refactor: slice groups should use custom backpermite instead of converting unboxed vectors.

Author: mchav

src/DataFrame/Internal/Column.hs

@@ -380,13 +380,13 @@ atIndicesStable indexes (BoxedColumn column) =
     BoxedColumn $
         VB.generate
             (VU.length indexes)
-            (\i -> column `VB.unsafeIndex` (indexes `VU.unsafeIndex` i))
+            ((column `VB.unsafeIndex`) . (indexes `VU.unsafeIndex`))
 atIndicesStable indexes (UnboxedColumn column) = UnboxedColumn $ VU.unsafeBackpermute column indexes
 atIndicesStable indexes (OptionalColumn column) =
     OptionalColumn $
         VB.generate
             (VU.length indexes)
-            (\i -> column `VB.unsafeIndex` (indexes `VU.unsafeIndex` i))
+            ((column `VB.unsafeIndex`) . (indexes `VU.unsafeIndex`))
 {-# INLINE atIndicesStable #-}
 
 {- | Like 'atIndicesStable' but treats negative indices as null,

src/DataFrame/Internal/Interpreter.hs

@@ -213,15 +213,21 @@ input — the sort happens once, not per-group.
 sliceGroups :: Column -> VU.Vector Int -> VU.Vector Int -> V.Vector Column
 sliceGroups col os indices = case col of
     BoxedColumn vec ->
-        let !sorted = V.unsafeBackpermute vec (V.convert indices)
+        let !sorted =
+                V.generate
+                    (VU.length indices)
+                    ((vec `V.unsafeIndex`) . (indices `VU.unsafeIndex`))
          in V.generate nGroups $ \i ->
                 BoxedColumn (V.unsafeSlice (start i) (len i) sorted)
     UnboxedColumn vec ->
         let !sorted = VU.unsafeBackpermute vec indices
          in V.generate nGroups $ \i ->
                 UnboxedColumn (VU.unsafeSlice (start i) (len i) sorted)
     OptionalColumn vec ->
-        let !sorted = V.unsafeBackpermute vec (V.convert indices)
+        let !sorted =
+                V.generate
+                    (VU.length indices)
+                    ((vec `V.unsafeIndex`) . (indices `VU.unsafeIndex`))
          in V.generate nGroups $ \i ->
                 OptionalColumn (V.unsafeSlice (start i) (len i) sorted)
   where