Implement an nth-index cache.

MozReview-Commit-ID: Ee0um3QXkxl

components/selectors/nth_index_cache.rs

@@ -0,0 +1,56 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use fnv::FnvHashMap;
+use tree::OpaqueElement;
+
+/// A cache to speed up matching of nth-index-like selectors.
+///
+/// See [1] for some discussion around the design tradeoffs.
+///
+/// [1] https://bugzilla.mozilla.org/show_bug.cgi?id=1401855#c3
+#[derive(Default)]
+pub struct NthIndexCache {
+    nth: NthIndexCacheInner,
+    nth_last: NthIndexCacheInner,
+    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,
+        }
+    }
+}
+
+/// The concrete per-pseudo-class cache.
+#[derive(Default)]
+pub struct NthIndexCacheInner(FnvHashMap<OpaqueElement, i32>);
+
+impl NthIndexCacheInner {
+    /// Does a lookup for a given element in the cache.
+    pub fn lookup(&mut self, el: OpaqueElement) -> Option<i32> {
+        self.0.get(&el).map(|x| *x)
+    }
+
+    /// Inserts an entry into the cache.
+    pub fn insert(&mut self, element: OpaqueElement, index: i32) {
+        self.0.insert(element, index);
+    }
+
+    /// Returns whether the cache is empty.
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+}