chatformat: reject a non-positive split limit instead of hanging

taskuary/chatformat.py

@@ -171,6 +171,11 @@ def split(text: str, limit: int = HARD) -> list:
     """One section that will not fit, broken where a reader would break it: a blank line, then a
     line end, and only as a last resort a space. The old splitter took max() of those three
     positions, so the space always won and every break landed mid-sentence."""
+    if limit < 1:
+        # A non-positive limit makes every cut 0 (or -1): text[:0] is appended, text never gets
+        # shorter, and the loop grows a list for ever (issue #60). Callers pass sensible limits,
+        # so this is a bug in the caller - say so instead of hanging the process.
+        raise ValueError(f'limit must be at least 1, got {limit}')
     out = []
     while len(text) > limit:
         window = text[:limit]

tests/test_chatformat.py

@@ -170,6 +170,21 @@ def test_blocks_leave_the_spelling_to_the_door(self):
         self.assertIn('## Top', got[0])
 
 
+class SplitLimitTests(unittest.TestCase):
+    """split() is pure and its callers pass sensible limits, so a limit <= 0 was latent - but the
+    arithmetic made it hang: every cut was 0 (or -1), text[:0] was appended and text never got
+    shorter, so the loop grew a list for ever. Reject it at the door."""
+
+    def test_a_non_positive_limit_raises_instead_of_hanging(self):
+        with self.assertRaises(ValueError):
+            cf.split('abc', 0)
+        with self.assertRaises(ValueError):
+            cf.split('abc', -5)
+
+    def test_limit_one_still_splits_one_character_at_a_time(self):
+        self.assertEqual(cf.split('ab', 1), ['a', 'b'])
+
+
 if __name__ == '__main__':
     unittest.main()