LZW压缩算法 C#源码

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];//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, 0, 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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