lzwencoder.cs

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using System;
using System.IO;

namespace LanMsg.Gif.Components
{
	public class LZWEncoder 
	{

		private static readonly int EOF = -1;

		private int imgW, imgH;
		private byte[] pixAry;
		private int initCodeSize;
		private int remaining;
		private int curPixel;

		// GIFCOMPR.C       - GIF Image compression routines
		//
		// Lempel-Ziv compression based on 'compress'.  GIF modifications by
		// David Rowley ([e-mail])

		// General DEFINEs

		static readonly int BITS = 12;

		static readonly int HSIZE = 5003; // 80% occupancy

		// GIF Image compression - modified 'compress'
		//
		// Based on: compress.c - File compression ala IEEE Computer, June 1984.
		//
		// By Authors:  Spencer W. Thomas      (decvax!harpo!utah-cs!utah-gr!thomas)
		//              Jim McKie              (decvax!mcvax!jim)
		//              Steve Davies           (decvax!vax135!petsd!peora!srd)
		//              Ken Turkowski          (decvax!decwrl!turtlevax!ken)
		//              James A. Woods         (decvax!ihnp4!ames!jaw)
		//              Joe Orost              (decvax!vax135!petsd!joe)

		int n_bits; // number of bits/code
		int maxbits = BITS; // user settable max # bits/code
		int maxcode; // maximum code, given n_bits
		int maxmaxcode = 1 << BITS; // should NEVER generate this code

		int[] htab = new int[HSIZE];
		int[] codetab = new int[HSIZE];

		int hsize = HSIZE; // for dynamic table sizing

		int free_ent = 0; // first unused entry

		// block compression parameters -- after all codes are used up,
		// and compression rate changes, start over.
		bool clear_flg = false;

		// Algorithm:  use open addressing double hashing (no chaining) on the
		// prefix code / next character combination.  We do a variant of Knuth's
		// algorithm D (vol. 3, sec. 6.4) along with G. Knott's relatively-prime
		// secondary probe.  Here, the modular division first probe is gives way
		// to a faster exclusive-or manipulation.  Also do block compression with
		// an adaptive reset, whereby the code table is cleared when the compression
		// ratio decreases, but after the table fills.  The variable-length output
		// codes are re-sized at this point, and a special CLEAR code is generated
		// for the decompressor.  Late addition:  construct the table according to
		// file size for noticeable speed improvement on small files.  Please direct
		// questions about this implementation to ames!jaw.

		int g_init_bits;

		int ClearCode;
		int EOFCode;

		// output
		//
		// Output the given code.
		// Inputs:
		//      code:   A n_bits-bit integer.  If == -1, then EOF.  This assumes
		//              that n_bits =< wordsize - 1.
		// Outputs:
		//      Outputs code to the file.
		// Assumptions:
		//      Chars are 8 bits long.
		// Algorithm:
		//      Maintain a BITS character long buffer (so that 8 codes will
		// fit in it exactly).  Use the VAX insv instruction to insert each
		// code in turn.  When the buffer fills up empty it and start over.

		int cur_accum = 0;
		int cur_bits = 0;

		int [] masks =
		{
			0x0000,
			0x0001,
			0x0003,
			0x0007,
			0x000F,
			0x001F,
			0x003F,
			0x007F,
			0x00FF,
			0x01FF,
			0x03FF,
			0x07FF,
			0x0FFF,
			0x1FFF,
			0x3FFF,
			0x7FFF,
			0xFFFF };

		// Number of characters so far in this 'packet'
		int a_count;

		// Define the storage for the packet accumulator
		byte[] accum = new byte[256];

		//----------------------------------------------------------------------------
		public LZWEncoder(int width, int height, byte[] pixels, int color_depth) 
		{
			imgW = width;
			imgH = height;
			pixAry = pixels;
			initCodeSize = Math.Max(2, color_depth);
		}
	
		// Add a character to the end of the current packet, and if it is 254
		// characters, flush the packet to disk.
		void Add(byte c, Stream outs)
		{
			accum[a_count++] = c;
			if (a_count >= 254)
				Flush(outs);
		}
	
		// Clear out the hash table

		// table clear for block compress
		void ClearTable(Stream outs)
		{
			ResetCodeTable(hsize);
			free_ent = ClearCode + 2;
			clear_flg = true;

