Fix: Parsing of negative integer slices

src/pystencils/backend/kernelcreation/ast_factory.py

@@ -138,6 +138,13 @@ class AstFactory:
                 self._typify(self.parse_index(iter_slice) + self.parse_index(1))
             )
             step = self.parse_index(1)
+
+            if normalize_to is not None:
+                upper_limit = self.parse_index(normalize_to)
+                if isinstance(start, PsConstantExpr) and start.constant.value < 0:
+                    start = fold(self._typify(upper_limit.clone() + start))
+                    stop = fold(self._typify(upper_limit.clone() + stop))
+
         else:
             start = self._parse_any_index(
                 iter_slice.start if iter_slice.start is not None else 0
@@ -156,21 +163,21 @@ class AstFactory:
                     f"Invalid value for `slice.step`: {step.constant.value}"
                 )
 
-        if normalize_to is not None:
-            upper_limit = self.parse_index(normalize_to)
-            if isinstance(start, PsConstantExpr) and start.constant.value < 0:
-                start = fold(self._typify(upper_limit.clone() + start))
+            if normalize_to is not None:
+                upper_limit = self.parse_index(normalize_to)
+                if isinstance(start, PsConstantExpr) and start.constant.value < 0:
+                    start = fold(self._typify(upper_limit.clone() + start))
 
-            if stop is None:
-                stop = upper_limit
-            elif isinstance(stop, PsConstantExpr) and stop.constant.value < 0:
-                stop = fold(self._typify(upper_limit.clone() + stop))
+                if stop is None:
+                    stop = upper_limit
+                elif isinstance(stop, PsConstantExpr) and stop.constant.value < 0:
+                    stop = fold(self._typify(upper_limit.clone() + stop))
+
+            elif stop is None:
+                raise ValueError(
+                    "Cannot parse a slice with `stop == None` if no normalization limit is given"
+                )
 
-        elif stop is None:
-            raise ValueError(
-                "Cannot parse a slice with `stop == None` if no normalization limit is given"
-            )
-        
         assert stop is not None  # for mypy
 
         return start, stop, step

tests/nbackend/kernelcreation/test_iteration_space.py

@@ -129,6 +129,36 @@ def test_slices_with_negative_start():
     )
 
 
+def test_negative_singular_slices():
+    ctx = KernelCreationContext()
+    factory = AstFactory(ctx)
+
+    archetype_field = Field.create_generic("f", spatial_dimensions=2, layout="fzyx")
+    ctx.add_field(archetype_field)
+    archetype_arr = ctx.get_buffer(archetype_field)
+
+    islice = (-2, -1)
+    ispace = FullIterationSpace.create_from_slice(ctx, islice, archetype_field)
+
+    dims = ispace.dimensions
+
+    assert dims[0].start.structurally_equal(
+        PsExpression.make(archetype_arr.shape[0]) + factory.parse_index(-2)
+    )
+
+    assert dims[0].stop.structurally_equal(
+        PsExpression.make(archetype_arr.shape[0]) + factory.parse_index(-1)
+    )
+
+    assert dims[1].start.structurally_equal(
+        PsExpression.make(archetype_arr.shape[1]) + factory.parse_index(-1)
+    )
+
+    assert dims[1].stop.structurally_equal(
+        PsExpression.make(archetype_arr.shape[1])
+    )
+
+
 def test_field_independent_slices():
     ctx = KernelCreationContext()