LZW压缩算法 C#源码

概述usingSystem;usingSystem.IO;namespaceGif.Components{publicclassLZWEncoder{privatestaticreadonlyintEOF=-1;

using System;using System.IO;namespace 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 ([email protected]) // 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];//这个是放hash的筒子,在这里面可以很快的找到1个key 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    // 全部初始化为-1 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      //原始数据的字长,在gif文件中，原始数据的字长可以为1(单色图),4(16色)，和8(256色)      //开始的时候先加上1      //但是当原始数据长度为1的时候，开始为3      //因此原始长度1->3,4->5,8->9      //?为何原始数据字长为1的时候，开始长度为3呢？?      //如果+1=2，只能表示四种状态，加上clearcode和endcode就用完了。所以必须扩展到3  g_init_bits = init_bits;  // Set up the necessary values      //是否需要加清除标志      //GIF为了提高压缩率，采用的是变长的字长(VCL)。比如说原始数据是8位，那么开始先加上1位(8+1=9)      //当标号到2^9=512的时候，超过了当前长度9所能表现的最大值，此时后面的标号就必须用10位来表示      //以此类推，当标号到2^12的时候，因为最大为12,不能继续扩展了，需要在2^12=4096的位置上插入一个ClearCode,表示从这往后，从9位重新再来了      clear_flg = false;  n_bits = g_init_bits;      //获得n位数能表述的最大值(gif图像中开始一般为3,5,9，故maxcode一般为7,31,511)  maxcode = MaxCode(n_bits);      //表示从这里我重新开始构造字典字典了，以前的所有标记作废，      //开始使用新的标记。这个标号集的大小多少比较合适呢？据说理论上是越大压缩率越高（我个人感觉太大了也不见得就好），      //不过处理的开销也呈指数增长      //gif规定，clearcode的值为原始数据最大字长所能表达的数值+1;比如原始数据长度为8,则clearcode=1<<(9-1)=256  ClearCode = 1 << (init_bits - 1);      //结束标志为clearcode+1  EOFCode = ClearCode + 1;      //这个是解除结束的  free_ent = ClearCode + 2;      //清楚数量  a_count = 0; // clear packet      //从图像中获得下一个像素  ent = NextPixel();  hshift = 0;  for (fcode = hsize; fcode < 65536; fcode *= 2)  ++hshift;      //设置hash码范围  hshift = 8 - hshift; // set hash code range bound  hsize_reg = hsize;      //清除固定大小的hash表，用于存储标记，这个相当于字典  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               //嘿嘿,小样,又来了,我认识你    if (htab[i] == fcode)    {     ent = codetab[i];     continue;    }               //这小子,新来的    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);               //从这里可以看出,ent就是前缀（prefix）,而当前正在处理的字符标志就是后缀（suffix）    ent = c;               //判断终止结束符是否超过当前位数所能表述的范围    if (free_ent < maxmaxcode)    {                 //如果没有超     codetab[i] = free_ent++; // code -> hashtable                 //hash表里面建立相应索引     htab[i] = fcode;    }    else                 //说明超过了当前所能表述的范围,清空字典,重新再来     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      //这个图像包含多少个像素  remaining = imgW * imgH; // reset navigation variables      //当前处理的像素索引  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,a_count);  a_count = 0;  } }        /// <summary>    /// 获得n位数所能表达的最大数值    /// </summary>    /// <param name="n_bits">位数，一般情况下n_bits = 9</param>    /// <returns>最大值,例如n_bits=8,则返回值就为2^8-1=255</returns> int MaxCode(int n_bits) {  return (1 << n_bits) - 1; }  //---------------------------------------------------------------------------- // Return the next pixel from the image //----------------------------------------------------------------------------    /// <summary>    /// 从图像中获得下一个像素    /// </summary>    /// <returns></returns> private int NextPixel() {      //还剩多少个像素没有处理      //如果没有了,返回结束标志  if (remaining == 0)  return EOF;      //否则处理下一个,并将未处理像素数目-1  --remaining;      //当前处理的像素  int temp = curPixel + 1;      //如果当前处理像素在像素范围之内  if ( temp < pixAry.GetUpperBound( 0 ))  {        //下一个像素  byte pix = pixAry[curPixeL++];  return pix & 0xff;  }  return 0xff; }   /// <summary>   /// 输出字到输出流   /// </summary>   /// <param name="code">要输出的字</param>   /// <param name="outs">输出流</param> voID Output(int code,Stream outs) {      //得到当前标志位所能表示的最大标志值  cur_accum &= masks[cur_bits];  if (cur_bits > 0)  cur_accum |= (code << cur_bits);  else        //如果标志位为0,就将当前标号为输入流  cur_accum = code;      //当前能标志的最大字长度(9-10-11-12-9-10。。。。。。。)  cur_bits += n_bits;      //如果当前最大长度大于8  while (cur_bits >= 8)  {        //向流中输出一个字节  Add((byte) (cur_accum & 0xff),outs);        //将当前标号右移8位  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);  } } }}

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