Function FindTailLineStartIndex

pkg/logging/tail/tail.go:45–75

FindTailLineStartIndex returns the start of last nth line. * If n <= 0, return the beginning of the file. * If n > 0, return the beginning of last nth line. Notice that if the last line is incomplete (no end-of-line), it will not be counted as one line.

(f io.ReadSeeker, n uint)

// Notice that if the last line is incomplete (no end-of-line), it will not be counted
// as one line.
func FindTailLineStartIndex(f io.ReadSeeker, n uint) (int64, error) {
	if n <= 0 {
		return 0, nil
	}
	size, err := f.Seek(0, io.SeekEnd)
	if err != nil {
		return 0, err
	}
	var left, cnt int64
	buf := make([]byte, blockSize)
	for right := size; right > 0 && uint(cnt) <= n; right -= blockSize {
		left = right - blockSize
		if left < 0 {
			left = 0
			buf = make([]byte, right)
		}
		if _, err := f.Seek(left, io.SeekStart); err != nil {
			return 0, err
		}
		if _, err := f.Read(buf); err != nil {
			return 0, err
		}
		cnt += int64(bytes.Count(buf, eol))
	}
	for ; uint(cnt) > n; cnt-- {
		idx := bytes.Index(buf, eol) + 1
		buf = buf[idx:]
		left += int64(idx)
	}
	return left, nil
}