Invert the order of each compound selector.

MozReview-Commit-ID: JkBjqyYgbrB

components/selectors/builder.rs

@@ -2,15 +2,208 @@
  * 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 parser::{Component, SelectorImpl, SelectorIter};
+//! Helper module to build up a selector safely and efficiently.
+//!
+//! Our selector representation is designed to optimize matching, and has
+//! several requirements:
+//! * All simple selectors and combinators are stored inline in the same buffer
+//!   as Component instances.
+//! * We store the top-level compound selectors from right to left, i.e. in
+//!   matching order.
+//! * We store the simple selectors for each combinator from left to right, so
+//!   that we match the cheaper simple selectors first.
+//!
+//! Meeting all these constraints without extra memmove traffic during parsing
+//! is non-trivial. This module encapsulates those details and presents an
+//! easy-to-use API for the parser.
+
+use parser::{Combinator, Component, SelectorImpl};
+use servo_arc::{Arc, HeaderWithLength, ThinArc};
+use sink::Push;
+use smallvec::SmallVec;
 use std::cmp;
 use std::ops::Add;
+use std::ptr;
+use std::slice;
+
+/// Top-level SelectorBuilder struct. This should be stack-allocated by the
+/// consumer and never moved (because it contains a lot of inline data that
+/// would be slow to memmov).
+///
+/// After instantation, callers may call the push_simple_selector() and
+/// push_combinator() methods to append selector data as it is encountered
+/// (from left to right). Once the process is complete, callers should invoke
+/// build(), which transforms the contents of the SelectorBuilder into a heap-
+/// allocated Selector and leaves the builder in a drained state.
+pub struct SelectorBuilder<Impl: SelectorImpl> {
+    /// The entire sequence of simple selectors, from left to right, without combinators.
+    ///
+    /// We make this large because the result of parsing a selector is fed into a new
+    /// Arc-ed allocation, so any spilled vec would be a wasted allocation. Also,
+    /// Components are large enough that we don't have much cache locality benefit
+    /// from reserving stack space for fewer of them.
+    simple_selectors: SmallVec<[Component<Impl>; 32]>,
+    /// The combinators, and the length of the compound selector to their left.
+    combinators: SmallVec<[(Combinator, usize); 16]>,
+    /// The length of the current compount selector.
+    current_len: usize,
+}
+
+impl<Impl: SelectorImpl> Default for SelectorBuilder<Impl> {
+    #[inline(always)]
+    fn default() -> Self {
+        SelectorBuilder {
+            simple_selectors: SmallVec::new(),
+            combinators: SmallVec::new(),
+            current_len: 0,
+        }
+    }
+}
+
+impl<Impl: SelectorImpl> Push<Component<Impl>> for SelectorBuilder<Impl> {
+    fn push(&mut self, value: Component<Impl>) {
+        self.push_simple_selector(value);
+    }
+}
+
+impl<Impl: SelectorImpl> SelectorBuilder<Impl> {
+    /// Pushes a simple selector onto the current compound selector.
+    #[inline(always)]
+    pub fn push_simple_selector(&mut self, ss: Component<Impl>) {
+        debug_assert!(!ss.is_combinator());
+        self.simple_selectors.push(ss);
+        self.current_len += 1;
+    }
+
+    /// Completes the current compound selector and starts a new one, delimited
+    /// by the given combinator.
+    #[inline(always)]
+    pub fn push_combinator(&mut self, c: Combinator) {
+        self.combinators.push((c, self.current_len));
+        self.current_len = 0;
+    }
+
+    /// Returns true if no simple selectors have ever been pushed to this builder.
+    #[inline(always)]
+    pub fn is_empty(&self) -> bool {
+        self.simple_selectors.is_empty()
+    }
+
+    /// Consumes the builder, producing a Selector.
+    #[inline(always)]
+    pub fn build(&mut self, parsed_pseudo: bool) -> ThinArc<SpecificityAndFlags, Component<Impl>> {
+        // Compute the specificity and flags.
+        let mut spec = SpecificityAndFlags(specificity(self.simple_selectors.iter()));
+        if parsed_pseudo {
+            spec.0 |= HAS_PSEUDO_BIT;
+        }
+
+        self.build_with_specificity_and_flags(spec)
+    }
+
+
+    /// Builds with an explicit SpecificityAndFlags. This is separated from build() so
+    /// that unit tests can pass an explicit specificity.
+    #[inline(always)]
+    pub fn build_with_specificity_and_flags(&mut self, spec: SpecificityAndFlags)
+                                            -> ThinArc<SpecificityAndFlags, Component<Impl>> {
+        // First, compute the total number of Components we'll need to allocate
+        // space for.
+        let full_len = self.simple_selectors.len() + self.combinators.len();
+
+        // Create the header.
+        let header = HeaderWithLength::new(spec, full_len);
+
+        // Create the Arc using an iterator that drains our buffers.
+        let iter = SelectorBuilderIter::new(self, full_len);
+        Arc::into_thin(Arc::from_header_and_iter(header, iter))
+    }
+}
+
+struct SelectorBuilderIter<'a, Impl: SelectorImpl> {
+    builder: &'a mut SelectorBuilder<Impl>,
+    end: *const Component<Impl>,
+    base: *const Component<Impl>,
+    ptr: *const Component<Impl>,
+    full_len: usize,
+}
+
+impl<'a, Impl: SelectorImpl> SelectorBuilderIter<'a, Impl> {
+    fn new(builder: &'a mut SelectorBuilder<Impl>, full_len: usize) -> Self {
+        // Store a pointer to the end of the array of simple selectors,
+        // and set ourselves up to iterate the rightmost compound selector.
+        let sequence_base = &*builder.simple_selectors as *const [Component<Impl>] as *const Component<Impl>;
+        let end = unsafe { sequence_base.offset(builder.simple_selectors.len() as isize) };
+        let base = unsafe { end.offset(-(builder.current_len as isize)) };
+        let ptr = base;
+
+        // Next, tell the SmallVec to forget about its entries so that they
+        // won't be dropped when it frees its buffer. We're transferring
+        // ownership into the selector.
+        unsafe { builder.simple_selectors.set_len(0); }
+
+        SelectorBuilderIter {
+            builder: builder,
+            end: end,
+            base: base,
+            ptr: ptr,
+            full_len: full_len,
+        }
+    }
+}
+
+impl<'a, Impl: SelectorImpl> ExactSizeIterator for SelectorBuilderIter<'a, Impl> {
+    fn len(&self) -> usize {
+        self.full_len
+    }
+}
+
+impl<'a, Impl: SelectorImpl> Iterator for SelectorBuilderIter<'a, Impl> {
+    type Item = Component<Impl>;
+    #[inline(always)]
+    fn next(&mut self) -> Option<Self::Item> {
+        // If ptr is below end, continue walking this compound selector.
+        if self.ptr != self.end {
+            debug_assert!(self.ptr < self.end);
+            let result = unsafe { Some(ptr::read(self.ptr)) };
+            self.ptr = unsafe { self.ptr.offset(1) };
+            return result;
+        }
+
+        if let Some((combinator, len)) = self.builder.combinators.pop() {
+            // There's another compound selector. Reset the pointers to iterate it,
+            // and then return the combinator.
+            self.end = self.base;
+            self.base = unsafe { self.end.offset(-(len as isize)) };
+            self.ptr = self.base;
+            Some(Component::Combinator(combinator))
+        } else {
+            // All done.
+            None
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.len(), Some(self.len()))
+    }
+}
+
+#[cfg(debug_assertions)]
+impl<'a, Impl: SelectorImpl> Drop for SelectorBuilderIter<'a, Impl> {
+    fn drop(&mut self) {
+        // Failing to iterate the entire builder would cause us to leak (but not
+        // crash). Verify that this doesn't happen.
+        debug_assert!(self.builder.simple_selectors.len() == 0);
+        debug_assert!(self.builder.combinators.len() == 0);
+        debug_assert!(self.ptr == self.end);
+    }
+}
+
+pub const HAS_PSEUDO_BIT: u32 = 1 << 30;
 
