fix: guard unusable bpm and empty notes, pin line-break tie ordering

src/karaoke_maker/grid.py

@@ -15,8 +15,8 @@ def detect_bpm(instrumental_path: Path) -> float:
 
 def scale_bpm(bpm: float) -> float:
     """Fold a tempo into UltraStar's usual [200, 400) file-BPM range."""
-    if bpm <= 0:
-        raise ValueError(f"cannot scale non-positive bpm {bpm}")
+    if not math.isfinite(bpm) or bpm <= 0:
+        raise ValueError(f"cannot scale bpm {bpm}")
     while bpm < 200:
         bpm *= 2
     while bpm >= 400:
@@ -26,6 +26,8 @@ def scale_bpm(bpm: float) -> float:
 
 def to_ultrastar(notes: list[dict], breaks: list[float], file_bpm: float,
                  headers: list[tuple[str, str]]) -> UltraStarSong:
+    if not notes:
+        raise ValueError("no notes to place on the beat grid")
     gap_ms = round(notes[0]["start"] * 1000)
     spb = 60 / (4 * file_bpm)
 

tests/test_grid.py

@@ -36,9 +36,15 @@ def test_scale_bpm():
     assert scale_bpm(450.0) == 225.0
 
 
-def test_scale_bpm_rejects_non_positive():
+@pytest.mark.parametrize("bpm", [0.0, -120.0, float("inf"), float("nan")])
+def test_scale_bpm_rejects_unusable_tempo(bpm):
     with pytest.raises(ValueError):
-        scale_bpm(0.0)
+        scale_bpm(bpm)
+
+
+def test_to_ultrastar_rejects_empty_notes():
+    with pytest.raises(ValueError):
+        to_ultrastar([], [], 240.0, [])
 
 
 def test_to_ultrastar_conversion():
@@ -90,6 +96,15 @@ def test_line_break_pushed_past_the_preceding_note():
     assert_ordered(song.body)
 
 
+def test_line_break_on_a_note_start_precedes_that_note():
+    """No gap across a line boundary puts the break exactly on the next onset."""
+    notes = [note(0.0, 0.5, 0, "end"), note(0.5, 0.5, 2, "New")]
+    song = to_ultrastar(notes, [0.5], 240.0, [])
+    assert song.body[1] == LineBreak(beat=8)
+    assert [n.text for n in song.body if isinstance(n, Note)] == ["end", "New"]
+    assert_ordered(song.body)
+
+
 def test_line_break_not_after_the_following_note():
     notes = [note(0.0, 0.02, 0, "a"), note(0.03, 0.02, 1, "b")]
     song = to_ultrastar(notes, [0.025], 240.0, [])