use crate::innerlude::{VNode, VText};

// Diff the `old` node with the `new` node. Emits instructions to modify a
// physical DOM node that reflects `old` into something that reflects `new`.
//
// Upon entry to this function, the physical DOM node must be on the top of the
// change list stack:
//
//     [... node]
//
// The change list stack is in the same state when this function exits.
pub(crate) fn diff(
    // cached_set: &CachedSet,
    // change_list: &mut ChangeListBuilder,
    // registry: &mut EventsRegistry,
    old: &VNode,
    new: &VNode,
    // cached_roots: &mut FxHashSet<CacheId>,
) {
    match (old, new) {
        // This case occurs when we generate two text nodes that are the sa
        (VNode::Text(VText { text: old_text }), VNode::Text(VText { text: new_text })) => {
            //
            if old_text != new_text {}
        }

        // compare elements
        // if different, schedule different types of update
        (VNode::Element(_), VNode::Element(_)) => {}
        (VNode::Element(_), VNode::Text(_)) => {}
        (VNode::Element(_), VNode::Component(_)) => {}
        (VNode::Text(_), VNode::Element(_)) => {}
        (VNode::Text(_), VNode::Component(_)) => {}

        (VNode::Component(_), VNode::Element(_)) => {}
        (VNode::Component(_), VNode::Text(_)) => {}

        // No immediate change to dom. If props changed though, queue a "props changed" update
        // However, mark these for a
        (VNode::Component(_), VNode::Component(_)) => {}
        //
        (VNode::Suspended, _) | (_, VNode::Suspended) => {
            todo!("Suspended components not currently available")
        } // (VNode::Element(_), VNode::Suspended) => {}
          // (VNode::Text(_), VNode::Suspended) => {}
          // (VNode::Component(_), VNode::Suspended) => {}
          // (VNode::Suspended, VNode::Element(_)) => {}
          // (VNode::Suspended, VNode::Text(_)) => {}
          // (VNode::Suspended, VNode::Suspended) => {}
          // (VNode::Suspended, VNode::Component(_)) => {}
    }
    todo!()
    // match (&new.kind, &old.kind) {
    //     (
    //         &NodeKind::Text(TextNode { text: new_text }),
    //         &NodeKind::Text(TextNode { text: old_text }),
    //     ) => {
    //         if new_text != old_text {
    //             change_list.commit_traversal();
    //             change_list.set_text(new_text);
    //         }
    //     }

    //     (&NodeKind::Text(_), &NodeKind::Element(_)) => {
    //         change_list.commit_traversal();
    //         create(cached_set, change_list, registry, new, cached_roots);
    //         registry.remove_subtree(&old);
    //         change_list.replace_with();
    //     }

    //     (&NodeKind::Element(_), &NodeKind::Text(_)) => {
    //         change_list.commit_traversal();
    //         create(cached_set, change_list, registry, new, cached_roots);
    //         // Note: text nodes cannot have event listeners, so we don't need to
    //         // remove the old node's listeners from our registry her.
    //         change_list.replace_with();
    //     }

    //     (
    //         &NodeKind::Element(ElementNode {
    //             key: _,
    //             tag_name: new_tag_name,
    //             listeners: new_listeners,
    //             attributes: new_attributes,
    //             children: new_children,
    //             namespace: new_namespace,
    //         }),
    //         &NodeKind::Element(ElementNode {
    //             key: _,
    //             tag_name: old_tag_name,
    //             listeners: old_listeners,
    //             attributes: old_attributes,
    //             children: old_children,
    //             namespace: old_namespace,
    //         }),
    //     ) => {
    //         if new_tag_name != old_tag_name || new_namespace != old_namespace {
    //             change_list.commit_traversal();
    //             create(cached_set, change_list, registry, new, cached_roots);
    //             registry.remove_subtree(&old);
    //             change_list.replace_with();
    //             return;
    //         }
    //         diff_listeners(change_list, registry, old_listeners, new_listeners);
    //         diff_attributes(
    //             change_list,
    //             old_attributes,
    //             new_attributes,
    //             new_namespace.is_some(),
    //         );
    //         diff_children(
    //             cached_set,
    //             change_list,
    //             registry,
    //             old_children,
    //             new_children,
    //             cached_roots,
    //         );
    //     }

    //     // Both the new and old nodes are cached.
    //     (&NodeKind::Cached(ref new), &NodeKind::Cached(ref old)) => {
    //         cached_roots.insert(new.id);

