Fast line reading in bun

readlines.js

async function readlines(stream, callback, buffer_len = 16) {
  const reader = stream.getReader()
  const td = new TextDecoder('ascii')
  let overflow = null
  let buffer = []

  function flush_buffer() {
    for (let b of buffer) {
      callback(td.decode(b))
    }
    buffer = []
  }

  function add_to_buffer(b) {
    buffer.push(b)
    if (buffer.length >= buffer_len) {
      flush_buffer()
    }
  }

  function process(value) {
    let i = -1
    let last_nl = 0
    while ((i = value.indexOf('\n'.charCodeAt(0), i + 1)) >= 0) {
      const next_line = value.slice(last_nl, i)
      if (overflow && overflow.length) {
        const merge = new Uint8Array(overflow.length + next_line.length)
        merge.set(overflow)
        merge.set(next_line, overflow.length)
        add_to_buffer(merge)
        overflow = null
      } else {
        add_to_buffer(next_line)
      }
      if (i >= 0) {
        last_nl = i + 1
      }
    }
    if (last_nl != value.length) {
      overflow = value.slice(last_nl, value.length)
    }
  }

  while (true) {
    const b = await reader.read()
    if (!b.value) {
      break
    }
    process(b.value)
  }
  if (overflow) {
    add_to_buffer(overflow)
  }
  flush_buffer()
}


const t0 = performance.now()
const stream = Bun.file("input.txt").stream()

let out = []
await readlines(stream, (line) => {
  const [a, b, c] = line.split(',').map(Number)
  const n = 4 * a + b * b + (c / 3)
  if (out.length && out[out.length - 1] > n) {
    out.push(n - out[out.length - 1])
  }
  out.push(n)
})

const d = (performance.now() - t0) / 1e3
console.log("computed", out.length, "in", d, "seconds")
console.log(out.length / d, "lines/sec")

Comparison to node.js: https://gist.github.com/bwasti/25951fbda7a61e74ac48613e7295fbbb

Input file generated like this:

python -c 'import random; print("\n".join(f"{random.random()},{random.random()},{random.random()}" for i in range(10000000)))' > input.txt