java-topology/docs/tickets/simplex-chat-0003-introduce-remaining-notelem-linear-scan.md

simplex-chat-0003: introduceToRemaining notElem list scan on every member

Target: simplex-chat/simplex-chat File: src/Simplex/Chat/Library/Internal.hs Lines: 1073 Severity: HIGH CWE: CWE-407 (Inefficient Algorithmic Complexity)

Description

introduceToRemaining fetches two lists from the database:

• members :: [GroupMember] — all group members (M items)
• introducedGMIds :: [GroupMemberId] — already-introduced members (K items)

It then calls:

let recipients = filter (introduceMemP introducedGMIds) members
  where
    introduceMemP introducedGMIds mem =
      memberCurrent mem
        && groupMemberId' mem `notElem` introducedGMIds   -- O(K) per member
        && groupMemberId' mem /= groupMemberId' m

For each of the M members, notElem walks the full introducedGMIds list: O(M × K) total.

getIntroducedGroupMemberIds is declared as returning IO [GroupMemberId] (Store/Groups.hs:1740) — a plain list.

This function is called every time a new member joins a group and introductions to remaining un-introduced members must be sent — a hot path in group join flows that scales quadratically with group membership.

Fix

Convert introducedGMIds to a S.Set GroupMemberId before the filter:

  let introducedSet = S.fromList introducedGMIds
      recipients = filter (introduceMemP introducedSet) members
  where
    introduceMemP introducedSet mem =
      memberCurrent mem
        && groupMemberId' mem `S.notMember` introducedSet
        && groupMemberId' mem /= groupMemberId' m

S is already imported as qualified Data.Set as S in this module (line 46).

Complexity

Before	After
O(M × K) per join event	O(M + K) per join event

At M=1000, K=900 (near-full group): ~900× reduction in comparisons.

Patch

defects/simplex-chat/patch/simplex-chat-0003.patch

Unit Test

defects/simplex-chat/unit/SimplexChatTest.java (shared with -0001/-0002)