fix(perf): bind length/1 to variables before loop bodies in route paths and sidebar (CSM-029)

test/bds/csm029_length_in_guards_test.exs

@@ -0,0 +1,55 @@
+defmodule BDS.CSM029LengthInGuardsTest do
+  use ExUnit.Case, async: true
+
+  @outputs_path "lib/bds/generation/outputs.ex"
+  @sidebar_path "lib/bds/ui/sidebar.ex"
+
+  describe "source-level: length/1 bound before use in loops" do
+    test "outputs.ex route path functions bind length before passing to paginated_archive_paths" do
+      source = File.read!(@outputs_path)
+
+      refute source =~ ~r/paginated_archive_paths\([^)]*length\(posts\)/,
+             "length(posts) should be bound to a variable before passing to paginated_archive_paths"
+    end
+
+    test "sidebar.ex build_post_section binds length before map literal" do
+      source = File.read!(@sidebar_path)
+
+      lines =
+        source
+        |> String.split("\n")
+        |> Enum.with_index(1)
+
+      build_section_lines =
+        Enum.filter(lines, fn {line, _} -> line =~ "defp build_post_section" end)
+
+      for {_line, line_no} <- build_section_lines do
+        context =
+          lines
+          |> Enum.drop(line_no - 1)
+          |> Enum.take(30)
+          |> Enum.map(fn {l, _} -> l end)
+          |> Enum.join("\n")
+
+        refute context =~ ~r/count: length\(/,
+               "length() should be pre-bound, not inline in map literal"
+      end
+    end
+
+    test "no length/1 calls remain inline inside Enum.flat_map callbacks in route path functions" do
+      source = File.read!(@outputs_path)
+
+      route_fns = ["category_route_paths", "tag_route_paths", "date_route_paths"]
+
+      for fn_name <- route_fns do
+        fn_match = Regex.run(~r/def #{fn_name}\(.*?\n  end/s, source)
+        assert fn_match, "Expected to find #{fn_name} in outputs.ex"
+
+        [fn_body] = fn_match
+
+        refute fn_body =~ ~r/\blength\(posts\)(?!\s)/,
+               "#{fn_name} should use pre-bound post_count instead of inline length(posts)"
+      end
+    end
+  end
+end