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/* 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/. */
//! 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 crate::parser::{Combinator, Component, NonTSPseudoClass, SelectorImpl};
use crate::sink::Push;
use servo_arc::{Arc, HeaderWithLength, ThinArc};
use smallvec::{self, SmallVec};
use std::cmp;
use std::iter;
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.
#[derive(Debug)]
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>) {
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 combinators have ever been pushed to this builder.
#[inline(always)]
pub fn has_combinators(&self) -> bool {
!self.combinators.is_empty()
}
/// Consumes the builder, producing a Selector.
#[inline(always)]
pub fn build(
&mut self,
parsed_pseudo: bool,
parsed_slotted: bool,
parsed_part: bool,
) -> ThinArc<SpecificityAndFlags, Component<Impl>> {
// Compute the specificity and flags.
let specificity = specificity(self.simple_selectors.iter());
let mut flags = SelectorFlags::empty();
if parsed_pseudo {
flags |= SelectorFlags::HAS_PSEUDO;
}
if parsed_slotted {
flags |= SelectorFlags::HAS_SLOTTED;
}
if parsed_part {
flags |= SelectorFlags::HAS_PART;
}
self.build_with_specificity_and_flags(SpecificityAndFlags { specificity, flags })
}
/// 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.
// Use a raw pointer to be able to call set_len despite "borrowing" the slice.
// This is similar to SmallVec::drain, but we use a slice here because
// we’re gonna traverse it non-linearly.
let raw_simple_selectors: *const [Component<Impl>] = &*self.simple_selectors;
unsafe {
// Panic-safety: if SelectorBuilderIter is not iterated to the end,
// some simple selectors will safely leak.
self.simple_selectors.set_len(0)
}
let (rest, current) = split_from_end(unsafe { &*raw_simple_selectors }, self.current_len);
let iter = SelectorBuilderIter {
current_simple_selectors: current.iter(),
rest_of_simple_selectors: rest,
combinators: self.combinators.drain(..).rev(),
};
Arc::into_thin(Arc::from_header_and_iter(header, iter))
}
}
struct SelectorBuilderIter<'a, Impl: SelectorImpl> {
current_simple_selectors: slice::Iter<'a, Component<Impl>>,
rest_of_simple_selectors: &'a [Component<Impl>],
combinators: iter::Rev<smallvec::Drain<'a, [(Combinator, usize); 16]>>,
}
impl<'a, Impl: SelectorImpl> ExactSizeIterator for SelectorBuilderIter<'a, Impl> {
fn len(&self) -> usize {
self.current_simple_selectors.len() +
self.rest_of_simple_selectors.len() +
self.combinators.len()
}
}
impl<'a, Impl: SelectorImpl> Iterator for SelectorBuilderIter<'a, Impl> {
type Item = Component<Impl>;
#[inline(always)]
fn next(&mut self) -> Option<Self::Item> {
if let Some(simple_selector_ref) = self.current_simple_selectors.next() {
// Move a simple selector out of this slice iterator.
// This is safe because we’ve called SmallVec::set_len(0) above,
// so SmallVec::drop won’t drop this simple selector.
unsafe { Some(ptr::read(simple_selector_ref)) }
} else {
self.combinators.next().map(|(combinator, len)| {
let (rest, current) = split_from_end(self.rest_of_simple_selectors, len);
self.rest_of_simple_selectors = rest;
self.current_simple_selectors = current.iter();
Component::Combinator(combinator)
})
}
}
fn size_hint(&self) -> (usize, Option<usize>) {
(self.len(), Some(self.len()))
}
}
fn split_from_end<T>(s: &[T], at: usize) -> (&[T], &[T]) {
s.split_at(s.len() - at)
}
bitflags! {
/// Flags that indicate at which point of parsing a selector are we.
