From 368ebf9f96e4fded6b69ca609f9db53eb19b4589 Mon Sep 17 00:00:00 2001
From: Justin Bedo <cu@cua0.org>
Date: Fri, 27 Jan 2023 17:00:48 +1100
Subject: rewrite tree mutation function to avoid leaking memory

---
 src/PPL/Internal.hs | 29 +++++++++++------------------
 src/PPL/Sampling.hs | 21 ++++++++++-----------
 2 files changed, 21 insertions(+), 29 deletions(-)

diff --git a/src/PPL/Internal.hs b/src/PPL/Internal.hs
index b7d49ee..0ba6ae5 100644
--- a/src/PPL/Internal.hs
+++ b/src/PPL/Internal.hs
@@ -13,6 +13,8 @@ module PPL.Internal
     randomTree,
     samples,
     mutateTree,
+    splitTrees,
+    draw,
   )
 where
 
@@ -29,7 +31,10 @@ import qualified System.Random as R
 
 -- Reimplementation of the LazyPPL monads to avoid some dependencies
 
-data Tree = Tree !Double [Tree]
+data Tree = Tree
+  { draw :: !Double,
+    splitTrees :: [Tree]
+  }
 
 split :: Tree -> (Tree, Tree)
 split (Tree r (t : ts)) = (t, Tree r ts)
@@ -41,23 +46,11 @@ randomTree g = let (a, g') = random g in Tree a (randomTrees g')
     randomTrees g = let (g1, g2) = R.split g in randomTree g1 : randomTrees g2
 
 {-# INLINE mutateTree #-}
-mutateTree :: RandomGen g => Double -> Double -> g -> Tree -> Tree
-mutateTree p q g (Tree a ts) =
-  let (r, g1) = random g
-      (b, g2) = random g1
-   in Tree
-        ( if r >= p
-            then a
-            else
-              if r < p * q
-                then 1 - a
-                else b
-        )
-        (mutateTrees p q g2 ts)
-  where
-    mutateTrees p q g (t : ts) =
-      let (g1, g2) = R.split g
-       in mutateTree p q g1 t : mutateTrees p q g2 ts
+mutateTree :: Double -> Tree -> Tree -> Tree -> Tree
+mutateTree p (Tree r rs) b@(Tree _ bs) (Tree a ts) =
+  if r < p
+    then b
+    else Tree a $ zipWith3 (mutateTree p) rs bs ts
 
 newtype Prob a = Prob {runProb :: Tree -> a}
 
diff --git a/src/PPL/Sampling.hs b/src/PPL/Sampling.hs
index ad11837..84e5ada 100644
--- a/src/PPL/Sampling.hs
+++ b/src/PPL/Sampling.hs
@@ -9,26 +9,25 @@ import Data.Bifunctor
 import Data.Monoid
 import Numeric.Log
 import PPL.Distr
-import PPL.Internal hiding (split)
-import System.Random (StdGen, random, randoms, split)
+import PPL.Internal
+import System.Random (StdGen, random, randoms)
 import qualified Streaming as S
 import Streaming.Prelude (Stream, yield, Of)
 
-mh :: Monad m => StdGen -> Double -> Double -> Meas a -> Stream (Of (a, Log Double)) m ()
-mh g p q m = step g2 t x w
+mh :: Monad m => StdGen -> Double -> Meas a -> Stream (Of (a, Log Double)) m ()
+mh g p m = step t0 t x w
   where
-    t = randomTree g1
-    (g1,g2) = split g
+    (t0,t) = split $ randomTree g
     (x, w) = head $ samples m t
 
-    step !g !t !x !w = do
-      let (g1, g2) = split g
-          t' = mutateTree p q g1 t
+    step !t0 !t !x !w = do
+      let (t1:t2:t3:t4:_) = splitTrees t0
+          t' = mutateTree p t1 t2 t
           (x', w') = head $ samples m t'
           ratio = w' / w
-          (Exp . log -> r, g3) = random g2
+          (Exp . log -> r) = draw t3
           (t'', x'', w'') = if r < ratio
             then (t', x', w')
             else (t, x, w)
       yield (x'', w'')
-      step g3 t'' x'' w''
+      step t4 t'' x'' w''
-- 
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