style: Implement selector matching for :nth-child(An+B of selector list) and :nth-last-child(An+B of selector list)

Since we have been using a single hash map to cache all :nth-child
indices (with no selector list), each different selector will need its
own cache.

As a side note, this patch does not address invalidation.

Differential Revision: https://phabricator.services.mozilla.com/D166266

components/selectors/nth_index_cache.rs

@@ -2,7 +2,9 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-use crate::tree::OpaqueElement;
+use std::hash::Hash;
+
+use crate::{parser::Selector, tree::OpaqueElement, SelectorImpl};
 use fxhash::FxHashMap;
 
 /// A cache to speed up matching of nth-index-like selectors.
@@ -13,20 +15,68 @@ use fxhash::FxHashMap;
 #[derive(Default)]
 pub struct NthIndexCache {
     nth: NthIndexCacheInner,
+    nth_of_selectors: NthIndexOfSelectorsCaches,
     nth_last: NthIndexCacheInner,
+    nth_last_of_selectors: NthIndexOfSelectorsCaches,
     nth_of_type: NthIndexCacheInner,
     nth_last_of_type: NthIndexCacheInner,
 }
 
 impl NthIndexCache {
     /// Gets the appropriate cache for the given parameters.
-    pub fn get(&mut self, is_of_type: bool, is_from_end: bool) -> &mut NthIndexCacheInner {
-        match (is_of_type, is_from_end) {
-            (false, false) => &mut self.nth,
-            (false, true) => &mut self.nth_last,
-            (true, false) => &mut self.nth_of_type,
-            (true, true) => &mut self.nth_last_of_type,
+    pub fn get<Impl: SelectorImpl>(
+        &mut self,
+        is_of_type: bool,
+        is_from_end: bool,
+        selectors: &[Selector<Impl>],
+    ) -> &mut NthIndexCacheInner {
+        if is_of_type {
+            return if is_from_end {
+                &mut self.nth_last_of_type
+            } else {
+                &mut self.nth_of_type
+            };
         }
+        if !selectors.is_empty() {
+            return if is_from_end {
+                self.nth_last_of_selectors.lookup(selectors)
+            } else {
+                self.nth_of_selectors.lookup(selectors)
+            };
+        }
+        if is_from_end {
+            &mut self.nth_last
+        } else {
+            &mut self.nth
+        }
+    }
+}
+
+#[derive(Hash, Eq, PartialEq)]
+struct SelectorListCacheKey(usize);
+
+/// Use the selector list's pointer as the cache key
+impl SelectorListCacheKey {
+    // :nth-child of selectors are reference-counted with `ThinArc`, so we know their pointers are stable.
+    fn new<Impl: SelectorImpl>(selectors: &[Selector<Impl>]) -> Self {
+        Self(selectors.as_ptr() as usize)
+    }
+}
+
+/// Use a different map of cached indices per :nth-child's or :nth-last-child's selector list
+#[derive(Default)]
+pub struct NthIndexOfSelectorsCaches(FxHashMap<SelectorListCacheKey, NthIndexCacheInner>);
+
+/// Get or insert a map of cached incides for the selector list of this
+/// particular :nth-child or :nth-last-child pseudoclass
+impl NthIndexOfSelectorsCaches {
+    pub fn lookup<Impl: SelectorImpl>(
+        &mut self,
+        selectors: &[Selector<Impl>],
+    ) -> &mut NthIndexCacheInner {
+        self.0
+            .entry(SelectorListCacheKey::new(selectors))
+            .or_default()
     }
 }