {
  const result = _modeToNumber(mode, def);
  if (typeof result !== 'number' || isNaN(result)) throw new TypeError(ERRSTR.MODE_INT);
  return result;
}

function isFd(path): boolean {
  return path >>> 0 === path;
}

function validateFd(fd) {
  if (!isFd(fd)) throw TypeError(ERRSTR.FD);
}

// converts Date or number to a fractional UNIX timestamp
export function toUnixTimestamp(time) {
  // tslint:disable-next-line triple-equals
  if (typeof time === 'string' && +time == (time as any)) {
    return +time;
  }
  if (time instanceof Date) {
    return time.getTime() / 1000;
  }
  if (isFinite(time)) {
    if (time < 0) {
      return Date.now() / 1000;
    }
    return time;
  }
  throw new Error('Cannot parse time: ' + time);
}

function validateUid(uid: number) {
  if (typeof uid !== 'number') throw TypeError(ERRSTR.UID);
}

function validateGid(gid: number) {
  if (typeof gid !== 'number') throw TypeError(ERRSTR.GID);
}

// ---------------------------------------- Volume
type DirectoryContent = string | null;

export interface DirectoryJSON {
  [key: string]: DirectoryContent;
}
export interface NestedDirectoryJSON {
  [key: string]: DirectoryContent | NestedDirectoryJSON;
}

function flattenJSON(nestedJSON: NestedDirectoryJSON): DirectoryJSON {
  const flatJSON: DirectoryJSON = {};

  function flatten(pathPrefix: string, node: NestedDirectoryJSON) {
    for (const path in node) {
      const contentOrNode = node[path];

      const joinedPath = join(pathPrefix, path);

      if (typeof contentOrNode === 'string') {
        flatJSON[joinedPath] = contentOrNode;
      } else if (typeof contentOrNode === 'object' && contentOrNode !== null && Object.keys(contentOrNode).length > 0) {
        // empty directories need an explicit entry and therefore get handled in `else`, non-empty ones are implicitly considered

        flatten(joinedPath, contentOrNode);
      } else {
        // without this branch null, empty-object or non-object entries would not be handled in the same way
        // by both fromJSON() and fromNestedJSON()
        flatJSON[joinedPath] = null;
      }
    }
  }

  flatten('', nestedJSON);

  return flatJSON;
}

/**
 * `Volume` represents a file system.
 */
export class Volume {
  static fromJSON(json: DirectoryJSON, cwd?: string): Volume {
    const vol = new Volume();
    vol.fromJSON(json, cwd);
    return vol;
  }

  static fromNestedJSON(json: NestedDirectoryJSON, cwd?: string): Volume {
    const vol = new Volume();
    vol.fromNestedJSON(json, cwd);
    return vol;
  }

  /**
   * Global file descriptor counter. UNIX file descriptors start from 0 and go sequentially
   * up, so here, in order not to conflict with them, we choose some big number and descrease
   * the file descriptor of every new opened file.
   * @type {number}
   * @todo This should not be static, right?
   */
  static fd: number = 0x7fffffff;

  // Constructor function used to create new nodes.
  // NodeClass: new (...args) => TNode = Node as new (...args) => TNode;

  // Hard link to the root of this volume.
  // root: Node = new (this.NodeClass)(null, '', true);
  root: Link;

  // I-node number counter.
  ino: number = 0;

  // A mapping for i-node numbers to i-nodes (`Node`);
  inodes: { [ino: number]: Node } = {};

  // List of released i-node numbers, for reuse.
  releasedInos: number[] = [];

  // A mapping for file descriptors to `File`s.
  fds: { [fd: number]: File } = {};

  // A list of reusable (opened and closed) file descriptors, that should be
  // used first before creating a new file descriptor.
  releasedFds: number[] = [];

  // Max number of open files.
  maxFiles = 10000;

  // Current number of open files.
  openFiles = 0;

  StatWatcher: new () => StatWatcher;
  ReadStream: new (...args) => IReadStream;
  WriteStream: new (...args) => IWriteStream;
  FSWatcher: new () => FSWatcher;

  props: {
    Node: new (...args) => Node;
    Link: new (...args) => Link;
    File: new (...args) => File;
  };

  private promisesApi = createPromisesApi(this);

  get promises() {
    if (this.promisesApi === null) throw new Error('Promise is not supported in this environment.');
    return this.promisesApi;
  }

  constructor(props = {}) {
    this.props = Object.assign({ Node, Link, File }, props);

    const root = this.createLink();
    root.setNode(this.createNode(true));

    const self = this; // tslint:disable-line no-this-assignment

    this.StatWatcher = class extends StatWatcher {
      constructor() {
        super(self);
      }
    };

    const _ReadStream: new (...args) => IReadStream = FsReadStream as any;
    this.ReadStream = (class extends _ReadStream {
      constructor(...args) {
        super(self, ...args);
      }
    } as any) as new (...args) => IReadStream;

    const _WriteStream: new (...args) => IWriteStream = FsWriteStream as any;
    this.WriteStream = (class extends _WriteStream {
      constructor(...args) {
        super(self, ...args);
      }
    } as any) as new (...args) => IWriteStream;

    this.FSWatcher = class extends FSWatcher {
      constructor() {
        super(self);
      }
    };

    // root.setChild('.', root);
    // root.getNode().nlink++;

    // root.setChild('..', root);
    // root.getNode().nlink++;

    this.root = root;
  }

  createLink(): Link;
  createLink(parent: Link, name: string, isDirectory?: boolean, perm?: number): Link;
  createLink(parent?: Link, name?: string, isDirectory: boolean = false, perm?: number): Link {
    if (!parent) {
      return new this.props.Link(this, null, '');
    }

    if (!name) {
      throw new Error('createLink: name cannot be empty');
    }

    return parent.createChild(name, this.createNode(isDirectory, perm));
  }

  deleteLink(link: Link): boolean {
    const parent = link.parent;
    if (parent) {
      parent.deleteChild(link);

      return true;
    }
    return false;
  }

  private newInoNumber(): number {
    const releasedFd = this.releasedInos.pop();

    if (releasedFd) return releasedFd;
    else {
      this.ino = (this.ino + 1) % 0xffffffff;
      return this.ino;
    }
  }

  private newFdNumber(): number {
    const releasedFd = this.releasedFds.pop();
    return typeof releasedFd === 'number' ? releasedFd : Volume.fd--;
  }

  createNode(isDirectory: boolean = false, perm?: number): Node {
    const node = new this.props.Node(this.newInoNumber(), perm);
    if (isDirectory) node.setIsDirectory();
    this.inodes[node.ino] = node;
    return node;
  }

  private getNode(ino: number) {
    return this.inodes[ino];
  }

  private deleteNode(node: Node) {
    node.del();
    delete this.inodes[node.ino];
    this.releasedInos.push(node.ino);
  }

  // Generates 6 character long random string, used by `mkdtemp`.
  genRndStr() {
    const str = (Math.random() + 1).toString(36).substr(2, 6);
    if (str.length === 6) return str;
    else return this.genRndStr();
  }

  // Returns a `Link` (hard link) referenced by path "split" into steps.
  getLink(steps: string[]): Link | null {
    return this.root.walk(steps);
  }

  // Just link `getLink`, but throws a correct user error, if link to found.
  getLinkOrThrow(filename: string, funcName?: string): Link {
    const steps = filenameToSteps(filename);
    const link = this.getLink(steps);
    if (!link) throw createError(ENOENT, funcName, filename);
    return link;
  }

  // Just like `getLink`, but also dereference/resolves symbolic links.
  getResolvedLink(filenameOrSteps: string | string[]): Link | null {
    let steps: string[] = typeof filenameOrSteps === 'string' ? filenameToSteps(filenameOrSteps) : filenameOrSteps;

    let link: Link | undefined = this.root;
    let i = 0;
    while (i < steps.length) {
      const step = steps[i];
      link = link.getChild(step);
      if (!link) return null;

      const node = link.getNode();
      if (node.isSymlink()) {
        steps = node.symlink.concat(steps.slice(i + 1));
        link = this.root;
        i = 0;
        continue;
      }

      i++;
    }

    return link;
  }

  // Just like `getLinkOrThrow`, but also dereference/resolves symbolic links.
  getResolvedLinkOrThrow(filename: string, funcName?: string): Link {
    const link = this.getResolvedLink(filename);
    if (!link) throw createError(ENOENT, funcName, filename);
    return link;
  }

  resolveSymlinks(link: Link): Link | null {
    // let node: Node = link.getNode();
    // while(link && node.isSymlink()) {
    //     link = this.getLink(node.symlink);
    //     if(!link) return null;
    //     node = link.getNode();
    // }
    // return link;
    return this.getResolvedLink(link.steps.slice(1));
  }

  // Just like `getLinkOrThrow`, but also verifies that the link is a directory.
  private getLinkAsDirOrThrow(filename: string, funcName?: string): Link {
    const link = this.getLinkOrThrow(filename, funcName);
    if (!link.getNode().isDirectory()) throw createError(ENOTDIR, funcName, filename);
    return link;
  }

