294 lines
7.3 KiB
C#
294 lines
7.3 KiB
C#
using System;
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using System.IO;
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namespace uGIF
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{
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public class LZWEncoder
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{
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static readonly int EOF = -1;
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byte[] pixAry;
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int initCodeSize;
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int curPixel;
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// GIFCOMPR.C - GIF Image compression routines
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//
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// Lempel-Ziv compression based on 'compress'. GIF modifications by
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// David Rowley (mgardi@watdcsu.waterloo.edu)
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// General DEFINEs
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static readonly int BITS = 12;
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static readonly int HSIZE = 5003; // 80% occupancy
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// GIF Image compression - modified 'compress'
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//
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// Based on: compress.c - File compression ala IEEE Computer, June 1984.
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//
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// By Authors: Spencer W. Thomas (decvax!harpo!utah-cs!utah-gr!thomas)
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// Jim McKie (decvax!mcvax!jim)
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// Steve Davies (decvax!vax135!petsd!peora!srd)
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// Ken Turkowski (decvax!decwrl!turtlevax!ken)
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// James A. Woods (decvax!ihnp4!ames!jaw)
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// Joe Orost (decvax!vax135!petsd!joe)
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int n_bits; // number of bits/code
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int maxbits = BITS; // user settable max # bits/code
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int maxcode; // maximum code, given n_bits
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int maxmaxcode = 1 << BITS; // should NEVER generate this code
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int[] htab = new int[HSIZE];
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int[] codetab = new int[HSIZE];
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int hsize = HSIZE; // for dynamic table sizing
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int free_ent = 0; // first unused entry
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// block compression parameters -- after all codes are used up,
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// and compression rate changes, start over.
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bool clear_flg = false;
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// Algorithm: use open addressing double hashing (no chaining) on the
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// prefix code / next character combination. We do a variant of Knuth's
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// algorithm D (vol. 3, sec. 6.4) along with G. Knott's relatively-prime
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// secondary probe. Here, the modular division first probe is gives way
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// to a faster exclusive-or manipulation. Also do block compression with
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// an adaptive reset, whereby the code table is cleared when the compression
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// ratio decreases, but after the table fills. The variable-length output
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// codes are re-sized at this point, and a special CLEAR code is generated
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// for the decompressor. Late addition: construct the table according to
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// file size for noticeable speed improvement on small files. Please direct
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// questions about this implementation to ames!jaw.
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int g_init_bits;
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int ClearCode;
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int EOFCode;
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// output
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//
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// Output the given code.
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// Inputs:
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// code: A n_bits-bit integer. If == -1, then EOF. This assumes
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// that n_bits =< wordsize - 1.
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// Outputs:
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// Outputs code to the file.
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// Assumptions:
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// Chars are 8 bits long.
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// Algorithm:
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// Maintain a BITS character long buffer (so that 8 codes will
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// fit in it exactly). Use the VAX insv instruction to insert each
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// code in turn. When the buffer fills up empty it and start over.
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int cur_accum = 0;
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int cur_bits = 0;
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int[] masks =
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{
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0x0000,
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0x0001,
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0x0003,
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0x0007,
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0x000F,
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0x001F,
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0x003F,
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0x007F,
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0x00FF,
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0x01FF,
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0x03FF,
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0x07FF,
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0x0FFF,
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0x1FFF,
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0x3FFF,
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0x7FFF,
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0xFFFF };
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// Number of characters so far in this 'packet'
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int a_count;
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// Define the storage for the packet accumulator
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byte[] accum = new byte[256];
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//----------------------------------------------------------------------------
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public LZWEncoder (int width, int height, byte[] pixels, int color_depth)
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{
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pixAry = pixels;
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initCodeSize = Math.Max (2, color_depth);
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}
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// Add a character to the end of the current packet, and if it is 254
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// characters, flush the packet to disk.
