Function calculateUnreadInRanges

server/topic.go:4010–4043

calculateUnreadInRanges calculates how many unread messages are within the given ranges. unreadStart is the first unread message SeqId (readID + 1), unreadEnd is the last possible message SeqId. Assumes ranges are sorted by Low ascending.

(readID, lastID int, ranges []types.Range)

// unreadStart is the first unread message SeqId (readID + 1), unreadEnd is the last possible message SeqId.
// Assumes ranges are sorted by Low ascending.
func calculateUnreadInRanges(readID, lastID int, ranges []types.Range) int {
	if readID >= lastID {
		// No unread messages
		return 0
	}

	unreadStart := readID + 1
	unreadEnd := lastID

	// Sum up unread messages.
	count := 0

	for i := 0; i < len(ranges); i++ {
		rangeStart := ranges[i].Low
		rangeEnd := ranges[i].Hi
		if rangeEnd == 0 {
			rangeEnd = rangeStart + 1
		}
		// Find the first range where rangeEnd > readID
		if rangeEnd <= readID {
			continue
		}

		// Find intersection of [unreadStart, unreadEnd] and [rangeStart, rangeEnd)
		intersectionStart := max(unreadStart, rangeStart)
		intersectionEnd := min(unreadEnd+1, rangeEnd) // +1 because unreadEnd is inclusive

		if intersectionStart < intersectionEnd {
			count += intersectionEnd - intersectionStart
		}
	}

	return count
}