  // Get the immediate parent directory of the link.
  private getLinkParent(steps: string[]): Link | null {
    return this.root.walk(steps, steps.length - 1);
  }

  private getLinkParentAsDirOrThrow(filenameOrSteps: string | string[], funcName?: string): Link {
    const steps = filenameOrSteps instanceof Array ? filenameOrSteps : filenameToSteps(filenameOrSteps);
    const link = this.getLinkParent(steps);
    if (!link) throw createError(ENOENT, funcName, sep + steps.join(sep));
    if (!link.getNode().isDirectory()) throw createError(ENOTDIR, funcName, sep + steps.join(sep));
    return link;
  }

  private getFileByFd(fd: number): File {
    return this.fds[String(fd)];
  }

  private getFileByFdOrThrow(fd: number, funcName?: string): File {
    if (!isFd(fd)) throw TypeError(ERRSTR.FD);
    const file = this.getFileByFd(fd);
    if (!file) throw createError(EBADF, funcName);
    return file;
  }

  /**
   * @todo This is not used anymore. Remove.
   */
  /*
  private getNodeByIdOrCreate(id: TFileId, flags: number, perm: number): Node {
    if (typeof id === 'number') {
      const file = this.getFileByFd(id);
      if (!file) throw Error('File nto found');
      return file.node;
    } else {
      const steps = pathToSteps(id as PathLike);
      let link = this.getLink(steps);
      if (link) return link.getNode();

      // Try creating a node if not found.
      if (flags & O_CREAT) {
        const dirLink = this.getLinkParent(steps);
        if (dirLink) {
          const name = steps[steps.length - 1];
          link = this.createLink(dirLink, name, false, perm);
          return link.getNode();
        }
      }

      throw createError(ENOENT, 'getNodeByIdOrCreate', pathToFilename(id));
    }
  }
  */

  private wrapAsync(method: (...args) => void, args: any[], callback: TCallback<any>) {
    validateCallback(callback);
    setImmediate(() => {
      let result;
      try {
        result = method.apply(this, args);
      } catch (err) {
        callback(err);
        return;
      }
      callback(null, result);
    });
  }

  private _toJSON(link = this.root, json = {}, path?: string): DirectoryJSON {
    let isEmpty = true;

    let children = link.children;

    if (link.getNode().isFile()) {
      children = { [link.getName()]: link.parent.getChild(link.getName()) };
      link = link.parent;
    }

    for (const name in children) {
      isEmpty = false;

      const child = link.getChild(name);

      if (!child) {
        throw new Error('_toJSON: unexpected undefined');
      }
      const node = child.getNode();
      if (node.isFile()) {
        let filename = child.getPath();
        if (path) filename = relative(path, filename);
        json[filename] = node.getString();
      } else if (node.isDirectory()) {
        this._toJSON(child, json, path);
      }
    }

    let dirPath = link.getPath();

    if (path) dirPath = relative(path, dirPath);

    if (dirPath && isEmpty) {
      json[dirPath] = null;
    }

    return json;
  }

  toJSON(paths?: PathLike | PathLike[], json = {}, isRelative = false): DirectoryJSON {
    const links: Link[] = [];

    if (paths) {
      if (!(paths instanceof Array)) paths = [paths];
      for (const path of paths) {
        const filename = pathToFilename(path);
        const link = this.getResolvedLink(filename);
        if (!link) continue;
        links.push(link);
      }
    } else {
      links.push(this.root);
    }

    if (!links.length) return json;
    for (const link of links) this._toJSON(link, json, isRelative ? link.getPath() : '');
    return json;
  }

  fromJSON(json: DirectoryJSON, cwd: string = process.cwd()) {
    for (let filename in json) {
      const data = json[filename];

      filename = resolve(filename, cwd);

      if (typeof data === 'string') {
        const dir = dirname(filename);
        this.mkdirpBase(dir, MODE.DIR);

        this.writeFileSync(filename, data);
      } else {
        this.mkdirpBase(filename, MODE.DIR);
      }
    }
  }

  fromNestedJSON(json: NestedDirectoryJSON, cwd?: string) {
    this.fromJSON(flattenJSON(json), cwd);
  }

  reset() {
    this.ino = 0;
    this.inodes = {};
    this.releasedInos = [];
    this.fds = {};
    this.releasedFds = [];
    this.openFiles = 0;

    this.root = this.createLink();
    this.root.setNode(this.createNode(true));
  }

  // Legacy interface
  mountSync(mountpoint: string, json: DirectoryJSON) {
    this.fromJSON(json, mountpoint);
  }

  private openLink(link: Link, flagsNum: number, resolveSymlinks: boolean = true): File {
    if (this.openFiles >= this.maxFiles) {
      // Too many open files.
      throw createError(EMFILE, 'open', link.getPath());
    }

    // Resolve symlinks.
    let realLink: Link | null = link;
    if (resolveSymlinks) realLink = this.resolveSymlinks(link);
    if (!realLink) throw createError(ENOENT, 'open', link.getPath());

    const node = realLink.getNode();

    // Check whether node is a directory
    if (node.isDirectory()) {
      if ((flagsNum & (O_RDONLY | O_RDWR | O_WRONLY)) !== O_RDONLY) throw createError(EISDIR, 'open', link.getPath());
    } else {
      if (flagsNum & O_DIRECTORY) throw createError(ENOTDIR, 'open', link.getPath());
    }

    // Check node permissions
    if (!(flagsNum & O_WRONLY)) {
      if (!node.canRead()) {
        throw createError(EACCES, 'open', link.getPath());
      }
    }
    if (flagsNum & O_RDWR) {
    }

    const file = new this.props.File(link, node, flagsNum, this.newFdNumber());
    this.fds[file.fd] = file;
    this.openFiles++;

    if (flagsNum & O_TRUNC) file.truncate();

    return file;
  }

  private openFile(
    filename: string,
    flagsNum: number,
    modeNum: number | undefined,
    resolveSymlinks: boolean = true,
  ): File {
    const steps = filenameToSteps(filename);
    let link: Link | null = resolveSymlinks ? this.getResolvedLink(steps) : this.getLink(steps);

    // Try creating a new file, if it does not exist.
    if (!link && flagsNum & O_CREAT) {
      // const dirLink: Link = this.getLinkParent(steps);
      const dirLink: Link | null = this.getResolvedLink(steps.slice(0, steps.length - 1));
      // if(!dirLink) throw createError(ENOENT, 'open', filename);
      if (!dirLink) throw createError(ENOENT, 'open', sep + steps.join(sep));

      if (flagsNum & O_CREAT && typeof modeNum === 'number') {
        link = this.createLink(dirLink, steps[steps.length - 1], false, modeNum);
      }
    }

    if (link) return this.openLink(link, flagsNum, resolveSymlinks);
    throw createError(ENOENT, 'open', filename);
  }

  private openBase(filename: string, flagsNum: number, modeNum: number, resolveSymlinks: boolean = true): number {
    const file = this.openFile(filename, flagsNum, modeNum, resolveSymlinks);
    if (!file) throw createError(ENOENT, 'open', filename);
    return file.fd;
  }

  openSync(path: PathLike, flags: TFlags, mode: TMode = MODE.DEFAULT): number {
    // Validate (1) mode; (2) path; (3) flags - in that order.
    const modeNum = modeToNumber(mode);
    const fileName = pathToFilename(path);
    const flagsNum = flagsToNumber(flags);
    return this.openBase(fileName, flagsNum, modeNum);
  }

  open(path: PathLike, flags: TFlags, /* ... */ callback: TCallback<number>);
  open(path: PathLike, flags: TFlags, mode: TMode, callback: TCallback<number>);
  open(path: PathLike, flags: TFlags, a: TMode | TCallback<number>, b?: TCallback<number>) {
    let mode: TMode = a as TMode;
    let callback: TCallback<number> = b as TCallback<number>;

    if (typeof a === 'function') {
      mode = MODE.DEFAULT;
      callback = a;
    }
    mode = mode || MODE.DEFAULT;

    const modeNum = modeToNumber(mode);
    const fileName = pathToFilename(path);
    const flagsNum = flagsToNumber(flags);

    this.wrapAsync(this.openBase, [fileName, flagsNum, modeNum], callback);
  }

  private closeFile(file: File) {
    if (!this.fds[file.fd]) return;

    this.openFiles--;
    delete this.fds[file.fd];
    this.releasedFds.push(file.fd);
  }

  closeSync(fd: number) {
    validateFd(fd);
    const file = this.getFileByFdOrThrow(fd, 'close');
    this.closeFile(file);
  }

  close(fd: number, callback: TCallback<void>) {
    validateFd(fd);
    this.wrapAsync(this.closeSync, [fd], callback);
  }

  private openFileOrGetById(id: TFileId, flagsNum: number, modeNum?: number): File {
    if (typeof id === 'number') {
      const file = this.fds[id];
      if (!file) throw createError(ENOENT);
      return file;
    } else {
      return this.openFile(pathToFilename(id), flagsNum, modeNum);
    }
  }

  private readBase(fd: number, buffer: Buffer | Uint8Array, offset: number, length: number, position: number): number {
    const file = this.getFileByFdOrThrow(fd);
    return file.read(buffer, Number(offset), Number(length), position);
  }

  readSync(fd: number, buffer: Buffer | Uint8Array, offset: number, length: number, position: number): number {
    validateFd(fd);
    return this.readBase(fd, buffer, offset, length, position);
  }

  read(
    fd: number,
    buffer: Buffer | Uint8Array,
    offset: number,
    length: number,
    position: number,
    callback: (err?: Error | null, bytesRead?: number, buffer?: Buffer | Uint8Array) => void,
  ) {
    validateCallback(callback);