			Output(ClearCode, outs);
		}
	
		// reset code table
		void ResetCodeTable(int hsize) 
		{
			for (int i = 0; i < hsize; ++i)
				htab[i] = -1;
		}
	
		void Compress(int init_bits, Stream outs)
		{
			int fcode;
			int i /* = 0 */;
			int c;
			int ent;
			int disp;
			int hsize_reg;
			int hshift;

			// Set up the globals:  g_init_bits - initial number of bits
			g_init_bits = init_bits;

			// Set up the necessary values
			clear_flg = false;
			n_bits = g_init_bits;
			maxcode = MaxCode(n_bits);

			ClearCode = 1 << (init_bits - 1);
			EOFCode = ClearCode + 1;
			free_ent = ClearCode + 2;

			a_count = 0; // clear packet

			ent = NextPixel();

			hshift = 0;
			for (fcode = hsize; fcode < 65536; fcode *= 2)
				++hshift;
			hshift = 8 - hshift; // set hash code range bound

			hsize_reg = hsize;
			ResetCodeTable(hsize_reg); // clear hash table

			Output(ClearCode, outs);

			outer_loop : while ((c = NextPixel()) != EOF) 
						 {
							 fcode = (c << maxbits) + ent;
							 i = (c << hshift) ^ ent; // xor hashing

							 if (htab[i] == fcode) 
							 {
								 ent = codetab[i];
								 continue;
							 } 
							 else if (htab[i] >= 0) // non-empty slot
							 {
								 disp = hsize_reg - i; // secondary hash (after G. Knott)
								 if (i == 0)
									 disp = 1;
								 do 
								 {
									 if ((i -= disp) < 0)
										 i += hsize_reg;

									 if (htab[i] == fcode) 
									 {
										 ent = codetab[i];
										 goto outer_loop;
									 }
								 } while (htab[i] >= 0);
							 }
						 	Output(ent, outs);
							 ent = c;
							 if (free_ent < maxmaxcode) 
							 {
								 codetab[i] = free_ent++; // code -> hashtable
								 htab[i] = fcode;
							 } 
							 else
							 	ClearTable(outs);
						 }
			// Put out the final code.
			Output(ent, outs);
			Output(EOFCode, outs);
		}
	
		//----------------------------------------------------------------------------
		public void Encode( Stream os)
		{
			os.WriteByte( Convert.ToByte( initCodeSize) ); // write "initial code size" byte

			remaining = imgW * imgH; // reset navigation variables
			curPixel = 0;

			Compress(initCodeSize + 1, os); // compress and write the pixel data

			os.WriteByte(0); // write block terminator
		}
	
		// Flush the packet to disk, and reset the accumulator
		void Flush(Stream outs)
		{
			if (a_count > 0) 
			{
				outs.WriteByte( Convert.ToByte( a_count ));
				outs.Write(accum, 0, a_count);
				a_count = 0;
			}
		}
	
		int MaxCode(int n_bits) 
		{
			return (1 << n_bits) - 1;
		}
	
		//----------------------------------------------------------------------------
		// Return the next pixel from the image
		//----------------------------------------------------------------------------
		private int NextPixel() 
		{
			if (remaining == 0)
				return EOF;

			--remaining;

			int temp = curPixel + 1;
			if ( temp < pixAry.GetUpperBound( 0 ))
			{
				byte pix = pixAry[curPixel++];

				return pix & 0xff;
			}
			return 0xff;
		}
	
		void Output(int code, Stream outs)
		{
			cur_accum &= masks[cur_bits];

			if (cur_bits > 0)
				cur_accum |= (code << cur_bits);
			else
				cur_accum = code;

			cur_bits += n_bits;

			while (cur_bits >= 8) 
			{
				Add((byte) (cur_accum & 0xff), outs);
				cur_accum >>= 8;
				cur_bits -= 8;
			}

			// If the next entry is going to be too big for the code size,
			// then increase it, if possible.
			if (free_ent > maxcode || clear_flg) 
			{
				if (clear_flg) 
				{
					maxcode = MaxCode(n_bits = g_init_bits);
					clear_flg = false;
				} 
				else 
				{
					++n_bits;
					if (n_bits == maxbits)
						maxcode = maxmaxcode;
					else
						maxcode = MaxCode(n_bits);
				}
			}

			if (code == EOFCode) 
			{
				// At EOF, write the rest of the buffer.
				while (cur_bits > 0) 
				{
					Add((byte) (cur_accum & 0xff), outs);
					cur_accum >>= 8;
					cur_bits -= 8;
				}

				Flush(outs);
			}
		}
	}
}