#[derive(Default, ToShmem)]
pub (crate) struct SelectorFlags : u8 {
const HAS_PSEUDO = 1 << 0;
const HAS_SLOTTED = 1 << 1;
const HAS_PART = 1 << 2;
}
}
#[derive(Clone, Copy, Debug, Eq, PartialEq, ToShmem)]
pub struct SpecificityAndFlags {
/// There are two free bits here, since we use ten bits for each specificity
/// kind (id, class, element).
pub(crate) specificity: u32,
/// There's padding after this field due to the size of the flags.
pub(crate) flags: SelectorFlags,
}
impl SpecificityAndFlags {
#[inline]
pub fn specificity(&self) -> u32 {
self.specificity
}
#[inline]
pub fn has_pseudo_element(&self) -> bool {
self.flags.intersects(SelectorFlags::HAS_PSEUDO)
}
#[inline]
pub fn is_slotted(&self) -> bool {
self.flags.intersects(SelectorFlags::HAS_SLOTTED)
}
#[inline]
pub fn is_part(&self) -> bool {
self.flags.intersects(SelectorFlags::HAS_PART)
}
}
const MAX_10BIT: u32 = (1u32 << 10) - 1;
#[derive(Add, AddAssign, Clone, Copy, Default, Eq, Ord, PartialEq, PartialOrd)]
struct Specificity {
id_selectors: u32,
class_like_selectors: u32,
element_selectors: u32,
}
impl From<u32> for Specificity {
#[inline]
fn from(value: u32) -> Specificity {
assert!(value <= MAX_10BIT << 20 | MAX_10BIT << 10 | MAX_10BIT);
Specificity {
id_selectors: value >> 20,
class_like_selectors: (value >> 10) & MAX_10BIT,
element_selectors: value & MAX_10BIT,
}
}
}
impl From<Specificity> for u32 {
#[inline]
fn from(specificity: Specificity) -> u32 {
cmp::min(specificity.id_selectors, MAX_10BIT) << 20 |
cmp::min(specificity.class_like_selectors, MAX_10BIT) << 10 |
cmp::min(specificity.element_selectors, MAX_10BIT)
}
}
fn specificity<Impl>(iter: slice::Iter<Component<Impl>>) -> u32
where
Impl: SelectorImpl,
{
complex_selector_specificity(iter).into()
}
fn complex_selector_specificity<Impl>(iter: slice::Iter<Component<Impl>>) -> Specificity
where
Impl: SelectorImpl,
{
fn simple_selector_specificity<Impl>(
simple_selector: &Component<Impl>,
specificity: &mut Specificity,
) where
Impl: SelectorImpl,
{
match *simple_selector {
Component::Combinator(..) => {
unreachable!("Found combinator in simple selectors vector?");
},
Component::Part(..) | Component::PseudoElement(..) | Component::LocalName(..) => {
specificity.element_selectors += 1
},
Component::Slotted(ref selector) => {
specificity.element_selectors += 1;
// Note that due to the way ::slotted works we only compete with
// other ::slotted rules, so the above rule doesn't really
// matter, but we do it still for consistency with other
// pseudo-elements.
//
*specificity += Specificity::from(selector.specificity());
},
Component::Host(ref selector) => {
specificity.class_like_selectors += 1;
if let Some(ref selector) = *selector {
*specificity += Specificity::from(selector.specificity());
}
},
Component::ID(..) => {
specificity.id_selectors += 1;
},
Component::Class(..) |
Component::AttributeInNoNamespace { .. } |
Component::AttributeInNoNamespaceExists { .. } |
Component::AttributeOther(..) |
Component::FirstChild |
Component::LastChild |
Component::OnlyChild |
Component::Root |
Component::Empty |
Component::Scope |
Component::NthChild(..) |
Component::NthLastChild(..) |
Component::NthOfType(..) |
Component::NthLastOfType(..) |
Component::FirstOfType |
Component::LastOfType |
Component::OnlyOfType => {
specificity.class_like_selectors += 1;
},
Component::NonTSPseudoClass(ref pseudo) => {
if !pseudo.has_zero_specificity() {
specificity.class_like_selectors += 1;
}
},
Component::Is(ref list) => {
//
// The specificity of an :is() pseudo-class is replaced by the
// specificity of the most specific complex selector in its
// selector list argument.
let mut max = 0;
for selector in &**list {
max = std::cmp::max(selector.specificity(), max);
}
*specificity += Specificity::from(max);
},
Component::Where(..) |
Component::ExplicitUniversalType |
Component::ExplicitAnyNamespace |
Component::ExplicitNoNamespace |
Component::DefaultNamespace(..) |
Component::Namespace(..) => {
// Does not affect specificity
},
Component::Negation(ref negated) => {
for ss in negated.iter() {
simple_selector_specificity(&ss, specificity);
}
},
}
}
let mut specificity = Default::default();
for simple_selector in iter {
simple_selector_specificity(&simple_selector, &mut specificity);
}
specificity
}