From b36bdaa076c4429d584def69486a33eac0db8fee Mon Sep 17 00:00:00 2001
From: Simon Bruder <simon@sbruder.de>
Date: Tue, 27 Dec 2022 20:02:46 +0100
Subject: [PATCH] Add simple property tests for Vertex

---
 genstar/Main.hs |  5 +++++
 genstar/Test.hs | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+)
 create mode 100644 genstar/Test.hs

diff --git a/genstar/Main.hs b/genstar/Main.hs
index 114f935..799dc8b 100644
--- a/genstar/Main.hs
+++ b/genstar/Main.hs
@@ -15,6 +15,11 @@ 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).
diff --git a/genstar/Test.hs b/genstar/Test.hs
new file mode 100644
index 0000000..ffa8f8f
--- /dev/null
+++ b/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