Modular exponentiation in AssemblyScript/Typescript.

powMod.ts

/** 
Modular exponentiation in AssemblyScript/TypeScript. Computes `a^b %n`.
Note that you'll likely need to change the int type. I'm using
the u128 (128-bit unsigned integer) from as-bignum.

@param a The base.
@param b The power.
@param n The modulus.
*/
export function powMod(a: u128, b: u128, n: u128): u128 {
    a = a % n;
    
    let result = 1;
    let x = a;
    
    while(b > 0) {
        let leastSignificantBit = b % 2;
        b = (b / 2) as u128;

        if(leastSignificantBit == 1) {
            result = result * x;
            result = result % n;      
        }

        x = x * x;
        x = x % n;    
    }

    return result;
}