From 9917c402e96a501aebbdd5bcdfcefa4bd25a6602 Mon Sep 17 00:00:00 2001
From: Matthew Leon <ml@matthewleon.com>
Date: Mon, 6 Nov 2017 17:48:38 -0500
Subject: [PATCH] more efficient genBoundedEnum

Should reduce time and space complexity to that of chooseInt for most
BoundedEnum instances.
---
 src/Data/Enum/Gen.purs | 25 ++++++++++++++++---------
 1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/src/Data/Enum/Gen.purs b/src/Data/Enum/Gen.purs
index 86caebd..5e3acc8 100644
--- a/src/Data/Enum/Gen.purs
+++ b/src/Data/Enum/Gen.purs
@@ -2,17 +2,24 @@ module Data.Enum.Gen where
 
 import Prelude
 
-import Control.Monad.Gen (class MonadGen, elements)
-import Data.Enum (class BoundedEnum, succ, enumFromTo)
-import Data.Maybe (Maybe(..))
+import Control.Monad.Gen (class MonadGen, chooseInt, elements)
+import Data.Enum (class BoundedEnum, Cardinality, cardinality, enumFromTo, fromEnum, succ, toEnum)
+import Data.Maybe (Maybe(..), fromJust)
+import Data.Newtype (unwrap)
 import Data.NonEmpty ((:|))
+import Partial.Unsafe (unsafePartial)
 
 -- | Create a random generator for a finite enumeration.
 genBoundedEnum :: forall m a. MonadGen m => BoundedEnum a => m a
 genBoundedEnum =
-  case succ bottom of
-    Just a →
-      let possibilities = enumFromTo a top :: Array a
-      in elements (bottom :| possibilities)
-    Nothing →
-      pure bottom
+  let topInt = fromEnum (top :: a)
+      bottomInt = fromEnum (bottom :: a)
+      enumRange = topInt - bottomInt
+  in if enumRange == unwrap (cardinality :: Cardinality a)
+       then unsafePartial $ fromJust <<< toEnum <$> chooseInt bottomInt topInt
+       else case succ bottom of
+         Just a →
+           let possibilities = enumFromTo a top :: Array a
+           in elements (bottom :| possibilities)
+         Nothing →
+           pure bottom