/**

Do the inverse of a positive action

?- notsamedirection(-1, 2, -1, 8, -1, 1, -1, 4, Result).

Result = 1 ;

No
?- notsamedirection(-1, 2, -1, 8, 1, 1, 1, 4, Result).

Result = 0 ;

notsamedirection(SignWindX, WindX, SignWindY, WindY, SignKiteX, KiteX, SignKiteY, KiteY, Result)
Returns a result of 0 if the kite on the ground is pointing in the same direction as the wind.  In the following, the wind vector is the direction a √2 m-length flag is pointing, the x-component (the distance of the flag in the x-direction) is equal to 1 m, and the y-component (the distance of the flag in the y-direction) is equal to 1 m. The sign of a value is +1 if it is in the positive half of the x or y axis and -1 if it is in the negative half of the x or y axis.

SignWindX - Sign of the wind vector's X component (1 or -1)
WindX - Wind vectors's X component
SignWindY - Sign of the wind vector's Y component (1 or -1)
WindY - Wind vector's Y component
SignKiteX - Sign of the kite vector's X component (1 or -1)
KiteX - Kite vector's X component
SignKiteY - Sign of the Kite vector's Y component (1 or -1)
KiteY - Kite vector's Y component
Result - 0 if the kite is pointing in the same direction as the wind

**/

notsamedirection(SignWindX, WindX, SignWindY, WindY, SignKiteX, KiteX, SignKiteY, KiteY, 0) :-
        not(samedirection(SignWindX, WindX, SignWindY, WindY, SignKiteX, KiteX, SignKiteY, KiteY)).
notsamedirection(SignWindX, WindX, SignWindY, WindY, SignKiteX, KiteX, SignKiteY, KiteY, 1) :-
        samedirection(SignWindX, WindX, SignWindY, WindY, SignKiteX, KiteX, SignKiteY, KiteY).

/**

samedirection(SignWindX, WindX, SignWindY, WindY, SignKiteX, KiteX, SignKiteY, KiteY
checks whether the kite is pointing in the same direction as the wind.
Variables - same as some of those for notsamedirection/9.

**/

samedirection(SignX, 0, SignY, _WindY, SignX, 0, SignY, _KiteY) :- !.
samedirection(SignX, WindX, SignY, WindY, SignX, KiteX, SignY, KiteY) :-
        WindGradient is (SignY * WindY) / (SignX * WindX),
        KiteGradient is (SignY * KiteY) / (SignX * KiteX),
        WindGradient is KiteGradient.