Explain method _updateDeletedRanges()

[XuanTung95]

Trying to understand this method but have no clue.
Could you explain the purpose of the method?

  /// Calculate ranges of missing messages
  void _updateDeletedRanges() {
    var ranges = <DataMessage>[];
    DataMessage prev;

    // Check for gap in the beginning, before the first message.
    var first = _messages.length > 0 ? _messages.getAt(0) : null;

    if (first != null && _minSeq > 1 && !_noEarlierMsgs) {
      // Some messages are missing in the beginning.
      if (first.hi != null && (first.hi ?? 0) > 0) {
        // The first message already represents a gap.
        if ((first.seq ?? 0) > 1) {
          first.seq = 1;
        }
        if ((first.hi ?? 0) < _minSeq - 1) {
          first.hi = _minSeq - 1;
        }
        prev = first;
      } else {
        // Create new gap.
        prev = DataMessage(seq: 1, hi: _minSeq - 1);
        ranges.add(prev);
      }
    } else {
      // No gap in the beginning.
      prev = DataMessage(seq: 0, hi: 0);
    }

    // Find gaps in the list of received messages. The list contains messages-proper as well
    // as placeholders for deleted ranges.
    // The messages are iterated by seq ID in ascending order.
    _messages.forEach((data, i) {
      // Do not create a gap between the last sent message and the first unsent.
      if (data.seq! >= _configService.appSettings.localSeqId) {
        return;
      }

      // New message is reducing the existing gap
      if (data.seq == ((prev.hi != null && prev.hi! > 0) ? prev.hi : prev.seq)! + 1) {
        // No new gap. Replace previous with current.
        prev = data;
        return;
      }

      // Found a new gap.
      if (prev.hi != null && prev.hi != 0) {
        // Previous is also a gap, alter it.
        prev.hi = data.hi! > 0 ? data.hi : data.seq;
        return;
      }

      // Previous is not a gap. Create a new gap.
      prev = DataMessage(
        seq: (data.hi! > 0 ? data.hi! : data.seq)! + 1,
        hi: data.hi! > 0 ? data.hi : data.seq,
      );
      ranges.add(prev);
    }, null, null);

    // Check for missing messages at the end.
    // All messages could be missing or it could be a new topic with no messages.
    var last = _messages.length > 0 ? _messages.getLast() : null;
    var maxSeq = max(seq!, _maxSeq);
    if ((maxSeq > 0 && last == null) || (last != null && (((last.hi != null && last.hi! > 0) ? last.hi : last.seq)! < maxSeq))) {
      if (last != null && (last.hi != null && last.hi! > 0)) {
        // Extend existing gap
        last.hi = maxSeq;
      } else {
        // Create new gap.
        ranges.add(DataMessage(seq: last != null ? last.seq! + 1 : 1, hi: maxSeq));
      }
    }

    // Insert new gaps into cache.
    ranges.map((gap) {
      _messages.put([gap]);
    });
  }