gifdecoder.cs

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						bits += 8;
						bi++;
						count--;
						continue;
					}

					//  Get the next code.

					code = datum & code_mask;
					datum >>= code_size;
					bits -= code_size;

					//  Interpret the code

					if ((code > available) || (code == end_of_information))
						break;
					if (code == clear) 
					{
						//  Reset decoder.
						code_size = data_size + 1;
						code_mask = (1 << code_size) - 1;
						available = clear + 2;
						old_code = NullCode;
						continue;
					}
					if (old_code == NullCode) 
					{
						pixelStack[top++] = suffix[code];
						old_code = code;
						first = code;
						continue;
					}
					in_code = code;
					if (code == available) 
					{
						pixelStack[top++] = (byte) first;
						code = old_code;
					}
					while (code > clear) 
					{
						pixelStack[top++] = suffix[code];
						code = prefix[code];
					}
					first = ((int) suffix[code]) & 0xff;

					//  Add a new string to the string table,

					if (available >= MaxStackSize)
						break;
					pixelStack[top++] = (byte) first;
					prefix[available] = (short) old_code;
					suffix[available] = (byte) first;
					available++;
					if (((available & code_mask) == 0)
						&& (available < MaxStackSize)) 
					{
						code_size++;
						code_mask += available;
					}
					old_code = in_code;
				}

				//  Pop a pixel off the pixel stack.

				top--;
				pixels[pi++] = pixelStack[top];
				i++;
			}

			for (i = pi; i < npix; i++) 
			{
				pixels[i] = 0; // clear missing pixels
			}

		}

		/**
		 * Returns true if an error was encountered during reading/decoding
		 */
		protected bool Error() 
		{
			return status != STATUS_OK;
		}

		/**
		 * Initializes or re-initializes reader
		 */
		protected void Init() 
		{
			status = STATUS_OK;
			frameCount = 0;
			frames = new ArrayList();
			gct = null;
			lct = null;
		}

		/**
		 * Reads a single byte from the input stream.
		 */
		protected int Read() 
		{
			int curByte = 0;
			try 
			{
				curByte = inStream.ReadByte();
			} 
			catch (IOException e) 
			{
				status = STATUS_FORMAT_ERROR;
			}
			return curByte;
		}

		/**
		 * Reads next variable length block from input.
		 *
		 * @return number of bytes stored in "buffer"
		 */
		protected int ReadBlock() 
		{
			blockSize = Read();
			int n = 0;
			if (blockSize > 0) 
			{
				try 
				{
					int count = 0;
					while (n < blockSize) 
					{
						count = inStream.Read(block, n, blockSize - n);
						if (count == -1) 
							break;
						n += count;
					}
				} 
				catch (IOException e) 
				{
				}

				if (n < blockSize) 
				{
					status = STATUS_FORMAT_ERROR;
				}
			}
			return n;
		}

		/**
		 * Reads color table as 256 RGB integer values
		 *
		 * @param ncolors int number of colors to read
		 * @return int array containing 256 colors (packed ARGB with full alpha)
		 */
		protected int[] ReadColorTable(int ncolors) 
		{
			int nbytes = 3 * ncolors;
			int[] tab = null;
			byte[] c = new byte[nbytes];
			int n = 0;
			try 
			{
				n = inStream.Read(c, 0, c.Length );
			} 
			catch (IOException e) 
			{
			}
			if (n < nbytes) 
			{
				status = STATUS_FORMAT_ERROR;
			} 
			else 
			{
				tab = new int[256]; // max size to avoid bounds checks
				int i = 0;
				int j = 0;
				while (i < ncolors) 
				{
					int r = ((int) c[j++]) & 0xff;
					int g = ((int) c[j++]) & 0xff;
					int b = ((int) c[j++]) & 0xff;
					tab[i++] = ( int ) ( 0xff000000 | (r << 16) | (g << 8) | b );
				}
			}
			return tab;
		}

		/**
		 * Main file parser.  Reads GIF content blocks.
		 */
		protected void ReadContents() 
		{
			// read GIF file content blocks
			bool done = false;
			while (!(done || Error())) 
			{
				int code = Read();
				switch (code) 
				{

					case 0x2C : // image separator
						ReadImage();
						break;

					case 0x21 : // extension
						code = Read();
					switch (code) 
					{
						case 0xf9 : // graphics control extension
							ReadGraphicControlExt();
							break;

						case 0xff : // application extension
							ReadBlock();
							String app = "";
							for (int i = 0; i < 11; i++) 
							{
								app += (char) block[i];
							}
							if (app.Equals("NETSCAPE2.0")) 
							{
								ReadNetscapeExt();
							}
							else
								Skip(); // don't care
							break;

