yak(mutation-testing-with-muex-blogex-pure-app-code-1341): mutation testing with muex (blogex + pure app code)

blogex/test/mutation_scope_test.exs

@@ -0,0 +1,64 @@
+defmodule Blogex.MutationScopeTest do
+  # The mutation scope files are the single source of truth for what `mix
+  # mutate` / `mix mutate.fast` are allowed to touch. These pins keep the
+  # sensor honest: a scope that quietly shrinks to an empty (or stale) set
+  # would report a score for a check it never ran.
+  use ExUnit.Case
+
+  @root Path.expand("..", __DIR__)
+  @full "mutation_scope.txt"
+  @fast "mutation_scope_fast.txt"
+
+  # Pure modules only. These are excluded deliberately:
+  #   blogex.ex      facade + moduledoc, nothing to mutate
+  #   blogex/blog.ex the `__using__` macro (NimblePublisher expansion)
+  #   router.ex      Phoenix router DSL
+  #   components.ex  Phoenix component DSL
+  #   layout.ex      Phoenix/LiveView layout DSL
+  @shell_modules [
+    "lib/blogex.ex",
+    "lib/blogex/blog.ex",
+    "lib/blogex/router.ex",
+    "lib/blogex/components.ex",
+    "lib/blogex/layout.ex"
+  ]
+
+  test "full scope lists only existing files" do
+    missing = Enum.reject(scope(@full), &File.exists?(Path.join(@root, &1)))
+
+    assert missing == [],
+           "mutation scope names files that do not exist: #{inspect(missing)}"
+  end
+
+  test "full scope is not empty" do
+    assert scope(@full) != []
+  end
+
+  test "fast scope is not empty" do
+    assert scope(@fast) != []
+  end
+
+  test "fast scope is a subset of full scope" do
+    out_of_scope = MapSet.difference(MapSet.new(scope(@fast)), MapSet.new(scope(@full)))
+
+    assert MapSet.size(out_of_scope) == 0,
+           "mutate.fast names files outside the audited scope: #{inspect(MapSet.to_list(out_of_scope))}"
+  end
+
+  test "scope excludes the macro and shell modules" do
+    scoped = MapSet.new(scope(@full) ++ scope(@fast))
+    present = Enum.filter(@shell_modules, &MapSet.member?(scoped, &1))
+
+    assert present == [],
+           "shell/macro modules belong outside the mutation scope: #{inspect(present)}"
+  end
+
+  defp scope(file) do
+    @root
+    |> Path.join(file)
+    |> File.read!()
+    |> String.split("\n", trim: true)
+    |> Enum.map(&String.trim/1)
+    |> Enum.reject(&(&1 == ""))
+  end
+end