spdy/core/hpack/hpack_huffman_table.h

// Copyright 2014 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 QUICHE_SPDY_CORE_HPACK_HPACK_HUFFMAN_TABLE_H_
#define QUICHE_SPDY_CORE_HPACK_HPACK_HUFFMAN_TABLE_H_

#include <cstddef>
#include <cstdint>
#include <vector>

#include "absl/strings/string_view.h"
#include "common/platform/api/quiche_export.h"
#include "spdy/core/hpack/hpack_constants.h"

namespace spdy {

namespace test {
class HpackHuffmanTablePeer;
}  // namespace test

class HpackOutputStream;

// HpackHuffmanTable encodes string literals using a constructed canonical
// Huffman code. Once initialized, an instance is read only and may be accessed
// only through its const interface.
class QUICHE_EXPORT_PRIVATE HpackHuffmanTable {
 public:
  friend class test::HpackHuffmanTablePeer;

  typedef HpackHuffmanSymbol Symbol;

  HpackHuffmanTable();
  ~HpackHuffmanTable();

  // Prepares HpackHuffmanTable to encode the canonical Huffman code as
  // determined by the given symbols. Must be called exactly once.
  // Returns false if the input symbols define an invalid coding, and true
  // otherwise. Symbols must be presented in ascending ID order with no gaps,
  // and |symbol_count| must fit in a uint16_t.
  bool Initialize(const Symbol* input_symbols, size_t symbol_count);

  // Returns whether Initialize() has been successfully called.
  bool IsInitialized() const;

  // Encodes the input string to the output stream using the table's Huffman
  // context.
  void EncodeString(absl::string_view in, HpackOutputStream* out) const;

  // Returns the encoded size of the input string.
  size_t EncodedSize(absl::string_view in) const;

  // Returns the estimate of dynamically allocated memory in bytes.
  size_t EstimateMemoryUsage() const;

 private:
  // Expects symbols ordered on ID ascending.
  void BuildEncodeTable(const std::vector<Symbol>& symbols);

  // Symbol code and code length, in ascending symbol ID order.
  // Codes are stored in the most-significant bits of the word.
  std::vector<uint32_t> code_by_id_;
  std::vector<uint8_t> length_by_id_;

  // The first 8 bits of the longest code. Applied when generating padding bits.
  uint8_t pad_bits_;

  // If initialization fails, preserve the symbol ID which failed validation
  // for examination in tests.
  uint16_t failed_symbol_id_;
};

}  // namespace spdy

#endif  // QUICHE_SPDY_CORE_HPACK_HPACK_HUFFMAN_TABLE_H_