    // This `if` branch is from Node.js
    if (length === 0) {
      return process.nextTick(() => {
        if (callback) callback(null, 0, buffer);
      });
    }

    setImmediate(() => {
      try {
        const bytes = this.readBase(fd, buffer, offset, length, position);
        callback(null, bytes, buffer);
      } catch (err) {
        callback(err);
      }
    });
  }

  private readFileBase(id: TFileId, flagsNum: number, encoding: BufferEncoding): Buffer | string {
    let result: Buffer | string;

    const isUserFd = typeof id === 'number';
    const userOwnsFd: boolean = isUserFd && isFd(id);
    let fd: number;

    if (userOwnsFd) fd = id as number;
    else {
      const filename = pathToFilename(id as PathLike);
      const steps = filenameToSteps(filename);
      const link: Link | null = this.getResolvedLink(steps);

      if (link) {
        const node = link.getNode();
        if (node.isDirectory()) throw createError(EISDIR, 'open', link.getPath());
      }

      fd = this.openSync(id as PathLike, flagsNum);
    }

    try {
      result = bufferToEncoding(this.getFileByFdOrThrow(fd).getBuffer(), encoding);
    } finally {
      if (!userOwnsFd) {
        this.closeSync(fd);
      }
    }

    return result;
  }

  readFileSync(file: TFileId, options?: IReadFileOptions | string): TDataOut {
    const opts = getReadFileOptions(options);
    const flagsNum = flagsToNumber(opts.flag);
    return this.readFileBase(file, flagsNum, opts.encoding as BufferEncoding);
  }

  readFile(id: TFileId, callback: TCallback<TDataOut>);
  readFile(id: TFileId, options: IReadFileOptions | string, callback: TCallback<TDataOut>);
  readFile(id: TFileId, a: TCallback<TDataOut> | IReadFileOptions | string, b?: TCallback<TDataOut>) {
    const [opts, callback] = optsAndCbGenerator<IReadFileOptions, TCallback<TDataOut>>(getReadFileOptions)(a, b);
    const flagsNum = flagsToNumber(opts.flag);
    this.wrapAsync(this.readFileBase, [id, flagsNum, opts.encoding], callback);
  }

  private writeBase(fd: number, buf: Buffer, offset?: number, length?: number, position?: number): number {
    const file = this.getFileByFdOrThrow(fd, 'write');
    return file.write(buf, offset, length, position);
  }

  writeSync(fd: number, buffer: Buffer | Uint8Array, offset?: number, length?: number, position?: number): number;
  writeSync(fd: number, str: string, position?: number, encoding?: BufferEncoding): number;
  writeSync(fd: number, a: string | Buffer | Uint8Array, b?: number, c?: number | BufferEncoding, d?: number): number {
    validateFd(fd);

    let encoding: BufferEncoding | undefined;
    let offset: number | undefined;
    let length: number | undefined;
    let position: number | undefined;

    const isBuffer = typeof a !== 'string';
    if (isBuffer) {
      offset = (b || 0) | 0;
      length = c as number;
      position = d;
    } else {
      position = b;
      encoding = c as BufferEncoding;
    }

    const buf: Buffer = dataToBuffer(a, encoding);

    if (isBuffer) {
      if (typeof length === 'undefined') {
        length = buf.length;
      }
    } else {
      offset = 0;
      length = buf.length;
    }

    return this.writeBase(fd, buf, offset, length, position);
  }

  write(fd: number, buffer: Buffer | Uint8Array, callback: (...args) => void);
  write(fd: number, buffer: Buffer | Uint8Array, offset: number, callback: (...args) => void);
  write(fd: number, buffer: Buffer | Uint8Array, offset: number, length: number, callback: (...args) => void);
  write(
    fd: number,
    buffer: Buffer | Uint8Array,
    offset: number,
    length: number,
    position: number,
    callback: (...args) => void,
  );
  write(fd: number, str: string, callback: (...args) => void);
  write(fd: number, str: string, position: number, callback: (...args) => void);
  write(fd: number, str: string, position: number, encoding: BufferEncoding, callback: (...args) => void);
  write(fd: number, a?, b?, c?, d?, e?) {
    validateFd(fd);

    let offset: number;
    let length: number | undefined;
    let position: number;
    let encoding: BufferEncoding | undefined;
    let callback: ((...args) => void) | undefined;

    const tipa = typeof a;
    const tipb = typeof b;
    const tipc = typeof c;
    const tipd = typeof d;

    if (tipa !== 'string') {
      if (tipb === 'function') {
        callback = b;
      } else if (tipc === 'function') {
        offset = b | 0;
        callback = c;
      } else if (tipd === 'function') {
        offset = b | 0;
        length = c;
        callback = d;
      } else {
        offset = b | 0;
        length = c;
        position = d;
        callback = e;
      }
    } else {
      if (tipb === 'function') {
        callback = b;
      } else if (tipc === 'function') {
        position = b;
        callback = c;
      } else if (tipd === 'function') {
        position = b;
        encoding = c;
        callback = d;
      }
    }

    const buf: Buffer = dataToBuffer(a, encoding);

    if (tipa !== 'string') {
      if (typeof length === 'undefined') length = buf.length;
    } else {
      offset = 0;
      length = buf.length;
    }

    const cb = validateCallback(callback);

    setImmediate(() => {
      try {
        const bytes = this.writeBase(fd, buf, offset, length, position);
        if (tipa !== 'string') {
          cb(null, bytes, buf);
        } else {
          cb(null, bytes, a);
        }
      } catch (err) {
        cb(err);
      }
    });
  }

  private writeFileBase(id: TFileId, buf: Buffer, flagsNum: number, modeNum: number) {
    // console.log('writeFileBase', id, buf, flagsNum, modeNum);
    // const node = this.getNodeByIdOrCreate(id, flagsNum, modeNum);
    // node.setBuffer(buf);

    const isUserFd = typeof id === 'number';
    let fd: number;

    if (isUserFd) fd = id as number;
    else {
      fd = this.openBase(pathToFilename(id as PathLike), flagsNum, modeNum);
      // fd = this.openSync(id as PathLike, flagsNum, modeNum);
    }

    let offset = 0;
    let length = buf.length;
    let position = flagsNum & O_APPEND ? undefined : 0;
    try {
      while (length > 0) {
        const written = this.writeSync(fd, buf, offset, length, position);
        offset += written;
        length -= written;
        if (position !== undefined) position += written;
      }
    } finally {
      if (!isUserFd) this.closeSync(fd);
    }
  }

  writeFileSync(id: TFileId, data: TData, options?: IWriteFileOptions) {
    const opts = getWriteFileOptions(options);
    const flagsNum = flagsToNumber(opts.flag);
    const modeNum = modeToNumber(opts.mode);
    const buf = dataToBuffer(data, opts.encoding);
    this.writeFileBase(id, buf, flagsNum, modeNum);
  }

  writeFile(id: TFileId, data: TData, callback: TCallback<void>);
  writeFile(id: TFileId, data: TData, options: IWriteFileOptions | string, callback: TCallback<void>);
  writeFile(id: TFileId, data: TData, a: TCallback<void> | IWriteFileOptions | string, b?: TCallback<void>) {
    let options: IWriteFileOptions | string = a as IWriteFileOptions;
    let callback: TCallback<void> | undefined = b;

    if (typeof a === 'function') {
      options = writeFileDefaults;
      callback = a;
    }

    const cb = validateCallback(callback);

    const opts = getWriteFileOptions(options);
    const flagsNum = flagsToNumber(opts.flag);
    const modeNum = modeToNumber(opts.mode);
    const buf = dataToBuffer(data, opts.encoding);
    this.wrapAsync(this.writeFileBase, [id, buf, flagsNum, modeNum], cb);
  }

  private linkBase(filename1: string, filename2: string) {
    const steps1 = filenameToSteps(filename1);
    const link1 = this.getLink(steps1);
    if (!link1) throw createError(ENOENT, 'link', filename1, filename2);

    const steps2 = filenameToSteps(filename2);

    // Check new link directory exists.
    const dir2 = this.getLinkParent(steps2);
    if (!dir2) throw createError(ENOENT, 'link', filename1, filename2);

    const name = steps2[steps2.length - 1];

