Add simple property tests for Vertex

genstar/Main.hs

@@ -15,14 +15,19 @@ instance Arbitrary Vertex where
     y <- arbitrary
     return (Point (x, y))
 
+-- | 'vertexDistance' @p@
+-- returns the distance of a vertex to the origin
+vertexDistance :: Vertex -> Double
+vertexDistance (Point (x, y)) = sqrt (x ** 2 + y ** 2)
+
 -- | 'fromAngle' @angle@
 -- creates a point on the unit circle at the specified angle
 -- (specified in radians).
 fromAngle angle = Point (sin angle, cos angle)
 
 -- | 'scale' @s@ @p@ scales the vector to @p@ by the factor @s@.
-scale :: Integer -> Vertex -> Vertex
-scale s (Point (x, y)) = Point (fromInteger s * x, fromInteger s * y)
+scale :: Double -> Vertex -> Vertex
+scale s (Point (x, y)) = Point (s * x, s * y)
 
 -- | 'move' @p1@ @p2@ adds the coordinates of @p1@ and @p2@,
 -- effectively moving @p1@ by @p2@ (or vice-versa).
@@ -43,7 +48,7 @@ instance Show PolygonPath where
 -- with the center at @pos@
 -- that is modified in the sense that it takes @rf@,
 -- which is used to set the radius for each individual point.
-regularPolygonMod :: Integer -> (Integer -> Integer) -> Vertex -> PolygonPath
+regularPolygonMod :: Integer -> (Integer -> Double) -> Vertex -> PolygonPath
 regularPolygonMod n rf pos = PolygonPath (map (\step -> move pos $ scale (rf step) $ fromAngle (fromInteger step * 2 * pi / fromInteger n)) [0 .. (n - 1)])
 
 -- | 'regularPolygon' @n@ @r@ @pos@
@@ -51,7 +56,7 @@ regularPolygonMod n rf pos = PolygonPath (map (\step -> move pos $ scale (rf ste
 -- with the radius @r@
 -- and the center at @pos@.
 regularPolygon :: Integer -> Integer -> Vertex -> PolygonPath
-regularPolygon n r = regularPolygonMod n (const r)
+regularPolygon n r = regularPolygonMod n (const (fromInteger r))
 
 -- | 'star' @n@ @r1@ @r2@ @pos@
 -- creates a star
@@ -59,7 +64,7 @@ regularPolygon n r = regularPolygonMod n (const r)
 -- the inner radius @r1@,
 -- the outer radius @r2@
 -- and the center @pos@.
-star :: Integer -> Integer -> Integer -> Vertex -> PolygonPath
+star :: Integer -> Double -> Double -> Vertex -> PolygonPath
 star n r1 r2 = regularPolygonMod (n * 2) ((take (fromInteger n * 2) (cycle [r1, r2]) !!) . fromInteger)
 
 main :: IO ()

genstar/Test.hs

@@ -0,0 +1,33 @@
+{-# LANGUAGE TemplateHaskell #-}
+
+module Test where
+
+import Main (Vertex (Point), fromAngle, move, scale, vertexDistance)
+import Test.QuickCheck (discard, quickCheck, quickCheckAll)
+
+-- | the largest acceptable difference between two doubles so they are considered equal
+epsilon :: Double
+epsilon = 1e-10
+
+-- | checks whether the distance of a vertex constructed by 'fromAngle' to the origin is 1
+prop_VertexFromAngleDistance :: Double -> Bool
+prop_VertexFromAngleDistance angle = abs (1 - vertexDistance (fromAngle angle)) < epsilon
+
+-- | checks whether the distance of a vertex scaled by an factor is multiplied by that factor (absolute).
+prop_VertexScaleDistance :: (Vertex, Double) -> Bool
+prop_VertexScaleDistance (p, s) = abs (abs s * vertexDistance p - vertexDistance (scale s p)) < epsilon
+
+-- | checks whether moving a vertex by itself equals scaling it by 2
+prop_MoveSelfEqualsScale2 :: Vertex -> Bool
+prop_MoveSelfEqualsScale2 p = move p p == scale 2 p
+
+-- | checks whether moving a vertex by itself scaled by -1 equals the origin
+prop_MoveNegativeSelfEqualsOrigin :: Vertex -> Bool
+prop_MoveNegativeSelfEqualsOrigin p = move p (scale (-1) p) == Point (0, 0)
+
+return []
+
+check = $quickCheckAll
+
+main :: IO Bool
+main = check