Add support for negative indexes

CHANGELOG.md

@@ -12,6 +12,7 @@ Versioning].
 - Improve performance by reducing the AES plaintext size. Note: this changes the
   ordering of integers for existing seeds, which may be a breaking change depending
   on the use case.
+- Add support for negative indexes.
 
 ## [1.1.0] - 2022-12-29
 

src/shuffled/core.py

@@ -46,6 +46,8 @@ def __getitem__(self, index: slice) -> Sequence[int]: ...
     def __getitem__(self, index: Union[int, slice]) -> Union[int, Sequence[int]]:
         if isinstance(index, slice):
             raise NotImplementedError
+        if index < 0:
+            index += self._size
         if index < 0 or index >= self._size:
             raise IndexError
         return self._encryptor.encrypt(index)

tests/test_shuffled.py

@@ -31,3 +31,15 @@ def test_normal(self, range_size):
     )
     def test_seed(self, seed, expected):
         assert list(Shuffled(20, seed)) == expected
+
+    def test_negative_index(self):
+        s = Shuffled(10, seed=b"\x00")
+
+        assert s[-1] == s[9]
+        assert s[-10] == s[0]
+
+    def test_negative_index_out_of_bounds(self):
+        s = Shuffled(10, seed=b"\x00")
+
+        with pytest.raises(IndexError):
+            s[-11]