    // Check if new file already exists.
    if (dir2.getChild(name)) throw createError(EEXIST, 'link', filename1, filename2);

    const node = link1.getNode();
    node.nlink++;
    dir2.createChild(name, node);
  }

  private copyFileBase(src: string, dest: string, flags: number) {
    const buf = this.readFileSync(src) as Buffer;

    if (flags & COPYFILE_EXCL) {
      if (this.existsSync(dest)) {
        throw createError(EEXIST, 'copyFile', src, dest);
      }
    }

    if (flags & COPYFILE_FICLONE_FORCE) {
      throw createError(ENOSYS, 'copyFile', src, dest);
    }

    this.writeFileBase(dest, buf, FLAGS.w, MODE.DEFAULT);
  }

  copyFileSync(src: PathLike, dest: PathLike, flags?: TFlagsCopy) {
    const srcFilename = pathToFilename(src);
    const destFilename = pathToFilename(dest);

    return this.copyFileBase(srcFilename, destFilename, (flags || 0) | 0);
  }

  copyFile(src: PathLike, dest: PathLike, callback: TCallback<void>);
  copyFile(src: PathLike, dest: PathLike, flags: TFlagsCopy, callback: TCallback<void>);
  copyFile(src: PathLike, dest: PathLike, a, b?) {
    const srcFilename = pathToFilename(src);
    const destFilename = pathToFilename(dest);

    let flags: TFlagsCopy;
    let callback: TCallback<void>;

    if (typeof a === 'function') {
      flags = 0;
      callback = a;
    } else {
      flags = a;
      callback = b;
    }

    validateCallback(callback);

    this.wrapAsync(this.copyFileBase, [srcFilename, destFilename, flags], callback);
  }

  linkSync(existingPath: PathLike, newPath: PathLike) {
    const existingPathFilename = pathToFilename(existingPath);
    const newPathFilename = pathToFilename(newPath);
    this.linkBase(existingPathFilename, newPathFilename);
  }

  link(existingPath: PathLike, newPath: PathLike, callback: TCallback<void>) {
    const existingPathFilename = pathToFilename(existingPath);
    const newPathFilename = pathToFilename(newPath);
    this.wrapAsync(this.linkBase, [existingPathFilename, newPathFilename], callback);
  }

  private unlinkBase(filename: string) {
    const steps = filenameToSteps(filename);
    const link = this.getLink(steps);
    if (!link) throw createError(ENOENT, 'unlink', filename);

    // TODO: Check if it is file, dir, other...

    if (link.length) throw Error('Dir not empty...');

    this.deleteLink(link);

    const node = link.getNode();
    node.nlink--;

    // When all hard links to i-node are deleted, remove the i-node, too.
    if (node.nlink <= 0) {
      this.deleteNode(node);
    }
  }

  unlinkSync(path: PathLike) {
    const filename = pathToFilename(path);
    this.unlinkBase(filename);
  }

  unlink(path: PathLike, callback: TCallback<void>) {
    const filename = pathToFilename(path);
    this.wrapAsync(this.unlinkBase, [filename], callback);
  }

  private symlinkBase(targetFilename: string, pathFilename: string): Link {
    const pathSteps = filenameToSteps(pathFilename);

    // Check if directory exists, where we about to create a symlink.
    const dirLink = this.getLinkParent(pathSteps);
    if (!dirLink) throw createError(ENOENT, 'symlink', targetFilename, pathFilename);

    const name = pathSteps[pathSteps.length - 1];

    // Check if new file already exists.
    if (dirLink.getChild(name)) throw createError(EEXIST, 'symlink', targetFilename, pathFilename);

    // Create symlink.
    const symlink: Link = dirLink.createChild(name);
    symlink.getNode().makeSymlink(filenameToSteps(targetFilename));
    return symlink;
  }

  // `type` argument works only on Windows.
  symlinkSync(target: PathLike, path: PathLike, type?: symlink.Type) {
    const targetFilename = pathToFilename(target);
    const pathFilename = pathToFilename(path);
    this.symlinkBase(targetFilename, pathFilename);
  }

  symlink(target: PathLike, path: PathLike, callback: TCallback<void>);
  symlink(target: PathLike, path: PathLike, type: symlink.Type, callback: TCallback<void>);
  symlink(target: PathLike, path: PathLike, a: symlink.Type | TCallback<void>, b?: TCallback<void>) {
    const callback: TCallback<void> = validateCallback(typeof a === 'function' ? a : b);
    const targetFilename = pathToFilename(target);
    const pathFilename = pathToFilename(path);
    this.wrapAsync(this.symlinkBase, [targetFilename, pathFilename], callback);
  }

  private realpathBase(filename: string, encoding: TEncodingExtended | undefined): TDataOut {
    const steps = filenameToSteps(filename);
    const realLink = this.getResolvedLink(steps);
    if (!realLink) throw createError(ENOENT, 'realpath', filename);

    return strToEncoding(realLink.getPath(), encoding);
  }

  realpathSync(path: PathLike, options?: IRealpathOptions | string): TDataOut {
    return this.realpathBase(pathToFilename(path), getRealpathOptions(options).encoding);
  }

  realpath(path: PathLike, callback: TCallback<TDataOut>);
  realpath(path: PathLike, options: IRealpathOptions | string, callback: TCallback<TDataOut>);
  realpath(path: PathLike, a: TCallback<TDataOut> | IRealpathOptions | string, b?: TCallback<TDataOut>) {
    const [opts, callback] = getRealpathOptsAndCb(a, b);
    const pathFilename = pathToFilename(path);
    this.wrapAsync(this.realpathBase, [pathFilename, opts.encoding], callback);
  }

  private lstatBase(filename: string, bigint: false, throwIfNoEntry: true): Stats<number>;
  private lstatBase(filename: string, bigint: true, throwIfNoEntry: true): Stats<bigint>;
  private lstatBase(filename: string, bigint: true, throwIfNoEntry: false): Stats<bigint> | undefined;
  private lstatBase(filename: string, bigint: false, throwIfNoEntry: false): Stats<number> | undefined;
  private lstatBase(filename: string, bigint = false, throwIfNoEntry = false): Stats | undefined {
    const link = this.getLink(filenameToSteps(filename));

    if (link) {
      return Stats.build(link.getNode(), bigint);
    } else if (!throwIfNoEntry) {
      return undefined;
    } else {
      throw createError(ENOENT, 'lstat', filename);
    }
  }

  lstatSync(path: PathLike): Stats<number>;
  lstatSync(path: PathLike, options: { throwIfNoEntry?: true | undefined }): Stats<number>;
  lstatSync(path: PathLike, options: { bigint: false; throwIfNoEntry?: true | undefined }): Stats<number>;
  lstatSync(path: PathLike, options: { bigint: true; throwIfNoEntry?: true | undefined }): Stats<bigint>;
  lstatSync(path: PathLike, options: { throwIfNoEntry: false }): Stats<number> | undefined;
  lstatSync(path: PathLike, options: { bigint: false; throwIfNoEntry: false }): Stats<number> | undefined;
  lstatSync(path: PathLike, options: { bigint: true; throwIfNoEntry: false }): Stats<bigint> | undefined;
  lstatSync(path: PathLike, options?: IStatOptions): Stats | undefined {
    const { throwIfNoEntry = true, bigint = false } = getStatOptions(options);

    return this.lstatBase(pathToFilename(path), bigint as any, throwIfNoEntry as any);
  }

  lstat(path: PathLike, callback: TCallback<Stats>): void;
  lstat(path: PathLike, options: IStatOptions, callback: TCallback<Stats>): void;
  lstat(path: PathLike, a: TCallback<Stats> | IStatOptions, b?: TCallback<Stats>): void {
    const [{ throwIfNoEntry = true, bigint = false }, callback] = getStatOptsAndCb(a, b);
    this.wrapAsync(this.lstatBase, [pathToFilename(path), bigint, throwIfNoEntry], callback);
  }

  private statBase(filename: string): Stats<number>;
  private statBase(filename: string, bigint: false, throwIfNoEntry: true): Stats<number>;
  private statBase(filename: string, bigint: true, throwIfNoEntry: true): Stats<bigint>;
  private statBase(filename: string, bigint: true, throwIfNoEntry: false): Stats<bigint> | undefined;
  private statBase(filename: string, bigint: false, throwIfNoEntry: false): Stats<number> | undefined;
  private statBase(filename: string, bigint = false, throwIfNoEntry = true): Stats | undefined {
    const link = this.getResolvedLink(filenameToSteps(filename));

    if (link) {
      return Stats.build(link.getNode(), bigint);
    } else if (!throwIfNoEntry) {
      return undefined;
    } else {
      throw createError(ENOENT, 'stat', filename);
    }
  }

  statSync(path: PathLike): Stats<number>;
  statSync(path: PathLike, options: { throwIfNoEntry?: true }): Stats<number>;
  statSync(path: PathLike, options: { throwIfNoEntry: false }): Stats<number> | undefined;
  statSync(path: PathLike, options: { bigint: false; throwIfNoEntry?: true }): Stats<number>;
  statSync(path: PathLike, options: { bigint: true; throwIfNoEntry?: true }): Stats<bigint>;
  statSync(path: PathLike, options: { bigint: false; throwIfNoEntry: false }): Stats<number> | undefined;
  statSync(path: PathLike, options: { bigint: true; throwIfNoEntry: false }): Stats<bigint> | undefined;
  statSync(path: PathLike, options?: IStatOptions): Stats | undefined {
    const { bigint = true, throwIfNoEntry = true } = getStatOptions(options);

    return this.statBase(pathToFilename(path), bigint as any, throwIfNoEntry as any);
  }

  stat(path: PathLike, callback: TCallback<Stats>): void;
  stat(path: PathLike, options: IStatOptions, callback: TCallback<Stats>): void;
  stat(path: PathLike, a: TCallback<Stats> | IStatOptions, b?: TCallback<Stats>): void {
    const [{ bigint = false, throwIfNoEntry = true }, callback] = getStatOptsAndCb(a, b);

