1 files changed, 102 insertions, 0 deletions
diff --git a/servo/components/selectors/nth_index_cache.rs b/servo/components/selectors/nth_index_cache.rs
new file mode 100644
index 0000000000..b4b41578d0
--- /dev/null
+++ b/servo/components/selectors/nth_index_cache.rs
@@ -0,0 +1,102 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+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.
+///
+/// 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_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<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()
+    }
+}
+
+/// The concrete per-pseudo-class cache.
+#[derive(Default)]
+pub struct NthIndexCacheInner(FxHashMap<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).copied()
+    }
+
+    /// 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()
+    }
+}