Add interactive watch command for running tests based on git status

The `git ls-files` command allows us to get a list of all files that
have been added or changed since the last commit. This is a convenient
way to determine what tests to run.

This commit adds a "d" keypress command to the watch mode. When the "d"
key is pressed, we check git for new and changed files, then find the
corresponding test files, and run those.

lib/mighty_test/file_system.rb

@@ -1,3 +1,5 @@
+require "open3"
+
 module MightyTest
   class FileSystem
     def listen(&)
@@ -17,5 +19,14 @@ def find_test_paths(directory="test")
       glob = File.join(directory, "**/*_test.rb")
       Dir[glob]
     end
+
+    def find_new_and_changed_paths
+      out, _err, status = Open3.capture3(*%w[git ls-files --deduplicate -m -o --exclude-standard test app lib])
+      return [] unless status.success?
+
+      out.lines(chomp: true).reject(&:empty?).uniq
+    rescue SystemCallError
+      []
+    end
   end
 end

lib/mighty_test/watcher.rb

@@ -12,7 +12,7 @@ def initialize(console: Console.new, extra_args: [], file_system: FileSystem.new
       @system_proc = system_proc
     end
 
-    def run(iterations: :indefinitely) # rubocop:disable Metrics/MethodLength
+    def run(iterations: :indefinitely) # rubocop:disable Metrics/MethodLength, Metrics/AbcSize
       start_file_system_listener
       start_keypress_listener
       puts WATCHING_FOR_CHANGES
@@ -28,6 +28,16 @@ def run(iterations: :indefinitely) # rubocop:disable Metrics/MethodLength
           console.clear
           puts "Running all tests...\n\n"
           mt
+        in [:keypress, "d"]
+          console.clear
+          if (paths = find_matching_tests_for_new_and_changed_paths).any?
+            puts paths.join("\n")
+            puts
+            mt(*paths)
+          else
+            puts "No affected test files detected since the last git commit."
+            puts WATCHING_FOR_CHANGES
+          end
         in [:keypress, "q"]
           break
         else
@@ -43,6 +53,11 @@ def run(iterations: :indefinitely) # rubocop:disable Metrics/MethodLength
 
     attr_reader :console, :extra_args, :file_system, :listener, :system_proc
 
+    def find_matching_tests_for_new_and_changed_paths
+      new_changed = file_system.find_new_and_changed_paths
+      new_changed.flat_map { |path| file_system.find_matching_test_path(path) }.uniq
+    end
+
     def mt(*test_paths)
       command = ["mt", *extra_args]
       command.append("--", *test_paths.flatten) if test_paths.any?

test/mighty_test/file_system_test.rb

@@ -71,6 +71,47 @@ def test_find_test_paths_returns_test_files_in_specific_directory
       )
     end
 
+    def test_find_new_and_changed_paths_returns_empty_array_if_git_exits_with_error
+      status = Minitest::Mock.new
+      status.expect(:success?, false)
+
+      paths = Open3.stub(:capture3, ["", "oh no!", status]) do
+        FileSystem.new.find_new_and_changed_paths
+      end
+
+      assert_empty paths
+    end
+
+    def test_find_new_and_changed_paths_returns_empty_array_if_system_call_fails
+      paths = Open3.stub(:capture3, ->(*) { raise SystemCallError, "oh no!" }) do
+        FileSystem.new.find_new_and_changed_paths
+      end
+
+      assert_empty paths
+    end
+
+    def test_find_new_and_changed_paths_returns_array_based_on_git_output
+      git_output = <<~OUT
+        lib/mighty_test/file_system.rb
+        test/mighty_test/file_system_test.rb
+      OUT
+
+      status = Minitest::Mock.new
+      status.expect(:success?, true)
+
+      paths = Open3.stub(:capture3, [git_output, "", status]) do
+        FileSystem.new.find_new_and_changed_paths
+      end
+
+      assert_equal(
+        %w[
+          lib/mighty_test/file_system.rb
+          test/mighty_test/file_system_test.rb
+        ],
+        paths
+      )
+    end
+
     private
 
     def find_matching_test_path(path, in: ".")

test/mighty_test/watcher_test.rb

@@ -124,6 +124,43 @@ def test_watcher_runs_all_tests_when_enter_key_is_pressed
       EXPECTED
     end
 
+    def test_watcher_runs_new_and_changed_files_according_to_git_when_d_key_is_pressed
+      system_proc do |*args|
+        puts "[SYSTEM] #{args.join(' ')}"
+        true
+      end
+
+      file_system = FileSystem.new
+      stdout, = file_system.stub(:find_new_and_changed_paths, %w[lib/example.rb]) do
+        run_watcher(file_system:, stdin: "dq", in: fixtures_path.join("example_project"))
+      end
+
+      assert_includes(stdout, <<~EXPECTED)
+        [CLEAR]
+        test/example_test.rb
+
+        [SYSTEM] mt -- test/example_test.rb
+      EXPECTED
+    end
+
+    def test_watcher_shows_a_message_if_d_key_is_pressed_and_there_are_no_changes
+      system_proc do |*args|
+        puts "[SYSTEM] #{args.join(' ')}"
+        true
+      end
+
+      file_system = FileSystem.new
+      stdout, = file_system.stub(:find_new_and_changed_paths, []) do
+        run_watcher(file_system:, stdin: "dq", in: fixtures_path.join("example_project"))
+      end
+
+      assert_includes(stdout, <<~EXPECTED)
+        [CLEAR]
+        No affected test files detected since the last git commit.
+        Watching for changes to source and test files. Press "q" to quit.
+      EXPECTED
+    end
+
     private
 
     class Listener
@@ -151,12 +188,11 @@ def paused?
       end
     end
 
-    def run_watcher(iterations: :indefinitely, in: ".", extra_args: [], stdin: nil)
+    def run_watcher(iterations: :indefinitely, in: ".", extra_args: [], stdin: nil, file_system: FileSystem.new)
       listen_thread = @listen_thread
       console = Console.new(stdin: stdin.nil? ? File::NULL : StringIO.new(stdin))
       console.define_singleton_method(:clear) { puts "[CLEAR]" }
       console.define_singleton_method(:play_sound) { |sound| puts "[SOUND] #{sound.inspect}" }
-      file_system = FileSystem.new
       file_system.define_singleton_method(:listen) { |&callback| Listener.new(listen_thread, callback) }
 
       capture_io do