    //         if new.id == old.id {
    //             // This is the same cached node, so nothing has changed!
    //             return;
    //         }

    //         let (new, new_template) = cached_set.get(new.id);
    //         let (old, old_template) = cached_set.get(old.id);
    //         if new_template == old_template {
    //             // If they are both using the same template, then just diff the
    //             // subtrees.
    //             diff(cached_set, change_list, registry, old, new, cached_roots);
    //         } else {
    //             // Otherwise, they are probably different enough that
    //             // re-constructing the subtree from scratch should be faster.
    //             // This doubly holds true if we have a new template.
    //             change_list.commit_traversal();
    //             create_and_replace(
    //                 cached_set,
    //                 change_list,
    //                 registry,
    //                 new_template,
    //                 old,
    //                 new,
    //                 cached_roots,
    //             );
    //         }
    //     }

    //     // New cached node when the old node was not cached. In this scenario,
    //     // we assume that they are pretty different, and it isn't worth diffing
    //     // the subtrees, so we just create the new cached node afresh.
    //     (&NodeKind::Cached(ref c), _) => {
    //         change_list.commit_traversal();
    //         cached_roots.insert(c.id);
    //         let (new, new_template) = cached_set.get(c.id);
    //         create_and_replace(
    //             cached_set,
    //             change_list,
    //             registry,
    //             new_template,
    //             old,
    //             new,
    //             cached_roots,
    //         );
    //     }

    //     // Old cached node and new non-cached node. Again, assume that they are
    //     // probably pretty different and create the new non-cached node afresh.
    //     (_, &NodeKind::Cached(_)) => {
    //         change_list.commit_traversal();
    //         create(cached_set, change_list, registry, new, cached_roots);
    //         registry.remove_subtree(&old);
    //         change_list.replace_with();
    //     }
    // }
}

#[cfg(predicate)]
mod todo_diff {

    // Diff event listeners between `old` and `new`.
    //
    // The listeners' node must be on top of the change list stack:
    //
    //     [... node]
    //
    // The change list stack is left unchanged.
    fn diff_listeners(
        change_list: &mut ChangeListBuilder,
        registry: &mut EventsRegistry,
        old: &[Listener],
        new: &[Listener],
    ) {
        if !old.is_empty() || !new.is_empty() {
            change_list.commit_traversal();
        }

        'outer1: for new_l in new {
            unsafe {
                // Safety relies on removing `new_l` from the registry manually at
                // the end of its lifetime. This happens below in the `'outer2`
                // loop, and elsewhere in diffing when removing old dom trees.
                registry.add(new_l);
            }

            for old_l in old {
                if new_l.event == old_l.event {
                    change_list.update_event_listener(new_l);
                    continue 'outer1;
                }
            }

            change_list.new_event_listener(new_l);
        }

        'outer2: for old_l in old {
            registry.remove(old_l);

            for new_l in new {
                if new_l.event == old_l.event {
                    continue 'outer2;
                }
            }
            change_list.remove_event_listener(old_l.event);
        }
    }

    // Diff a node's attributes.
    //
    // The attributes' node must be on top of the change list stack:
    //
    //     [... node]
    //
    // The change list stack is left unchanged.
    fn diff_attributes(
        change_list: &mut ChangeListBuilder,
        old: &[Attribute],
        new: &[Attribute],
        is_namespaced: bool,
    ) {
        // Do O(n^2) passes to add/update and remove attributes, since
        // there are almost always very few attributes.
        'outer: for new_attr in new {
            if new_attr.is_volatile() {
                change_list.commit_traversal();
                change_list.set_attribute(new_attr.name, new_attr.value, is_namespaced);
            } else {
                for old_attr in old {
                    if old_attr.name == new_attr.name {
                        if old_attr.value != new_attr.value {
                            change_list.commit_traversal();
                            change_list.set_attribute(new_attr.name, new_attr.value, is_namespaced);
                        }
                        continue 'outer;
                    }
                }

                change_list.commit_traversal();
                change_list.set_attribute(new_attr.name, new_attr.value, is_namespaced);
            }
        }

        'outer2: for old_attr in old {
            for new_attr in new {
                if old_attr.name == new_attr.name {
                    continue 'outer2;
                }
            }

            change_list.commit_traversal();
            change_list.remove_attribute(old_attr.name);
        }
    }

