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sclass FreeList {
  MultiSetMap<Int, IntRange> byLength = multiSetMap_innerCustomCompactTreeSet_outerTreeMap(intRangeComparator());
  CompactTreeSet<IntRange> inOrder = new(intRangeComparator());
  int totalFree;
  
  void add(IntRange r) {
    if (empty(r)) ret;
    // 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);
    totalFree += r.length();
  }
  
  void remove(IntRange r) {
    if (empty(r)) ret;
    IntRange r2 = inOrder.floor(r);
    if (r2 == null) fail("User error");
    if (eq(r, r2)) {
      inOrder.remove(r2);
      byLength.remove(r2.length(), r2);
      totalFree -= r2.length();
      ret;
    }
    
    // remove full range, add back left and right remainders
    remove(r2);
    add(IntRange(r2.start, r.start));
    add(IntRange(r.end, r2.end));
  }
  
  IntRange findFreeSpace(int minSize) {
    Map.Entry<Int, Set<IntRange>> e = ((NavigableMap) byLength.data).ceilingEntry(minSize);
    ret e == null ? null : first(e.getValue());
  }
  
  int totalFree() { ret totalFree; }
  
  void clear {
    byLength.clear();
    inOrder.clear();
    totalFree = 0;
  }
  
  IntRange highestFreeRange() { ret last(inOrder); }
  
  L<IntRange> asList() { ret cloneList(inOrder); }
}