我正在尝试使用 Wave 到 Base 64 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();
在编码块中写入Base64编码算法
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长度的长字符串,但通过位操作生成的索引有时会超出范围(0到63)。
为什么!!!
解决这个问题的方法是正确处理位操作。
对
char
8 位进行操作,然后转换为 unsigned int
引入了额外的 24 位,需要设置为 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)];
..等等处理遮罩