j2me解压GZIP
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GZIP最早由Jean-loup Gailly和Mark Adler创建，用于UNIX系统的文件压缩。我们在Linux中经常会用到后缀为.gz的文件，它们就是GZIP格式的。现今已经成为Internet上使用非常普遍的一种数据压缩格式，或者说一种文件格式。HTTP协议上的GZIP编码是一种用来改进WEB应用程序性能的技术。大流量的WEB站点常常使用GZIP压缩技术来让用户感受更快的速度。
GZIP本身只是一种文件格式，其内部通常采用DEFLATE数据格式，而DEFLATE采用LZ77压缩算法来压缩数据。
GZIP文件由1到多个“块”组成，实际上通常只有1块。每个块包含头、数据和尾三部分。块的概貌如下：
+---+---+---+---+---+---+---+---+---+---+========//========+===========//==========+---+---+---+---+---+---+---+---+
|ID1|ID2| CM|FLG| MTIME |XFL| OS| 额外的头字段 | 压缩的数据 | CRC32 | ISIZE |
+---+---+---+---+---+---+---+---+---+---+========//========+===========//==========+---+---+---+---+---+---+---+---+
1. 头部分
ID1与ID2：各1字节。固定值，ID1 = 31 (0x1F)，ID2 = 139(0x8B)，指示GZIP格式。
CM：1字节。压缩方法。目前只有一种：CM = 8，指示DEFLATE方法。
FLG：1字节。标志。
bit 0 FTEXT - 指示文本数据
bit 1 FHCRC - 指示存在CRC16头校验字段
bit 2 FEXTRA - 指示存在可选项字段
bit 3 FNAME - 指示存在原文件名字段
bit 4 FCOMMENT - 指示存在注释字段
bit 5-7 保留
MTIME：4字节。更改时间。UINX格式。
XFL：1字节。附加的标志。当CM = 8时，XFL = 2 - 最大压缩但最慢的算法；XFL = 4 - 最快但最小压缩的算法
OS：1字节。操作系统，确切地说应该是文件系统。有下列定义：
0 - FAT文件系统 (MS-DOS, OS/2, NT/Win32)
1 - Amiga
2 - VMS/OpenVMS
3 - Unix
4 - VM/CMS
5 - Atari TOS
6 - HPFS文件系统 (OS/2, NT)
7 - Macintosh
8 - Z-System
9 - CP/M
10 - TOPS-20
11 - NTFS文件系统 (NT)
12 - QDOS
13 - Acorn RISCOS
255 - 未知
额外的头字段：
(若 FLG.FEXTRA = 1)
+---+---+---+---+===============//================+
|SI1|SI2| XLEN | 长度为XLEN字节的可选项 |
+---+---+---+---+===============//================+
(若 FLG.FNAME = 1)
+=======================//========================+
| 原文件名(以NULL结尾) |
+=======================//========================+
(若 FLG.FCOMMENT = 1)
+=======================//========================+
| 注释文字(只能使用iso-8859-1字符，以NULL结尾) |
+=======================//========================+
(若 FLG.FHCRC = 1)
+---+---+
| CRC16 |
+---+---+
存在额外的可选项时，SI1与SI2指示可选项ID，XLEN指示可选项字节数。如 SI1 = 0x41 ('A')，SI2 = 0x70 ('P')，表示可选项是Apollo文件格式的额外数据。
2. 数据部分
DEFLATE数据格式，包含一系列子数据块。子块概貌如下：
+......+......+......+=============//============+
|BFINAL| BTYPE | 数据 |
+......+......+......+=============//============+
BFINAL：1比特。0 - 还有后续子块；1 - 该子块是最后一块。
BTYPE：2比特。00 - 不压缩；01 - 静态Huffman编码压缩；10 - 动态Huffman编码压缩；11 - 保留。
各种情形的处理过程，请参考后面列出的RFC文档。
3. 尾部分
CRC32：4字节。原始(未压缩)数据的32位校验和。
ISIZE：4字节。原始(未压缩)数据的长度的低32位。
GZIP中字节排列顺序是LSB方式，即Little-Endian，与ZLIB中的相反。
在j2se中java有java.util.zip.*包来实现对GIP的解压，但是j2me中没有。自己实现GZIP的解压缩算法如下：
package com.DriverBook.mtraffic;
/*
* GZIP.java
*
* Created on 2007年10月17日, 下午3:37
*
* To change this template, choose Tools | Template Manager
* and open the template in the editor.
