From 7653e357f04aa39c1e96037bf1ea2e4338f8ae76 Mon Sep 17 00:00:00 2001
From: Justin Bedo <cu@cua0.org>
Date: Tue, 24 Jan 2023 16:41:53 +1100
Subject: reimplement sampler in streaming framework

---
 package.yaml        |  1 +
 ppl.cabal           |  1 +
 src/PPL/Sampling.hs | 48 +++++++++++++++---------------------------------
 3 files changed, 17 insertions(+), 33 deletions(-)

diff --git a/package.yaml b/package.yaml
index 7c3a3e8..703c5a2 100644
--- a/package.yaml
+++ b/package.yaml
@@ -11,6 +11,7 @@ dependencies:
   - random
   - template-haskell
   - containers
+  - streaming
 
 library:
   source-dirs: src
diff --git a/ppl.cabal b/ppl.cabal
index b3b5533..5d8db40 100644
--- a/ppl.cabal
+++ b/ppl.cabal
@@ -26,6 +26,7 @@ library
     , containers
     , log-domain
     , random
+    , streaming
     , template-haskell
     , transformers
   default-language: Haskell2010
diff --git a/src/PPL/Sampling.hs b/src/PPL/Sampling.hs
index b3b8a90..ad11837 100644
--- a/src/PPL/Sampling.hs
+++ b/src/PPL/Sampling.hs
@@ -1,4 +1,5 @@
 {-# LANGUAGE ViewPatterns #-}
+{-# LANGUAGE BangPatterns #-}
 
 module PPL.Sampling where
 
@@ -9,44 +10,25 @@ import Data.Monoid
 import Numeric.Log
 import PPL.Distr
 import PPL.Internal hiding (split)
-import System.Random (getStdGen, newStdGen, random, randoms, split)
+import System.Random (StdGen, random, randoms, split)
+import qualified Streaming as S
+import Streaming.Prelude (Stream, yield, Of)
 
-importance :: MonadIO m => Int -> Meas a -> m [a]
-importance n m = do
-  newStdGen
-  g <- getStdGen
-  let ys = take n $ accumulate xs
-      max = snd $ last ys
-      xs = samples m $ randomTree g1
-      (g1, g2) = split g
-  let rs = randoms g2
-  pure $ flip map rs $ \r -> fst . head $ flip filter ys $ \(x, w) -> w >= Exp (log r) * max
+mh :: Monad m => StdGen -> Double -> Double -> Meas a -> Stream (Of (a, Log Double)) m ()
+mh g p q m = step g2 t x w
   where
-    cumsum = tail . scanl (+) 0
-    accumulate = uncurry zip . second cumsum . unzip
+    t = randomTree g1
+    (g1,g2) = split g
+    (x, w) = head $ samples m t
 
-mh :: MonadIO m => Double -> Double -> Meas a -> m [(a, Log Double)]
-mh p q m = do
-  newStdGen
-  g <- getStdGen
-  let (g1, g2) = split g
-      (x, w) = head $ samples m t
-      t = randomTree g1
-  pure $ map (\(_, x, w) -> (x, w)) $ evalState (iterateM step (t, x, w)) g2
-  where
-    step (t, x, w) = do
-      g <- get
+    step !g !t !x !w = do
       let (g1, g2) = split g
           t' = mutateTree p q g1 t
           (x', w') = head $ samples m t'
           ratio = w' / w
           (Exp . log -> r, g3) = random g2
-      put g3
-      pure $!
-        if r < ratio
-          then (t', x', w')
-          else (t, x, w)
-
-    iterateM f x = do
-      y <- f x
-      (y :) <$> iterateM f y
+          (t'', x'', w'') = if r < ratio
+            then (t', x', w')
+            else (t, x, w)
+      yield (x'', w'')
+      step g3 t'' x'' w''
-- 
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