						default : // uninteresting extension
							Skip();
							break;
					}
						break;

					case 0x3b : // terminator
						done = true;
						break;

					case 0x00 : // bad byte, but keep going and see what happens
						break;

					default :
						status = STATUS_FORMAT_ERROR;
						break;
				}
			}
		}

		/**
		 * Reads Graphics Control Extension values
		 */
		protected void ReadGraphicControlExt() 
		{
			Read(); // block size
			int packed = Read(); // packed fields
			dispose = (packed & 0x1c) >> 2; // disposal method
			if (dispose == 0) 
			{
				dispose = 1; // elect to keep old image if discretionary
			}
			transparency = (packed & 1) != 0;
			delay = ReadShort() * 10; // delay in milliseconds
			transIndex = Read(); // transparent color index
			Read(); // block terminator
		}

		/**
		 * Reads GIF file header information.
		 */
		protected void ReadHeader() 
		{
			String id = "";
			for (int i = 0; i < 6; i++) 
			{
				id += (char) Read();
			}
			if (!id.StartsWith("GIF")) 
			{
				status = STATUS_FORMAT_ERROR;
				return;
			}

			ReadLSD();
			if (gctFlag && !Error()) 
			{
				gct = ReadColorTable(gctSize);
				bgColor = gct[bgIndex];
			}
		}

		/**
		 * Reads next frame image
		 */
		protected void ReadImage() 
		{
			ix = ReadShort(); // (sub)image position & size
			iy = ReadShort();
			iw = ReadShort();
			ih = ReadShort();

			int packed = Read();
			lctFlag = (packed & 0x80) != 0; // 1 - local color table flag
			interlace = (packed & 0x40) != 0; // 2 - interlace flag
			// 3 - sort flag
			// 4-5 - reserved
			lctSize = 2 << (packed & 7); // 6-8 - local color table size

			if (lctFlag) 
			{
				lct = ReadColorTable(lctSize); // read table
				act = lct; // make local table active
			} 
			else 
			{
				act = gct; // make global table active
				if (bgIndex == transIndex)
					bgColor = 0;
			}
			int save = 0;
			if (transparency) 
			{
				save = act[transIndex];
				act[transIndex] = 0; // set transparent color if specified
			}

			if (act == null) 
			{
				status = STATUS_FORMAT_ERROR; // no color table defined
			}

			if (Error()) return;

			DecodeImageData(); // decode pixel data
			Skip();

			if (Error()) return;

			frameCount++;

			// create new image to receive frame data
			//		image =
			//			new BufferedImage(width, height, BufferedImage.TYPE_INT_ARGB_PRE);

			bitmap = new Bitmap( width, height );
			image = bitmap;
			SetPixels(); // transfer pixel data to image

			frames.Add(new GifFrame(bitmap, delay)); // add image to frame list

			if (transparency) 
			{
				act[transIndex] = save;
			}
			ResetFrame();

		}

		/**
		 * Reads Logical Screen Descriptor
		 */
		protected void ReadLSD() 
		{

			// logical screen size
			width = ReadShort();
			height = ReadShort();

			// packed fields
			int packed = Read();
			gctFlag = (packed & 0x80) != 0; // 1   : global color table flag
			// 2-4 : color resolution
			// 5   : gct sort flag
			gctSize = 2 << (packed & 7); // 6-8 : gct size

			bgIndex = Read(); // background color index
			pixelAspect = Read(); // pixel aspect ratio
		}

		/**
		 * Reads Netscape extenstion to obtain iteration count
		 */
		protected void ReadNetscapeExt() 
		{
			do 
			{
				ReadBlock();
				if (block[0] == 1) 
				{
					// loop count sub-block
					int b1 = ((int) block[1]) & 0xff;
					int b2 = ((int) block[2]) & 0xff;
					loopCount = (b2 << 8) | b1;
				}
			} while ((blockSize > 0) && !Error());
		}

		/**
		 * Reads next 16-bit value, LSB first
		 */
		protected int ReadShort() 
		{
			// read 16-bit value, LSB first
			return Read() | (Read() << 8);
		}

		/**
		 * Resets frame state for reading next image.
		 */
		protected void ResetFrame() 
		{
			lastDispose = dispose;
			lastRect = new Rectangle(ix, iy, iw, ih);
			lastImage = image;
			lastBgColor = bgColor;
			//		int dispose = 0;
			bool transparency = false;
			int delay = 0;
			lct = null;
		}

		/**
		 * Skips variable length blocks up to and including
		 * next zero length block.
		 */
		protected void Skip() 
		{
			do 
			{
				ReadBlock();
			} while ((blockSize > 0) && !Error());
		}
	}
}