    this.wrapAsync(this.statBase, [pathToFilename(path), bigint, throwIfNoEntry], callback);
  }

  private fstatBase(fd: number): Stats<number>;
  private fstatBase(fd: number, bigint: false): Stats<number>;
  private fstatBase(fd: number, bigint: true): Stats<bigint>;
  private fstatBase(fd: number, bigint: boolean = false): Stats {
    const file = this.getFileByFd(fd);
    if (!file) throw createError(EBADF, 'fstat');
    return Stats.build(file.node, bigint);
  }

  fstatSync(fd: number): Stats<number>;
  fstatSync(fd: number, options: { bigint: false }): Stats<number>;
  fstatSync(fd: number, options: { bigint: true }): Stats<bigint>;
  fstatSync(fd: number, options?: IFStatOptions): Stats {
    return this.fstatBase(fd, getStatOptions(options).bigint as any);
  }

  fstat(fd: number, callback: TCallback<Stats>): void;
  fstat(fd: number, options: IFStatOptions, callback: TCallback<Stats>): void;
  fstat(fd: number, a: TCallback<Stats> | IFStatOptions, b?: TCallback<Stats>): void {
    const [opts, callback] = getStatOptsAndCb(a, b);
    this.wrapAsync(this.fstatBase, [fd, opts.bigint], callback);
  }

  private renameBase(oldPathFilename: string, newPathFilename: string) {
    const link = this.getLink(filenameToSteps(oldPathFilename));
    if (!link) throw createError(ENOENT, 'rename', oldPathFilename, newPathFilename);

    // TODO: Check if it is directory, if non-empty, we cannot move it, right?

    const newPathSteps = filenameToSteps(newPathFilename);

    // Check directory exists for the new location.
    const newPathDirLink = this.getLinkParent(newPathSteps);
    if (!newPathDirLink) throw createError(ENOENT, 'rename', oldPathFilename, newPathFilename);

    // TODO: Also treat cases with directories and symbolic links.
    // TODO: See: http://man7.org/linux/man-pages/man2/rename.2.html

    // Remove hard link from old folder.
    const oldLinkParent = link.parent;
    if (oldLinkParent) {
      oldLinkParent.deleteChild(link);
    }

    // Rename should overwrite the new path, if that exists.
    const name = newPathSteps[newPathSteps.length - 1];
    link.steps = [...newPathDirLink.steps, name];
    newPathDirLink.setChild(link.getName(), link);
  }

  renameSync(oldPath: PathLike, newPath: PathLike) {
    const oldPathFilename = pathToFilename(oldPath);
    const newPathFilename = pathToFilename(newPath);
    this.renameBase(oldPathFilename, newPathFilename);
  }

  rename(oldPath: PathLike, newPath: PathLike, callback: TCallback<void>) {
    const oldPathFilename = pathToFilename(oldPath);
    const newPathFilename = pathToFilename(newPath);
    this.wrapAsync(this.renameBase, [oldPathFilename, newPathFilename], callback);
  }

  private existsBase(filename: string): boolean {
    return !!this.statBase(filename);
  }

  existsSync(path: PathLike): boolean {
    try {
      return this.existsBase(pathToFilename(path));
    } catch (err) {
      return false;
    }
  }

  exists(path: PathLike, callback: (exists: boolean) => void) {
    const filename = pathToFilename(path);

    if (typeof callback !== 'function') throw Error(ERRSTR.CB);

    setImmediate(() => {
      try {
        callback(this.existsBase(filename));
      } catch (err) {
        callback(false);
      }
    });
  }

  private accessBase(filename: string, mode: number) {
    const link = this.getLinkOrThrow(filename, 'access');

    // TODO: Verify permissions
  }

  accessSync(path: PathLike, mode: number = F_OK) {
    const filename = pathToFilename(path);
    mode = mode | 0;
    this.accessBase(filename, mode);
  }

  access(path: PathLike, callback: TCallback<void>);
  access(path: PathLike, mode: number, callback: TCallback<void>);
  access(path: PathLike, a: TCallback<void> | number, b?: TCallback<void>) {
    let mode: number = F_OK;
    let callback: TCallback<void>;

    if (typeof a !== 'function') {
      mode = a | 0; // cast to number
      callback = validateCallback(b);
    } else {
      callback = a;
    }

    const filename = pathToFilename(path);

    this.wrapAsync(this.accessBase, [filename, mode], callback);
  }

  appendFileSync(id: TFileId, data: TData, options: IAppendFileOptions | string = appendFileDefaults) {
    const opts = getAppendFileOpts(options);

    // force append behavior when using a supplied file descriptor
    if (!opts.flag || isFd(id)) opts.flag = 'a';

    this.writeFileSync(id, data, opts);
  }

  appendFile(id: TFileId, data: TData, callback: TCallback<void>);
  appendFile(id: TFileId, data: TData, options: IAppendFileOptions | string, callback: TCallback<void>);
  appendFile(id: TFileId, data: TData, a, b?) {
    const [opts, callback] = getAppendFileOptsAndCb(a, b);

    // force append behavior when using a supplied file descriptor
    if (!opts.flag || isFd(id)) opts.flag = 'a';

    this.writeFile(id, data, opts, callback);
  }

  private readdirBase(filename: string, options: IReaddirOptions): TDataOut[] | Dirent[] {
    const steps = filenameToSteps(filename);
    const link: Link | null = this.getResolvedLink(steps);
    if (!link) throw createError(ENOENT, 'readdir', filename);

    const node = link.getNode();
    if (!node.isDirectory()) throw createError(ENOTDIR, 'scandir', filename);

    if (options.withFileTypes) {
      const list: Dirent[] = [];
      for (const name in link.children) {
        const child = link.getChild(name);

        if (!child) {
          continue;
        }

        list.push(Dirent.build(child, options.encoding));
      }
      if (!isWin && options.encoding !== 'buffer')
        list.sort((a, b) => {
          if (a.name < b.name) return -1;
          if (a.name > b.name) return 1;
          return 0;
        });
      return list;
    }

    const list: TDataOut[] = [];
    for (const name in link.children) {
      list.push(strToEncoding(name, options.encoding));
    }

    if (!isWin && options.encoding !== 'buffer') list.sort();

    return list;
  }

  readdirSync(path: PathLike, options?: IReaddirOptions | string): TDataOut[] | Dirent[] {
    const opts = getReaddirOptions(options);
    const filename = pathToFilename(path);
    return this.readdirBase(filename, opts);
  }

  readdir(path: PathLike, callback: TCallback<TDataOut[] | Dirent[]>);
  readdir(path: PathLike, options: IReaddirOptions | string, callback: TCallback<TDataOut[] | Dirent[]>);
  readdir(path: PathLike, a?, b?) {
    const [options, callback] = getReaddirOptsAndCb(a, b);
    const filename = pathToFilename(path);
    this.wrapAsync(this.readdirBase, [filename, options], callback);
  }

  private readlinkBase(filename: string, encoding: TEncodingExtended | undefined): TDataOut {
    const link = this.getLinkOrThrow(filename, 'readlink');
    const node = link.getNode();

    if (!node.isSymlink()) throw createError(EINVAL, 'readlink', filename);

    const str = sep + node.symlink.join(sep);
    return strToEncoding(str, encoding);
  }

  readlinkSync(path: PathLike, options?: IOptions): TDataOut {
    const opts = getDefaultOpts(options);
    const filename = pathToFilename(path);
    return this.readlinkBase(filename, opts.encoding);
  }

  readlink(path: PathLike, callback: TCallback<TDataOut>);
  readlink(path: PathLike, options: IOptions, callback: TCallback<TDataOut>);
  readlink(path: PathLike, a: TCallback<TDataOut> | IOptions, b?: TCallback<TDataOut>) {
    const [opts, callback] = getDefaultOptsAndCb(a, b);
    const filename = pathToFilename(path);
    this.wrapAsync(this.readlinkBase, [filename, opts.encoding], callback);
  }

  private fsyncBase(fd: number) {
    this.getFileByFdOrThrow(fd, 'fsync');
  }

  fsyncSync(fd: number) {
    this.fsyncBase(fd);
  }

  fsync(fd: number, callback: TCallback<void>) {
    this.wrapAsync(this.fsyncBase, [fd], callback);
  }

  private fdatasyncBase(fd: number) {
    this.getFileByFdOrThrow(fd, 'fdatasync');
  }

  fdatasyncSync(fd: number) {
    this.fdatasyncBase(fd);
  }

  fdatasync(fd: number, callback: TCallback<void>) {
    this.wrapAsync(this.fdatasyncBase, [fd], callback);
  }

  private ftruncateBase(fd: number, len?: number) {
    const file = this.getFileByFdOrThrow(fd, 'ftruncate');
    file.truncate(len);
  }

  ftruncateSync(fd: number, len?: number) {
    this.ftruncateBase(fd, len);
  }

  ftruncate(fd: number, callback: TCallback<void>);
  ftruncate(fd: number, len: number, callback: TCallback<void>);
  ftruncate(fd: number, a: TCallback<void> | number, b?: TCallback<void>) {
    const len: number = typeof a === 'number' ? a : 0;
    const callback: TCallback<void> = validateCallback(typeof a === 'number' ? b : a);

