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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 */
//! Applicable declarations management.
use crate::properties::PropertyDeclarationBlock;
use crate::rule_tree::{CascadeLevel, StyleSource};
use crate::shared_lock::Locked;
use servo_arc::Arc;
use smallvec::SmallVec;
/// List of applicable declarations. This is a transient structure that shuttles
/// declarations between selector matching and inserting into the rule tree, and
/// therefore we want to avoid heap-allocation where possible.
/// In measurements on wikipedia, we pretty much never have more than 8 applicable
/// declarations, so we could consider making this 8 entries instead of 16.
/// However, it may depend a lot on workload, and stack space is cheap.
pub type ApplicableDeclarationList = SmallVec<[ApplicableDeclarationBlock; 16]>;
/// Blink uses 18 bits to store source order, and does not check overflow [1].
/// That's a limit that could be reached in realistic webpages, so we use
/// 24 bits and enforce defined behavior in the overflow case.
/// RuleSet.h?l=128&rcl=90140ab80b84d0f889abc253410f44ed54ae04f3
const SOURCE_ORDER_SHIFT: usize = 0;
const SOURCE_ORDER_BITS: usize = 24;
const SOURCE_ORDER_MAX: u32 = (1 << SOURCE_ORDER_BITS) - 1;
/// We pack the cascade level in a single byte, see CascadeLevel::to_byte_lossy
/// for the different trade-offs there.
/// Stores the source order of a block, the cascade level it belongs to, and the
/// counter needed to handle Shadow DOM cascade order properly.
#[derive(Clone, Copy, Debug, Eq, MallocSizeOf, PartialEq)]
struct ApplicableDeclarationBits(u32);
impl ApplicableDeclarationBits {
fn new(source_order: u32, cascade_level: CascadeLevel) -> Self {
(source_order & SOURCE_ORDER_MASK) |
((cascade_level.to_byte_lossy() as u32) << CASCADE_LEVEL_SHIFT),
fn source_order(&self) -> u32 {
fn level(&self) -> CascadeLevel {
CascadeLevel::from_byte((self.0 >> CASCADE_LEVEL_SHIFT) as u8)
/// A property declaration together with its precedence among rules of equal
/// specificity so that we can sort them.
/// This represents the declarations in a given declaration block for a given
/// importance.
#[derive(Clone, Debug, MallocSizeOf, PartialEq)]
pub struct ApplicableDeclarationBlock {
/// The style source, either a style rule, or a property declaration block.
#[ignore_malloc_size_of = "Arc"]
pub source: StyleSource,
/// The bits containing the source order, cascade level, and shadow cascade
/// order.
bits: ApplicableDeclarationBits,
/// The specificity of the selector this block is represented by.
pub specificity: u32,
impl ApplicableDeclarationBlock {
/// Constructs an applicable declaration block from a given property
/// declaration block and importance.
pub fn from_declarations(
declarations: Arc<Locked<PropertyDeclarationBlock>>,
level: CascadeLevel,
) -> Self {
ApplicableDeclarationBlock {
source: StyleSource::from_declarations(declarations),
bits: ApplicableDeclarationBits::new(0, level),
specificity: 0,
/// Constructs an applicable declaration block from the given components
pub fn new(source: StyleSource, order: u32, level: CascadeLevel, specificity: u32) -> Self {
ApplicableDeclarationBlock {
bits: ApplicableDeclarationBits::new(order, level),
/// Returns the source order of the block.
pub fn source_order(&self) -> u32 {
/// Returns the cascade level of the block.
pub fn level(&self) -> CascadeLevel {
/// Convenience method to consume self and return the right thing for the
/// rule tree to iterate over.
pub fn for_rule_tree(self) -> (StyleSource, CascadeLevel) {
let level = self.level();
(self.source, level)