From 6e8c2b8df3898d0f50463f3151831f5d7c9306a2 Mon Sep 17 00:00:00 2001
From: Tom Smeding <tom@tomsmeding.com>
Date: Sun, 2 Aug 2026 20:19:10 +0100
Subject: Index logs in parallel

---
 src/Parallel.hs | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)
 create mode 100644 src/Parallel.hs

(limited to 'src/Parallel.hs')

diff --git a/src/Parallel.hs b/src/Parallel.hs
new file mode 100644
index 0000000..cc6eb22
--- /dev/null
+++ b/src/Parallel.hs
@@ -0,0 +1,30 @@
+module Parallel where
+
+import Control.Concurrent
+import Control.Monad (replicateM_)
+import Data.IORef
+
+
+-- | Does not return results in-order.
+parallelForM :: [a] -> (a -> IO b) -> IO [b]
+parallelForM inputList action = do
+  nthread <- getNumCapabilities
+
+  listref <- newIORef inputList
+  outref <- newIORef []
+  donechan <- newChan
+  replicateM_ nthread $ forkIO $
+    let loop = do
+          mitem <- atomicModifyIORef' listref (\case l@[] -> (l, Nothing)
+                                                     item : l -> (l, Just item))
+          case mitem of
+            Just item -> do
+              res <- action item
+              atomicModifyIORef' outref (\l -> (res : l, ()))
+              loop
+            Nothing -> do
+              writeChan donechan ()
+    in loop
+
+  replicateM_ nthread $ readChan donechan
+  reverse <$> readIORef outref
-- 
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