    // Diff the given set of old and new children.
    //
    // The parent must be on top of the change list stack when this function is
    // entered:
    //
    //     [... parent]
    //
    // the change list stack is in the same state when this function returns.
    fn diff_children(
        cached_set: &CachedSet,
        change_list: &mut ChangeListBuilder,
        registry: &mut EventsRegistry,
        old: &[Node],
        new: &[Node],
        cached_roots: &mut FxHashSet<CacheId>,
    ) {
        if new.is_empty() {
            if !old.is_empty() {
                change_list.commit_traversal();
                remove_all_children(change_list, registry, old);
            }
            return;
        }

        if new.len() == 1 {
            match (old.first(), &new[0]) {
                (
                    Some(&Node {
                        kind: NodeKind::Text(TextNode { text: old_text }),
                    }),
                    &Node {
                        kind: NodeKind::Text(TextNode { text: new_text }),
                    },
                ) if old_text == new_text => {
                    // Don't take this fast path...
                }
                (
                    _,
                    &Node {
                        kind: NodeKind::Text(TextNode { text }),
                    },
                ) => {
                    change_list.commit_traversal();
                    change_list.set_text(text);
                    for o in old {
                        registry.remove_subtree(o);
                    }
                    return;
                }
                (_, _) => {}
            }
        }

        if old.is_empty() {
            if !new.is_empty() {
                change_list.commit_traversal();
                create_and_append_children(cached_set, change_list, registry, new, cached_roots);
            }
            return;
        }

        let new_is_keyed = new[0].key().is_some();
        let old_is_keyed = old[0].key().is_some();

        debug_assert!(
            new.iter().all(|n| n.key().is_some() == new_is_keyed),
            "all siblings must be keyed or all siblings must be non-keyed"
        );
        debug_assert!(
            old.iter().all(|o| o.key().is_some() == old_is_keyed),
            "all siblings must be keyed or all siblings must be non-keyed"
        );

        if new_is_keyed && old_is_keyed {
            let t = change_list.next_temporary();
            diff_keyed_children(cached_set, change_list, registry, old, new, cached_roots);
            change_list.set_next_temporary(t);
        } else {
            diff_non_keyed_children(cached_set, change_list, registry, old, new, cached_roots);
        }
    }

    // Diffing "keyed" children.
    //
    // With keyed children, we care about whether we delete, move, or create nodes
    // versus mutate existing nodes in place. Presumably there is some sort of CSS
    // transition animation that makes the virtual DOM diffing algorithm
    // observable. By specifying keys for nodes, we know which virtual DOM nodes
    // must reuse (or not reuse) the same physical DOM nodes.
    //
    // This is loosely based on Inferno's keyed patching implementation. However, we
    // have to modify the algorithm since we are compiling the diff down into change
    // list instructions that will be executed later, rather than applying the
    // changes to the DOM directly as we compare virtual DOMs.
    //
    // https://github.com/infernojs/inferno/blob/36fd96/packages/inferno/src/DOM/patching.ts#L530-L739
    //
    // When entering this function, the parent must be on top of the change list
    // stack:
    //
    //     [... parent]
    //
    // Upon exiting, the change list stack is in the same state.
    fn diff_keyed_children(
        cached_set: &CachedSet,
        change_list: &mut ChangeListBuilder,
        registry: &mut EventsRegistry,
        old: &[Node],
        new: &[Node],
        cached_roots: &mut FxHashSet<CacheId>,
    ) {
        if cfg!(debug_assertions) {
            let mut keys = FxHashSet::default();
            let mut assert_unique_keys = |children: &[Node]| {
                keys.clear();
                for child in children {
                    let key = child.key();
                    debug_assert!(
                        key.is_some(),
                        "if any sibling is keyed, all siblings must be keyed"
                    );
                    keys.insert(key);
                }
                debug_assert_eq!(
                    children.len(),
                    keys.len(),
                    "keyed siblings must each have a unique key"
                );
            };
            assert_unique_keys(old);
            assert_unique_keys(new);
        }

        // First up, we diff all the nodes with the same key at the beginning of the
        // children.
        //
        // `shared_prefix_count` is the count of how many nodes at the start of
        // `new` and `old` share the same keys.
        let shared_prefix_count =
            match diff_keyed_prefix(cached_set, change_list, registry, old, new, cached_roots) {
                KeyedPrefixResult::Finished => return,
                KeyedPrefixResult::MoreWorkToDo(count) => count,
            };

