Remove VTable::child; derive children from slots at the call sites

Every encoding used the default "idx-th non-None slot" definition, so the
trait method was not a real customization point. Inline that definition
into the DynArrayData children/nth_child/named_children implementations
and drop the method from the vtable.

Signed-off-by: "Joe Isaacs" <joe.isaacs@live.co.uk>

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XbjnDVZQ9KqXgAAhxy9g7D

Author: claude

vortex-array/src/array/mod.rs

@@ -268,8 +268,8 @@ impl<V: VTable> DynArrayData for ArrayData<V> {
     }
 
     fn children(&self, this: &ArrayRef) -> Vec<ArrayRef> {
-        let view = unsafe { ArrayView::new_unchecked(this, &self.data) };
-        (0..V::nchildren(view)).map(|i| V::child(view, i)).collect()
+        let view: ArrayView<'_, V> = unsafe { ArrayView::new_unchecked(this, &self.data) };
+        view.slots().iter().filter_map(|s| s.clone()).collect()
     }
 
     fn nchildren(&self, this: &ArrayRef) -> usize {
@@ -278,8 +278,12 @@ impl<V: VTable> DynArrayData for ArrayData<V> {
     }
 
     fn nth_child(&self, this: &ArrayRef, idx: usize) -> Option<ArrayRef> {
-        let view = unsafe { ArrayView::new_unchecked(this, &self.data) };
-        (idx < V::nchildren(view)).then(|| V::child(view, idx))
+        let view: ArrayView<'_, V> = unsafe { ArrayView::new_unchecked(this, &self.data) };
+        view.slots()
+            .iter()
+            .filter_map(|s| s.as_ref())
+            .nth(idx)
+            .cloned()
     }
 
     fn children_names(&self, this: &ArrayRef) -> Vec<String> {
@@ -290,9 +294,12 @@ impl<V: VTable> DynArrayData for ArrayData<V> {
     }
 
     fn named_children(&self, this: &ArrayRef) -> Vec<(String, ArrayRef)> {
-        let view = unsafe { ArrayView::new_unchecked(this, &self.data) };
-        (0..V::nchildren(view))
-            .map(|i| (V::child_name(view, i), V::child(view, i)))
+        let view: ArrayView<'_, V> = unsafe { ArrayView::new_unchecked(this, &self.data) };
+        view.slots()
+            .iter()
+            .filter_map(|s| s.as_ref())
+            .enumerate()
+            .map(|(i, child)| (V::child_name(view, i), child.clone()))
             .collect()
     }
 

vortex-array/src/array/vtable/mod.rs

@@ -122,21 +122,6 @@ pub trait VTable: 'static + Clone + Sized + Send + Sync + Debug {
         array.slots().iter().filter(|s| s.is_some()).count()
     }
 
-    /// Returns the child at the given index.
-    ///
-    /// The default returns the `idx`-th non-None slot.
-    ///
-    /// # Panics
-    /// Panics if `idx >= nchildren(array)`.
-    fn child(array: ArrayView<'_, Self>, idx: usize) -> ArrayRef {
-        array
-            .slots()
-            .iter()
-            .filter_map(|s| s.clone())
-            .nth(idx)
-            .vortex_expect("child index out of bounds")
-    }
-
     /// Returns the name of the child at the given index.
     ///
     /// The default returns the slot name of the `idx`-th non-None slot.