/**
 * Way data is stored for this database
 * For a Node.js/Node Webkit database it's the file system
 * For a browser-side database it's localforage which chooses the best option depending on user browser (IndexedDB then WebSQL then localStorage)
 *
 * This version is the Node.js/Node Webkit version
 * It's essentially fs, mkdirp and crash safe write and read functions
 */
const fs = require('fs')
const path = require('path')
const async = require('async')
const storage = {}

// eslint-disable-next-line node/no-callback-literal
storage.exists = (path, cb) => fs.access(path, fs.constants.F_OK, (err) => { cb(!err) })
storage.rename = fs.rename
storage.writeFile = fs.writeFile
storage.unlink = fs.unlink
storage.appendFile = fs.appendFile
storage.readFile = fs.readFile
storage.mkdir = fs.mkdir

/**
 * Explicit name ...
 */
storage.ensureFileDoesntExist = function (file, callback) {
  storage.exists(file, function (exists) {
    if (!exists) { return callback(null) }

    storage.unlink(file, function (err) { return callback(err) })
  })
}

/**
 * Flush data in OS buffer to storage if corresponding option is set
 * @param {String} options.filename
 * @param {Boolean} options.isDir Optional, defaults to false
 * If options is a string, it is assumed that the flush of the file (not dir) called options was requested
 */
storage.flushToStorage = function (options, callback) {
  let filename
  let flags
  if (typeof options === 'string') {
    filename = options
    flags = 'r+'
  } else {
    filename = options.filename
    flags = options.isDir ? 'r' : 'r+'
  }

  // Windows can't fsync (FlushFileBuffers) directories. We can live with this as it cannot cause 100% dataloss
  // except in the very rare event of the first time database is loaded and a crash happens
  if (flags === 'r' && (process.platform === 'win32' || process.platform === 'win64')) { return callback(null) }

  fs.open(filename, flags, function (err, fd) {
    if (err) { return callback(err) }
    fs.fsync(fd, function (errFS) {
      fs.close(fd, function (errC) {
        if (errFS || errC) {
          const e = new Error('Failed to flush to storage')
          e.errorOnFsync = errFS
          e.errorOnClose = errC
          return callback(e)
        } else {
          return callback(null)
        }
      })
    })
  })
}

/**
 * Fully write or rewrite the datafile, immune to crashes during the write operation (data will not be lost)
 * @param {String} filename
 * @param {String} data
 * @param {Function} cb Optional callback, signature: err
 */
storage.crashSafeWriteFile = function (filename, data, cb) {
  const callback = cb || function () {}
  const tempFilename = filename + '~'

  async.waterfall([
    async.apply(storage.flushToStorage, { filename: path.dirname(filename), isDir: true }),
    function (cb) {
      storage.exists(filename, function (exists) {
        if (exists) {
          storage.flushToStorage(filename, function (err) { return cb(err) })
        } else {
          return cb()
        }
      })
    },
    function (cb) {
      storage.writeFile(tempFilename, data, function (err) { return cb(err) })
    },
    async.apply(storage.flushToStorage, tempFilename),
    function (cb) {
      storage.rename(tempFilename, filename, function (err) { return cb(err) })
    },
    async.apply(storage.flushToStorage, { filename: path.dirname(filename), isDir: true })
  ], function (err) { return callback(err) })
}

/**
 * Ensure the datafile contains all the data, even if there was a crash during a full file write
 * @param {String} filename
 * @param {Function} callback signature: err
 */
storage.ensureDatafileIntegrity = function (filename, callback) {
  const tempFilename = filename + '~'

  storage.exists(filename, function (filenameExists) {
    // Write was successful
    if (filenameExists) { return callback(null) }

    storage.exists(tempFilename, function (oldFilenameExists) {
      // New database
      if (!oldFilenameExists) {
        return storage.writeFile(filename, '', 'utf8', function (err) { callback(err) })
      }

      // Write failed, use old version
      storage.rename(tempFilename, filename, function (err) { return callback(err) })
    })
  })
}

// Interface
module.exports = storage