sclass HuffmanTreeMaker {
  delegate Node, Branch, Leaf to HuffmanTree.
  
  gettable new HuffmanTree tree;
  new TreeSet<Head> pool;
  int indexCounter;
  
  class Head is Comparable<Head> {
    int index = indexCounter++;
    settable Node node;
    settable double freq;
    
    public int compareTo(Head h) {
      if (this == h) ret 0;
      var c = cmp(freq, h.freq);
      if (c != 0) ret c;
      ret cmp(index, h.index);
    }
  }
  
  selfType importByteHistogram(int[] histogram) {
    int n = l(histogram);
    for i to n: {
      double freq = histogram[i];
      if (freq != 0)
        pool.add(new Head().freq(freq).node(new Leaf(i)));
    }
    this;
  }
  
  run {
    while ping (l(pool) > 1) {
      var it = iterator(pool);
      var a = it.next();
      it.remove();
      var b = it.next();
      it.remove();
      pool.add(new Head()
        .freq(a.freq+b.freq)
        .node(new Branch(a.node, b.node)));
    }
  }
  
  HuffmanTree get() {
    run();
    if (nempty(pool))
      tree.root(removeFirst(pool).node());
    ret tree;
  }
}