split file by maximum size, but never break lines

linesplit.go

package main

import (
	"bufio"
	"flag"
	"fmt"
	"os"
)

func main() {
	maxSize := flag.Int("s", 1024*1024, "chunk size")
	flag.Parse()

	if flag.NArg() < 2 {
		fmt.Fprintf(os.Stderr, "usage: %s [-s chunksize] input outputfmt\n", os.Args[0])
		fmt.Fprintf(os.Stderr, "\texample:\n\t%s -s 1024 users.txt newusers%%02d.txt\n", os.Args[0])
		os.Exit(1)
	}

	var srcPath = flag.Args()[0]
	var dstBase = flag.Args()[1]
	var currentSize int
	var currentFileno int

	inFile, err := os.Open(srcPath)
	if err != nil {
		panic(err)
	}

	out, err := os.Create(fmt.Sprintf(dstBase, 0))
	if err != nil {
		panic(err)
	}

	scanner := bufio.NewScanner(inFile)
	for scanner.Scan() {
		line := scanner.Text() + "\n"
		currentSize += len(line)
		if currentSize >= *maxSize {
			out.Close()
			currentFileno++
			currentSize = len(line)
			out, err = os.Create(fmt.Sprintf(dstBase, currentFileno))
			if err != nil {
				panic(err)
			}
		}
		out.WriteString(line)
	}
	out.Close()
	inFile.Close()
}