Split nodes do not shrink correctly

[irevoire]

This test runs on the main branch / latest release of arroy and shouldn’t:

#[test]
fn create_root_split_node_with_empty_child() {
    let handle = create_database::<Euclidean>();
    let mut rng = rng();
    let mut wtxn = handle.env.write_txn().unwrap();
    let writer = Writer::new(handle.database, 0, 2);

    for i in 0..6 {
        writer.add_item(&mut wtxn, i, &[i as f32, 0.]).unwrap();
    }
    writer.builder(&mut rng).n_trees(1).build(&mut wtxn).unwrap();
    wtxn.commit().unwrap();

    insta::assert_snapshot!(handle, @r###"
    ==================
    Dumping index 0
    Root: Metadata { dimensions: 2, items: RoaringBitmap<[0, 1, 2, 3, 4, 5]>, roots: [4], distance: "euclidean" }
    Tree 0: Descendants(Descendants { descendants: [1, 3] })
    Tree 1: SplitPlaneNormal(SplitPlaneNormal<euclidean> { left: Tree(0), right: Item(2), normal: [0.0000, 0.0000] })
    Tree 2: Descendants(Descendants { descendants: [4, 5] })
    Tree 3: SplitPlaneNormal(SplitPlaneNormal<euclidean> { left: Tree(1), right: Tree(2), normal: [0.0000, 0.0000] })
    Tree 4: SplitPlaneNormal(SplitPlaneNormal<euclidean> { left: Item(0), right: Tree(3), normal: [1.0000, 0.0000] })
    Item 0: Leaf(Leaf { header: NodeHeaderEuclidean { bias: 0.0 }, vector: [0.0000, 0.0000] })
    Item 1: Leaf(Leaf { header: NodeHeaderEuclidean { bias: 0.0 }, vector: [1.0000, 0.0000] })
    Item 2: Leaf(Leaf { header: NodeHeaderEuclidean { bias: 0.0 }, vector: [2.0000, 0.0000] })
    Item 3: Leaf(Leaf { header: NodeHeaderEuclidean { bias: 0.0 }, vector: [3.0000, 0.0000] })
    Item 4: Leaf(Leaf { header: NodeHeaderEuclidean { bias: 0.0 }, vector: [4.0000, 0.0000] })
    Item 5: Leaf(Leaf { header: NodeHeaderEuclidean { bias: 0.0 }, vector: [5.0000, 0.0000] })
    "###);

    let mut wtxn = handle.env.write_txn().unwrap();
    let writer = Writer::new(handle.database, 0, 2);

    // if we delete the 1 and 5 the tree node 4 should remove itself and be replaced by the 3rd one
    writer.del_item(&mut wtxn, 4).unwrap();
    writer.del_item(&mut wtxn, 5).unwrap();
    writer.builder(&mut rng).n_trees(1).build(&mut wtxn).unwrap();
    wtxn.commit().unwrap();

    insta::assert_snapshot!(handle, @r###"
    ==================
    Dumping index 0
    Root: Metadata { dimensions: 2, items: RoaringBitmap<[0, 1, 2, 3]>, roots: [4], distance: "euclidean" }
    Tree 0: Descendants(Descendants { descendants: [1, 3] })
    Tree 1: SplitPlaneNormal(SplitPlaneNormal<euclidean> { left: Tree(0), right: Item(2), normal: [0.0000, 0.0000] })
    Tree 2: Descendants(Descendants { descendants: [] })
    Tree 3: SplitPlaneNormal(SplitPlaneNormal<euclidean> { left: Tree(1), right: Tree(2), normal: [0.0000, 0.0000] })
    Tree 4: SplitPlaneNormal(SplitPlaneNormal<euclidean> { left: Item(0), right: Tree(3), normal: [1.0000, 0.0000] })
    Item 0: Leaf(Leaf { header: NodeHeaderEuclidean { bias: 0.0 }, vector: [0.0000, 0.0000] })
    Item 1: Leaf(Leaf { header: NodeHeaderEuclidean { bias: 0.0 }, vector: [1.0000, 0.0000] })
    Item 2: Leaf(Leaf { header: NodeHeaderEuclidean { bias: 0.0 }, vector: [2.0000, 0.0000] })
    Item 3: Leaf(Leaf { header: NodeHeaderEuclidean { bias: 0.0 }, vector: [3.0000, 0.0000] })
    "###);
}

(drop it under tests/writer.rs if you want to test it yourself)

As you can see on the second snapshot, the tree node number 2 contains an empty descendant because the split node number 3 didn’t shrink properly.
Instead of pointing to an empty descendant, it should delete itself and points directly to the tree number 1.
(The case where the whole split nodes fits in a single descendant is already handled correctly).