VersionConstraint difference()

in lib/src/version_union.dart [126:198]
51 lines of code
19 McCabe index (conditional complexity)

  VersionConstraint difference(VersionConstraint other) {
    var ourRanges = ranges.iterator;
    var theirRanges = _rangesFor(other).iterator;

    var newRanges = <VersionRange>[];
    ourRanges.moveNext();
    theirRanges.moveNext();
    var current = ourRanges.current;

    bool theirNextRange() {
      if (theirRanges.moveNext()) return true;

      // If there are no more of their ranges, none of the rest of our ranges
      // need to be subtracted so we can add them as-is.
      newRanges.add(current);
      while (ourRanges.moveNext()) {
        newRanges.add(ourRanges.current);
      }
      return false;
    }

    bool ourNextRange({bool includeCurrent = true}) {
      if (includeCurrent) newRanges.add(current);
      if (!ourRanges.moveNext()) return false;
      current = ourRanges.current;
      return true;
    }

    for (;;) {
      // If the current ranges are disjoint, move the lowest one forward.
      if (strictlyLower(theirRanges.current, current)) {
        if (!theirNextRange()) break;
        continue;
      }

      if (strictlyHigher(theirRanges.current, current)) {
        if (!ourNextRange()) break;
        continue;
      }

      // If we're here, we know [theirRanges.current] overlaps [current].
      var difference = current.difference(theirRanges.current);
      if (difference is VersionUnion) {
        // If their range split [current] in half, we only need to continue
        // checking future ranges against the latter half.
        assert(difference.ranges.length == 2);
        newRanges.add(difference.ranges.first);
        current = difference.ranges.last;

        // Since their range split [current], it definitely doesn't allow higher
        // versions, so we should move their ranges forward.
        if (!theirNextRange()) break;
      } else if (difference.isEmpty) {
        if (!ourNextRange(includeCurrent: false)) break;
      } else {
        current = difference as VersionRange;

        // Move the constraint with the lower max value forward. This ensures
        // that we keep both lists in sync as much as possible, and that large
        // ranges have a chance to subtract or be subtracted by multiple small
        // ranges that they contain.
        if (allowsHigher(current, theirRanges.current)) {
          if (!theirNextRange()) break;
        } else {
          if (!ourNextRange()) break;
        }
      }
    }

    if (newRanges.isEmpty) return VersionConstraint.empty;
    if (newRanges.length == 1) return newRanges.single;
    return VersionUnion.fromRanges(newRanges);
  }