*/
/**
*
* @author hedonist
*/
/*
* GZIP library for j2me applications.
*
* Copyright (c) 2004-2006 Carlos Araiz (caraiz@java4ever.com)
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
*/
import java.io.*;
public class GZIP {
// M醩caras para el flag.
/** FLG：1字节。标志。
bit 0 FTEXT - 指示文本数据
bit 1 FHCRC - 指示存在CRC16头校验字段
bit 2 FEXTRA - 指示存在可选项字段
bit 3 FNAME - 指示存在原文件名字段
bit 4 FCOMMENT - 指示存在注释字段 **/
private static final int FTEXT_MASK = 1;
private static final int FHCRC_MASK = 2;
private static final int FEXTRA_MASK = 4;
private static final int FNAME_MASK = 8;
private static final int FCOMMENT_MASK = 16;
// Tipos de bloques.
//BTYPE：2比特。00 - 不压缩；01 - 静态Huffman编码压缩；10 - 动态Huffman编码压缩；11 - 保留
private static final int BTYPE_NONE = 0;
private static final int BTYPE_FIXED = 1;
private static final int BTYPE_DYNAMIC = 2;
private static final int BTYPE_RESERVED = 3;
// L韒ites.
private static final int MAX_BITS = 16;
private static final int MAX_CODE_LITERALS = 287;
private static final int MAX_CODE_DISTANCES = 31;
private static final int MAX_CODE_LENGTHS = 18;
private static final int EOB_CODE = 256;
// Datos prefijados (LENGTH: 257..287 / DISTANCE: 0..29 / DYNAMIC_LENGTH_ORDER: 0..18).
private static final int LENGTH_EXTRA_BITS[] = {0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5, 0, 99, 99};
private static final int LENGTH_VALUES[] = {3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 15, 17, 19, 23, 27, 31, 35, 43, 51, 59, 67, 83, 99, 115, 131, 163, 195, 227, 258, 0, 0};
private static final int DISTANCE_EXTRA_BITS[] = {0, 0, 0, 0, 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8, 9, 9, 10, 10, 11, 11, 12, 12, 13, 13};
private static final int DISTANCE_VALUES[] = {1, 2, 3, 4, 5, 7, 9, 13, 17, 25, 33, 49, 65, 97, 129, 193, 257, 385, 513, 769, 1025, 1537, 2049, 3073, 4097, 6145, 8193, 12289, 16385, 24577};
private static final int DYNAMIC_LENGTH_ORDER[] = {16, 17, 18, 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15};
/*************************************************************************/
// Variables para la lectura de datos comprimidos.
private static int gzipIndex, gzipByte, gzipBit;
/*************************************************************************/
/*************************************************************************/
/**
* Descomprime un fichero GZIP.
*
* @param gzip Array con los datos del fichero comprimido
*
* @return Array con los datos descomprimidos
*/
/**GZIP的数据结构
* +---+---+---+---+---+---+---+---+---+---+========//========+===========//==========+---+---+---+---+---+---+---+---+
|ID1|ID2| CM|FLG| MTIME |XFL| OS| 额外的头字段 | 压缩的数据 | CRC32 | ISIZE |
+---+---+---+---+---+---+---+---+---+---+========//========+===========//==========+---+---+---+---+---+---+---+---+
*/
public static byte[] inflate(byte gzip[]) throws IOException {
// Inicializa.
gzipIndex = gzipByte = gzipBit = 0;
// Cabecera.
//ID1与ID2：各1字节。固定值，ID1 = 31 (0x1F)，ID2 = 139(0x8B)，指示GZIP格式。
if (readBits(gzip, 16) != 0x8B1F || readBits(gzip, 8) != 8) {
throw new IOException("Invalid GZIP format");
// Flag.
