使用按位运算符的C++中的压缩

Question

我们已经给出了一个赋值，其中我们必须将4个字节压缩为3个字节。它希望我们通过将字符打包成6位而不是8位来压缩25％。它应该是25％的确切压缩率，但是我的程序约占50％。 Code Book是我自己的“ASCII”排序表，按位运算符用于执行压缩。有没有人知道为什么它将它压缩了50％而不是25％？我知道命名空间标准不是一个好习惯，但我们被要求使用它。谢谢`

#include <iostream> 
#include <fstream> 
#include <string> 

using namespace std; 

unsigned char CodeBook[53][2]; 

unsigned char FindCharacterCode(unsigned char C) 
{ 
    for (int j = 0; j < 53; j++) 
    if (CodeBook[j][0] == C) 
    return CodeBook[j][1]; 
    return 0; 
} 

void MyCodeBook() 
{ 
    CodeBook[0][0] = 'a'; CodeBook[0][1] = 1; 
    CodeBook[1][0] = 'b'; CodeBook[1][1] = 2; 
    CodeBook[2][0] = 'c'; CodeBook[2][1] = 3; 
    CodeBook[3][0] = 'd'; CodeBook[3][1] = 4; 
    CodeBook[4][0] = 'e'; CodeBook[4][1] = 5; 
    CodeBook[5][0] = 'f'; CodeBook[5][1] = 6; 
    CodeBook[6][0] = 'g'; CodeBook[6][1] = 7; 
    CodeBook[7][0] = 'h'; CodeBook[7][1] = 8; 
    CodeBook[8][0] = 'i'; CodeBook[8][1] = 9; 
    CodeBook[9][0] = 'j'; CodeBook[9][1] = 10; 
    CodeBook[10][0] = 'k'; CodeBook[10][1] = 11; 
    CodeBook[11][0] = 'l'; CodeBook[11][1] = 12; 
    CodeBook[12][0] = 'm'; CodeBook[12][1] = 13; 
    CodeBook[13][0] = 'n'; CodeBook[13][1] = 14; 
    CodeBook[14][0] = 'o'; CodeBook[14][1] = 15; 
    CodeBook[15][0] = 'p'; CodeBook[15][1] = 16; 
    CodeBook[16][0] = 'q'; CodeBook[16][1] = 17; 
    CodeBook[17][0] = 'r'; CodeBook[17][1] = 18; 
    CodeBook[18][0] = 's'; CodeBook[18][1] = 19; 
    CodeBook[19][0] = 't'; CodeBook[19][1] = 20; 
    CodeBook[20][0] = 'u'; CodeBook[20][1] = 21; 
    CodeBook[21][0] = 'v'; CodeBook[21][1] = 22; 
    CodeBook[2][0] = 'w'; CodeBook[22][1] = 23; 
    CodeBook[23][0] = 'x'; CodeBook[23][1] = 24; 
    CodeBook[24][0] = 'y'; CodeBook[24][1] = 25; 
    CodeBook[25][0] = 'z'; CodeBook[25][1] = 26; 
    CodeBook[26][0] = '0'; CodeBook[26][1] = 27; 
    CodeBook[27][0] = '1'; CodeBook[27][1] = 28; 
    CodeBook[28][0] = '2'; CodeBook[28][1] = 29; 
    CodeBook[29][0] = '3'; CodeBook[29][1] = 30; 
    CodeBook[30][0] = '4'; CodeBook[30][1] = 31; 
    CodeBook[31][0] = '5'; CodeBook[31][1] = 32; 
    CodeBook[32][0] = '6'; CodeBook[32][1] = 33; 
    CodeBook[33][0] = '7'; CodeBook[33][1] = 34; 
    CodeBook[34][0] = '8'; CodeBook[34][1] = 35; 
    CodeBook[35][0] = '9'; CodeBook[35][1] = 36; 
    CodeBook[36][0] = '!'; CodeBook[36][1] = 37; 
    CodeBook[37][0] = '$'; CodeBook[37][1] = 38; 
    CodeBook[38][0] = '('; CodeBook[38][1] = 39; 
    CodeBook[39][0] = ')'; CodeBook[39][1] = 40; 
    CodeBook[40][0] = '#'; CodeBook[40][1] = 41; 
    CodeBook[41][0] = '&'; CodeBook[41][1] = 42; 
    CodeBook[42][0] = '%'; CodeBook[42][1] = 43; 
    CodeBook[43][0] = '-'; CodeBook[43][1] = 44; 
    CodeBook[44][0] = '.'; CodeBook[44][1] = 45; 
    CodeBook[45][0] = ','; CodeBook[45][1] = 46; 
    CodeBook[46][0] = '\''; CodeBook[46][1] = 47; 
    CodeBook[47][0] = ';'; CodeBook[47][1] = 48; 
    CodeBook[48][0] = ':'; CodeBook[48][1] = 49; 
    CodeBook[49][0] = '?'; CodeBook[49][1] = 50; 
    CodeBook[50][0] = ' '; CodeBook[50][1] = 51; 
    CodeBook[51][0] = ' '; CodeBook[51][1] = 52; 
    CodeBook[52][0] = '¶'; CodeBook[52][1] = 53; 

} 

int main() 
{ 

    MyCodeBook(); 

    ifstream In; 
    ofstream Out; 

    In.open("C://Users//osama//Desktop//code.txt"); 
    Out.open("C://Users//osama//Desktop//compressed.txt"); 

    unsigned char Data[4] = { 0 }, 
    Compressed[3] = { 0 }, Code[4] = { 0 }; 
    int i; 

    while (!In.eof()) 
    { 

     if (In.is_open()) 
     { 
      for (i = 0; i < 4; i++) 
      { 
       In >> Data[i]; 
       Code[i] = FindCharacterCode(Data[i]); 
      } 
     } 
     else 
     { 
      cout << "Not open!" << endl << endl; 
      return -1; 
     } 

     Compressed[0] = Code[0] << 2; 
     Compressed[0] = (Compressed[0] | Code[1] >> 4); 

     Compressed[1] = (Code[1] << 4); 
     Compressed[1] = (Compressed[1] | Code[2] >> 2); 

     Compressed[2] = (Code[2] << 6); 
     Compressed[2] = Compressed[2] | Code[3]; 

     for (i = 0; i < 3; i++) 
     { 
       Out << Compressed[i]; 
     } 

    } 

    In.close(); 
    Out.close(); 



    return 0; 
} 

Answer 1

提示：

为了您的压缩算法中的任何给定的输入您将获得1和53之间的数字（姑且称之为0和53）。

我们需要多少位来适合这么大的数字？

1？有一点我们可以存储在0和1之间 - 太小。

2？在2位中，我们可以存储0和3之间的值 - 太小。

...

5？在5位中，我们可以存储0和31之间的值 - 太小。

6？在6位我们可以存储0和63之间 - 足够大。

输入是字符或字节 - 8位。

6/8 * 100 = 75％

这是您的25％压缩。

现在你需要弄清楚如何将4位6位的8位编码成3个字节。

这就是你的位运算符（和一些位移）将进来，