C++实现Base64编码

可以参考以下博文，写得挺详细的。

https://www.cnblogs.com/zhangmingda/p/14124282.html

大概理解就是：

编码的时候：每3个字节，也就是24个bit分为分配到4个字节中去，充当每个字节的后6个bit位，每个字节的前2个bit都是0。

如过不是3字节的整数倍，差几个字节，就末尾补充几个字节0，最后编码的结果，就替换末尾几个字节为几个字节的'='。

解码的时候：首先看末尾有几个==，最后去掉几个字节的数据即可。

解码就是把4字节重新转换为3字节而已。规则类似。

下面是本人的实现，效率不太高的样子，也就练练手。

仅仅测试1M文件的例子，和php编码、解码一样，就认为没问题了。

1 #include<iostream>
  2 #include<string.h>
  3 #include<map>
  4 #include<time.h>
  5 using namespace std;
  6 char base64str[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
  7 map<unsigned char,unsigned char> m;
  8 void test()
  9 {
 10     unsigned char h = 'h';
 11     unsigned char e = 'e';
 12     unsigned char l = 'l';
 13     char s[10];
 14 
 15     itoa(h,s,2);
 16     cout<<"h = "<<s<<endl;
 17     itoa(e,s,2);
 18     cout<<"e = "<<s<<endl;
 19     itoa(l,s,2);
 20     cout<<"l = "<<s<<endl;
 21     unsigned char tmp = h>>2;
 22     itoa(tmp,s,2);
 23     tmp = (h<<6);
 24     tmp >>=2;
 25     itoa(tmp,s,2);
 26     tmp = ((h<<6)>>2)^(e>>4);
 27     itoa(tmp,s,2);
 28 
 29     cout<<(((h<<6)>>2)==(h<<4))<<endl;
 30 }
 31 void decode(unsigned char str[],int len)
 32 {
 33     int extra = 0;
 34     if(str[len-1]=='=')
 35     {
 36         extra++;
 37         if(str[len-2]=='=')
 38         {
 39             extra++;
 40         }
 41         cout<<"extra = "<<extra<<endl;
 42     }
 43     len-=4;
 44     string s = "";
 45     unsigned char temp[4];
 46     for(int i=0;i<=len;i+=4)
 47     {
 48         memcpy(temp,str+i,4);
 49         unsigned char t;
 50         temp[0] = m[temp[0]];
 51         temp[1] = m[temp[1]];
 52         temp[2] = m[temp[2]];
 53         temp[3] = m[temp[3]];
 54         t = temp[0]<<2;
 55         unsigned char t2;
 56         t2 = temp[1]<<2;
 57         t2>>=6;
 58         t^=t2;
 59         s+=t;
 60 
 61         t = temp[1]<<4;
 62         t2 = temp[2]>>2;
 63         t2<<=4;
 64         t2>>=4;
 65         t^=t2;
 66         s+=t;
 67 
 68         t = temp[2]<<6;
 69         t2 = temp[3]<<2;
 70         t2>>=2;
 71         t^=t2;
 72         s+=t;
 73 
 74     }
 75     FILE* fp;
 76     fp = fopen("C:\\Users\\17724\\Desktop\\10_1\\c++_old","wb");
 77     if(fp==NULL)
 78     {
 79         cout<<"cannot write!"<<endl;
 80         return;
 81     }
 82     fwrite(s.c_str(),1,s.length()-extra,fp);
 83     fclose(fp);
 84 }
 85 void encode(unsigned char str[],int len)
 86 {
 87     cout<<"no"<<endl;
 88     char a[10];
 89     int left = 3-len%3;
 90     cout<<"left = "<<left<<endl;
 91     if(left==0)
 92     {
 93         len-=3;
 94     }
 95     else
 96     {
 97         unsigned char* tmp;
 98         tmp = (unsigned char*)malloc(len+left);
 99         memset(tmp,0,len+left);
100         memcpy(tmp,str,len);
101         str = (unsigned char*)tmp;
102     }
103     string s = "";
104     unsigned char tmp[3];
105     unsigned char t[4];
106     for(int i=0;i<=len;i+=3)
107     {
108         memcpy(tmp,str+i,3);
109 
110         memset(t,0,4);
111         itoa(tmp[0],a,2);
112         itoa(tmp[1],a,2);
113         itoa(tmp[2],a,2);
114         t[0] = tmp[0]>>2;
115         itoa(t[0],a,2);
116 
117         unsigned char f1 = tmp[0]<<6;
118         f1>>=2;
119         t[1] = (f1)^(tmp[1]>>4);
120         itoa(t[1],a,2);
121 
122         f1 = tmp[1]<<4;
123         f1>>=2;
124         t[2] = (f1)^(tmp[2]>>6);
125         itoa(t[2],a,2);
126 
127         f1 = tmp[2]<<2;
128         f1 >>=2;
129         t[3] = f1;
130         itoa(t[3],a,2);
131 
132         t[0] = base64str[t[0]];
133         t[1] = base64str[t[1]];
134         t[2] = base64str[t[2]];
135         t[3] = base64str[t[3]];
136         for(int i=0;i<4;i++)
137         {
138             s+=t[i];
139         }
140     }
141     cout<<s.length()<<endl;
142     for(int i=0;i<left;i++)
143     {
144         s[s.length()-1-i] = '=';
145     }
146     FILE* fp;
147     fp = fopen("C:\\Users\\17724\\Desktop\\10_1\\c++","wb");
148     if(fp==NULL)
149     {
150         cout<<"cannot write!"<<endl;
151         return;
152     }
153     fwrite(s.c_str(),1,s.length(),fp);
154     fclose(fp);
155     cout<<"endl"<<endl;
156 }
157 int main()
158 {
159     for(int i=0;i<64;i++)
160     {
161         m[base64str[i]]=i;
162     }
163 //    unsigned char str[] = "hello";
164     FILE* fp;
165     fp = fopen("C:\\Users\\17724\\Desktop\\10_1\\0.mp4","rb");
166     fseek(fp,0,SEEK_END);
167     int len = ftell(fp);
168     cout<<"len = "<<len<<endl;
169     fseek(fp,0,SEEK_SET);
170     unsigned char *str;
171     str = (unsigned char*)malloc(len);
172     fread(str,1,len,fp);
173     fclose(fp);
174     clock_t t1 = clock();
175     encode(str,len);
176     clock_t t2 = clock();
177     cout<<"use time = "<<(t2-t1)<<endl;
178     return 0;
179 }

原文链接: https://www.cnblogs.com/dayq/p/15361487.html

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