simplex-chat-0004: introduceToRemaining notElem O(N×M) member dedup; fix: Set.notMember O(log N)

defects/simplex-chat/patch/simplex-chat-0004-introduce-to-remaining-notelem.md

@@ -0,0 +1,51 @@
+# Defect: simplex-chat-0004 — introduceToRemaining `notElem` O(N×M) member dedup
+
+**Project:** simplex-chat  
+**File:** `src/Simplex/Chat/Library/Internal.hs`  
+**Function:** `introduceToRemaining` / `introduceMemP`  
+**Complexity:** O(|members| × |introducedGMIds|) per member join event  
+**Severity:** MEDIUM  
+**Fix:** Convert `introducedGMIds :: [GroupMemberId]` to `Set GroupMemberId` before filter
+
+## Pattern
+
+```haskell
+-- DEFECT: O(|members| × |introducedGMIds|)
+introduceToRemaining vr user gInfo m = do
+  (members, introducedGMIds) <-
+    withStore' $ \db -> (,) <$> getGroupMembers db vr user gInfo
+                            <*> getIntroducedGroupMemberIds db m
+  let recipients = filter (introduceMemP introducedGMIds) members
+  ...
+  where
+    introduceMemP introducedGMIds mem =
+      memberCurrent mem
+        && groupMemberId' mem `notElem` introducedGMIds   -- O(|introducedGMIds|) scan
+        && groupMemberId' mem /= groupMemberId' m
+```
+
+## Fix
+
+```haskell
+-- FIX: O(|members| × log|introducedGMIds|)
+import qualified Data.Set as S
+
+introduceToRemaining vr user gInfo m = do
+  (members, introducedGMIds) <-
+    withStore' $ \db -> (,) <$> getGroupMembers db vr user gInfo
+                            <*> getIntroducedGroupMemberIds db m
+  let introducedSet = S.fromList introducedGMIds
+      recipients = filter (introduceMemP introducedSet) members
+  ...
+  where
+    introduceMemP introducedSet mem =
+      memberCurrent mem
+        && groupMemberId' mem `S.notMember` introducedSet   -- O(log N)
+        && groupMemberId' mem /= groupMemberId' m
+```
+
+## Impact
+
+In a group with N members where M introductions have been made:
+- Per new member join: O(N×M) → O(N log M)
+- At N=1000 members, M=500 introductions: 500,000 → 9,000 comparisons (**55× overhead**)

defects/simplex-chat/patch/simplex-chat-0004-introduce-to-remaining-notelem.patch

@@ -0,0 +1,25 @@
+# UNDF: (pending)
+--- a/src/Simplex/Chat/Library/Internal.hs
++++ b/src/Simplex/Chat/Library/Internal.hs
+@@ -30,6 +30,7 @@ import Control.Monad.Except
+ import Data.Bifunctor (bimap, first)
+ import Data.Fixed (Fixed (..))
+ import Data.List (find, foldl', mapAccumL, partition)
++import qualified Data.Set as S
+ import Data.Map.Strict (Map)
+ import qualified Data.Map.Strict as M
+ ...
+@@ -1064,9 +1064,11 @@ introduceToRemaining vr user gInfo m = do
+   (members, introducedGMIds) <-
+     withStore' $ \db -> (,) <$> getGroupMembers db vr user gInfo <*> getIntroducedGroupMemberIds db m
+-  let recipients = filter (introduceMemP introducedGMIds) members
++  let introducedSet = S.fromList introducedGMIds
++      recipients = filter (introduceMemP introducedSet) members
+   introduceMember vr user gInfo m recipients Nothing
+   where
+-    introduceMemP introducedGMIds mem =
++    introduceMemP introducedSet mem =
+       memberCurrent mem
+-        && groupMemberId' mem `notElem` introducedGMIds
++        && groupMemberId' mem `S.notMember` introducedSet
+         && groupMemberId' mem /= groupMemberId' m