Implement better HPACK Huffman code decoder.

net/spdy/hpack/hpack_input_stream.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 NET_SPDY_HPACK_INPUT_STREAM_H_
 #define NET_SPDY_HPACK_INPUT_STREAM_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <string>
+#include <utility>
 
 #include "base/macros.h"
 #include "base/strings/string_piece.h"
 #include "net/base/net_export.h"
 #include "net/spdy/hpack/hpack_constants.h"
 #include "net/spdy/hpack/hpack_huffman_table.h"
 
 // All section references below are to
 // http://tools.ietf.org/html/draft-ietf-httpbis-header-compression-08
 
 namespace net {
 
+typedef std::pair<size_t, uint32_t> InitialPeekResult;
+
 // An HpackInputStream handles all the low-level details of decoding
 // header fields.
 class NET_EXPORT_PRIVATE HpackInputStream {
  public:
   // |max_string_literal_size| is the largest that any one string
   // literal (header name or header value) can be.
   HpackInputStream(uint32_t max_string_literal_size, base::StringPiece buffer);
   ~HpackInputStream();
 
   // Returns whether or not there is more data to process.
   bool HasMoreData() const;
 
   // If the next bits of input match |prefix|, consumes them and returns true.
   // Otherwise, consumes nothing and returns false.
   bool MatchPrefixAndConsume(HpackPrefix prefix);
 
   // The Decode* functions return true and fill in their arguments if
   // decoding was successful, or false if an error was encountered.
 
   bool DecodeNextUint32(uint32_t* I);
   bool DecodeNextIdentityString(base::StringPiece* str);
-  bool DecodeNextHuffmanString(const HpackHuffmanTable& table,
-                               std::string* str);
+  bool DecodeNextHuffmanString(std::string* str);
 
   // Stores input bits into the most-significant, unfilled bits of |out|.
   // |peeked_count| is the number of filled bits in |out| which have been
   // previously peeked. PeekBits() will fill some number of remaining bits,
   // returning the new total number via |peeked_count|. Returns true if one
-  // or more additional bits could be peeked, and false otherwise.
+  // or more additional bits were added to |out|, and false otherwise.
   bool PeekBits(size_t* peeked_count, uint32_t* out) const;
 
+  // Similar to PeekBits, but intended to be used when starting to decode a
+  // Huffman encoded string. Returns a pair containing the peeked_count and
+  // out values as described for PeekBits, with the bits from the first N bytes
+  // of buffer_, where N == min(4, buffer_.size()), starting with the high
+  // order bits.
+  // Should only be called when first peeking at bits from the input stream as
+  // it does not take peeked_count as an input, so doesn't know how many bits
+  // have already been returned by previous calls to InitializePeekBits and
+  // PeekBits.
+  InitialPeekResult InitializePeekBits();
+
   // Consumes |count| bits of input. Generally paired with PeekBits().
   void ConsumeBits(size_t count);
 
   // If not currently on a byte boundary, consumes and discards
   // remaining bits in the current byte.
   void ConsumeByteRemainder();
 
   // Accessors for testing.
 
   void SetBitOffsetForTest(size_t bit_offset) { bit_offset_ = bit_offset; }
 
  private:
   const uint32_t max_string_literal_size_;
   base::StringPiece buffer_;
   size_t bit_offset_;
 
   bool PeekNextOctet(uint8_t* next_octet);
 
   bool DecodeNextOctet(uint8_t* next_octet);
 
   DISALLOW_COPY_AND_ASSIGN(HpackInputStream);
 };
 
 }  // namespace net
 
 #endif  // NET_SPDY_HPACK_INPUT_STREAM_H_