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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 http://mozilla.org/MPL/2.0/. */
//! Applicable declarations management.
use properties::PropertyDeclarationBlock;
use rule_tree::{CascadeLevel, ShadowCascadeOrder, StyleSource};
use servo_arc::Arc;
use shared_lock::Locked;
use smallvec::SmallVec;
use std::fmt::{self, Debug};
/// 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.
///
/// [1] https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/css/
/// 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;
const SOURCE_ORDER_MASK: u32 = SOURCE_ORDER_MAX << SOURCE_ORDER_SHIFT;
/// We store up-to-15 shadow order levels.
///
/// You'd need an element slotted across 16 components with ::slotted rules to
/// trigger this as of this writing, which looks... Unlikely.
const SHADOW_CASCADE_ORDER_SHIFT: usize = SOURCE_ORDER_BITS;
const SHADOW_CASCADE_ORDER_BITS: usize = 4;
const SHADOW_CASCADE_ORDER_MAX: u8 = (1 << SHADOW_CASCADE_ORDER_BITS) - 1;
const SHADOW_CASCADE_ORDER_MASK: u32 = (SHADOW_CASCADE_ORDER_MAX as u32) << SHADOW_CASCADE_ORDER_SHIFT;
const CASCADE_LEVEL_SHIFT: usize = SOURCE_ORDER_BITS + SHADOW_CASCADE_ORDER_BITS;
const CASCADE_LEVEL_BITS: usize = 4;
const CASCADE_LEVEL_MAX: u8 = (1 << CASCADE_LEVEL_BITS) - 1;
const CASCADE_LEVEL_MASK: u32 = (CASCADE_LEVEL_MAX as u32) << CASCADE_LEVEL_SHIFT;
/// 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, Eq, MallocSizeOf, PartialEq)]
struct ApplicableDeclarationBits(u32);
impl ApplicableDeclarationBits {
fn new(
source_order: u32,
cascade_level: CascadeLevel,
shadow_cascade_order: ShadowCascadeOrder,
) -> Self {
debug_assert!(
cascade_level as u8 <= CASCADE_LEVEL_MAX,
"Gotta find more bits!"
);
let mut bits = ::std::cmp::min(source_order, SOURCE_ORDER_MAX);
bits |= ((shadow_cascade_order & SHADOW_CASCADE_ORDER_MAX) as u32) << SHADOW_CASCADE_ORDER_SHIFT;
bits |= (cascade_level as u8 as u32) << CASCADE_LEVEL_SHIFT;
ApplicableDeclarationBits(bits)
}
fn source_order(&self) -> u32 {
(self.0 & SOURCE_ORDER_MASK) >> SOURCE_ORDER_SHIFT
}
fn shadow_cascade_order(&self) -> ShadowCascadeOrder {
((self.0 & SHADOW_CASCADE_ORDER_MASK) >> SHADOW_CASCADE_ORDER_SHIFT) as ShadowCascadeOrder
}
fn level(&self) -> CascadeLevel {
let byte = ((self.0 & CASCADE_LEVEL_MASK) >> CASCADE_LEVEL_SHIFT) as u8;
unsafe { CascadeLevel::from_byte(byte) }
}
}
impl Debug for ApplicableDeclarationBits {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
f.debug_struct("ApplicableDeclarationBits")
.field("source_order", &self.source_order())
.field("shadow_cascade_order", &self.shadow_cascade_order())
.field("level", &self.level())
.finish()
}
}
/// 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.
#[inline]
pub fn from_declarations(
declarations: Arc<Locked<PropertyDeclarationBlock>>,
level: CascadeLevel,
) -> Self {
ApplicableDeclarationBlock {
source: StyleSource::from_declarations(declarations),
bits: ApplicableDeclarationBits::new(0, level, 0),
specificity: 0,
}
}
/// Constructs an applicable declaration block from the given components
#[inline]
pub fn new(
source: StyleSource,
order: u32,
level: CascadeLevel,
specificity: u32,
shadow_cascade_order: ShadowCascadeOrder,
) -> Self {
ApplicableDeclarationBlock {
source,
bits: ApplicableDeclarationBits::new(order, level, shadow_cascade_order),
specificity,
}
}
/// Returns the source order of the block.
#[inline]
pub fn source_order(&self) -> u32 {
self.bits.source_order()
}
/// Returns the cascade level of the block.
#[inline]
pub fn level(&self) -> CascadeLevel {
self.bits.level()
}
/// Convenience method to consume self and return the right thing for the
/// rule tree to iterate over.
#[inline]
pub fn for_rule_tree(self) -> (StyleSource, CascadeLevel, ShadowCascadeOrder) {
let level = self.level();
let cascade_order = self.bits.shadow_cascade_order();
(self.source, level, cascade_order)
}
}
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