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];//這個是放hash的,在這裏面可以很快的找到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
// 全部初始化爲-1
void ResetCodeTable(int hsize)
{
for (int i = 0; i < hsize; ++i)
htab = -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爲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爲1的時候，開始爲3
//因此原始長度1->3,4->5,8->9
//?爲何原始數據字長爲1的時候，開始長度爲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
//是否需要加清除標誌
//GIF爲了提高壓縮率，採用的是變長的字長(VCL)。比如說原始數據是8位，那麼開始先加上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，此時後面的標號就必須用10位來表示
//以此類推，當標號到2^12的時候，因爲最大爲12,不能繼續擴展了，需要在2^12=4096的位置上插入一個ClearCode,表示從這往後，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圖像中開始一般爲3,5,9，故maxcode一般爲7,31,511)
maxcode = MaxCode(n_bits);
//表示從這裏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的值爲原始數據最大字長所能表達的數值+1;比如原始數據長度爲8,則clearcode=1<<(9-1)=256
ClearCode = 1 << (init_bits - 1);
//結束標誌爲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碼範圍
hshift = 8 - hshift; // set hash code range bound
hsize_reg = hsize;
//清除固定大小的hash表，用於存儲標記，這個相當於字典
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
//
if (htab == fcode)
{
ent = codetab;
continue;
}
//
else if (htab >= 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 == fcode)
{
ent = codetab;
goto outer_loop;
}
} while (htab >= 0);
}
Output(ent, outs);
//從這裏可以看出,ent就是前綴（prefix）,而當前正在處理的字符標誌就是後綴（suffix）
ent = c;
//判斷終止結束符是否超過當前位數所能表述的範圍
if (free_ent < maxmaxcode)
{
//如果沒有超
codetab = free_ent++; // code -> hashtable
//hash表裏面建立相應索引
htab = fcode;
}
else
//說明超過了當前所能表述的範圍,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,則返回值就爲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,返回結束標誌
if (remaining == 0)
return EOF;
//否則處理下一個,並將未處理像素數目-1
--remaining;
//當前處理的像素
int temp = curPixel + 1;
//如果當前處理像素在像素範圍之內
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)
{
//得到當前標誌位所能表示的最大標誌值
cur_accum &= masks[cur_bits];
if (cur_bits > 0)
cur_accum |= (code << cur_bits);
else
//如果標誌位爲0,就將當前標號爲輸入流
cur_accum = code;
//當前能標誌的最大字長度(9-10-11-12-9-10。。。。。。。)
cur_bits += n_bits;
//如果當前最大長度大於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);
}
}
}
}
