Prime.sol

contract Prime {

    function isPrime(uint256 n) public view returns (bool){
        if (n < 2) {
            return false;
        }
        if (n == 2) {
            return true;
        }
        return expmod(2, n - 1, n) == 1;
    }

    // use precompile expmod to calculate modular exponentiation
    function expmod(uint256 base, uint256 e, uint256 m) internal view returns (uint o) {

        assembly {
        // define pointer
            let p := mload(0x40)
        // store data assembly-favouring ways
            mstore(p, 0x20)             // Length of Base
            mstore(add(p, 0x20), 0x20)  // Length of Exponent
            mstore(add(p, 0x40), 0x20)  // Length of Modulus
            mstore(add(p, 0x60), base)  // Base
            mstore(add(p, 0x80), e)     // Exponent
            mstore(add(p, 0xa0), m)     // Modulus
            if iszero(staticcall(sub(gas, 2000), 0x05, p, 0xc0, p, 0x20)) {
                revert(0, 0)
            }
        // data
            o := mload(p)
        }}

}

@eucliss
Call 0x05 precompile (modular exponentation) w/ the following Args.
This is precompiled contract for ModeExp (https://www.rareskills.io/post/solidity-precompiles)

         *      args and revert on failure.
         
         *      Args:
         *      gas,
         *      precomipled contract address,
         *      memory pointer of begin of calldata,
         *      size of call data (callDataSize),
         *      pointer for where to copy return,
         *      size of return data
         */