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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.         a <<= 1;
  16.     }
  17.     return d;
  18. }
  19.  
  20. int main() {
  21. 	unsigned long long a = 3, b = 0x7FFFFFFFFFFFFFFFULL, m = 0xFFFFFFFFFFFFFFFFULL;
  22. 	unsigned long long result = mul_mod(a, b, m);
  23. 	printf("%llX * %llX mod %llX = %llX\n", a, b, m, result);
  24. 	return 0;
  25. }
  26.  
Success #stdin #stdout 0s 5528KB
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Standard input is empty
stdout
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3 * 7FFFFFFFFFFFFFFF mod FFFFFFFFFFFFFFFF = 7FFFFFFFFFFFFFFD
https://ideone.com/R3TGoE
language:
C++ (gcc 8.3)
created:
4 years ago
visibility:
public

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