        // Next, we find out how many of the nodes at the end of the children have
        // the same key. We do _not_ diff them yet, since we want to emit the change
        // list instructions such that they can be applied in a single pass over the
        // DOM. Instead, we just save this information for later.
        //
        // `shared_suffix_count` is the count of how many nodes at the end of `new`
        // and `old` share the same keys.
        let shared_suffix_count = old[shared_prefix_count..]
            .iter()
            .rev()
            .zip(new[shared_prefix_count..].iter().rev())
            .take_while(|&(old, new)| old.key() == new.key())
            .count();

        let old_shared_suffix_start = old.len() - shared_suffix_count;
        let new_shared_suffix_start = new.len() - shared_suffix_count;

        // Ok, we now hopefully have a smaller range of children in the middle
        // within which to re-order nodes with the same keys, remove old nodes with
        // now-unused keys, and create new nodes with fresh keys.
        diff_keyed_middle(
            cached_set,
            change_list,
            registry,
            &old[shared_prefix_count..old_shared_suffix_start],
            &new[shared_prefix_count..new_shared_suffix_start],
            cached_roots,
            shared_prefix_count,
            shared_suffix_count,
            old_shared_suffix_start,
        );

        // Finally, diff the nodes at the end of `old` and `new` that share keys.
        let old_suffix = &old[old_shared_suffix_start..];
        let new_suffix = &new[new_shared_suffix_start..];
        debug_assert_eq!(old_suffix.len(), new_suffix.len());
        if !old_suffix.is_empty() {
            diff_keyed_suffix(
                cached_set,
                change_list,
                registry,
                old_suffix,
                new_suffix,
                cached_roots,
                new_shared_suffix_start,
            );
        }
    }

    enum KeyedPrefixResult {
        // Fast path: we finished diffing all the children just by looking at the
        // prefix of shared keys!
        Finished,
        // There is more diffing work to do. Here is a count of how many children at
        // the beginning of `new` and `old` we already processed.
        MoreWorkToDo(usize),
    }

    // Diff the prefix of children in `new` and `old` that share the same keys in
    // the same order.
    //
    // Upon entry of this function, the change list stack must be:
    //
    //     [... parent]
    //
    // Upon exit, the change list stack is the same.
    fn diff_keyed_prefix(
        cached_set: &CachedSet,
        change_list: &mut ChangeListBuilder,
        registry: &mut EventsRegistry,
        old: &[Node],
        new: &[Node],
        cached_roots: &mut FxHashSet<CacheId>,
    ) -> KeyedPrefixResult {
        change_list.go_down();
        let mut shared_prefix_count = 0;

        for (i, (old, new)) in old.iter().zip(new.iter()).enumerate() {
            if old.key() != new.key() {
                break;
            }

            change_list.go_to_sibling(i);
            diff(cached_set, change_list, registry, old, new, cached_roots);
            shared_prefix_count += 1;
        }

        // If that was all of the old children, then create and append the remaining
        // new children and we're finished.
        if shared_prefix_count == old.len() {
            change_list.go_up();
            change_list.commit_traversal();
            create_and_append_children(
                cached_set,
                change_list,
                registry,
                &new[shared_prefix_count..],
                cached_roots,
            );
            return KeyedPrefixResult::Finished;
        }

        // And if that was all of the new children, then remove all of the remaining
        // old children and we're finished.
        if shared_prefix_count == new.len() {
            change_list.go_to_sibling(shared_prefix_count);
            change_list.commit_traversal();
            remove_self_and_next_siblings(change_list, registry, &old[shared_prefix_count..]);
            return KeyedPrefixResult::Finished;
        }

        change_list.go_up();
        KeyedPrefixResult::MoreWorkToDo(shared_prefix_count)
    }

    // The most-general, expensive code path for keyed children diffing.
    //
    // We find the longest subsequence within `old` of children that are relatively
    // ordered the same way in `new` (via finding a longest-increasing-subsequence
    // of the old child's index within `new`). The children that are elements of
    // this subsequence will remain in place, minimizing the number of DOM moves we
    // will have to do.
    //
    // Upon entry to this function, the change list stack must be:
    //
    //     [... parent]
    //
    // Upon exit from this function, it will be restored to that same state.
    fn diff_keyed_middle(
        cached_set: &CachedSet,
        change_list: &mut ChangeListBuilder,
        registry: &mut EventsRegistry,
        old: &[Node],
        mut new: &[Node],
        cached_roots: &mut FxHashSet<CacheId>,
        shared_prefix_count: usize,
        shared_suffix_count: usize,
        old_shared_suffix_start: usize,
    ) {
        // Should have already diffed the shared-key prefixes and suffixes.
        debug_assert_ne!(new.first().map(|n| n.key()), old.first().map(|o| o.key()));
        debug_assert_ne!(new.last().map(|n| n.key()), old.last().map(|o| o.key()));

