Linux Tail

import fs from 'fs'
import os from 'os'
import events from 'events'

const INITIAL_READ_LINES = 10
const READ_CHUNK_SIZE = 16
const POLLING_INTERVAL = 10000

export default class TailFile extends events.EventEmitter {
  private filename: fs.PathLike
  private descriptor: number | null
  private eof: number | null
  private inode: number | null
  private timer?: NodeJS.Timeout

  constructor(filename: fs.PathLike) {
    super()
    this.filename = filename
    this.descriptor = null
    this.eof = null
    this.inode = null
  }

  start() {
    this._openFile()
    this._readInitialLines()
    this._scheduleTimer()
  }

  private _openFile() {
    this.descriptor = fs.openSync(this.filename, 'r')
    const stats = fs.fstatSync(this.descriptor)
    this.eof = stats.size
    this.inode = stats.ino
  }

  private _closeFile() {
    if (this.descriptor !== null) {
      fs.closeSync(this.descriptor)
    }
    this.eof = this.inode = this.descriptor = null
  }

  private _readInitialLines() {
    if (this.descriptor === null || this.eof === null) {
      return
    }

    let bufferSize = Math.min(READ_CHUNK_SIZE, this.eof)
    let buffer = Buffer.alloc(bufferSize)
    let stringTail = ''
    let position = this.eof

    while (position > 0) {
      position -= bufferSize
      if (position < 0) {
        bufferSize += position
        buffer = buffer.subarray(0, position)
        position = 0
      }

      fs.readSync(this.descriptor, buffer, 0, bufferSize, position)
      stringTail = buffer.toString() + stringTail

      const newLineChars = stringTail.match(os.EOL)?.length ?? 0
      if (newLineChars >= INITIAL_READ_LINES || position === 0) {
        stringTail = stringTail.split(os.EOL).slice(-INITIAL_READ_LINES).join(os.EOL)
        this.emit('data', stringTail)
        break
      }
    }
  }

  private _pollFileForChanges() {
    if (this.descriptor === null || this.eof === null || this.inode === null) {
      return
    }

    const stats = fs.statSync(this.filename)

    if (stats.ino !== this.inode) {
      this._readRemainderFromDescriptor()
      this._openFile()
      this._readFromDescriptor(0, this.eof)
    } else if (stats.size < this.eof) {
      this._readFromDescriptor(0, this.eof)
    } else if (stats.size > this.eof) {
      this._readFromDescriptor(this.eof, stats.size)
    }

    this.eof = stats.size
    this._scheduleTimer()
  }

  private _readFromDescriptor(start: number, end: number) {
    if (this.descriptor === null) {
      return
    }

    const bufferSize = end - start
    const buffer = Buffer.alloc(bufferSize)

    fs.readSync(this.descriptor, buffer, 0, bufferSize, start)
    this.emit('data', buffer.toString())
  }

  private _readRemainderFromDescriptor() {
    if (this.descriptor === null || this.eof === null) {
      return
    }

    const stats = fs.fstatSync(this.descriptor)
    const bufferSize = stats.size - this.eof
    const buffer = Buffer.alloc(bufferSize)

    fs.readSync(this.descriptor, buffer, 0, bufferSize, this.eof)
    this.emit('data', buffer.toString())
    this._closeFile()
  }

  private _scheduleTimer() {
    clearTimeout(this.timer)
    this.timer = setTimeout(this._pollFileForChanges.bind(this), POLLING_INTERVAL)
  }
}