feat: automatic stereo mixdown

1 files changed, 47 insertions, 2 deletions

diff --git a/lib/project/tests/test_pipeline.py b/lib/project/tests/test_pipeline.py
index fe04519..0bc2cef 100644
--- a/lib/project/tests/test_pipeline.py
+++ b/lib/project/tests/test_pipeline.py
@@ -64,13 +64,13 @@ def test_existing_report_log_is_replaced_without_overwrite(wav, tmp_path, bypass
 @pytest.mark.parametrize("kind,message", [
     ("empty", "empty"), ("silence", "silent or too quiet"),
     ("quiet", "silent or too quiet"), ("surround", "Only mono or stereo"),
-    ("nonfinite", "non-finite"), ("stereo", "downmix to mono"),
+    ("nonfinite", "non-finite"),
 ])
 def test_unusable_inputs_leave_no_outputs(wav, tmp_path, bypass, progress, ffmpeg, kind, message):
     samples = {
         "empty": np.empty(0), "silence": np.zeros(48000),
         "quiet": np.full(48000, 1e-6), "surround": np.ones((4800, 3)) * 0.1,
-        "nonfinite": np.array([0.1, np.nan]), "stereo": np.ones((4800, 2)) * 0.1,
+        "nonfinite": np.array([0.1, np.nan]),
     }[kind]
     source = wav(samples)
     target = tmp_path / "outputs" / "master.wav"
@@ -79,6 +79,51 @@ def test_unusable_inputs_leave_no_outputs(wav, tmp_path, bypass, progress, ffmpe
     assert not target.parent.exists()
 
 
+def test_downmix_to_mono_strategies(wav, tmp_path, progress):
+    from producer.audio import downmix_to_mono
+    time = np.arange(48000 * 3) / 48000
+    voice = 0.1 * np.sin(2 * np.pi * 440 * time)
+    # Identical channels: a plain average is lossless and keeps full level.
+    mono = tmp_path / "same.wav"
+    method = downmix_to_mono(wav(np.column_stack([voice, voice])), mono, progress)
+    assert method == "the channels were averaged"
+    assert sf.info(mono).channels == 1
+    assert np.abs(sf.read(mono)[0] - voice).max() == pytest.approx(0, abs=1e-7)
+    # Inverted channels: the phase-cancel case the old error guarded against.
+    mono = tmp_path / "inverted.wav"
+    method = downmix_to_mono(wav(np.column_stack([voice, -voice])), mono, progress)
+    assert "inverted" in method
+    assert np.abs(sf.read(mono)[0] - voice).max() == pytest.approx(0, abs=1e-7)
+    # One silent channel: keep the live channel at unity gain, not -6 dB.
+    mono = tmp_path / "silent.wav"
+    method = downmix_to_mono(wav(np.column_stack([voice, np.zeros_like(voice)])), mono, progress)
+    assert "right channel is silent" in method
+    assert np.abs(sf.read(mono)[0] - voice).max() == pytest.approx(0, abs=1e-7)
+    method = downmix_to_mono(wav(np.column_stack([np.zeros_like(voice), voice])),
+                             tmp_path / "silent-left.wav", progress)
+    assert "left channel is silent" in method
+    with pytest.raises(ValueError, match="non-finite"):
+        downmix_to_mono(wav(np.array([[0.1, np.nan]])), tmp_path / "bad.wav", progress)
+
+
+def test_stereo_input_is_downmixed_not_rejected(wav, tmp_path, bypass, progress, ffmpeg):
+    time = np.arange(48000 * 3) / 48000
+    voice = 0.1 * np.sin(2 * np.pi * 440 * time)
+    source = wav(np.column_stack([voice, -voice]))
+    original = source.read_bytes()
+    target = tmp_path / "master.wav"
+    report = pipeline.process(source, target, bypass, progress)
+    assert any("downmixed to mono" in warning and "inverted" in warning
+               for warning in report["warnings"])
+    stats = sf.info(target)
+    assert stats.channels == 1 and stats.frames == 144000
+    # A naive (L + R) / 2 mix of these channels is digital silence; the master
+    # must carry real signal instead.
+    assert report["master"]["sample_peak_dbfs"] > -30
+    assert source.read_bytes() == original
+    assert not list(target.parent.glob(".producer-*"))
+
+
 def test_measure_reports_channels_peaks_clipping_and_progress(wav):
     samples = np.tile([1.0, -0.25], (4800, 1))
     events = []