        // The algorithm below relies upon using `u32::MAX` as a sentinel
        // value, so if we have that many new nodes, it won't work. This
        // check is a bit academic (hence only enabled in debug), since
        // wasm32 doesn't have enough address space to hold that many nodes
        // in memory.
        debug_assert!(new.len() < u32::MAX as usize);

        // Map from each `old` node's key to its index within `old`.
        let mut old_key_to_old_index = FxHashMap::default();
        old_key_to_old_index.reserve(old.len());
        old_key_to_old_index.extend(old.iter().enumerate().map(|(i, o)| (o.key(), i)));

        // The set of shared keys between `new` and `old`.
        let mut shared_keys = FxHashSet::default();
        // Map from each index in `new` to the index of the node in `old` that
        // has the same key.
        let mut new_index_to_old_index = Vec::with_capacity(new.len());
        new_index_to_old_index.extend(new.iter().map(|n| {
            let key = n.key();
            if let Some(&i) = old_key_to_old_index.get(&key) {
                shared_keys.insert(key);
                i
            } else {
                u32::MAX as usize
            }
        }));

        // If none of the old keys are reused by the new children, then we
        // remove all the remaining old children and create the new children
        // afresh.
        if shared_suffix_count == 0 && shared_keys.is_empty() {
            if shared_prefix_count == 0 {
                change_list.commit_traversal();
                remove_all_children(change_list, registry, old);
            } else {
                change_list.go_down_to_child(shared_prefix_count);
                change_list.commit_traversal();
                remove_self_and_next_siblings(change_list, registry, &old[shared_prefix_count..]);
            }
            create_and_append_children(cached_set, change_list, registry, new, cached_roots);
            return;
        }

        // Save each of the old children whose keys are reused in the new
        // children.
        let mut old_index_to_temp = vec![u32::MAX; old.len()];
        let mut start = 0;
        loop {
            let end = (start..old.len())
                .find(|&i| {
                    let key = old[i].key();
                    !shared_keys.contains(&key)
                })
                .unwrap_or(old.len());

            if end - start > 0 {
                change_list.commit_traversal();
                let mut t = change_list.save_children_to_temporaries(
                    shared_prefix_count + start,
                    shared_prefix_count + end,
                );
                for i in start..end {
                    old_index_to_temp[i] = t;
                    t += 1;
                }
            }

            debug_assert!(end <= old.len());
            if end == old.len() {
                break;
            } else {
                start = end + 1;
            }
        }

        // Remove any old children whose keys were not reused in the new
        // children. Remove from the end first so that we don't mess up indices.
        let mut removed_count = 0;
        for (i, old_child) in old.iter().enumerate().rev() {
            if !shared_keys.contains(&old_child.key()) {
                registry.remove_subtree(old_child);
                change_list.commit_traversal();
                change_list.remove_child(i + shared_prefix_count);
                removed_count += 1;
            }
        }

        // If there aren't any more new children, then we are done!
        if new.is_empty() {
            return;
        }

        // The longest increasing subsequence within `new_index_to_old_index`. This
        // is the longest sequence on DOM nodes in `old` that are relatively ordered
        // correctly within `new`. We will leave these nodes in place in the DOM,
        // and only move nodes that are not part of the LIS. This results in the
        // maximum number of DOM nodes left in place, AKA the minimum number of DOM
        // nodes moved.
        let mut new_index_is_in_lis = FxHashSet::default();
        new_index_is_in_lis.reserve(new_index_to_old_index.len());
        let mut predecessors = vec![0; new_index_to_old_index.len()];
        let mut starts = vec![0; new_index_to_old_index.len()];
        longest_increasing_subsequence::lis_with(
            &new_index_to_old_index,
            &mut new_index_is_in_lis,
            |a, b| a < b,
            &mut predecessors,
            &mut starts,
        );