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void Add (byte c, Stream outs)
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{
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accum [a_count++] = c;
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if (a_count >= 254)
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Flush (outs);
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}
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// Clear out the hash table
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// table clear for block compress
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void ClearTable (Stream outs)
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{
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ResetCodeTable (hsize);
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free_ent = ClearCode + 2;
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clear_flg = true;
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Output (ClearCode, outs);
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}
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// reset code table
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void ResetCodeTable (int hsize)
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{
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for (int i = 0; i < hsize; ++i)
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htab [i] = -1;
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}
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void Compress (int init_bits, Stream outs)
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{
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int fcode;
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int i /* = 0 */;
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int c;
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int ent;
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int disp;
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int hsize_reg;
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int hshift;
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// Set up the globals: g_init_bits - initial number of bits
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g_init_bits = init_bits;
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// Set up the necessary values
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clear_flg = false;
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n_bits = g_init_bits;
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maxcode = MaxCode (n_bits);
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ClearCode = 1 << (init_bits - 1);
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EOFCode = ClearCode + 1;
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free_ent = ClearCode + 2;
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a_count = 0; // clear packet
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ent = NextPixel ();
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hshift = 0;
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for (fcode = hsize; fcode < 65536; fcode *= 2)
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++hshift;
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hshift = 8 - hshift; // set hash code range bound
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hsize_reg = hsize;
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ResetCodeTable (hsize_reg); // clear hash table
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Output (ClearCode, outs);
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outer_loop :
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while ((c = NextPixel()) != EOF) {
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fcode = (c << maxbits) + ent;
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i = (c << hshift) ^ ent; // xor hashing
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if (htab [i] == fcode) {
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ent = codetab [i];
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continue;
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} else if (htab [i] >= 0) { // non-empty slot
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disp = hsize_reg - i; // secondary hash (after G. Knott)
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if (i == 0)
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disp = 1;
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do {
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if ((i -= disp) < 0)
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i += hsize_reg;
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if (htab [i] == fcode) {
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ent = codetab [i];
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goto outer_loop;
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}
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} while (htab[i] >= 0);
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}
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Output (ent, outs);
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ent = c;
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if (free_ent < maxmaxcode) {
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codetab [i] = free_ent++; // code -> hashtable
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htab [i] = fcode;
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} else
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ClearTable (outs);
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}
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// Put out the final code.
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Output (ent, outs);
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Output (EOFCode, outs);
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}
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//----------------------------------------------------------------------------
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public void Encode (Stream os)
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{
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os.WriteByte (Convert.ToByte (initCodeSize)); // write "initial code size" byte
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curPixel = 0;
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Compress (initCodeSize + 1, os); // compress and write the pixel data
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os.WriteByte (0); // write block terminator
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}
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// Flush the packet to disk, and reset the accumulator
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void Flush (Stream outs)
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{
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if (a_count > 0) {
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outs.WriteByte (Convert.ToByte (a_count));
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outs.Write (accum, 0, a_count);
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a_count = 0;
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}
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}
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int MaxCode (int n_bits)
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{
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return (1 << n_bits) - 1;
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}
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//----------------------------------------------------------------------------
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// Return the next pixel from the image
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//----------------------------------------------------------------------------
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int NextPixel ()
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{
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if (curPixel == pixAry.Length)
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return EOF;
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curPixel++;
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return pixAry [curPixel - 1] & 0xff;
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}
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void Output (int code, Stream outs)
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{
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cur_accum &= masks [cur_bits];
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if (cur_bits > 0)
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cur_accum |= (code << cur_bits);
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else
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cur_accum = code;
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cur_bits += n_bits;
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while (cur_bits >= 8) {
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Add ((byte)(cur_accum & 0xff), outs);
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cur_accum >>= 8;
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cur_bits -= 8;
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}
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// If the next entry is going to be too big for the code size,
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// then increase it, if possible.
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if (free_ent > maxcode || clear_flg) {
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if (clear_flg) {
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maxcode = MaxCode (n_bits = g_init_bits);
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clear_flg = false;
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} else {
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++n_bits;
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if (n_bits == maxbits)
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maxcode = maxmaxcode;
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else
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maxcode = MaxCode (n_bits);
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}
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}
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if (code == EOFCode) {
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// At EOF, write the rest of the buffer.
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while (cur_bits > 0) {
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Add ((byte)(cur_accum & 0xff), outs);
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cur_accum >>= 8;
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cur_bits -= 8;
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}
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Flush (outs);
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}
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}
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}
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} |