    this.wrapAsync(this.ftruncateBase, [fd, len], callback);
  }

  private truncateBase(path: PathLike, len?: number) {
    const fd = this.openSync(path, 'r+');
    try {
      this.ftruncateSync(fd, len);
    } finally {
      this.closeSync(fd);
    }
  }

  truncateSync(id: TFileId, len?: number) {
    if (isFd(id)) return this.ftruncateSync(id as number, len);

    this.truncateBase(id as PathLike, len);
  }

  truncate(id: TFileId, callback: TCallback<void>);
  truncate(id: TFileId, len: number, callback: TCallback<void>);
  truncate(id: TFileId, a: TCallback<void> | number, b?: TCallback<void>) {
    const len: number = typeof a === 'number' ? a : 0;
    const callback: TCallback<void> = validateCallback(typeof a === 'number' ? b : a);

    if (isFd(id)) return this.ftruncate(id as number, len, callback);

    this.wrapAsync(this.truncateBase, [id, len], callback);
  }

  private futimesBase(fd: number, atime: number, mtime: number) {
    const file = this.getFileByFdOrThrow(fd, 'futimes');
    const node = file.node;
    node.atime = new Date(atime * 1000);
    node.mtime = new Date(mtime * 1000);
  }

  futimesSync(fd: number, atime: TTime, mtime: TTime) {
    this.futimesBase(fd, toUnixTimestamp(atime), toUnixTimestamp(mtime));
  }

  futimes(fd: number, atime: TTime, mtime: TTime, callback: TCallback<void>) {
    this.wrapAsync(this.futimesBase, [fd, toUnixTimestamp(atime), toUnixTimestamp(mtime)], callback);
  }

  private utimesBase(filename: string, atime: number, mtime: number) {
    const fd = this.openSync(filename, 'r+');
    try {
      this.futimesBase(fd, atime, mtime);
    } finally {
      this.closeSync(fd);
    }
  }

  utimesSync(path: PathLike, atime: TTime, mtime: TTime) {
    this.utimesBase(pathToFilename(path), toUnixTimestamp(atime), toUnixTimestamp(mtime));
  }

  utimes(path: PathLike, atime: TTime, mtime: TTime, callback: TCallback<void>) {
    this.wrapAsync(this.utimesBase, [pathToFilename(path), toUnixTimestamp(atime), toUnixTimestamp(mtime)], callback);
  }

  private mkdirBase(filename: string, modeNum: number) {
    const steps = filenameToSteps(filename);

    // This will throw if user tries to create root dir `fs.mkdirSync('/')`.
    if (!steps.length) {
      throw createError(EEXIST, 'mkdir', filename);
    }

    const dir = this.getLinkParentAsDirOrThrow(filename, 'mkdir');

    // Check path already exists.
    const name = steps[steps.length - 1];
    if (dir.getChild(name)) throw createError(EEXIST, 'mkdir', filename);

    dir.createChild(name, this.createNode(true, modeNum));
  }

  /**
   * Creates directory tree recursively.
   * @param filename
   * @param modeNum
   */
  private mkdirpBase(filename: string, modeNum: number) {
    const steps = filenameToSteps(filename);
    let link = this.root;
    for (let i = 0; i < steps.length; i++) {
      const step = steps[i];

      if (!link.getNode().isDirectory()) throw createError(ENOTDIR, 'mkdir', link.getPath());

      const child = link.getChild(step);
      if (child) {
        if (child.getNode().isDirectory()) link = child;
        else throw createError(ENOTDIR, 'mkdir', child.getPath());
      } else {
        link = link.createChild(step, this.createNode(true, modeNum));
      }
    }
  }

  mkdirSync(path: PathLike, options?: TMode | IMkdirOptions) {
    const opts = getMkdirOptions(options);
    const modeNum = modeToNumber(opts.mode, 0o777);
    const filename = pathToFilename(path);
    if (opts.recursive) this.mkdirpBase(filename, modeNum);
    else this.mkdirBase(filename, modeNum);
  }

  mkdir(path: PathLike, callback: TCallback<void>);
  mkdir(path: PathLike, mode: TMode | IMkdirOptions, callback: TCallback<void>);
  mkdir(path: PathLike, a: TCallback<void> | TMode | IMkdirOptions, b?: TCallback<void>) {
    const opts: TMode | IMkdirOptions = getMkdirOptions(a);
    const callback: TCallback<void> = validateCallback(typeof a === 'function' ? a : b);
    const modeNum = modeToNumber(opts.mode, 0o777);
    const filename = pathToFilename(path);
    if (opts.recursive) this.wrapAsync(this.mkdirpBase, [filename, modeNum], callback);
    else this.wrapAsync(this.mkdirBase, [filename, modeNum], callback);
  }

  // legacy interface
  mkdirpSync(path: PathLike, mode?: TMode) {
    this.mkdirSync(path, { mode, recursive: true });
  }

  mkdirp(path: PathLike, callback: TCallback<void>);
  mkdirp(path: PathLike, mode: TMode, callback: TCallback<void>);
  mkdirp(path: PathLike, a: TCallback<void> | TMode, b?: TCallback<void>) {
    const mode: TMode | undefined = typeof a === 'function' ? undefined : a;
    const callback: TCallback<void> = validateCallback(typeof a === 'function' ? a : b);
    this.mkdir(path, { mode, recursive: true }, callback);
  }

  private mkdtempBase(prefix: string, encoding?: TEncodingExtended, retry: number = 5): TDataOut {
    const filename = prefix + this.genRndStr();
    try {
      this.mkdirBase(filename, MODE.DIR);
      return strToEncoding(filename, encoding);
    } catch (err) {
      if (err.code === EEXIST) {
        if (retry > 1) return this.mkdtempBase(prefix, encoding, retry - 1);
        else throw Error('Could not create temp dir.');
      } else throw err;
    }
  }

  mkdtempSync(prefix: string, options?: IOptions): TDataOut {
    const { encoding } = getDefaultOpts(options);

    if (!prefix || typeof prefix !== 'string') throw new TypeError('filename prefix is required');

    nullCheck(prefix);

    return this.mkdtempBase(prefix, encoding);
  }

  mkdtemp(prefix: string, callback: TCallback<void>);
  mkdtemp(prefix: string, options: IOptions, callback: TCallback<void>);
  mkdtemp(prefix: string, a: TCallback<void> | IOptions, b?: TCallback<void>) {
    const [{ encoding }, callback] = getDefaultOptsAndCb(a, b);

    if (!prefix || typeof prefix !== 'string') throw new TypeError('filename prefix is required');

    if (!nullCheck(prefix)) return;

    this.wrapAsync(this.mkdtempBase, [prefix, encoding], callback);
  }

  private rmdirBase(filename: string, options?: IRmdirOptions) {
    const opts = getRmdirOptions(options);
    const link = this.getLinkAsDirOrThrow(filename, 'rmdir');

    // Check directory is empty.
    if (link.length && !opts.recursive) throw createError(ENOTEMPTY, 'rmdir', filename);

    this.deleteLink(link);
  }

  rmdirSync(path: PathLike, options?: IRmdirOptions) {
    this.rmdirBase(pathToFilename(path), options);
  }

  rmdir(path: PathLike, callback: TCallback<void>);
  rmdir(path: PathLike, options: IRmdirOptions, callback: TCallback<void>);
  rmdir(path: PathLike, a: TCallback<void> | IRmdirOptions, b?: TCallback<void>) {
    const opts: IRmdirOptions = getRmdirOptions(a);
    const callback: TCallback<void> = validateCallback(typeof a === 'function' ? a : b);
    this.wrapAsync(this.rmdirBase, [pathToFilename(path), opts], callback);
  }

  private rmBase(filename: string, options: IRmOptions = {}): void {
    const link = this.getResolvedLink(filename);
    if (!link) {
      // "stat" is used to match Node's native error message.
      if (!options.force) throw createError(ENOENT, 'stat', filename);
      return;
    }
    if (link.getNode().isDirectory()) {
      if (!options.recursive) {
        throw createError(ERR_FS_EISDIR, 'rm', filename);
      }
    }
    this.deleteLink(link);
  }

  public rmSync(path: PathLike, options?: IRmOptions): void {
    this.rmBase(pathToFilename(path), options);
  }

  public rm(path: PathLike, callback: TCallback<void>): void;
  public rm(path: PathLike, options: IRmOptions, callback: TCallback<void>): void;
  public rm(path: PathLike, a: TCallback<void> | IRmOptions, b?: TCallback<void>): void {
    const [opts, callback] = getRmOptsAndCb(a, b);
    this.wrapAsync(this.rmBase, [pathToFilename(path), opts], callback);
  }

  private fchmodBase(fd: number, modeNum: number) {
    const file = this.getFileByFdOrThrow(fd, 'fchmod');
    file.chmod(modeNum);
  }

  fchmodSync(fd: number, mode: TMode) {
    this.fchmodBase(fd, modeToNumber(mode));
  }

