Fix slicing MapSet with stepped range (#12667)

Author: sabiwara

lib/elixir/lib/enum.ex

@@ -3009,13 +3009,16 @@ defmodule Enum do
 
     if first < count and amount > 0 do
       amount = Kernel.min(amount, count - first)
-      amount = if step == 1, do: amount, else: div(amount - 1, step) + 1
+      amount = amount_with_step(amount, step)
       fun.(first, amount, step)
     else
       []
     end
   end
 
+  defp amount_with_step(amount, 1), do: amount
+  defp amount_with_step(amount, step), do: div(amount - 1, step) + 1
+
   @doc """
   Returns a subset list of the given `enumerable`, from `start_index` (zero-based)
   with `amount` number of elements if available.
@@ -4440,15 +4443,15 @@ defmodule Enum do
 
     if start >= 0 do
       amount = Kernel.min(amount, count - start)
-      amount = if step == 1, do: amount, else: div(amount - 1, step) + 1
+      amount = amount_with_step(amount, step)
       fun.(start, amount, step)
     else
       []
     end
   end
 
   defp slice_forward(list, start, amount, step) when is_list(list) do
-    amount = if step == 1, do: amount, else: div(amount - 1, step) + 1
+    amount = amount_with_step(amount, step)
     slice_list(list, start, amount, step)
   end
 
@@ -4458,7 +4461,7 @@ defmodule Enum do
         []
 
       {:ok, count, fun} when is_function(fun, 1) ->
-        amount = Kernel.min(amount, count - start)
+        amount = Kernel.min(amount, count - start) |> amount_with_step(step)
         enumerable |> fun.() |> slice_exact(start, amount, step, count)
 
       # TODO: Deprecate me in Elixir v1.18.
@@ -4473,8 +4476,7 @@ defmodule Enum do
         end
 
       {:ok, count, fun} when is_function(fun, 3) ->
-        amount = Kernel.min(amount, count - start)
-        amount = if step == 1, do: amount, else: div(amount - 1, step) + 1
+        amount = Kernel.min(amount, count - start) |> amount_with_step(step)
         fun.(start, amount, step)
 
       {:error, module} ->

lib/elixir/test/elixir/enum_test.exs

@@ -1102,6 +1102,16 @@ defmodule EnumTest do
              [2, 1, 3]
   end
 
+  test "slice on MapSets" do
+    assert MapSet.new(1..10) |> Enum.slice(0, 2) |> Enum.count() == 2
+    assert MapSet.new(1..3) |> Enum.slice(0, 10) |> Enum.count() == 3
+    assert MapSet.new(1..10) |> Enum.slice(0..1) |> Enum.count() == 2
+    assert MapSet.new(1..3) |> Enum.slice(0..10) |> Enum.count() == 3
+
+    assert MapSet.new(1..10) |> Enum.slice(0..4//2) |> Enum.count() == 3
+    assert MapSet.new(1..10) |> Enum.slice(0..5//2) |> Enum.count() == 3
+  end
+
   test "sort/1" do
     assert Enum.sort([5, 3, 2, 4, 1]) == [1, 2, 3, 4, 5]
   end