Bugfix for range subsetting when 'indices' is also a range.

If 'indices' has a negative 'stop', the previous conversion of 'indices' to
slice would translate the negative stop to a reverse index (i.e., the last
element) rather than its actual interpretation as 'one before the start'. This
is wrong when trying to reach the first element for a negative 'step'.

src/biocutils/subset_sequence.py

@@ -30,6 +30,10 @@ def _subset_sequence_list(x: list, indices: Sequence) -> list:
 @subset_sequence.register
 def _subset_sequence_range(x: range, indices: Sequence) -> Union[list, range]:
     if isinstance(indices, range):
-        return x[slice(indices.start, indices.stop, indices.step)]
+        return range(
+            x.start + x.step * indices.start,
+            x.start + x.step * indices.stop,
+            x.step * indices.step
+        )
     else:
         return [x[i] for i in indices]

tests/test_subset_sequence.py

@@ -25,3 +25,14 @@ def test_subset_range():
     x = range(10, 20)
     assert subset_sequence(x, range(2, 8, 2)) == range(12, 18, 2)
     assert subset_sequence(x, [0, 1, 5, 9]) == [10, 11, 15, 19]
+    assert subset_sequence(x, range(9, -1, -1)) == range(19, 9, -1)
+
+    x = range(10, 30, 3)
+    assert subset_sequence(x, range(2, 7, 2)) == x[2:7:2]
+    assert subset_sequence(x, range(5, 0, -2)) == x[5:0:-2]
+    assert subset_sequence(x, range(len(x) - 1, -1, -1)) == x[::-1]
+
+    x = range(100, 21, -6)
+    assert subset_sequence(x, range(3, 10, 2)) == x[3:10:2]
+    assert subset_sequence(x, range(7, 1, -1)) == x[7:1:-1]
+    assert subset_sequence(x, range(len(x) - 1, -1, -1)) == x[::-1]