Use level-wise algorithm for BF unfolds

`unfoldTreeM_BF` and `unfoldForestM_BF` previously used a
queue-based algorithm from a note by Okasaki. That same
note indicates that the level-wise approach tends to be slightly
faster, and my own informal tests suggest the same. Both solutions
suffer from a potential space leak, retaining the seed lists
longer than necessary in order to later use their lengths. It
remains to be seen whether this leak can be plugged without
a speed penalty.

Author: treeowl

Data/Tree.hs

@@ -34,18 +34,14 @@ module Data.Tree(
     unfoldTreeM_BF, unfoldForestM_BF,
     ) where
 
-#if MIN_VERSION_base(4,8,0)
-import Data.Foldable (toList)
-#else
+#if !(MIN_VERSION_base(4,8,0))
 import Control.Applicative (Applicative(..), (<$>))
-import Data.Foldable (Foldable(foldMap), toList)
+import Data.Foldable (Foldable(foldMap, toList))
 import Data.Monoid (Monoid(..))
 import Data.Traversable (Traversable(traverse))
 #endif
 
 import Control.Monad (liftM)
-import Data.Sequence (Seq, empty, singleton, (<|), (|>), fromList,
-            ViewL(..), ViewR(..), viewl, viewr)
 import Data.Typeable
 import Control.DeepSeq (NFData(rnf))
 
@@ -163,37 +159,23 @@ unfoldForestM :: Monad m => (b -> m (a, [b])) -> [b] -> m (Forest a)
 #endif
 unfoldForestM f = Prelude.mapM (unfoldTreeM f)
 
--- | Monadic tree builder, in breadth-first order,
--- using an algorithm adapted from
--- /Breadth-First Numbering: Lessons from a Small Exercise in Algorithm Design/,
--- by Chris Okasaki, /ICFP'00/.
+-- | Monadic tree builder, in breadth-first order.
 unfoldTreeM_BF :: Monad m => (b -> m (a, [b])) -> b -> m (Tree a)
-unfoldTreeM_BF f b = liftM getElement $ unfoldForestQ f (singleton b)
-  where
-    getElement xs = case viewl xs of
-        x :< _ -> x
-        EmptyL -> error "unfoldTreeM_BF"
-
--- | Monadic forest builder, in breadth-first order,
--- using an algorithm adapted from
--- /Breadth-First Numbering: Lessons from a Small Exercise in Algorithm Design/,
--- by Chris Okasaki, /ICFP'00/.
-unfoldForestM_BF :: Monad m => (b -> m (a, [b])) -> [b] -> m (Forest a)
-unfoldForestM_BF f = liftM toList . unfoldForestQ f . fromList
-
--- takes a sequence (queue) of seeds
--- produces a sequence (reversed queue) of trees of the same length
-unfoldForestQ :: Monad m => (b -> m (a, [b])) -> Seq b -> m (Seq (Tree a))
-unfoldForestQ f aQ = case viewl aQ of
-    EmptyL -> return empty
-    a :< aQ' -> do
-        (b, as) <- f a
-        tQ <- unfoldForestQ f (Prelude.foldl (|>) aQ' as)
-        let (tQ', ts) = splitOnto [] as tQ
-        return (Node b ts <| tQ')
+unfoldTreeM_BF f b0 = do
+  (a, bs) <- f b0
+  Node a `liftM` unfoldForestM_BF f bs
+
+-- | Monadic forest builder, in breadth-first order.
+unfoldForestM_BF :: Monad m
+                 => (b -> m (a, [b])) -> [b] -> m (Forest a)
+unfoldForestM_BF _f [] = return []
+unfoldForestM_BF f bs = do
+  asbss' <- mapM f bs
+  rebuild asbss' `liftM` unfoldForestM_BF f (concatMap snd asbss')
+
   where
-    splitOnto :: [a'] -> [b'] -> Seq a' -> (Seq a', [a'])
-    splitOnto as [] q = (q, as)
-    splitOnto as (_:bs) q = case viewr q of
-        q' :> a -> splitOnto (a:as) bs q'
-        EmptyR -> error "unfoldForestQ"
+    rebuild :: [(a, [any])] -> [Tree a] -> [Tree a]
+    rebuild [] ts = ts
+    rebuild ((a, bs') : xs) ts =
+      case splitAt (length bs') ts of
+        (us, ts') -> Node a us : rebuild xs ts'