Remove VTable::nchildren; count non-None slots at the call sites

Same treatment as VTable::child: the only override (PythonVTable)
matched the default, so the method was not a real customization point.

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

@@ -273,8 +273,8 @@ impl<V: VTable> DynArrayData for ArrayData<V> {
     }
 
     fn nchildren(&self, this: &ArrayRef) -> usize {
-        let view = unsafe { ArrayView::new_unchecked(this, &self.data) };
-        V::nchildren(view)
+        let view: ArrayView<'_, V> = unsafe { ArrayView::new_unchecked(this, &self.data) };
+        view.slots().iter().filter(|s| s.is_some()).count()
     }
 
     fn nth_child(&self, this: &ArrayRef, idx: usize) -> Option<ArrayRef> {
@@ -287,9 +287,12 @@ impl<V: VTable> DynArrayData for ArrayData<V> {
     }
 
     fn children_names(&self, this: &ArrayRef) -> Vec<String> {
-        let view = unsafe { ArrayView::new_unchecked(this, &self.data) };
-        (0..V::nchildren(view))
-            .map(|i| V::child_name(view, i))
+        let view: ArrayView<'_, V> = unsafe { ArrayView::new_unchecked(this, &self.data) };
+        view.slots()
+            .iter()
+            .filter(|s| s.is_some())
+            .enumerate()
+            .map(|(i, _)| V::child_name(view, i))
             .collect()
     }
 

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

@@ -115,19 +115,12 @@ pub trait VTable: 'static + Clone + Sized + Send + Sync + Debug {
         buffers: &[BufferHandle],
     ) -> VortexResult<ArrayParts<Self>>;
 
-    /// Returns the number of children in the array.
-    ///
-    /// The default counts non-None slots.
-    fn nchildren(array: ArrayView<'_, Self>) -> usize {
-        array.slots().iter().filter(|s| s.is_some()).count()
-    }
-
     /// Returns the name of the child at the given index.
     ///
     /// The default returns the slot name of the `idx`-th non-None slot.
     ///
     /// # Panics
-    /// Panics if `idx >= nchildren(array)`.
+    /// Panics if `idx` is not less than the number of non-None slots.
     fn child_name(array: ArrayView<'_, Self>, idx: usize) -> String {
         array
             .slots()

vortex-python/src/arrays/py/vtable.rs

@@ -92,10 +92,6 @@ impl VTable for PythonVTable {
         with_empty_buffers(self, array, buffers)
     }
 
-    fn nchildren(_array: ArrayView<'_, Self>) -> usize {
-        0
-    }
-
     fn child_name(_array: ArrayView<'_, Self>, idx: usize) -> String {
         vortex_panic!("PythonArray child_name index {idx} out of bounds")
     }