clockwise

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Chapter 3 - Excercices -10 11 12

Hi guys,

I have found a trouble. For u in function directOf when sing > 0 return RIGHT.
This are extract from Cross Product from http://en.wikipedia.org/wiki/Graham_scan and in this case when sign > 0 you must return LEFT. For understand it better you can draw a three points, after that join P3 and P1 if u are doing the same direction that a clock you are going to Right, else you are going to Left (less when u cant build the triangle). Then the function should be:

directionOf a v b
| sign > 0 = LEFT
| sign < 0 = RIGHT
| otherwise = STRAIGHT
where sign = (x v - x a) * (y b - y a) - (y v - y a) * (x b - x a)