Golang：如何有效确定文件中的行数？

In Golang, I am looking for an efficient way to determine the number of lines a file has.

Of course, I can always loop through the entire file, but does not seem very efficient.

file, _ := os.Open("/path/to/filename")
fileScanner := bufio.NewScanner(file)
lineCount := 0
for fileScanner.Scan() {
    lineCount++
}
fmt.Println("number of lines:", lineCount)

Is there a better (quicker, less expensive) way to find out how many lines a file has?

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{'
'}

    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

There is no approach that is significantly faster than yours as there is no meta-data on how many lines a file has. You could reach a little speed up by manually looking for newline-characters:

func lineCount(r io.Reader) (int n, error err) {
    buf := make([]byte, 8192)

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

        for _, b := range buf[:c] {
            if b == '
' {
                n++
            }
        }
    }

    if err == io.EOF {
        err = nil
    }
}

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 readSize int
    var err error
    var count int

    buf := make([]byte, 1024)

    for {
        readSize, err = r.Read(buf)
        if err != nil {
            break
        }

        var buffPosition int
        for {
            i := bytes.IndexByte(buf[buffPosition:], '
')
            if i == -1 || readSize == buffPosition {
                break
            }
            buffPosition += i + 1
            count++
        }
    }
    if readSize > 0 && count == 0 || count > 0 {
        count++
    }
    if err == io.EOF {
        return count, nil
    }

    return count, err
}

Benchmark

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