From a09720d0853fa49c91d0ab921e87bca32551fbc4 Mon Sep 17 00:00:00 2001
From: preciz <barna.kovacs@tutanota.com>
Date: Sat, 13 Jun 2026 22:04:40 +0200
Subject: [PATCH 1/3] Optimize MapSet.symmetric_difference/2 when sizes
 mismatched

By folding over the smaller set and using the larger set as the starting accumulator, the time complexity is reduced from O(large) to O(small) iterations. This provides a over 100x speedup when set sizes are mismatched.
---
 lib/elixir/lib/map_set.ex | 24 ++++++++++++++----------
 1 file changed, 14 insertions(+), 10 deletions(-)

diff --git a/lib/elixir/lib/map_set.ex b/lib/elixir/lib/map_set.ex
index fbfd9c303c1..0e6aba1274f 100644
--- a/lib/elixir/lib/map_set.ex
+++ b/lib/elixir/lib/map_set.ex
@@ -173,16 +173,20 @@ defmodule MapSet do
   def symmetric_difference(%MapSet{map: set1} = map_set1, %MapSet{map: set2} = _map_set2) do
     {small, large} = if :sets.size(set1) <= :sets.size(set2), do: {set1, set2}, else: {set2, set1}
 
-    disjointer_fun = fn elem, {small, acc} ->
-      if :sets.is_element(elem, small) do
-        {:sets.del_element(elem, small), acc}
-      else
-        {small, [elem | acc]}
-      end
-    end
-
-    {new_small, list} = :sets.fold(disjointer_fun, {small, []}, large)
-    %{map_set1 | map: :sets.union(new_small, :sets.from_list(list, version: 2))}
+    map =
+      :sets.fold(
+        fn elem, acc ->
+          if :sets.is_element(elem, acc) do
+            :sets.del_element(elem, acc)
+          else
+            :sets.add_element(elem, acc)
+          end
+        end,
+        large,
+        small
+      )
+
+    %{map_set1 | map: map}
   end
 
   @doc """

From 68a89c94ada891f52a0e20187cb2b47f3e6e1171 Mon Sep 17 00:00:00 2001
From: preciz <barna.kovacs@tutanota.com>
Date: Sun, 14 Jun 2026 10:43:13 +0200
Subject: [PATCH 2/3] Further optimize MapSet.symmetric_difference

---
 lib/elixir/lib/map_set.ex | 21 ++++++++-------------
 1 file changed, 8 insertions(+), 13 deletions(-)

diff --git a/lib/elixir/lib/map_set.ex b/lib/elixir/lib/map_set.ex
index 0e6aba1274f..b036e313fd0 100644
--- a/lib/elixir/lib/map_set.ex
+++ b/lib/elixir/lib/map_set.ex
@@ -171,20 +171,15 @@ defmodule MapSet do
   @doc since: "1.14.0"
   @spec symmetric_difference(t(val1), t(val2)) :: t(val1 | val2) when val1: value, val2: value
   def symmetric_difference(%MapSet{map: set1} = map_set1, %MapSet{map: set2} = _map_set2) do
-    {small, large} = if :sets.size(set1) <= :sets.size(set2), do: {set1, set2}, else: {set2, set1}
-
     map =
-      :sets.fold(
-        fn elem, acc ->
-          if :sets.is_element(elem, acc) do
-            :sets.del_element(elem, acc)
-          else
-            :sets.add_element(elem, acc)
-          end
-        end,
-        large,
-        small
-      )
+      if :sets.is_disjoint(set1, set2) do
+        :sets.union(set1, set2)
+      else
+        {small, large} =
+          if :sets.size(set1) <= :sets.size(set2), do: {set1, set2}, else: {set2, set1}
+
+        :sets.union(:sets.subtract(large, small), :sets.subtract(small, large))
+      end
 
     %{map_set1 | map: map}
   end

From 17c2c9cd8080d28707c0a0026224487015d3f0d5 Mon Sep 17 00:00:00 2001
From: preciz <barna.kovacs@tutanota.com>
Date: Sun, 14 Jun 2026 11:00:35 +0200
Subject: [PATCH 3/3] No comparison required

---
 lib/elixir/lib/map_set.ex | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/lib/elixir/lib/map_set.ex b/lib/elixir/lib/map_set.ex
index b036e313fd0..8826ba8cff1 100644
--- a/lib/elixir/lib/map_set.ex
+++ b/lib/elixir/lib/map_set.ex
@@ -175,10 +175,7 @@ defmodule MapSet do
       if :sets.is_disjoint(set1, set2) do
         :sets.union(set1, set2)
       else
-        {small, large} =
-          if :sets.size(set1) <= :sets.size(set2), do: {set1, set2}, else: {set2, set1}
-
-        :sets.union(:sets.subtract(large, small), :sets.subtract(small, large))
+        :sets.union(:sets.subtract(set1, set2), :sets.subtract(set2, set1))
       end
 
     %{map_set1 | map: map}