Function ReadLastNLineOffsets

pkg/util/readutil/readutil.go:62–105

readLastNLineOffsets reads all line offsets from the reader, keeping only the last maxLines in a sliding window. keepFirst indicates whether offset 0 should be included (true if starting from file beginning). Returns the offsets and the total number of lines found.

(rs io.ReadSeeker, maxLines int, keepFirst bool)

// keepFirst indicates whether offset 0 should be included (true if starting from file beginning).
// Returns the offsets and the total number of lines found.
func ReadLastNLineOffsets(rs io.ReadSeeker, maxLines int, keepFirst bool) ([]int64, int, error) {
	if _, err := rs.Seek(0, io.SeekStart); err != nil {
		return nil, 0, err
	}

	var offsets []int64
	reader := bufio.NewReader(rs)
	var currentPos int64 = 0
	totalLines := 0

	if keepFirst {
		offsets = append(offsets, 0)
		totalLines = 1
	}

	for {
		line, err := reader.ReadBytes('\n')

		if len(line) > 0 {
			currentPos += int64(len(line))
			offsets = append(offsets, currentPos)
			totalLines++
			// Keep maxLines+1 for sliding window (extra slot for EOF position)
			if len(offsets) > maxLines+1 {
				offsets = offsets[1:]
			}
		}

		if err == io.EOF {
			break
		}
		if err != nil {
			return nil, 0, err
		}
	}

	// Trim the final EOF offset if we have one
	if len(offsets) > 0 {
		offsets = offsets[:len(offsets)-1]
		totalLines--
	}

	return offsets, totalLines, nil
}

// readTailLinesInternal reads the last lineCount lines from the reader, excluding the last lineOffset lines.
// For example, lineCount=10 and lineOffset=5 would return lines -15 through -6 (the 10 lines before the last 5).