Golang: How do I determine the number of lines in a file efficiently?

Here's a faster line counter using bytes.Count to find the newline characters.

It's faster because it takes away all the extra logic and buffering required to return whole lines, and takes advantage of some assembly optimized functions offered by the bytes package to search characters in a byte slice.

Larger buffers also help here, especially with larger files. On my system, with the file I used for testing, a 32k buffer was fastest.

func lineCounter(r io.Reader) (int, error) {
    buf := make([]byte, 32*1024)
    count := 0
    lineSep := []byte{'\n'}

    for {
        c, err := r.Read(buf)
        count += bytes.Count(buf[:c], lineSep)

        switch {
        case err == io.EOF:
            return count, nil

        case err != nil:
            return count, err
        }
    }
}

and the benchmark output:

BenchmarkBuffioScan   500      6408963 ns/op     4208 B/op    2 allocs/op
BenchmarkBytesCount   500      4323397 ns/op     8200 B/op    1 allocs/op
BenchmarkBytes32k     500      3650818 ns/op     65545 B/op   1 allocs/op

The most efficient way I found is using IndexByte of the byte packet, it is at least four times faster than using bytes.Count and depending on the size of the buffer it uses much less memory.

func LineCounter(r io.Reader) (int, error) {

    var count int
    const lineBreak = '\n'

    buf := make([]byte, bufio.MaxScanTokenSize)

    for {
        bufferSize, err := r.Read(buf)
        if err != nil && err != io.EOF {
            return 0, err
        }

        var buffPosition int
        for {
            i := bytes.IndexByte(buf[buffPosition:], lineBreak)
            if i == -1 || bufferSize == buffPosition {
                break
            }
            buffPosition += i + 1
            count++
        }
        if err == io.EOF {
            break
        }
    }

    return count, nil
}

Benchmark

BenchmarkIndexByteWithBuffer  2000000          653 ns/op        1024 B/op          1 allocs/op
BenchmarkBytes32k             500000          3189 ns/op       32768 B/op          1 allocs/op