C++ Tricks

实现类似Python的Range

#define crange(fir, ...) std::views::iota(0  __VA_OPT__(+(fir)), __VA_OPT__(__VA_ARGS__ - (fir) +)  (fir))

const auto j = 10;
for (auto i : crange(3, j))
{
    // ok
}

Gosper's Hack

生成C_n^k个n元集合所有k元子集的算法.

void GospersHack(int k, int n)
{
    int cur = (1 << k) - 1;
    int limit = (1 << n);
    while (cur < limit)
    {
        // do something
        int lb = cur & -cur;
        int r = cur + lb;
        cur = ((r ^ cur) >> __builtin_ctz(lb) + 2) | r;
        // 或：cur = (((r ^ cur) >> 2) / lb) | r;
    }
}

// void GospersHack(3, 5)
// 00111
// 01011
// 01101
// 01110
// 10011
// 10101
// 10110
// 11001
// 11010
// 11100

Kahan Sum

用以低损失的求一列32位浮点数的和

float kahanSum(vector<float> nums) {
  float sum = 0.0f;
  float c = 0.0f;
  for (auto num : nums) {
    float y = num - c;
    float t = sum + y;
    c = (t - sum) - y;
    sum = t;
  }
  return sum;
}

简单求某个值在结构体的偏移位

#define OFFSET(STRU, VAR) (unsigned)((void *)(&(((STRU*)(0x8))->VAR)) - 0x8)
#include <cstdio>
int main()
{
    struct alignas(8) stu{
        long a;
        unsigned x;
        char t[3];
    };
    printf("%d", OFFSET(stu, t[2]));
}