hull.hs

module Hull where

data Direction = L | R | S

instance Show Direction where
    show L = "Left"
    show R = "Right"
    show S = "Straight"

data Point = Point Float Float

slope :: Point -> Point -> Float
slope (Point ax ay) (Point bx by) = (by - ay) / (bx-ax)

crossProduct :: Point -> Point -> Float
crossProduct (Point ax ay) (Point bx by) = (ax * by) - (ay * bx)

turn :: Point -> Point -> Point ->Direction
turn (Point ax ay) (Point bx by) (Point cx cy) =
    let p1 = Point (ax - bx) (ay - by) -- Move dots so that point B is in origion
        p3 = Point (cx - bx) (cy - by)
        c = crossProduct p1 p3
    in case compare c 0 of
        GT -> R
        LT -> L
        _  -> S