 #[derive(Copy, Clone, Debug, Eq, PartialEq)]
 pub struct SpecificityAndFlags(pub u32);
 
-pub const HAS_PSEUDO_BIT: u32 = 1 << 30;
-
 impl SpecificityAndFlags {
     pub fn specificity(&self) -> u32 {
         self.0 & !HAS_PSEUDO_BIT
@@ -73,13 +266,13 @@ impl From<Specificity> for u32 {
     }
 }
 
-pub fn specificity<Impl>(iter: SelectorIter<Impl>) -> u32
+fn specificity<Impl>(iter: slice::Iter<Component<Impl>>) -> u32
     where Impl: SelectorImpl
 {
     complex_selector_specificity(iter).into()
 }
 
-fn complex_selector_specificity<Impl>(mut iter: SelectorIter<Impl>)
+fn complex_selector_specificity<Impl>(mut iter: slice::Iter<Component<Impl>>)
                                       -> Specificity
     where Impl: SelectorImpl
 {
@@ -129,13 +322,8 @@ fn complex_selector_specificity<Impl>(mut iter: SelectorIter<Impl>)
     }
 
     let mut specificity = Default::default();
-    loop {
-        for simple_selector in &mut iter {
-            simple_selector_specificity(&simple_selector, &mut specificity);
-        }
-        if iter.next_sequence().is_none() {
-            break;
-        }
+    for simple_selector in &mut iter {
+        simple_selector_specificity(&simple_selector, &mut specificity);
     }
     specificity
 }