test: add codegen determinism regression test

Author: Torkan

test/ash_typescript/rpc/codegen_determinism_test.exs

@@ -0,0 +1,38 @@
+# SPDX-FileCopyrightText: 2025 ash_typescript contributors <https://github.com/ash-project/ash_typescript/graphs/contributors>
+#
+# SPDX-License-Identifier: MIT
+
+defmodule AshTypescript.Rpc.CodegenDeterminismTest do
+  @moduledoc """
+  Regression test to ensure TypeScript code generation is deterministic.
+
+  Runs the codegen multiple times and verifies that the output is identical
+  across all runs. Non-determinism typically comes from iterating over maps,
+  MapSets, or unsorted collections derived from Ash.Info.domains().
+  """
+  use ExUnit.Case, async: false
+
+  @moduletag :ash_typescript
+
+  setup do
+    Application.put_env(:ash_typescript, :enable_namespace_files, false)
+    :ok
+  end
+
+  describe "codegen determinism" do
+    test "generating TypeScript types 10 times produces identical output" do
+      hashes =
+        Enum.map(1..10, fn _i ->
+          {:ok, typescript} =
+            AshTypescript.Rpc.Codegen.generate_typescript_types(:ash_typescript)
+
+          :crypto.hash(:sha256, typescript) |> Base.encode16()
+        end)
+
+      unique_hashes = Enum.uniq(hashes)
+
+      assert length(unique_hashes) == 1,
+             "Expected all 10 runs to produce identical output, but got #{length(unique_hashes)} distinct outputs"
+    end
+  end
+end