Read random lines from a file

rl.go

// inspired by https://github.com/miku/randlines
package main

import (
	"errors"
	"io"
	"log"
	"math/rand"
	"os"
	"strconv"
)

func main() {
	if len(os.Args) != 3 {
		log.Fatalf("Usage: %s /file #lines\nExample: e.g. %s /some/file 100", os.Args[0], os.Args[0])
	}

	N, err := strconv.Atoi(os.Args[2])
	if err != nil {
		log.Fatal(err)
	}
	if N < 1 {
		log.Fatalf("n must be > 0 but was %d", N)
	}

	path := os.Args[1]
	f, err := os.Open(path)
	if err != nil {
		log.Fatal(err)
	}
	defer f.Close()

	buf := make([]byte, 1<<10)
	var cnt int64
	var newlines []int64
	for {
		n, err := f.Read(buf)
		if err != nil {
			if n == 0 && errors.Is(err, io.EOF) {
				break
			}
			log.Fatal(err)
		}
		for i := int64(0); i < int64(n); i++ {
			if buf[i] == '\n' {
				newlines = append(newlines, cnt+i+1)
			}
		}
		cnt += int64(n)
	}

	for N > 0 {
		i := 1 + rand.Intn(len(newlines)-1)

		start, end := newlines[i-1], newlines[i]
		_, err = f.Seek(start, 0)
		if err != nil {
			log.Fatal(err)
		}
		_, err = io.CopyN(os.Stdout, f, end-start)
		if err != nil {
			log.Fatal(err)
		}
		N--
	}
}