// Builds a HuffmanTree from a byte histogram
sclass HuffmanTreeMaker {
  delegate Node, Branch, Leaf to HuffmanTree.
  
  // output
  gettable new HuffmanTree tree;
  
  // internal
  new TreeSet<Head> pool; // combinable trees sorted by incidence (symbol frequency)
  int indexCounter; // counter to keep order deterministic
  
  class Head is Comparable<Head> {
    int index = indexCounter++;
    settable Node node;
    settable double freq; // using floating point to keep things futureproof

    // sort by frequency first, then by index (order of creation)
    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);
    }
    
    toString { ret freq + "x " + node; }
  }
  
  // histogram[0] = incidence for byte 00
  // histogram[1] = incidence for byte 01
  // etc. up to 255 (or less)
  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(tree.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();
      //print("Merging frequencies: " + commaCombine(a.freq, b.freq));
      var newHead = new Head()
        .freq(a.freq+b.freq)
        .node(tree.new Branch().zero(b.node).one(a.node));
      //print("Merging: " + commaCombine(a, b) + " to " + newHead);
      pool.add(newHead);
    }
  }
  
  HuffmanTree get() {
    run();
    if (nempty(pool))
      tree.root(popFirst(pool).node());
    ret tree;
  }
}