        // Now we will iterate from the end of the new children back to the
        // beginning, diffing old children we are reusing and if they aren't in the
        // LIS moving them to their new destination, or creating new children. Note
        // that iterating in reverse order lets us use `Node.prototype.insertBefore`
        // to move/insert children.
        //
        // But first, we ensure that we have a child on the change list stack that
        // we can `insertBefore`. We handle this once before looping over `new`
        // children, so that we don't have to keep checking on every loop iteration.
        if shared_suffix_count > 0 {
            // There is a shared suffix after these middle children. We will be
            // inserting before that shared suffix, so add the first child of that
            // shared suffix to the change list stack.
            //
            // [... parent]
            change_list.go_down_to_child(old_shared_suffix_start - removed_count);
        // [... parent first_child_of_shared_suffix]
        } else {
            // There is no shared suffix coming after these middle children.
            // Therefore we have to process the last child in `new` and move it to
            // the end of the parent's children if it isn't already there.
            let last_index = new.len() - 1;
            let last = new.last().unwrap_throw();
            new = &new[..new.len() - 1];
            if shared_keys.contains(&last.key()) {
                let old_index = new_index_to_old_index[last_index];
                let temp = old_index_to_temp[old_index];
                // [... parent]
                change_list.go_down_to_temp_child(temp);
                // [... parent last]
                diff(
                    cached_set,
                    change_list,
                    registry,
                    &old[old_index],
                    last,
                    cached_roots,
                );
                if new_index_is_in_lis.contains(&last_index) {
                    // Don't move it, since it is already where it needs to be.
                } else {
                    change_list.commit_traversal();
                    // [... parent last]
                    change_list.append_child();
                    // [... parent]
                    change_list.go_down_to_temp_child(temp);
                    // [... parent last]
                }
            } else {
                change_list.commit_traversal();
                // [... parent]
                create(cached_set, change_list, registry, last, cached_roots);
                // [... parent last]
                change_list.append_child();
                // [... parent]
                change_list.go_down_to_reverse_child(0);
                // [... parent last]
            }
        }

        for (new_index, new_child) in new.iter().enumerate().rev() {
            let old_index = new_index_to_old_index[new_index];
            if old_index == u32::MAX as usize {
                debug_assert!(!shared_keys.contains(&new_child.key()));
                change_list.commit_traversal();
                // [... parent successor]
                create(cached_set, change_list, registry, new_child, cached_roots);
                // [... parent successor new_child]
                change_list.insert_before();
            // [... parent new_child]
            } else {
                debug_assert!(shared_keys.contains(&new_child.key()));
                let temp = old_index_to_temp[old_index];
                debug_assert_ne!(temp, u32::MAX);

                if new_index_is_in_lis.contains(&new_index) {
                    // [... parent successor]
                    change_list.go_to_temp_sibling(temp);
                // [... parent new_child]
                } else {
                    change_list.commit_traversal();
                    // [... parent successor]
                    change_list.push_temporary(temp);
                    // [... parent successor new_child]
                    change_list.insert_before();
                    // [... parent new_child]
                }

                diff(
                    cached_set,
                    change_list,
                    registry,
                    &old[old_index],
                    new_child,
                    cached_roots,
                );
            }
        }

        // [... parent child]
        change_list.go_up();
        // [... parent]
    }

    // Diff the suffix of keyed children that share the same keys in the same order.
    //
    // The parent must be on the change list stack when we enter this function:
    //
    //     [... parent]
    //
    // When this function exits, the change list stack remains the same.
    fn diff_keyed_suffix(
        cached_set: &CachedSet,
        change_list: &mut ChangeListBuilder,
        registry: &mut EventsRegistry,
        old: &[Node],
        new: &[Node],
        cached_roots: &mut FxHashSet<CacheId>,
        new_shared_suffix_start: usize,
    ) {
        debug_assert_eq!(old.len(), new.len());
        debug_assert!(!old.is_empty());

        // [... parent]
        change_list.go_down();
        // [... parent new_child]

        for (i, (old_child, new_child)) in old.iter().zip(new.iter()).enumerate() {
            change_list.go_to_sibling(new_shared_suffix_start + i);
            diff(
                cached_set,
                change_list,
                registry,
                old_child,
                new_child,
                cached_roots,
            );
        }

