OptionalComparator.swift

struct OptionalComparator<T: Comparable>: SortComparator {
    typealias Compared = T?
    var order: SortOrder = .forward

    func compare(_ lhs: T?, _ rhs: T?) -> ComparisonResult {
        return switch (lhs, rhs) {
        case (_, nil):
            order == .forward ? .orderedDescending : .orderedAscending
        default:
            order == .forward ? .orderedAscending : .orderedDescending
        }
    }
}