Account for "pseudo max" on ranges

If `step > 1`, it's possible for `range.last`
to be pastthe bounds but for the `range`
to still be a subset of the columns.

lib/explorer/shared.ex

@@ -207,16 +207,18 @@ defmodule Explorer.Shared do
     names
   end
 
-  def to_existing_columns(%{names: names} = df, %Range{} = columns, raise?) do
+  def to_existing_columns(%{names: names} = df, first..last//step = columns, raise?) do
     if raise? do
       n_cols = Explorer.DataFrame.n_columns(df)
 
       # With `Enum.slice/2`, negative indices are counted from the end.
-      [slice_min, slice_max] =
-        [columns.first, columns.last]
+      [slice_min, slice_pseudo_max] =
+        [first, last]
         |> Enum.map(&if(&1 < 0, do: n_cols + &1, else: &1))
         |> Enum.sort()
 
+      slice_max = slice_min + step * (Range.size(slice_min..slice_pseudo_max//step) - 1)
+
       if slice_min < 0 or slice_max >= n_cols do
         raise ArgumentError,
               "range #{inspect(columns)} is out of bounds for a dataframe with #{n_cols} column(s)"

test/explorer/data_frame_test.exs

@@ -2690,6 +2690,7 @@ defmodule Explorer.DataFrameTest do
     assert DF.to_columns(df[[:a, :c]]) == %{"a" => [1, 2, 3], "c" => [4.0, 5.1, 6.2]}
     assert DF.to_columns(df[0..-2//1]) == %{"a" => [1, 2, 3], "b" => ["a", "b", "c"]}
     assert DF.to_columns(df[-3..-1]) == DF.to_columns(df)
+    assert DF.to_columns(df[1..3//3]) == %{"b" => ["a", "b", "c"]}
     assert DF.to_columns(df[..]) == DF.to_columns(df)
 
     assert %Series{} = s1 = df[0]