is it adjacent square?

How that check was done before:
((xdiff * xdiff == 1) != (ydiff * ydiff == 1))

How it should be done:
(xdiff * xdiff + ydiff * ydiff == 1)
Test case: x = -1; y = 4.

was: [(-1 * -1 == 1) != (4 * 4 == 1)] - true but it's not an adjacent square.

It's easy to check the behavior of multi-square stuff using shutter doors in firetruck.