Fix Huffman tree construction.

net/tools/transport_security_state_generator/huffman/huffman_builder.h

 // Copyright (c) 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef NET_TOOLS_TRANSPORT_SECURITY_STATE_GENERATOR_HUFFMAN_BUILDER_H_
 #define NET_TOOLS_TRANSPORT_SECURITY_STATE_GENERATOR_HUFFMAN_BUILDER_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 21 matching lines @@
 
 // This class tracks the number of times each character is used and calculates
 // a space efficient way to represent all tracked characters by constructing a
 // Huffman tree based on the number of times each character is seen.
 class HuffmanBuilder {
  public:
   HuffmanBuilder();
   ~HuffmanBuilder();
 
   // Will increase the count for |character| by one, indicating it has been
-  // used.
+  // used. |character| must be in the range 0-127.
   void RecordUsage(uint8_t character);
 
   // Returns a HuffmanRepresentationTable based on the usage data collected
   // through RecordUsage().
   HuffmanRepresentationTable ToTable();
 
   // Outputs the Huffman representation as a vector of uint8_t's in a format
   // Chromium can use to reconstruct the tree.
   //
   // The nodes of the tree are pairs of uint8s. The last node in the array is
   // the root of the tree. Each pair is two uint8_t values, the first is "left"
   // and the second is "right". If a uint8_t value has the MSB set then it
   // represents a literal leaf value. Otherwise it's a pointer to the n'th
   // element of the array.
   std::vector<uint8_t> ToVector();
 
  private:
   // Determines the Huffman representation of the characters under |node| and
   // inserts them into |*table|. |bits| and |number_of_bits| are used as a
   // prefix.
   void TreeToTable(HuffmanNode* node,
                    uint32_t bits,
                    uint32_t number_of_bits,
                    HuffmanRepresentationTable* table);
 
   // Converts the tree under |*node| into a byte representation in |*vector|.
   // See ToVector() for more information on the format.
   uint32_t WriteToVector(HuffmanNode* node, std::vector<uint8_t>* vector);
 
-  // Constructs a Huffman tree based on |counts_|.
+  // Constructs a Huffman tree based on |counts_|. Appends additional nodes to
+  // the tree until it contains at least 2 leafs.
   std::unique_ptr<HuffmanNode> BuildTree();
 
   // Holds usage information for the tracked characters. Maps the character to
   // the number of times its usage has been recorded through RecordUsage().
   std::map<uint8_t, uint32_t> counts_;
 };
 
 }  // namespace transport_security_state
 
 }  // namespace net
 
 #endif  // NET_TOOLS_TRANSPORT_SECURITY_STATE_GENERATOR_HUFFMAN_BUILDER_H_