sclass FreeList {
  MultiSetMap<Int, IntRange> byLength = multiSetMap_innerCustomCompactTreeSet_outerTreeMap(intRangeComparatorByStart());
  TreeSet<IntRange> inOrder = new(intRangeComparatorByStart());
  
  void add(IntRange r) {
    // only special case here is merging with previous and/or following entry
    IntRange before = inOrder.floor(r), after = inOrder.ceiling(r);
    if (before != null && before.end == r.start) {
      r = IntRange(before.start, r.end);
      remove(before);
    }
    if (after != null && after.start == r.end) {
      r = IntRange(r.start, after.end);
      remove(after);
    }
      
    inOrder.add(r);
    byLength.add(r.length(), r);
  }
  
  void remove(IntRange r) {
    IntRange r2 = inOrder.floor(r);
    if (r2 == null) fail("User error");
    if (eq(r, r2)) {
      inOrder.remove(r2);
      byLength.remove(r2.length(), r2);
      ret;
    }
    
    remove(r2);
    IntRange left = new(r2.start, r.start);
    IntRange right = new(r.end, r2.end);
    if (nempty(left)) add(left);
    if (nempty(right)) add(right);
  }
  
  IntRange findFreeSpace(int minSize) {
    Map.Entry<Int, Set<IntRange>> e = byLength.data.ceilingEntry(minSize);
    ret e == null ? null : first(e.getValue());
  }
}