Implement shuffle

CCard.hs

@@ -31,7 +31,7 @@ data Card =
     | Maple0 | Maple1 | MapleBlue | Deer
     | Lightning | WillowRed | Swallow | RainMan
     | Paulownia0 | Paulownia1 | Sand | Phoenix
-  deriving (Eq, Ord, Enum)
+  deriving (Eq, Ord, Enum, Show)
 
 type Pack = Word64
 

KoiKoi.hs

@@ -2,6 +2,7 @@ module KoiKoi where
 
 import CCard
 import Data.Bits (popCount, (.&.))
+import System.Random (randomRIO)
 
 type Points = Int
 
@@ -55,3 +56,19 @@ yakus = [
       | n > 0 = Just (yaku, n)
       | otherwise = Nothing
     is set yaku pack = if set == pack then Just (yaku, 5) else Nothing
+
+shuffle :: [a] -> IO [a]
+shuffle l =
+  aux (length l) l
+  where
+    aux n [] = return []
+    aux n [a] = return [a]
+    aux n l = do
+      cut <- randomRIO (0, n - 1)
+      let (top, bottom) = splitAt cut l
+      shuffledTop <- aux cut top
+      shuffledBottom <- aux (n - cut) bottom
+      reorder <- randomRIO (False, True)
+      return $ if reorder
+        then shuffledTop ++ shuffledBottom
+        else shuffledBottom ++ shuffledTop