        // [... parent]
        change_list.go_up();
    }

    // Diff children that are not keyed.
    //
    // The parent must be on the top of the change list stack when entering this
    // function:
    //
    //     [... parent]
    //
    // the change list stack is in the same state when this function returns.
    fn diff_non_keyed_children(
        cached_set: &CachedSet,
        change_list: &mut ChangeListBuilder,
        registry: &mut EventsRegistry,
        old: &[Node],
        new: &[Node],
        cached_roots: &mut FxHashSet<CacheId>,
    ) {
        // Handled these cases in `diff_children` before calling this function.
        debug_assert!(!new.is_empty());
        debug_assert!(!old.is_empty());

        //     [... parent]
        change_list.go_down();
        //     [... parent child]

        for (i, (new_child, old_child)) in new.iter().zip(old.iter()).enumerate() {
            // [... parent prev_child]
            change_list.go_to_sibling(i);
            // [... parent this_child]
            diff(
                cached_set,
                change_list,
                registry,
                old_child,
                new_child,
                cached_roots,
            );
        }

        match old.len().cmp(&new.len()) {
            Ordering::Greater => {
                // [... parent prev_child]
                change_list.go_to_sibling(new.len());
                // [... parent first_child_to_remove]
                change_list.commit_traversal();
                remove_self_and_next_siblings(change_list, registry, &old[new.len()..]);
                // [... parent]
            }
            Ordering::Less => {
                // [... parent last_child]
                change_list.go_up();
                // [... parent]
                change_list.commit_traversal();
                create_and_append_children(
                    cached_set,
                    change_list,
                    registry,
                    &new[old.len()..],
                    cached_roots,
                );
            }
            Ordering::Equal => {
                // [... parent child]
                change_list.go_up();
                // [... parent]
            }
        }
    }

    // Create the given children and append them to the parent node.
    //
    // The parent node must currently be on top of the change list stack:
    //
    //     [... parent]
    //
    // When this function returns, the change list stack is in the same state.
    fn create_and_append_children(
        cached_set: &CachedSet,
        change_list: &mut ChangeListBuilder,
        registry: &mut EventsRegistry,
        new: &[Node],
        cached_roots: &mut FxHashSet<CacheId>,
    ) {
        debug_assert!(change_list.traversal_is_committed());
        for child in new {
            create(cached_set, change_list, registry, child, cached_roots);
            change_list.append_child();
        }
    }

    // Remove all of a node's children.
    //
    // The change list stack must have this shape upon entry to this function:
    //
    //     [... parent]
    //
    // When this function returns, the change list stack is in the same state.
    fn remove_all_children(
        change_list: &mut ChangeListBuilder,
        registry: &mut EventsRegistry,
        old: &[Node],
    ) {
        debug_assert!(change_list.traversal_is_committed());
        for child in old {
            registry.remove_subtree(child);
        }
        // Fast way to remove all children: set the node's textContent to an empty
        // string.
        change_list.set_text("");
    }

    // Remove the current child and all of its following siblings.
    //
    // The change list stack must have this shape upon entry to this function:
    //
    //     [... parent child]
    //
    // After the function returns, the child is no longer on the change list stack:
    //
    //     [... parent]
    fn remove_self_and_next_siblings(
        change_list: &mut ChangeListBuilder,
        registry: &mut EventsRegistry,
        old: &[Node],
    ) {
        debug_assert!(change_list.traversal_is_committed());
        for child in old {
            registry.remove_subtree(child);
        }
        change_list.remove_self_and_next_siblings();
    }

    // Emit instructions to create the given virtual node.
    //
    // The change list stack may have any shape upon entering this function:
    //
    //     [...]
    //
    // When this function returns, the new node is on top of the change list stack:
    //
    //     [... node]
    fn create(
        cached_set: &CachedSet,
        change_list: &mut ChangeListBuilder,
        registry: &mut EventsRegistry,
        node: &Node,
        cached_roots: &mut FxHashSet<CacheId>,
    ) {
        debug_assert!(change_list.traversal_is_committed());
        match node.kind {
            NodeKind::Text(TextNode { text }) => {
                change_list.create_text_node(text);
            }
            NodeKind::Element(&ElementNode {
                key: _,
                tag_name,
                listeners,
                attributes,
                children,
                namespace,
            }) => {
                if let Some(namespace) = namespace {
                    change_list.create_element_ns(tag_name, namespace);
                } else {
                    change_list.create_element(tag_name);
                }

                for l in listeners {
                    unsafe {
                        registry.add(l);
                    }
                    change_list.new_event_listener(l);
                }

                for attr in attributes {
                    change_list.set_attribute(&attr.name, &attr.value, namespace.is_some());
                }