}
int flg = readBits(gzip, 8);
// Fecha(4) / XFL(1) / OS(1).
gzipIndex += 6;
// Comprueba los flags.
if ((flg & FEXTRA_MASK) != 0) {
gzipIndex += readBits(gzip, 16);
}
if ((flg & FNAME_MASK) != 0) {
while (gzip[gzipIndex++] != 0);
}
if ((flg & FCOMMENT_MASK) != 0) {
while (gzip[gzipIndex++] != 0);
}
if ((flg & FHCRC_MASK) != 0) {
gzipIndex += 2;
// Tama駉 de los datos descomprimidos.
}
int index = gzipIndex;
gzipIndex = gzip.length - 4;
byte uncompressed[] = new byte[readBits(gzip, 16) | (readBits(gzip, 16) << 16)];
int uncompressedIndex = 0;
gzipIndex = index;
// Bloque con datos comprimidos.
int bfinal = 0, btype = 0;
do {
// Lee la cabecera del bloque.
bfinal = readBits(gzip, 1);
btype = readBits(gzip, 2);
// Comprueba el tipo de compresi髇.
if (btype == BTYPE_NONE) {
// Ignora los bits dentro del byte actual.
gzipBit = 0;
// LEN.
int len = readBits(gzip, 16);
// NLEN.
int nlen = readBits(gzip, 16);
// Lee los datos.
System.arraycopy(gzip, gzipIndex, uncompressed, uncompressedIndex, len);
gzipIndex += len;
// Actualiza el 韓dice de los datos descomprimidos.
uncompressedIndex += len;
} else {
int literalTree[], distanceTree[];
if (btype == BTYPE_DYNAMIC) {
// N鷐ero de datos de cada tipo.
int hlit = readBits(gzip, 5) + 257;
int hdist = readBits(gzip, 5) + 1;
int hclen = readBits(gzip, 4) + 4;
// Lee el n鷐ero de bits para cada c骴igo de longitud.
byte lengthBits[] = new byte[MAX_CODE_LENGTHS + 1];
for (int i = 0; i < hclen; i++) {
lengthBits[DYNAMIC_LENGTH_ORDER[i]] = (byte) readBits(gzip, 3);
}
// Crea los c骴igos para la longitud.
int lengthTree[] = createHuffmanTree(lengthBits, MAX_CODE_LENGTHS);
// Genera los 醨boles.
literalTree = createHuffmanTree(decodeCodeLengths(gzip, lengthTree, hlit), hlit - 1);
distanceTree = createHuffmanTree(decodeCodeLengths(gzip, lengthTree, hdist), hdist - 1);
} else {
byte literalBits[] = new byte[MAX_CODE_LITERALS + 1];
for (int i = 0; i < 144; i++) {
literalBits[i] = 8;
}
for (int i = 144; i < 256; i++) {
literalBits[i] = 9;
}
for (int i = 256; i < 280; i++) {
literalBits[i] = 7;
}
for (int i = 280; i < 288; i++) {
literalBits[i] = 8;
}
literalTree = createHuffmanTree(literalBits, MAX_CODE_LITERALS);
//
byte distanceBits[] = new byte[MAX_CODE_DISTANCES + 1];
for (int i = 0; i < distanceBits.length; i++) {
distanceBits[i] = 5;
}
distanceTree = createHuffmanTree(distanceBits, MAX_CODE_DISTANCES);
}
// Descomprime el bloque.
int code = 0, leb = 0, deb = 0;
while ((code = readCode(gzip, literalTree)) != EOB_CODE) {
if (code > EOB_CODE) {
code -= 257;
int length = LENGTH_VALUES[code];
if ((leb = LENGTH_EXTRA_BITS[code]) > 0) {
length += readBits(gzip, leb);
}
code = readCode(gzip, distanceTree);
int distance = DISTANCE_VALUES[code];
if ((deb = DISTANCE_EXTRA_BITS[code]) > 0) {
distance += readBits(gzip, deb);
// Repite la informaci髇.
