Improve GIF decoding performance

Source/core/platform/image-decoders/gif/GIFImageReader.h

 /* -*- Mode: C; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* ***** BEGIN LICENSE BLOCK *****
  * Version: MPL 1.1/GPL 2.0/LGPL 2.1
  *
  * The contents of this file are subject to the Mozilla Public License Version
  * 1.1 (the "License"); you may not use this file except in compliance with
  * the License. You may obtain a copy of the License at
  * http://www.mozilla.org/MPL/
  *
  * Software distributed under the License is distributed on an "AS IS" basis,
@@ skipping 28 matching lines @@
 #define GIFImageReader_h
 
 // Define ourselves as the clientPtr.  Mozilla just hacked their C++ callback class into this old C decoder,
 // so we will too.
 #include "platform/SharedBuffer.h"
 #include "core/platform/image-decoders/gif/GIFImageDecoder.h"
 #include "wtf/OwnPtr.h"
 #include "wtf/PassOwnPtr.h"
 #include "wtf/Vector.h"
 
-#define MAX_LZW_BITS          12
-#define MAX_BYTES           4097 /* 2^MAX_LZW_BITS+1 */
-#define MAX_COLORS           256
-#define GIF_COLORS             3
+#define MAX_DICTIONARY_ENTRY_BITS 12
+#define MAX_DICTIONARY_ENTRIES    4096 // 2^MAX_DICTIONARY_ENTRY_BITS
+#define MAX_COLORS                256
+#define BYTES_PER_COLORMAP_ENTRY  3
 
 const int cLoopCountNotSeen = -2;
 
 // List of possible parsing states.
 enum GIFState {
     GIFType,
     GIFGlobalHeader,
     GIFGlobalColormap,
     GIFImageStart,
     GIFImageHeader,
@@ skipping 25 matching lines @@
         , codemask(0)
         , clearCode(0)
         , avail(0)
         , oldcode(0)
         , firstchar(0)
         , bits(0)
         , datum(0)
         , ipass(0)
         , irow(0)
         , rowsRemaining(0)
-        , stackp(0)
         , rowIter(0)
         , m_client(client)
         , m_frameContext(frameContext)
     { }
 
     bool prepareToDecode();
-    bool outputRow();
+    bool outputRow(GIFRow::const_iterator rowBegin);
     bool doLZW(const unsigned char* block, size_t bytesInBlock);
     bool hasRemainingRows() { return rowsRemaining; }
 
 private:
     // LZW decoding states and output states.
     int codesize;
     int codemask;
     int clearCode; // Codeword used to trigger dictionary reset.
     int avail; // Index of next available slot in dictionary.
     int oldcode;
     unsigned char firstchar;
     int bits; // Number of unread bits in "datum".
     int datum; // 32-bit input buffer.
     int ipass; // Interlace pass; Ranges 1-4 if interlaced.
     size_t irow; // Current output row, starting at zero.
     size_t rowsRemaining; // Rows remaining to be output.
 
-    unsigned short prefix[MAX_BYTES];
-    unsigned char suffix[MAX_BYTES];
-    unsigned char stack[MAX_BYTES];
-    Vector<unsigned char> rowBuffer; // Single scanline temporary buffer.
-
-    size_t stackp; // Current stack pointer.
-    Vector<unsigned char>::iterator rowIter;
+    unsigned short prefix[MAX_DICTIONARY_ENTRIES];
+    unsigned char suffix[MAX_DICTIONARY_ENTRIES];
+    unsigned short suffixLength[MAX_DICTIONARY_ENTRIES];
+    GIFRow rowBuffer; // Single scanline temporary buffer.
+    GIFRow::iterator rowIter;
 
     // Initialized during construction and read-only.
     WebCore::GIFImageDecoder* m_client;
     const GIFFrameContext* m_frameContext;
 };
 
 // Data structure for one LZW block.
 struct GIFLZWBlock {
     WTF_MAKE_FAST_ALLOCATED;
 public:
@@ skipping 218 matching lines @@
     GIFColorMap m_globalColorMap;
     int m_loopCount; // Netscape specific extension block to control the number of animation loops a GIF renders.
 
     Vector<OwnPtr<GIFFrameContext> > m_frames;
 
     RefPtr<WebCore::SharedBuffer> m_data;
     bool m_parseCompleted;
 };
 
 #endif