大疆hexToBase64编码转换，笔试题，2022-8-21

#include
#include
#include
using namespace std;int numToBase64(int num) { //按数字转为Base64编码char c;if (num >= 62) {if (num == 62) {c = '+';}else {c = '/';}}else if (0 <= num && num <= 25) {c = num + 'A';}else if (26 <= num && num <= 51) {c = num - 26 + 'a';}else {c = num - 52 + '0';}return c;
}void printB(int num) {  //显示二进制数int i = 0b10000000;while (i) {if (num & i) {cout << '1';}else {cout << '0';}i = i >> 1;}cout << endl;
}int main() {string input = "254ASDF1";string res;int n = input.size();int pre = 0;int cur = 0;for (int i = 0; i < n; ++i) {cur = input[i];printB(cur);if (i % 3 == 0) {pre = cur & 3;  //剩2位cur = (cur & 0b11111100) >> 2; // 用6位res.push_back(numToBase64(cur));}else if (i % 3 == 1) {//cur = pre * 16 + cur / 16;cur = (pre << 4) + (cur >> 4);  //需要注意的问题：左移和右移运算优先级低于+-符号的运算，因此此处会先计算加法res.push_back(numToBase64(cur)); //用4位pre = input[i] & 0b00001111; //剩4位}else {cur = pre * 4 + cur / 64;  //用2位pre = input[i] & 0b00111111; //剩6位res.push_back(numToBase64(cur));  cur = pre; //用6位pre = 0;res.push_back(numToBase64(cur)); }cout << "输出:" << cur << "    ";printB(cur);cout << "保留:" << pre << "    ";printB(pre);}if (n % 3 == 1) {  // 只剩一个字节，上面先处理了1个base64数，还差3个cur = pre * 16;pre = 0;res.push_back(numToBase64(cur));res.push_back('=');res.push_back('=');}else if(n % 3 == 2) {  // 只剩2个字节，上面处理了2个base64数，还差2个cur = pre * 4;pre = 0;res.push_back(numToBase64(cur));res.push_back('=');}cout << res <<endl;//下面为测试运算符优先级问题int temp = 1;int temp2 = 4;//int temp3 = temp << 1 + temp2 >> 1;   // 先计算1+temp2 = 5， temp左移5位，再右移1位；cout << temp3;return 0;
}

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