  fchmod(fd: number, mode: TMode, callback: TCallback<void>) {
    this.wrapAsync(this.fchmodBase, [fd, modeToNumber(mode)], callback);
  }

  private chmodBase(filename: string, modeNum: number) {
    const fd = this.openSync(filename, 'r+');
    try {
      this.fchmodBase(fd, modeNum);
    } finally {
      this.closeSync(fd);
    }
  }

  chmodSync(path: PathLike, mode: TMode) {
    const modeNum = modeToNumber(mode);
    const filename = pathToFilename(path);
    this.chmodBase(filename, modeNum);
  }

  chmod(path: PathLike, mode: TMode, callback: TCallback<void>) {
    const modeNum = modeToNumber(mode);
    const filename = pathToFilename(path);
    this.wrapAsync(this.chmodBase, [filename, modeNum], callback);
  }

  private lchmodBase(filename: string, modeNum: number) {
    const fd = this.openBase(filename, O_RDWR, 0, false);
    try {
      this.fchmodBase(fd, modeNum);
    } finally {
      this.closeSync(fd);
    }
  }

  lchmodSync(path: PathLike, mode: TMode) {
    const modeNum = modeToNumber(mode);
    const filename = pathToFilename(path);
    this.lchmodBase(filename, modeNum);
  }

  lchmod(path: PathLike, mode: TMode, callback: TCallback<void>) {
    const modeNum = modeToNumber(mode);
    const filename = pathToFilename(path);
    this.wrapAsync(this.lchmodBase, [filename, modeNum], callback);
  }

  private fchownBase(fd: number, uid: number, gid: number) {
    this.getFileByFdOrThrow(fd, 'fchown').chown(uid, gid);
  }

  fchownSync(fd: number, uid: number, gid: number) {
    validateUid(uid);
    validateGid(gid);
    this.fchownBase(fd, uid, gid);
  }

  fchown(fd: number, uid: number, gid: number, callback: TCallback<void>) {
    validateUid(uid);
    validateGid(gid);
    this.wrapAsync(this.fchownBase, [fd, uid, gid], callback);
  }

  private chownBase(filename: string, uid: number, gid: number) {
    const link = this.getResolvedLinkOrThrow(filename, 'chown');
    const node = link.getNode();
    node.chown(uid, gid);

    // if(node.isFile() || node.isSymlink()) {
    //
    // } else if(node.isDirectory()) {
    //
    // } else {
    // TODO: What do we do here?
    // }
  }

  chownSync(path: PathLike, uid: number, gid: number) {
    validateUid(uid);
    validateGid(gid);
    this.chownBase(pathToFilename(path), uid, gid);
  }

  chown(path: PathLike, uid: number, gid: number, callback: TCallback<void>) {
    validateUid(uid);
    validateGid(gid);
    this.wrapAsync(this.chownBase, [pathToFilename(path), uid, gid], callback);
  }

  private lchownBase(filename: string, uid: number, gid: number) {
    this.getLinkOrThrow(filename, 'lchown')
      .getNode()
      .chown(uid, gid);
  }

  lchownSync(path: PathLike, uid: number, gid: number) {
    validateUid(uid);
    validateGid(gid);
    this.lchownBase(pathToFilename(path), uid, gid);
  }

  lchown(path: PathLike, uid: number, gid: number, callback: TCallback<void>) {
    validateUid(uid);
    validateGid(gid);
    this.wrapAsync(this.lchownBase, [pathToFilename(path), uid, gid], callback);
  }

  private statWatchers: Record<string, StatWatcher> = {};

  watchFile(path: PathLike, listener: (curr: Stats, prev: Stats) => void): StatWatcher;
  watchFile(path: PathLike, options: IWatchFileOptions, listener: (curr: Stats, prev: Stats) => void): StatWatcher;
  watchFile(path: PathLike, a, b?): StatWatcher {
    const filename = pathToFilename(path);

    let options: IWatchFileOptions | null = a;
    let listener: (curr: Stats, prev: Stats) => void = b;

    if (typeof options === 'function') {
      listener = a;
      options = null;
    }

    if (typeof listener !== 'function') {
      throw Error('"watchFile()" requires a listener function');
    }

    let interval = 5007;
    let persistent = true;

    if (options && typeof options === 'object') {
      if (typeof options.interval === 'number') interval = options.interval;
      if (typeof options.persistent === 'boolean') persistent = options.persistent;
    }

    let watcher: StatWatcher = this.statWatchers[filename];

    if (!watcher) {
      watcher = new this.StatWatcher();
      watcher.start(filename, persistent, interval);
      this.statWatchers[filename] = watcher;
    }

    watcher.addListener('change', listener);
    return watcher;
  }

  unwatchFile(path: PathLike, listener?: (curr: Stats, prev: Stats) => void) {
    const filename = pathToFilename(path);
    const watcher = this.statWatchers[filename];
    if (!watcher) return;

    if (typeof listener === 'function') {
      watcher.removeListener('change', listener);
    } else {
      watcher.removeAllListeners('change');
    }

    if (watcher.listenerCount('change') === 0) {
      watcher.stop();
      delete this.statWatchers[filename];
    }
  }

  createReadStream(path: PathLike, options?: IReadStreamOptions | string): IReadStream {
    return new this.ReadStream(path, options);
  }

  createWriteStream(path: PathLike, options?: IWriteStreamOptions | string): IWriteStream {
    return new this.WriteStream(path, options);
  }

  // watch(path: PathLike): FSWatcher;
  // watch(path: PathLike, options?: IWatchOptions | string): FSWatcher;
  watch(
    path: PathLike,
    options?: IWatchOptions | string,
    listener?: (eventType: string, filename: string) => void,
  ): FSWatcher {
    const filename = pathToFilename(path);
    let givenOptions: typeof options | null = options;

    if (typeof options === 'function') {
      listener = options;
      givenOptions = null;
    }

    // tslint:disable-next-line prefer-const
    let { persistent, recursive, encoding }: IWatchOptions = getDefaultOpts(givenOptions);
    if (persistent === undefined) persistent = true;
    if (recursive === undefined) recursive = false;

    const watcher = new this.FSWatcher();
    watcher.start(filename, persistent, recursive, encoding as BufferEncoding);

    if (listener) {
      watcher.addListener('change', listener);
    }

    return watcher;
  }
}

function emitStop(self) {
  self.emit('stop');
}

export class StatWatcher extends EventEmitter {
  vol: Volume;
  filename: string;
  interval: number;
  timeoutRef?;
  setTimeout: TSetTimeout;
  prev: Stats;

  constructor(vol: Volume) {
    super();
    this.vol = vol;
  }

  private loop() {
    this.timeoutRef = this.setTimeout(this.onInterval, this.interval);
  }

  private hasChanged(stats: Stats): boolean {
    // if(!this.prev) return false;
    if (stats.mtimeMs > this.prev.mtimeMs) return true;
    if (stats.nlink !== this.prev.nlink) return true;
    return false;
  }

  private onInterval = () => {
    try {
      const stats = this.vol.statSync(this.filename);
      if (this.hasChanged(stats)) {
        this.emit('change', stats, this.prev);
        this.prev = stats;
      }
    } finally {
      this.loop();
    }
  };

  start(path: string, persistent: boolean = true, interval: number = 5007) {
    this.filename = pathToFilename(path);
    this.setTimeout = persistent ? setTimeout.bind(typeof globalThis !== 'undefined' ? globalThis : global) : setTimeoutUnref;
    this.interval = interval;
    this.prev = this.vol.statSync(this.filename);
    this.loop();
  }

  stop() {
    clearTimeout(this.timeoutRef);
    process.nextTick(emitStop, this);
  }
}

/* tslint:disable no-var-keyword prefer-const */
// ---------------------------------------- ReadStream

export interface IReadStream extends Readable {
  new (path: PathLike, options: IReadStreamOptions);
  open();
  close(callback: TCallback<void>);
  bytesRead: number;
  path: string;
}

var pool;

function allocNewPool(poolSize) {
  pool = bufferAllocUnsafe(poolSize);
  pool.used = 0;
}

util.inherits(FsReadStream, Readable);
exports.ReadStream = FsReadStream;
function FsReadStream(vol, path, options) {
  if (!(this instanceof FsReadStream)) return new (FsReadStream as any)(vol, path, options);

  this._vol = vol;

  // a little bit bigger buffer and water marks by default
  options = Object.assign({}, getOptions(options, {}));
  if (options.highWaterMark === undefined) options.highWaterMark = 64 * 1024;

  Readable.call(this, options);

  this.path = pathToFilename(path);
  this.fd = options.fd === undefined ? null : options.fd;
  this.flags = options.flags === undefined ? 'r' : options.flags;
  this.mode = options.mode === undefined ? 0o666 : options.mode;

  this.start = options.start;
  this.end = options.end;
  this.autoClose = options.autoClose === undefined ? true : options.autoClose;
  this.pos = undefined;
  this.bytesRead = 0;

  if (this.start !== undefined) {
    if (typeof this.start !== 'number') {
      throw new TypeError('"start" option must be a Number');
    }
    if (this.end === undefined) {
      this.end = Infinity;
    } else if (typeof this.end !== 'number') {
      throw new TypeError('"end" option must be a Number');
    }

    if (this.start > this.end) {
      throw new Error('"start" option must be <= "end" option');
    }

    this.pos = this.start;
  }

  if (typeof this.fd !== 'number') this.open();

  this.on('end', function() {
    if (this.autoClose) {
      if (this.destroy) this.destroy();
    }
  });
}

