fix: negative start and stop parameter values in Series.str.slice() (#2104)

Author: GarrettWu

tests/system/small/operations/test_strings.py

@@ -236,7 +236,20 @@ def test_reverse(scalars_dfs):
 
 
 @pytest.mark.parametrize(
-    ["start", "stop"], [(0, 1), (3, 5), (100, 101), (None, 1), (0, 12), (0, None)]
+    ["start", "stop"],
+    [
+        (0, 1),
+        (3, 5),
+        (100, 101),
+        (None, 1),
+        (0, 12),
+        (0, None),
+        (None, -1),
+        (-1, None),
+        (-5, -1),
+        (1, -1),
+        (-10, 10),
+    ],
 )
 def test_slice(scalars_dfs, start, stop):
     scalars_df, scalars_pandas_df = scalars_dfs

third_party/bigframes_vendored/ibis/expr/rewrites.py

@@ -206,21 +206,26 @@ def replace_parameter(_, params, **kwargs):
 @replace(p.StringSlice)
 def lower_stringslice(_, **kwargs):
     """Rewrite StringSlice in terms of Substring."""
-    if _.end is None:
-        return ops.Substring(_.arg, start=_.start)
     if _.start is None:
-        return ops.Substring(_.arg, start=0, length=_.end)
-    if (
-        isinstance(_.start, ops.Literal)
-        and isinstance(_.start.value, int)
-        and isinstance(_.end, ops.Literal)
-        and isinstance(_.end.value, int)
-    ):
-        # optimization for constant values
-        length = _.end.value - _.start.value
+        real_start = 0
     else:
-        length = ops.Subtract(_.end, _.start)
-    return ops.Substring(_.arg, start=_.start, length=length)
+        real_start = ops.IfElse(
+            ops.GreaterEqual(_.start, 0),
+            _.start,
+            ops.Greatest((0, ops.Add(ops.StringLength(_.arg), _.start))),
+        )
+
+    if _.end is None:
+        real_end = ops.StringLength(_.arg)
+    else:
+        real_end = ops.IfElse(
+            ops.GreaterEqual(_.end, 0),
+            _.end,
+            ops.Greatest((0, ops.Add(ops.StringLength(_.arg), _.end))),
+        )
+
+    length = ops.Greatest((0, ops.Subtract(real_end, real_start)))
+    return ops.Substring(_.arg, start=real_start, length=length)
 
 
 @replace(p.Analytic)

third_party/bigframes_vendored/ibis/expr/types/strings.py

@@ -96,15 +96,6 @@ def __getitem__(self, key: slice | int | ir.IntegerScalar) -> StringValue:
 
             if isinstance(step, ir.Expr) or (step is not None and step != 1):
                 raise ValueError("Step can only be 1")
-            if start is not None and not isinstance(start, ir.Expr) and start < 0:
-                raise ValueError(
-                    "Negative slicing not yet supported, got start value "
-                    f"of {start:d}"
-                )
-            if stop is not None and not isinstance(stop, ir.Expr) and stop < 0:
-                raise ValueError(
-                    "Negative slicing not yet supported, got stop value " f"of {stop:d}"
-                )
             if start is None and stop is None:
                 return self
             return ops.StringSlice(self, start, stop).to_expr()