Extract helpers and test the behavior mutation testing was missing.

The switches stay intact. The new tests pin option parsing, file selection, normalization, and which forms each language compares, so the survivors that remain are equivalent mutants.

tests/test_cli.py

@@ -1,7 +1,9 @@
 from pathlib import Path
 
-from dryer.cli import main, parse_args, run
-from dryer.discover import language_of
+import pytest
+
+from dryer.cli import _changed_files, _count, _tracked_source, main, parse_args, run, select_files
+from dryer.discover import is_test_file, iter_source_files, language_of
 
 
 def write_source(root: Path, name: str, text: str) -> None:
@@ -135,3 +137,187 @@ def test_main_exits(monkeypatch):
     monkeypatch.setattr("dryer.cli.sys.exit", lambda code: codes.append(code))
     main(["--help"])
     assert codes == [0]
+
+
+def test_names_that_are_tests():
+    names = [
+        "pkg/foo_test.go",
+        "src/app_test.clj",
+        "src/app_test.cljc",
+        "src/app_test.cljs",
+        "src/app_test.cljd",
+        "src/app_test.bb",
+        "ui/a.test.ts",
+        "ui/a.spec.ts",
+        "ui/a.test.tsx",
+        "ui/a.spec.tsx",
+        "ui/a.test.mts",
+        "ui/a.spec.mts",
+        "tests/conftest.py",
+        "pkg/foo_test.py",
+        "pkg/test_foo.py",
+        "tests/app.py",
+    ]
+    for name in names:
+        assert is_test_file(name), name
+    assert not is_test_file("src/app.py")
+
+
+class _Git:
+    def __init__(self, returncode, stdout, stderr=""):
+        self.returncode = returncode
+        self.stdout = stdout
+        self.stderr = stderr
+
+
+def test_changed_files_reads_porcelain(tmp_path, monkeypatch):
+    stdout = ' M src/a.py\nR  old.py -> src/b.py\n?? "src/c d.py"\n\nX\n'
+    monkeypatch.setattr(
+        "dryer.cli.subprocess.run",
+        lambda *args, **kwargs: _Git(0, stdout),
+    )
+    assert _changed_files(tmp_path) == [
+        (tmp_path / "src/a.py").resolve(),
+        (tmp_path / "src/b.py").resolve(),
+        (tmp_path / "src/c d.py").resolve(),
+    ]
+
+
+def test_changed_files_reports_git_failure(tmp_path, monkeypatch, capsys):
+    monkeypatch.setattr(
+        "dryer.cli.subprocess.run",
+        lambda *args, **kwargs: _Git(1, "", "fatal: not a git repository\n"),
+    )
+    assert _changed_files(tmp_path) == []
+    assert capsys.readouterr().err == "fatal: not a git repository\n"
+
+    monkeypatch.setattr(
+        "dryer.cli.subprocess.run",
+        lambda *args, **kwargs: _Git(1, "", "  \n"),
+    )
+    assert _changed_files(tmp_path) == []
+    assert capsys.readouterr().err == "git status failed\n"
+
+
+def test_changed_limits_the_report(tmp_path, monkeypatch, capsys):
+    body = "def alpha(xs):\n    ys = filter(xs, odd)\n    zs = map(ys, inc)\n    return list(zs)\n"
+    other = "def beta(items):\n    kept = filter(items, even)\n    out = map(kept, dec)\n    return list(out)\n"
+    write_source(tmp_path, "src/a.py", body)
+    write_source(tmp_path, "src/b.py", other)
+    write_source(tmp_path, "src/c.py", body)
+    write_source(tmp_path, "tests/test_a.py", other)
+    stdout = " M src/a.py\n M src/b.py\n M tests/test_a.py\n ?? README.md\n"
+    monkeypatch.setattr(
+        "dryer.cli.subprocess.run",
+        lambda *args, **kwargs: _Git(0, stdout),
+    )
+    assert run(["--root", str(tmp_path), "--changed", "--min-lines", "3", "--min-nodes", "1"]) == 0
+    out = capsys.readouterr().out
+    assert "src/a.py" in out
+    assert "src/b.py" in out
+    assert "src/c.py" not in out
+    assert "test_a.py" not in out
+
+
+def test_an_option_without_a_value_is_a_usage_error(capsys):
+    assert run(["--threshold"]) == 1
+    assert "--threshold requires a value" in capsys.readouterr().err
+
+
+def test_an_empty_value_is_rejected(capsys):