}
int offset = uncompressedIndex - distance;
while (distance < length) {
System.arraycopy(uncompressed, offset, uncompressed, uncompressedIndex, distance);
uncompressedIndex += distance;
length -= distance;
distance <<= 1;
}
System.arraycopy(uncompressed, offset, uncompressed, uncompressedIndex, length);
uncompressedIndex += length;
} else {
uncompressed[uncompressedIndex++] = (byte) code;
}
}
}
} while (bfinal == 0);
//
return uncompressed;
}
/**
* Lee un n鷐ero de bits
*
* @param n N鷐ero de bits [0..16]
*/
private static int readBits(byte gzip[], int n) {
// Asegura que tenemos un byte.
int data = (gzipBit == 0 ? (gzipByte = (gzip[gzipIndex++] & 0xFF)) : (gzipByte >> gzipBit));
// Lee hasta completar los bits.
for (int i = (8 - gzipBit); i < n; i += 8) {
gzipByte = (gzip[gzipIndex++] & 0xFF);
data |= (gzipByte << i);
}
// Ajusta la posici髇 actual.
gzipBit = (gzipBit + n) & 7;
// Devuelve el dato.
return (data & ((1 << n) - 1));
}
/**
* Lee un c骴igo.
*/
private static int readCode(byte gzip[], int tree[]) {
int node = tree[0];
while (node >= 0) {
// Lee un byte si es necesario.
if (gzipBit == 0) {
gzipByte = (gzip[gzipIndex++] & 0xFF);
// Accede al nodo correspondiente.
}
node = (((gzipByte & (1 << gzipBit)) == 0) ? tree[node >> 16] : tree[node & 0xFFFF]);
// Ajusta la posici髇 actual.
gzipBit = (gzipBit + 1) & 7;
}
return (node & 0xFFFF);
}
/**
* Decodifica la longitud de c骴igos (usado en bloques comprimidos con c骴igos din醡icos).
*/
private static byte[] decodeCodeLengths(byte gzip[], int lengthTree[], int count) {
byte bits[] = new byte[count];
for (int i = 0, code = 0, last = 0; i < count;) {
code = readCode(gzip, lengthTree);
if (code >= 16) {
int repeat = 0;
if (code == 16) {
repeat = 3 + readBits(gzip, 2);
code = last;
} else {
if (code == 17) {
repeat = 3 + readBits(gzip, 3);
} else {
repeat = 11 + readBits(gzip, 7);
}
code = 0;
}
while (repeat-- > 0) {
bits[i++] = (byte) code;
}
} else {
bits[i++] = (byte) code;
//
}
last = code;
}
return bits;
}
/**
* Crea el 醨bol para los c骴igos Huffman.
*/
private static int[] createHuffmanTree(byte bits[], int maxCode) {
// N鷐ero de c骴igos por cada longitud de c骴igo.
int bl_count[] = new int[MAX_BITS + 1];
for (int i = 0; i < bits.length; i++) {
bl_count[bits[i]]++;
}
// M韓imo valor num閞ico del c骴igo para cada longitud de c骴igo.
int code = 0;
bl_count[0] = 0;
int next_code[] = new int[MAX_BITS + 1];
for (int i = 1; i <= MAX_BITS; i++) {
next_code[i] = code = (code + bl_count[i - 1]) << 1;
}
// Genera el 醨bol.
// Bit 31 => Nodo (0) o c骴igo (1).
// (Nodo) bit 16..30 => 韓dice del nodo de la izquierda (0 si no tiene).
// (Nodo) bit 0..15 => 韓dice del nodo de la derecha (0 si no tiene).
// (C骴igo) bit 0..15
int tree[] = new int[(maxCode << 1) + MAX_BITS];
int treeInsert = 1;
for (int i = 0; i <= maxCode; i++) {
int len = bits[i];
if (len != 0) {
code = next_code[len]++;
// Lo mete en en 醨bol.
int node = 0;
for (int bit = len - 1; bit >= 0; bit--) {
int value = code & (1 << bit);
// Inserta a la izquierda.
if (value == 0) {
int left = tree[node] >> 16;
if (left == 0) {
tree[node] |= (treeInsert << 16);
node = treeInsert++;
} else {
node = left;
}
} // Inserta a la derecha.
else {
int right = tree[node] & 0xFFFF;
if (right == 0) {
tree[node] |= treeInsert;
node = treeInsert++;
} else {
node = right;
}
}
}
// Inserta el c骴igo.
tree[node] = 0x80000000 | i;
}
}
return tree;
}
}
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