我正在尝试以base 64为基础的转换器程序。
我正在尝试以下代码片段:
vector<char> in(3);
std::string out = "abcd"; //four letter garbage value as initializer
ifstream file_ptr(filename.c_str(), ios::in | ios::binary);
unsigned int threebytes = 0;
//Apply the Base 64 encoding algorithm
do {
threebytes = (unsigned int) file_ptr.rdbuf()->sgetn(&in[0], 3);
if (threebytes > 0) {
EncodeBlock(in, out, (int)threebytes); //Apply conversion algorithm to convert 3 bytes into 4
outbuff = outbuff + out; //Append the 4 bytes got from above step to the output
}
} while (threebytes == in.size());
file_ptr.close();
在编写Base64编码算法的编码块中
void EncodeBlock(const std::vector<char>& in, std::string& out, int len) {
using namespace std;
cb64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
out[0] = cb64[(int) (in[0] >> 2)];
out[1] = cb64[(int) (((in[0] << 6) >> 2) | (in[1] >> 4))];
out[2] = (len > 1) ?
cb64[(int) (((in[1] << 4) >> 2) | (in[2] >> 6))] :
'=';
out[3] = (len > 2) ?
cb64[(int) ((in[2] << 2) >> 2)] :
'=';
}
cb64
是一个64位长的字符串,但是通过位操作生成的索引有时不在范围内(0到63)。
为什么!!!
答案 0 :(得分:1)
解决方案是正确处理位操作。
先对char
的8位进行操作,然后将其强制转换为unsigned int
,这会额外增加24位,需要将其设置为0
。
所以,
out[0] = cb64[(unsigned int) ((in[0] >> 2) & 0x003f)];
out[1] = cb64[(unsigned int) ((((in[0] << 6) >> 2) | (in[1] >> 4))) & 0x003f)];
..依此类推处理遮罩