From 191aa9242e4d3547c2bf1e9d1b758bc0a3a4c8ee Mon Sep 17 00:00:00 2001
From: Julian Ospald <hasufell@posteo.de>
Date: Sat, 22 Jan 2022 16:03:43 +0100
Subject: [PATCH] Implement intercalate manually

---
 Data/ByteString/Short/Internal.hs | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/Data/ByteString/Short/Internal.hs b/Data/ByteString/Short/Internal.hs
index af79c7f55..7d1ce0662 100644
--- a/Data/ByteString/Short/Internal.hs
+++ b/Data/ByteString/Short/Internal.hs
@@ -136,7 +136,7 @@ module Data.ByteString.Short.Internal (
     useAsCStringLen,
   ) where
 
-import Data.ByteString.Internal (ByteString(..), accursedUnutterablePerformIO, memchr)
+import Data.ByteString.Internal (ByteString(..), accursedUnutterablePerformIO, memchr, checkedAdd)
 import qualified Data.ByteString.Internal as BS
 
 import Data.Bifunctor   ( first, bimap )
@@ -700,7 +700,24 @@ reverse = \sbs ->
 --
 -- @since 0.11.3.0
 intercalate :: ShortByteString -> [ShortByteString] -> ShortByteString
-intercalate s = concat . List.intersperse s
+intercalate _ [] = mempty
+intercalate _ [x] = x -- This branch exists for laziness, not speed
+intercalate inc (sbs:t) = create totalLen (\mba ->
+                                              let l = length sbs
+                                              in copyByteArray (asBA sbs) 0 mba 0 l >> go mba l t)
+ where
+  go :: MBA s -> Int -> [ShortByteString] -> ST s ()
+  go _ _ [] = pure ()
+  go mba !off (chunk:chunks) = do
+    let lc = length chunk
+    copyByteArray ba 0 mba off lba
+    copyByteArray (asBA chunk) 0 mba (off + lba) lc
+    go mba (off + lc + lba) chunks
+
+  totalLen = List.foldl' (\acc chunk -> acc +! length inc +! length chunk) (length sbs) t
+  (+!) = checkedAdd "intercalate"
+  ba = asBA inc
+  lba = length inc
 {-# INLINE intercalate #-}