                // Fast path: if there is a single text child, it is faster to
                // create-and-append the text node all at once via setting the
                // parent's `textContent` in a single change list instruction than
                // to emit three instructions to (1) create a text node, (2) set its
                // text content, and finally (3) append the text node to this
                // parent.
                if children.len() == 1 {
                    if let Node {
                        kind: NodeKind::Text(TextNode { text }),
                    } = children[0]
                    {
                        change_list.set_text(text);
                        return;
                    }
                }

                for child in children {
                    create(cached_set, change_list, registry, child, cached_roots);
                    change_list.append_child();
                }
            }
            NodeKind::Cached(ref c) => {
                cached_roots.insert(c.id);
                let (node, template) = cached_set.get(c.id);
                if let Some(template) = template {
                    create_with_template(
                        cached_set,
                        change_list,
                        registry,
                        template,
                        node,
                        cached_roots,
                    );
                } else {
                    create(cached_set, change_list, registry, node, cached_roots);
                }
            }
        }
    }

    // Get or create the template.
    //
    // Upon entering this function the change list stack may be in any shape:
    //
    //     [...]
    //
    // When this function returns, it leaves a freshly cloned copy of the template
    // on the top of the change list stack:
    //
    //     [... template]
    #[inline]
    fn get_or_create_template<'a>(
        cached_set: &'a CachedSet,
        change_list: &mut ChangeListBuilder,
        registry: &mut EventsRegistry,
        cached_roots: &mut FxHashSet<CacheId>,
        template_id: CacheId,
    ) -> (&'a Node<'a>, bool) {
        let (template, template_template) = cached_set.get(template_id);
        debug_assert!(
            template_template.is_none(),
            "templates should not be templated themselves"
        );

        // If we haven't already created and saved the physical DOM subtree for this
        // template, do that now.
        if change_list.has_template(template_id) {
            // Clone the template and push it onto the stack.
            //
            // [...]
            change_list.push_template(template_id);
            // [... template]

            (template, true)
        } else {
            // [...]
            create(cached_set, change_list, registry, template, cached_roots);
            // [... template]
            change_list.save_template(template_id);
            // [... template]

            (template, false)
        }
    }

    fn create_and_replace(
        cached_set: &CachedSet,
        change_list: &mut ChangeListBuilder,
        registry: &mut EventsRegistry,
        new_template: Option<CacheId>,
        old: &Node,
        new: &Node,
        cached_roots: &mut FxHashSet<CacheId>,
    ) {
        debug_assert!(change_list.traversal_is_committed());

        if let Some(template_id) = new_template {
            let (template, needs_listeners) = get_or_create_template(
                cached_set,
                change_list,
                registry,
                cached_roots,
                template_id,
            );
            change_list.replace_with();

            let mut old_forcing = None;
            if needs_listeners {
                old_forcing = Some(change_list.push_force_new_listeners());
            }

            diff(
                cached_set,
                change_list,
                registry,
                template,
                new,
                cached_roots,
            );

            if let Some(old) = old_forcing {
                change_list.pop_force_new_listeners(old);
            }

            change_list.commit_traversal();
        } else {
            create(cached_set, change_list, registry, new, cached_roots);
            change_list.replace_with();
        }
        registry.remove_subtree(old);
    }

    fn create_with_template(
        cached_set: &CachedSet,
        change_list: &mut ChangeListBuilder,
        registry: &mut EventsRegistry,
        template_id: CacheId,
        node: &Node,
        cached_roots: &mut FxHashSet<CacheId>,
    ) {
        debug_assert!(change_list.traversal_is_committed());

        // [...]
        let (template, needs_listeners) =
            get_or_create_template(cached_set, change_list, registry, cached_roots, template_id);
        // [... template]

        // Now diff the node with its template.
        //
        // We must force adding new listeners instead of updating existing ones,
        // since listeners don't get cloned in `cloneNode`.
        let mut old_forcing = None;
        if needs_listeners {
            old_forcing = Some(change_list.push_force_new_listeners());
        }

        diff(
            cached_set,
            change_list,
            registry,
            template,
            node,
            cached_roots,
        );

        if let Some(old) = old_forcing {
            change_list.pop_force_new_listeners(old);
        }

        // Make sure that we come back up to the level we were at originally.
        change_list.commit_traversal();
    }
}