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  1. #include <cstdio>
  2.  
  3. using namespace std;
  4.  
  5. unsigned long long mul_mod(unsigned long long a, unsigned long long b, unsigned long long m) {
  6.     unsigned long long d = 0, mp2 = m >> 1;
  7.     if (a >= m) a %= m;
  8.     if (b >= m) b %= m;
  9.     for (int i = 0; i < 64; i++)
  10.     {
  11.         d = (d > mp2) ? (d << 1) - m : d << 1;
  12.         // if (a & 0x8000000000000000ULL)
  13.         //     d += b;
  14.         // if (d >= m) d -= m;
  15.         unsigned long long x = -(a >> 63) & b;
  16.         if (d >= m - x) d -= m;
  17.         d += x;
  18.         a <<= 1;
  19.     }
  20.     return d;
  21. }
  22.  
  23. int main() {
  24. 	unsigned long long a = 3, b = 0x7FFFFFFFFFFFFFFFULL, m = 0xFFFFFFFFFFFFFFFFULL;
  25. 	unsigned long long result = mul_mod(a, b, m);
  26. 	printf("%llX * %llX mod %llX = %llX\n", a, b, m, result);
  27. 	return 0;
  28. }
  29.  
Success #stdin #stdout 0s 5648KB
comments (?)
stdin 
Standard input is empty
stdout 
3 * 7FFFFFFFFFFFFFFF mod FFFFFFFFFFFFFFFF = 7FFFFFFFFFFFFFFE
https://ideone.com/ezRtCi
language:
C++ (gcc 8.3)
created:
4 years ago
visibility:
public

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