read-lines-reverse.ts

import * as Fs from "node:fs/promises"

export async function* readLinesReverse(path: string, chunkSize = 1024) {
  const file = await Fs.open(path, "r")
  const stat = await file.stat()

  let pos = stat.size
  let buffer: Buffer | null = null
  while (true) {
    pos = Math.max(0, pos - chunkSize)

    let chunk = Buffer.alloc(chunkSize)
    const result = await file.read(chunk, 0, chunkSize, pos)
    pos += chunkSize - result.bytesRead

    if (buffer) {
      chunk = Buffer.concat([chunk.subarray(0, result.bytesRead), buffer])
      buffer = null
    } else {
      chunk = chunk.subarray(0, result.bytesRead)
    }

    while (true) {
      const newLinePos = chunk.lastIndexOf("\n")
      if (newLinePos === -1) {
        buffer = chunk
        break
      }
      yield chunk.subarray(newLinePos + 1, chunk.length)
      chunk = chunk.subarray(0, newLinePos)
    }

    if (pos === 0) {
      if (chunk.length > 0) yield chunk
      break
    }
  }
}