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