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 (mgardi@watdcsu.waterloo.edu) // 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];//這個是放hash的筒子,在這里面可以很快的找到1個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    // 全部初始化為-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      //原始數據的字長,在gif文件中，原始數據的字長可以為1(單色圖),4(16色)，和8(256色)      //開始的時候先加上1      //但是當原始數據長度為1的時候，開始為3      //因此原始長度1->3,4->5,8->9      //?為何原始數據字長為1的時候，開始長度為3呢？?      //如果+1=2，只能表示四種狀態，加上clearcode和endcode就用完了。所以必須擴展到3  g_init_bits = init_bits;  // Set up the necessary values      //是否需要加清除標志      //GIF為了提高壓縮率，采用的是變長的字長(VCL)。比如說原始數據是8位，那么開始先加上1位(8+1=9)      //當標號到2^9=512的時候，超過了當前長度9所能表現的最大值，此時后面的標號就必須用10位來表示      //以此類推，當標號到2^12的時候，因為最大為12,不能繼續擴展了，需要在2^12=4096的位置上插入一個ClearCode,表示從這往后，從9位重新再來了      clear_flg = false;  n_bits = g_init_bits;      //獲得n位數能表述的最大值(gif圖像中開始一般為3,5,9，故maxcode一般為7,31,511)  maxcode = MaxCode(n_bits);      //表示從這里我重新開始構造字典字典了，以前的所有標記作廢，      //開始使用新的標記。這個標號集的大小多少比較合適呢？據說理論上是越大壓縮率越高（我個人感覺太大了也不見得就好），      //不過處理的開銷也呈指數增長      //gif規定，clearcode的值為原始數據最大字長所能表達的數值+1;比如原始數據長度為8,則clearcode=1<<(9-1)=256  ClearCode = 1 << (init_bits - 1);      //結束標志為clearcode+1  EOFCode = ClearCode + 1;      //這個是解除結束的  free_ent = ClearCode + 2;      //清楚數量  a_count = 0; // clear packet      //從圖像中獲得下一個像素  ent = NextPixel();  hshift = 0;  for (fcode = hsize; fcode < 65536; fcode *= 2)  ++hshift;      //設置hash碼范圍  hshift = 8 - hshift; // set hash code range bound  hsize_reg = hsize;      //清除固定大小的hash表，用于存儲標記，這個相當于字典  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,小樣,又來了,我認識你    if (htab[i] == fcode)    {     ent = codetab[i];     continue;    }               //這小子,新來的    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);               //從這里可以看出,ent就是前綴（prefix）,而當前正在處理的字符標志就是后綴（suffix）    ent = c;               //判斷終止結束符是否超過當前位數所能表述的范圍    if (free_ent < maxmaxcode)    {                 //如果沒有超     codetab[i] = free_ent++; // code -> hashtable                 //hash表里面建立相應索引     htab[i] = fcode;    }    else                 //說明超過了當前所能表述的范圍,清空字典,重新再來     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      //這個圖像包含多少個像素  remaining = imgW * imgH; // reset navigation variables      //當前處理的像素索引  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, 0, a_count);  a_count = 0;  } }        /// <summary>    /// 獲得n位數所能表達的最大數值    /// </summary>    /// <param name="n_bits">位數，一般情況下n_bits = 9</param>    /// <returns>最大值,例如n_bits=8,則返回值就為2^8-1=255</returns> int MaxCode(int n_bits) {  return (1 << n_bits) - 1; }  //---------------------------------------------------------------------------- // Return the next pixel from the image //----------------------------------------------------------------------------    /// <summary>    /// 從圖像中獲得下一個像素    /// </summary>    /// <returns></returns> private int NextPixel() {      //還剩多少個像素沒有處理      //如果沒有了,返回結束標志  if (remaining == 0)  return EOF;      //否則處理下一個,并將未處理像素數目-1  --remaining;      //當前處理的像素  int temp = curPixel + 1;      //如果當前處理像素在像素范圍之內  if ( temp < pixAry.GetUpperBound( 0 ))  {        //下一個像素  byte pix = pixAry[curPixel++];  return pix & 0xff;  }  return 0xff; }   /// <summary>   /// 輸出字到輸出流   /// </summary>   /// <param name="code">要輸出的字</param>   /// <param name="outs">輸出流</param> void Output(int code, Stream outs) {      //得到當前標志位所能表示的最大標志值  cur_accum &= masks[cur_bits];  if (cur_bits > 0)  cur_accum |= (code << cur_bits);  else        //如果標志位為0,就將當前標號為輸入流  cur_accum = code;      //當前能標志的最大字長度(9-10-11-12-9-10。。。。。。。)  cur_bits += n_bits;      //如果當前最大長度大于8  while (cur_bits >= 8)  {        //向流中輸出一個字節  Add((byte) (cur_accum & 0xff), outs);        //將當前標號右移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);  } } }}