+    assert run(["file.py", "--min-lines", ""]) == 1
+    assert "--min-lines requires a value" in capsys.readouterr().err
+
+
+def test_a_flag_is_not_a_value(capsys):
+    assert run(["--min-lines", "--min-nodes", "1"]) == 1
+    assert "--min-lines requires a value" in capsys.readouterr().err
+
+
+def test_zero_is_a_valid_count():
+    options = parse_args(["--min-lines", "0", "--min-nodes", "0"])
+    assert options.action == "scan"
+    assert options.min_lines == 0
+    assert options.min_nodes == 0
+
+
+def test_a_negative_count_is_rejected():
+    with pytest.raises(ValueError, match="non-negative"):
+        _count("-1", "--min-lines")
+
+
+def test_a_count_must_be_an_integer(capsys):
+    assert run(["--min-lines", "nope"]) == 1
+    assert "--min-lines requires an integer" in capsys.readouterr().err
+
+
+def test_a_threshold_must_be_a_number(capsys):
+    assert run(["--threshold", "nope"]) == 1
+    assert "--threshold requires a number" in capsys.readouterr().err
+
+
+def test_parse_reads_the_process_arguments(monkeypatch):
+    monkeypatch.setattr("dryer.cli.sys.argv", ["dryer", "only-this"])
+    assert parse_args().positionals == ["only-this"]
+
+
+def test_text_format_wins_when_it_comes_last(tmp_path, capsys):
+    write_source(tmp_path, "one.py", "def alpha(xs):\n    return xs\n")
+    assert run(["--root", str(tmp_path), "--edn", "--text", "--min-lines", "1", "--min-nodes", "1"]) == 0
+    assert capsys.readouterr().out == "No duplicate candidates found.\n"
+
+
+def test_source_root_limits_the_walk(tmp_path):
+    write_source(tmp_path, "src/a.py", "x = 1\n")
+    write_source(tmp_path, "extra/b.py", "x = 1\n")
+    options = parse_args(["--root", str(tmp_path), "--source-root", "src"])
+    files = [path.relative_to(tmp_path).as_posix() for path in select_files(options)]
+    assert files == ["src/a.py"]
+
+
+def test_git_status_asks_for_text_without_checking(tmp_path, monkeypatch):
+    seen = {}
+
+    def fake_run(args, **kwargs):
+        seen["args"] = args
+        seen["kwargs"] = kwargs
+        return _Git(0, " M src/a.py\n")
+
+    monkeypatch.setattr("dryer.cli.subprocess.run", fake_run)
+    assert _changed_files(tmp_path) == [(tmp_path / "src/a.py").resolve()]
+    assert seen["args"] == ["git", "status", "--porcelain"]
+    assert seen["kwargs"]["check"] is False
+    assert seen["kwargs"]["capture_output"] is True
+    assert seen["kwargs"]["text"] is True
+    assert seen["kwargs"]["cwd"] == tmp_path
+
+
+def test_a_status_line_of_four_characters_is_a_path(tmp_path, monkeypatch):
+    monkeypatch.setattr(
+        "dryer.cli.subprocess.run",
+        lambda *args, **kwargs: _Git(0, " M a\n"),
+    )
+    assert _changed_files(tmp_path) == [(tmp_path / "a").resolve()]
+
+
+def test_changed_source_skips_unknown_and_test_files(tmp_path):
+    assert _tracked_source(tmp_path / "README.md") is False
+    assert _tracked_source(tmp_path / "tests" / "test_a.py") is False
+    assert _tracked_source(tmp_path / "src" / "a.py") is True
+
+
+def test_test_files_and_skipped_directories_are_left_out(tmp_path):
+    write_source(tmp_path, "src/app.py", "x = 1\n")
+    write_source(tmp_path, "src/foo_test.py", "x = 1\n")
+    write_source(tmp_path, "target/app.py", "x = 1\n")
+    write_source(tmp_path, "tests/app.py", "x = 1\n")
+    relative = [path.relative_to(tmp_path).as_posix() for path in iter_source_files([tmp_path])]
+    assert relative == ["src/app.py"]
+
+
+def test_a_second_report_replaces_the_snapshot(tmp_path, capsys):
+    write_source(tmp_path, "one.py", "def alpha(xs):\n    return xs\n")
+    args = ["--root", str(tmp_path), "--min-lines", "1", "--min-nodes", "1"]
+    assert run(args) == 0
+    assert run(args) == 0
+    assert (tmp_path / ".metrics" / "dry.edn").read_text(encoding="utf-8") == "{:candidates []}\n"