const blockSize … // 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. func findTailLineStartIndex(f io.ReadSeeker, n int64) (int64, error) { … }