Reject negative n in generate_n_keys_between

README.md

@@ -149,6 +149,12 @@ This is a Python port of the original JavaScript implementation by [@rocicorp](h
 
 ## Changelog
 
+### 1.0.1
+
+- `generate_n_keys_between()` now raises `FIError` for a negative `n`, instead of silently returning a single key
+  (one bound `None`) or hitting `RecursionError` (both bounds set). Deviation from the JS reference, which only
+  documents the `n >= 0` precondition.
+
 ### 0.2.0
 
 Brings the library to parity with [rocicorp/fractional-indexing](https://github.com/rocicorp/fractional-indexing)

fractional_indexing.py

@@ -411,14 +411,18 @@ def generate_n_keys_between(
 ) -> List[str]:
     """
     same preconditions as generate_key_between().
-    n >= 0.
+    n must be >= 0 (raises FIError otherwise).
     Returns an array of n distinct keys in sorted order.
     If a and b are both null, returns [a0, a1, ...]
     If one or the other is null, returns consecutive "integer"
     keys.  Otherwise, returns relatively short keys between `a` and `b`.
 
     """
     digits, int_digits = _resolve_alphabets(digits, int_digits)
+    if n < 0:
+        # Without this guard a negative n silently returns a single key when
+        # one bound is None, and recurses without bound when both are set.
+        raise FIError(f'n must be >= 0: {n}')
     if n == 0:
         return []
     if n == 1:

tests.py

@@ -247,6 +247,20 @@ def test_equal_bounds_rejected() -> None:
         generate_key_between('a0', 'a0')
 
 
+# A negative n must raise, not silently return a single key (one bound None)
+# or recurse without bound (both bounds set).
+@pytest.mark.parametrize(['a', 'b'], [
+    (None, None),
+    ('a0', None),
+    (None, 'a1'),
+    ('a0', 'a5'),
+])
+def test_negative_n_rejected(a: Optional[str], b: Optional[str]) -> None:
+    with pytest.raises(FIError) as e:
+        generate_n_keys_between(a, b, -1)
+    assert e.value.args[0] == 'n must be >= 0: -1'
+
+
 def test_readme_examples_single_key():
     # Insert at the beginning
     first = generate_key_between(None, None)