Find the next Even integer

Ask whether next even means the smallest even integer greater than x, or the smallest even integer greater than or equal to x. For the strict interpretation, return x plus two when x is even and x plus one when x is odd; for non-strict, return x when it is already even. In languages where modulo of negative values can be negative, parity is still determined by x modulo two being zero, but write conditions clearly rather than relying on a positive-only assumption. If x can equal the maximum representable integer, adding one or two may overflow; either use a wider type, return an error, or define wraparound only if the specification explicitly says so. Test zero, negative odd and even values, and the largest safe value. This is O(1) time and O(1) space. Do not overengineer it with loops, but do demonstrate that you noticed the API semantics hidden in the word next.