enum AllowedDirection {
    AllowedUp,
    AllowedDown
};

char cur = ...

if( (cur == StateA && prev == StateAB || cur == StateAB && prev == StateB) && allowed == AllowedDown ) {
    down();
    allowed = AllowedUp;
} else if( (cur == StateB && prev == StateAB || cur == StateAB && prev == StateA) && allowed == AllowedUp ) {
    up();
    allowed = AllowedDown;
)

if( cur == StateA )
    allowed = AllowedUp;
else if( cur == StateB )
    allowed = AllowedDown;

prev = cur;