how to check if bytes32 begins with certain bytes?

Question

how do i gas-efficiently check if a bytes32 begins with certain bytes?

the user gives me a bytes32 input, and i want to check if it start with 4 zero bytes.

thank you

Answer 1

Testing on remix the cheapest function I got uses 352 gas for execution

function g0(bytes32 x, bytes4 y) public pure returns (bool) {
    return bytes4(x) == y;
}

Answer 2

I can't speak to gas efficiency, unfortunately.

From the Solidity docs:

Index access: If x is of type bytesI, then x[k] for 0 <= k < I returns the k th byte (read-only).

So:

function fourLeadingZeroBytes(bytes32 b) public pure returns (bool) {
    return b[0] == 0 && b[1] == 0 && b[2] == 0 && b[3] == 0;
}