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      1 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
      2 // Use of this source code is governed by a BSD-style license that can be
      3 // found in the LICENSE file.
      4 
      5 // SdchFilter applies open_vcdiff content decoding to a datastream.
      6 // This decoding uses a pre-cached dictionary of text fragments to decode
      7 // (expand) the stream back to its original contents.
      8 //
      9 // This SdchFilter internally uses open_vcdiff/vcdec library to do decoding.
     10 //
     11 // SdchFilter is also a subclass of Filter. See the latter's header file
     12 // filter.h for sample usage.
     13 
     14 #ifndef NET_BASE_SDCH_FILTER_H_
     15 #define NET_BASE_SDCH_FILTER_H_
     16 
     17 #include <string>
     18 
     19 #include "base/scoped_ptr.h"
     20 #include "net/base/filter.h"
     21 #include "net/base/sdch_manager.h"
     22 
     23 class SafeOutputStringInterface;
     24 
     25 namespace open_vcdiff {
     26   class VCDiffStreamingDecoder;
     27 }
     28 
     29 class SdchFilter : public Filter {
     30  public:
     31   explicit SdchFilter(const FilterContext& filter_context);
     32 
     33   virtual ~SdchFilter();
     34 
     35   // Initializes filter decoding mode and internal control blocks.
     36   bool InitDecoding(Filter::FilterType filter_type);
     37 
     38   // Decode the pre-filter data and writes the output into |dest_buffer|
     39   // The function returns FilterStatus. See filter.h for its description.
     40   //
     41   // Upon entry, *dest_len is the total size (in number of chars) of the
     42   // destination buffer. Upon exit, *dest_len is the actual number of chars
     43   // written into the destination buffer.
     44   virtual FilterStatus ReadFilteredData(char* dest_buffer, int* dest_len);
     45 
     46  private:
     47   // Internal status.  Once we enter an error state, we stop processing data.
     48   enum DecodingStatus {
     49     DECODING_UNINITIALIZED,
     50     WAITING_FOR_DICTIONARY_SELECTION,
     51     DECODING_IN_PROGRESS,
     52     DECODING_ERROR,
     53     META_REFRESH_RECOVERY,  // Decoding error being handled by a meta-refresh.
     54     PASS_THROUGH,  // Non-sdch content being passed without alteration.
     55   };
     56 
     57   // Identify the suggested dictionary, and initialize underlying decompressor.
     58   Filter::FilterStatus InitializeDictionary();
     59 
     60   // Move data that was internally buffered (after decompression) to the
     61   // specified dest_buffer.
     62   int OutputBufferExcess(char* const dest_buffer, size_t available_space);
     63 
     64   // Tracks the status of decoding.
     65   // This variable is initialized by InitDecoding and updated only by
     66   // ReadFilteredData.
     67   DecodingStatus decoding_status_;
     68 
     69   // The underlying decoder that processes data.
     70   // This data structure is initialized by InitDecoding and updated in
     71   // ReadFilteredData.
     72   scoped_ptr<open_vcdiff::VCDiffStreamingDecoder> vcdiff_streaming_decoder_;
     73 
     74   // In case we need to assemble the hash piecemeal, we have a place to store
     75   // a part of the hash until we "get all 8 bytes plus a null."
     76   std::string dictionary_hash_;
     77 
     78   // After assembling an entire dictionary hash (the first 9 bytes of the
     79   // sdch payload, we check to see if it is plausible, meaning it has a null
     80   // termination, and has 8 characters that are possible in a net-safe base64
     81   // encoding.  If the hash is not plausible, then the payload is probably not
     82   // an SDCH encoded bundle, and various error recovery strategies can be
     83   // attempted.
     84   bool dictionary_hash_is_plausible_;
     85 
     86   // We hold an in-memory copy of the dictionary during the entire decoding, as
     87   // it is used directly by the VC-DIFF decoding system.
     88   // That char* data is part of the dictionary_ we hold a reference to.
     89   scoped_refptr<SdchManager::Dictionary> dictionary_;
     90 
     91   // The decoder may demand a larger output buffer than the target of
     92   // ReadFilteredData so we buffer the excess output between calls.
     93   std::string dest_buffer_excess_;
     94   // To avoid moving strings around too much, we save the index into
     95   // dest_buffer_excess_ that has the next byte to output.
     96   size_t dest_buffer_excess_index_;
     97 
     98   // To get stats on activities, we keep track of source and target bytes.
     99   // Visit about:histograms/Sdch to see histogram data.
    100   size_t source_bytes_;
    101   size_t output_bytes_;
    102 
    103   // Error recovery in content type may add an sdch filter type, in which case
    104   // we should gracefully perform pass through if the format is incorrect, or
    105   // an applicable dictionary can't be found.
    106   bool possible_pass_through_;
    107 
    108   // The URL that is currently being filtered.
    109   // This is used to restrict use of a dictionary to a specific URL or path.
    110   GURL url_;
    111 
    112   // To facilitate error recovery, allow filter to know if content is text/html
    113   // by checking within this mime type (we may do a meta-refresh via html).
    114   std::string mime_type_;
    115 
    116   DISALLOW_COPY_AND_ASSIGN(SdchFilter);
    117 };
    118 
    119 #endif  // NET_BASE_SDCH_FILTER_H_
    120