Finding binomial coefficient for large n and k modulo m

The binominal coefficient of (n, k) is calculated by the formula:

(n, k) = n! / k! / (n - k)!

To make this work for large numbers n and k modulo m observe that:

1. Factorial of a number modulo m can be calculated step-by-step, in each step taking the result % m. However, this will be far too slow with n up to 10^18. So there are faster methods where the complexity is bounded by the modulo, and you can use some of those.

2. The division (a / b) mod m is equal to (a * b^-1) mod m, where b^-1 is the inverse of b modulo m (that is, (b * b^-1 = 1) mod m).

This means that:

(n, k) mod m = (n! * (k!)^-1 * ((n - k)!)^-1) mod m

The inverse of a number can be efficiently found using the Extended Euclidean algorithm. Assuming you have the factorial calculation sorted out, the rest of the algorithm is straightforward, just watch out for integer overflows on multiplication. Here's reference code that works up to n=10^9. To handle for larger numbers the factorial computation should be replaced with a more efficient algorithm and the code should be slightly adapted to avoid integer overflows, but the main idea will remain the same:

#define MOD 1000000007

// Extended Euclidean algorithm
int xGCD(int a, int b, int &x, int &y) {
    if (b == 0) {
        x = 1;
        y = 0;
        return a;
    }

    int x1, y1, gcd = xGCD(b, a % b, x1, y1);
    x = y1;
    y = x1 - (long long)(a / b) * y1;
    return gcd;
}

// factorial of n modulo MOD
int modfact(int n) {
    int result = 1;
    while (n > 1) {
        result = (long long)result * n % MOD;
        n -= 1;
    }
    return result;
}

// multiply a and b modulo MOD
int modmult(int a, int b) {
    return (long long)a * b % MOD;
}

// inverse of a modulo MOD
int inverse(int a) {
    int x, y;
    xGCD(a, MOD, x, y);
    return x;
}

// binomial coefficient nCk modulo MOD
int bc(int n, int k)
{
    return modmult(modmult(modfact(n), inverse(modfact(k))), inverse(modfact(n - k)));
}

First, you don't need to pre-compute and store all the possible aCb values! they can be computed per case.

Second, for the special case when (k < m) and (n < m^2), the Lucas theorem easily reduces to the following result:

(n choose k) mod m = ((n mod m) choose k) mod m

then since (n mod m) < 10^9+7 you can simply use the code proposed by @kfx.

Just use the fact that

(n, k) = n! / k! / (n - k)! = n*(n-1)*...*(n-k+1)/[k*(k-1)*...*1]

so you actually have just 2*k=2*10^5 factors. For the inverse of a number you can use suggestion of kfx since your m is prime.