Caching HTMCollections.

We cache the state of any live HTMLCollection, keeping track of

a) the optional cached length of the collection, and
b) an optional cursor into the collection (a node in the collection plus its index).

The cache is invalidated based on the version number of the node.

We use these caches for speeding up random access to the collection.
When returning coll[i], we search from the cursor, if it exists,
and otherwise search from the front of the collection.
In particular, both a forward for-loop and a backward for-loop
through the collection will now have each access take O(1)
time rather than O(n) time.

This gets 1000x speed-up on the relevant Dromaeo DOM query tests.

components/script/dom/document.rs

@@ -26,7 +26,7 @@ use dom::bindings::refcounted::Trusted;
 use dom::bindings::reflector::{Reflectable, reflect_dom_object};
 use dom::bindings::trace::RootedVec;
 use dom::bindings::xmlname::XMLName::InvalidXMLName;
-use dom::bindings::xmlname::{validate_and_extract, xml_name_type};
+use dom::bindings::xmlname::{validate_and_extract, namespace_from_domstring, xml_name_type};
 use dom::comment::Comment;
 use dom::customevent::CustomEvent;
 use dom::documentfragment::DocumentFragment;
@@ -119,7 +119,6 @@ enum ParserBlockedByScript {
 pub struct Document {
     node: Node,
     window: JS<Window>,
-    idmap: DOMRefCell<HashMap<Atom, Vec<JS<Element>>>>,
     implementation: MutNullableHeap<JS<DOMImplementation>>,
     location: MutNullableHeap<JS<Location>>,
     content_type: DOMString,
@@ -128,6 +127,11 @@ pub struct Document {
     is_html_document: bool,
     url: Url,
     quirks_mode: Cell<QuirksMode>,
+    /// Caches for the getElement methods
+    id_map: DOMRefCell<HashMap<Atom, Vec<JS<Element>>>>,
+    tag_map: DOMRefCell<HashMap<Atom, JS<HTMLCollection>>>,
+    tagns_map: DOMRefCell<HashMap<QualName, JS<HTMLCollection>>>,
+    classes_map: DOMRefCell<HashMap<Vec<Atom>, JS<HTMLCollection>>>,
     images: MutNullableHeap<JS<HTMLCollection>>,
     embeds: MutNullableHeap<JS<HTMLCollection>>,
     links: MutNullableHeap<JS<HTMLCollection>>,
@@ -387,8 +391,8 @@ impl Document {
                                 to_unregister: &Element,
                                 id: Atom) {
         debug!("Removing named element from document {:p}: {:p} id={}", self, to_unregister, id);
-        let mut idmap = self.idmap.borrow_mut();
-        let is_empty = match idmap.get_mut(&id) {
+        let mut id_map = self.id_map.borrow_mut();
+        let is_empty = match id_map.get_mut(&id) {
             None => false,
             Some(elements) => {
                 let position = elements.iter()
@@ -399,7 +403,7 @@ impl Document {
             }
         };
         if is_empty {
-            idmap.remove(&id);
+            id_map.remove(&id);
         }
     }
 
@@ -411,12 +415,12 @@ impl Document {
         assert!(element.upcast::<Node>().is_in_doc());
         assert!(!id.is_empty());
 
-        let mut idmap = self.idmap.borrow_mut();
+        let mut id_map = self.id_map.borrow_mut();
 
         let root = self.GetDocumentElement().expect(
             "The element is in the document, so there must be a document element.");
 
