codebook/mod.cc at master · johnathan79717/codebook · GitHub

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#define MOD 1000000007
inline int add(int a, int b, int mod=MOD) { return a + b < mod ? a + b : a + b - mod; }
inline void addBy(int &a, int b, int mod=MOD) { a = add(a, b, mod); }
inline int sub(int a, int b, int mod=MOD) { return a - b < 0 ? a - b + mod : a - b; }
inline void subBy(int &a, int b, int mod=MOD) { a = sub(a, b, mod); }
inline int mul(int a, int b, int mod=MOD) { return (LL)a * b % mod; }
inline void mulBy(int &a, int b, int mod=MOD) { a = mul(a, b, mod); }

/* a*x ≡ 1 (mod m) */
int inverse(int a, int mod = MOD)
{
    int d = mod, x = 0, s = 1, q, r, t;
    while (a != 0)
    {
        q = d / a, r = d % a;
        d = a, a = r;
        t = x - q * s, x = s, s = t;
    }
    if (d != 1) return -1;
    return (x + mod) % mod;
}
inline int divide(int a, int b, int mod=MOD) { return mul(a, inverse(b, mod), mod); }

LL mul(LL a,LL b,LL mod=MOD){
  LL x = 0,y=a%mod;
  while(b > 0){
    if(b%2 == 1){
      x = x+y;
      if(x >= mod) x -= mod;
    }
    y = y*2;
    if(y >= mod) y -= mod;
    b /= 2;
  }
  return x%mod;
}

template<class T>
T pow(T a, T b, T c=MOD){
  T x=1,y=a; // ll is taken to avoid overflow of intermediate results
  while(b > 0){
    if(b%2 == 1){
      x=mul(x, y, c);
    }
    y = mul(y, y, c); // squaring the base
    b /= 2;
  }
  return x%c;
}

vector<int> factorial(1, 1);

int fact(int n) {
  while(factorial.size() <= n)
    factorial.push_back(mul(factorial.back(), factorial.size()));
  return factorial[n];
}

int choose(int n, int k) {
  if(n < k) return 0;
  return divide(fact(n), mul(fact(k), fact(n-k)));
}