2020编码大赛（4）LZW压缩算法-白红宇

2020编码大赛（4）LZW压缩算法

阅读量：607 次

发布时间：2019-03-12

本文共 3601 字，大约阅读时间需要 12 分钟。

LZW的原理：

初始化字典为255个字符，随着压缩的过程，字典一步步扩充，最终把全文本转换成字典的id序列，再用变长编码的方法，把id序列编码成二进制。

编码方法：0-254用8位表示，255到510用9位表示，......

当第一次需要编码9位的时候，插入255表示升位，此后0-510都用9位表示，......

因为代码是用string来操作的，所以额外加了对于/0的编码，保证算法的通用性。

压缩率：大概在1024:420左右

代码：

#include "compress.h"#include #define BUF_SIZE 1048576#define ASCII 256#define ASCIILEN 8using namespace std;char g_buff[BUF_SIZE + 5];char g_curByte;int g_bufSize;int g_bits;int g_byte;bool g_EOF;void writeBit(unsigned int data, int length){    if (!length) {        return;    }    if (!g_bits) {        g_buff[g_byte++] = g_curByte, g_curByte = 0;    }    g_bits = (g_bits + 7) % 8, g_curByte |= (((data >> length - 1) & 1) << g_bits);    writeBit(data, length - 1);}void writeInt(unsigned int data, int &length){    if (data >= unsigned((1 << length) - 1)) {        writeBit((1 << length) - 1, length);        length++;    }    writeBit(data, length);}void readBit(unsigned int &val){    if (!g_bits) {        g_EOF = (g_byte == g_bufSize), g_curByte = g_buff[g_byte++];    }    g_bits = (g_bits + 7) % 8, val = val * 2 + ((g_curByte >> g_bits) & 1);}unsigned int readInt(int length){    unsigned int val = 0;    while (length-- && !g_EOF) {        readBit(val);    }    return val;}void compress(ifstream &fin, ofstream &fout){    map
    
      m;    for (int i = 0; i < ASCII; i++) {        m[string("") + char(i)] = i;    }    int bitLen = ASCIILEN;    unsigned int index = ASCII;    char ch;    string str;    while (fin.get(ch)) {        if (!ch) {            if (m[str]) {                writeInt(m[str], bitLen);                str = "";            }            writeInt(0, bitLen);            continue;        }        if (m[str + ch]) {            str += ch;        } else {            writeInt(m[str], bitLen);            m[str + ch] = index++;            str = ch;        }    }    if (str != "") {        writeInt(m[str], bitLen);    }}void decompress(ofstream &fout, ifstream &fin){    map
     
       m;    for (int i = 0; i < ASCII; i++) {        m[i] = string("") + char(i);    }    int bitLen = ASCIILEN;    unsigned int index = ASCII;    char zero = 0;    string oldstr, newstr;    g_curByte = g_buff[0];    int code = g_curByte;    if (code == ASCII - 1) { // 以\255开头的文件        bitLen++;        code = readInt(bitLen);    }    if (code == 0) {        fout << zero;    } else {        oldstr = m[code];        fout << oldstr;    }    while (true) {        code = readInt(bitLen);        if (g_EOF) {            break;        }        if (code == (1 << bitLen) - 1) {            bitLen++;            code = readInt(bitLen);        }        if (code == 0) {            fout << zero;            oldstr = "";            continue;        }        if (m[code] != "") {            newstr = m[code];        } else {            newstr = oldstr + oldstr[0];        }        if (oldstr != "") {            m[index++] = oldstr + newstr[0];        }        oldstr = newstr;        fout << oldstr;    }}void compress(const string input, const string output){    ifstream fin(input.c_str(), ios::binary);    ofstream fout(output.c_str(), ios::binary);    g_bits = 8;    g_byte = 0;    g_curByte = 0;    compress(fin, fout);    fin.close();    g_buff[g_byte++] = g_curByte;    fout.write(g_buff, sizeof(char) * g_byte);    fout.close();}void decompress(const string input, const string output){    ifstream fin(input.c_str(), ios::binary);    ofstream fout(output.c_str(), ios::binary);    g_EOF = false;    g_bits = 0;    g_byte = 1;    fin.read(g_buff, sizeof(char) * BUF_SIZE);    g_bufSize = int(fin.gcount());    decompress(fout, fin);    fout.close();    fin.close();}

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