-        match idmap.entry(id) {
+        match id_map.entry(id) {
             Vacant(entry) => {
                 entry.insert(vec![JS::from_ref(element)]);
             }
@@ -1278,7 +1282,6 @@ impl Document {
         Document {
             node: Node::new_document_node(),
             window: JS::from_ref(window),
-            idmap: DOMRefCell::new(HashMap::new()),
             implementation: Default::default(),
             location: Default::default(),
             content_type: match content_type {
@@ -1297,6 +1300,10 @@ impl Document {
             // https://dom.spec.whatwg.org/#concept-document-encoding
             encoding_name: DOMRefCell::new(DOMString("UTF-8".to_owned())),
             is_html_document: is_html_document == IsHTMLDocument::HTMLDocument,
+            id_map: DOMRefCell::new(HashMap::new()),
+            tag_map: DOMRefCell::new(HashMap::new()),
+            tagns_map: DOMRefCell::new(HashMap::new()),
+            classes_map: DOMRefCell::new(HashMap::new()),
             images: Default::default(),
             embeds: Default::default(),
             links: Default::default(),
@@ -1386,7 +1393,7 @@ impl Document {
     }
 
     pub fn get_element_by_id(&self, id: &Atom) -> Option<Root<Element>> {
-        self.idmap.borrow().get(&id).map(|ref elements| Root::from_ref(&*(*elements)[0]))
+        self.id_map.borrow().get(&id).map(|ref elements| Root::from_ref(&*(*elements)[0]))
     }
 
     pub fn record_element_state_change(&self, el: &Element, which: ElementState) {
@@ -1504,18 +1511,47 @@ impl DocumentMethods for Document {
 
     // https://dom.spec.whatwg.org/#dom-document-getelementsbytagname
     fn GetElementsByTagName(&self, tag_name: DOMString) -> Root<HTMLCollection> {
-        HTMLCollection::by_tag_name(&self.window, self.upcast(), tag_name)
+        let tag_atom = Atom::from_slice(&tag_name);
+        match self.tag_map.borrow_mut().entry(tag_atom.clone()) {
+            Occupied(entry) => Root::from_ref(entry.get()),
+            Vacant(entry) => {
+                let mut tag_copy = tag_name;
+                tag_copy.make_ascii_lowercase();
+                let ascii_lower_tag = Atom::from_slice(&tag_copy);
+                let result = HTMLCollection::by_atomic_tag_name(&self.window, self.upcast(), tag_atom, ascii_lower_tag);
+                entry.insert(JS::from_rooted(&result));
+                result
+            }
+        }
     }
 
     // https://dom.spec.whatwg.org/#dom-document-getelementsbytagnamens
     fn GetElementsByTagNameNS(&self, maybe_ns: Option<DOMString>, tag_name: DOMString)
                               -> Root<HTMLCollection> {
-        HTMLCollection::by_tag_name_ns(&self.window, self.upcast(), tag_name, maybe_ns)
+        let ns = namespace_from_domstring(maybe_ns);
+        let local = Atom::from_slice(&tag_name);
+        let qname = QualName::new(ns, local);
+        match self.tagns_map.borrow_mut().entry(qname.clone()) {
+            Occupied(entry) => Root::from_ref(entry.get()),
+            Vacant(entry) => {
+                let result = HTMLCollection::by_qual_tag_name(&self.window, self.upcast(), qname);
+                entry.insert(JS::from_rooted(&result));
+                result
+            }
+        }
     }
 
     // https://dom.spec.whatwg.org/#dom-document-getelementsbyclassname
     fn GetElementsByClassName(&self, classes: DOMString) -> Root<HTMLCollection> {
-        HTMLCollection::by_class_name(&self.window, self.upcast(), classes)
+        let class_atoms: Vec<Atom> = split_html_space_chars(&classes).map(Atom::from_slice).collect();
+        match self.classes_map.borrow_mut().entry(class_atoms.clone())  {
+            Occupied(entry) => Root::from_ref(entry.get()),
+            Vacant(entry) => {
+                let result = HTMLCollection::by_atomic_class_name(&self.window, self.upcast(), class_atoms);
+                entry.insert(JS::from_rooted(&result));
+                result
+            }
+        }
     }
 
     // https://dom.spec.whatwg.org/#dom-nonelementparentnode-getelementbyid