FsReadStream.prototype.open = function() {
  var self = this; // tslint:disable-line no-this-assignment
  this._vol.open(this.path, this.flags, this.mode, (er, fd) => {
    if (er) {
      if (self.autoClose) {
        if (self.destroy) self.destroy();
      }
      self.emit('error', er);
      return;
    }

    self.fd = fd;
    self.emit('open', fd);
    // start the flow of data.
    self.read();
  });
};

FsReadStream.prototype._read = function(n) {
  if (typeof this.fd !== 'number') {
    return this.once('open', function() {
      this._read(n);
    });
  }

  if (this.destroyed) return;

  if (!pool || pool.length - pool.used < kMinPoolSpace) {
    // discard the old pool.
    allocNewPool(this._readableState.highWaterMark);
  }

  // Grab another reference to the pool in the case that while we're
  // in the thread pool another read() finishes up the pool, and
  // allocates a new one.
  var thisPool = pool;
  var toRead = Math.min(pool.length - pool.used, n);
  var start = pool.used;

  if (this.pos !== undefined) toRead = Math.min(this.end - this.pos + 1, toRead);

  // already read everything we were supposed to read!
  // treat as EOF.
  if (toRead <= 0) return this.push(null);

  // the actual read.
  var self = this; // tslint:disable-line no-this-assignment
  this._vol.read(this.fd, pool, pool.used, toRead, this.pos, onread);

  // move the pool positions, and internal position for reading.
  if (this.pos !== undefined) this.pos += toRead;
  pool.used += toRead;

  function onread(er, bytesRead) {
    if (er) {
      if (self.autoClose && self.destroy) {
        self.destroy();
      }
      self.emit('error', er);
    } else {
      var b = null;
      if (bytesRead > 0) {
        self.bytesRead += bytesRead;
        b = thisPool.slice(start, start + bytesRead);
      }

      self.push(b);
    }
  }
};

FsReadStream.prototype._destroy = function(err, cb) {
  this.close(err2 => {
    cb(err || err2);
  });
};

FsReadStream.prototype.close = function(cb) {
  if (cb) this.once('close', cb);

  if (this.closed || typeof this.fd !== 'number') {
    if (typeof this.fd !== 'number') {
      this.once('open', closeOnOpen);
      return;
    }
    return process.nextTick(() => this.emit('close'));
  }

  this.closed = true;

  this._vol.close(this.fd, er => {
    if (er) this.emit('error', er);
    else this.emit('close');
  });

  this.fd = null;
};

// needed because as it will be called with arguments
// that does not match this.close() signature
function closeOnOpen(fd) {
  this.close();
}

// ---------------------------------------- WriteStream

export interface IWriteStream extends Writable {
  bytesWritten: number;
  path: string;
  new (path: PathLike, options: IWriteStreamOptions);
  open();
  close();
}

util.inherits(FsWriteStream, Writable);
exports.WriteStream = FsWriteStream;
function FsWriteStream(vol, path, options) {
  if (!(this instanceof FsWriteStream)) return new (FsWriteStream as any)(vol, path, options);

  this._vol = vol;
  options = Object.assign({}, getOptions(options, {}));

  Writable.call(this, options);

  this.path = pathToFilename(path);
  this.fd = options.fd === undefined ? null : options.fd;
  this.flags = options.flags === undefined ? 'w' : options.flags;
  this.mode = options.mode === undefined ? 0o666 : options.mode;

  this.start = options.start;
  this.autoClose = options.autoClose === undefined ? true : !!options.autoClose;
  this.pos = undefined;
  this.bytesWritten = 0;

  if (this.start !== undefined) {
    if (typeof this.start !== 'number') {
      throw new TypeError('"start" option must be a Number');
    }
    if (this.start < 0) {
      throw new Error('"start" must be >= zero');
    }

    this.pos = this.start;
  }

  if (options.encoding) this.setDefaultEncoding(options.encoding);

  if (typeof this.fd !== 'number') this.open();

  // dispose on finish.
  this.once('finish', function() {
    if (this.autoClose) {
      this.close();
    }
  });
}

FsWriteStream.prototype.open = function() {
  this._vol.open(
    this.path,
    this.flags,
    this.mode,
    function(er, fd) {
      if (er) {
        if (this.autoClose && this.destroy) {
          this.destroy();
        }
        this.emit('error', er);
        return;
      }

      this.fd = fd;
      this.emit('open', fd);
    }.bind(this),
  );
};

FsWriteStream.prototype._write = function(data, encoding, cb) {
  if (!(data instanceof Buffer)) return this.emit('error', new Error('Invalid data'));

  if (typeof this.fd !== 'number') {
    return this.once('open', function() {
      this._write(data, encoding, cb);
    });
  }

  var self = this; // tslint:disable-line no-this-assignment
  this._vol.write(this.fd, data, 0, data.length, this.pos, (er, bytes) => {
    if (er) {
      if (self.autoClose && self.destroy) {
        self.destroy();
      }
      return cb(er);
    }
    self.bytesWritten += bytes;
    cb();
  });

  if (this.pos !== undefined) this.pos += data.length;
};

FsWriteStream.prototype._writev = function(data, cb) {
  if (typeof this.fd !== 'number') {
    return this.once('open', function() {
      this._writev(data, cb);
    });
  }

  const self = this; // tslint:disable-line no-this-assignment
  const len = data.length;
  const chunks = new Array(len);
  var size = 0;

  for (var i = 0; i < len; i++) {
    var chunk = data[i].chunk;

    chunks[i] = chunk;
    size += chunk.length;
  }

  const buf = Buffer.concat(chunks);
  this._vol.write(this.fd, buf, 0, buf.length, this.pos, (er, bytes) => {
    if (er) {
      if (self.destroy) self.destroy();
      return cb(er);
    }
    self.bytesWritten += bytes;
    cb();
  });

  if (this.pos !== undefined) this.pos += size;
};

FsWriteStream.prototype._destroy = FsReadStream.prototype._destroy;
FsWriteStream.prototype.close = FsReadStream.prototype.close;

// There is no shutdown() for files.
FsWriteStream.prototype.destroySoon = FsWriteStream.prototype.end;

// ---------------------------------------- FSWatcher

export class FSWatcher extends EventEmitter {
  _vol: Volume;
  _filename: string = '';
  _steps: string[];
  _filenameEncoded: TDataOut = '';
  // _persistent: boolean = true;
  _recursive: boolean = false;
  _encoding: BufferEncoding = ENCODING_UTF8;
  _link: Link;

  _timer; // Timer that keeps this task persistent.

  constructor(vol: Volume) {
    super();
    this._vol = vol;

    // TODO: Emit "error" messages when watching.
    // this._handle.onchange = function(status, eventType, filename) {
    //     if (status < 0) {
    //         self._handle.close();
    //         const error = !filename ?
    //             errnoException(status, 'Error watching file for changes:') :
    //             errnoException(status, `Error watching file ${filename} for changes:`);
    //         error.filename = filename;
    //         self.emit('error', error);
    //     } else {
    //         self.emit('change', eventType, filename);
    //     }
    // };
  }

  private _getName(): string {
    return this._steps[this._steps.length - 1];
  }

  private _onNodeChange = () => {
    this._emit('change');
  };

  private _onParentChild = (link: Link) => {
    if (link.getName() === this._getName()) {
      this._emit('rename');
    }
  };

  private _emit = (type: 'change' | 'rename') => {
    this.emit('change', type, this._filenameEncoded);
  };

  private _persist = () => {
    this._timer = setTimeout(this._persist, 1e6);
  };

  start(
    path: PathLike,
    persistent: boolean = true,
    recursive: boolean = false,
    encoding: BufferEncoding = ENCODING_UTF8,
  ) {
    this._filename = pathToFilename(path);
    this._steps = filenameToSteps(this._filename);
    this._filenameEncoded = strToEncoding(this._filename);
    // this._persistent = persistent;
    this._recursive = recursive;
    this._encoding = encoding;

    try {
      this._link = this._vol.getLinkOrThrow(this._filename, 'FSWatcher');
    } catch (err) {
      const error = new Error(`watch ${this._filename} ${err.code}`);
      (error as any).code = err.code;
      (error as any).errno = err.code;
      throw error;
    }

    this._link.getNode().on('change', this._onNodeChange);

    this._link.on('child:add', this._onNodeChange);
    this._link.on('child:delete', this._onNodeChange);

    const parent = this._link.parent;
    if (parent) {
      // parent.on('child:add', this._onParentChild);
      parent.setMaxListeners(parent.getMaxListeners() + 1);
      parent.on('child:delete', this._onParentChild);
    }

    if (persistent) this._persist();
  }

  close() {
    clearTimeout(this._timer);

    this._link.getNode().removeListener('change', this._onNodeChange);

    const parent = this._link.parent;
    if (parent) {
      // parent.removeListener('child:add', this._onParentChild);
      parent.removeListener('child:delete', this._onParentChild);
    }
  }
}