servo/components/style/selector_map.rs

/* 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/. */

//! A data structure to efficiently index structs containing selectors by local
//! name, ids and hash.

use {Atom, LocalName};
use applicable_declarations::ApplicableDeclarationList;
use context::QuirksMode;
use dom::TElement;
use fallible::FallibleVec;
use hash::{HashMap, HashSet};
use hash::map as hash_map;
use hashglobe::FailedAllocationError;
use precomputed_hash::PrecomputedHash;
use rule_tree::CascadeLevel;
use selector_parser::SelectorImpl;
use selectors::matching::{matches_selector, MatchingContext, ElementSelectorFlags};
use selectors::parser::{Component, Combinator, SelectorIter};
use smallvec::SmallVec;
use std::hash::{BuildHasherDefault, Hash, Hasher};
use stylist::Rule;

/// A hasher implementation that doesn't hash anything, because it expects its
/// input to be a suitable u32 hash.
pub struct PrecomputedHasher {
    hash: Option<u32>,
}

impl Default for PrecomputedHasher {
    fn default() -> Self {
        Self { hash: None }
    }
}

/// A simple alias for a hashmap using PrecomputedHasher.
pub type PrecomputedHashMap<K, V> = HashMap<K, V, BuildHasherDefault<PrecomputedHasher>>;

/// A simple alias for a hashset using PrecomputedHasher.
pub type PrecomputedHashSet<K> = HashSet<K, BuildHasherDefault<PrecomputedHasher>>;

impl Hasher for PrecomputedHasher {
    #[inline]
    fn write(&mut self, _: &[u8]) {
        unreachable!("Called into PrecomputedHasher with something that isn't \
                     a u32")
    }

    #[inline]
    fn write_u32(&mut self, i: u32) {
        debug_assert!(self.hash.is_none());
        self.hash = Some(i);
    }

    #[inline]
    fn finish(&self) -> u64 {
        self.hash.expect("PrecomputedHasher wasn't fed?") as u64
    }
}

/// A trait to abstract over a given selector map entry.
pub trait SelectorMapEntry : Sized + Clone {
    /// Gets the selector we should use to index in the selector map.
    fn selector(&self) -> SelectorIter<SelectorImpl>;
}

/// Map element data to selector-providing objects for which the last simple
/// selector starts with them.
///
/// e.g.,
/// "p > img" would go into the set of selectors corresponding to the
/// element "img"
/// "a .foo .bar.baz" would go into the set of selectors corresponding to
/// the class "bar"
///
/// Because we match selectors right-to-left (i.e., moving up the tree
/// from an element), we need to compare the last simple selector in the
/// selector with the element.
///
/// So, if an element has ID "id1" and classes "foo" and "bar", then all
/// the rules it matches will have their last simple selector starting
/// either with "#id1" or with ".foo" or with ".bar".
///
/// Hence, the union of the rules keyed on each of element's classes, ID,
/// element name, etc. will contain the Selectors that actually match that
/// element.
///
/// We use a 1-entry SmallVec to avoid a separate heap allocation in the case
/// where we only have one entry, which is quite common. See measurements in:
/// * https://bugzilla.mozilla.org/show_bug.cgi?id=1363789#c5
/// * https://bugzilla.mozilla.org/show_bug.cgi?id=681755
///
/// TODO: Tune the initial capacity of the HashMap
#[derive(Debug, MallocSizeOf)]
pub struct SelectorMap<T: 'static> {
    /// A hash from an ID to rules which contain that ID selector.
    pub id_hash: MaybeCaseInsensitiveHashMap<Atom, SmallVec<[T; 1]>>,
    /// A hash from a class name to rules which contain that class selector.
    pub class_hash: MaybeCaseInsensitiveHashMap<Atom, SmallVec<[T; 1]>>,
    /// A hash from local name to rules which contain that local name selector.
    pub local_name_hash: PrecomputedHashMap<LocalName, SmallVec<[T; 1]>>,
    /// Rules that don't have ID, class, or element selectors.
    pub other: SmallVec<[T; 1]>,
    /// The number of entries in this map.
    pub count: usize,
}

// FIXME(Manishearth) the 'static bound can be removed when
// our HashMap fork (hashglobe) is able to use NonZero,
// or when stdlib gets fallible collections
impl<T: 'static> SelectorMap<T> {
    /// Trivially constructs an empty `SelectorMap`.
    pub fn new() -> Self {
        SelectorMap {
            id_hash: MaybeCaseInsensitiveHashMap::new(),
            class_hash: MaybeCaseInsensitiveHashMap::new(),
            local_name_hash: HashMap::default(),
            other: SmallVec::new(),
            count: 0,
        }
    }

    /// Clears the hashmap retaining storage.
    pub fn clear(&mut self) {
        self.id_hash.clear();
        self.class_hash.clear();
        self.local_name_hash.clear();
        self.other.clear();
        self.count = 0;
    }

    /// Returns whether there are any entries in the map.
    pub fn is_empty(&self) -> bool {
        self.count == 0
    }

    /// Returns the number of entries.
    pub fn len(&self) -> usize {
        self.count
    }
}

impl SelectorMap<Rule> {
    /// Append to `rule_list` all Rules in `self` that match element.
    ///
    /// Extract matching rules as per element's ID, classes, tag name, etc..
    /// Sort the Rules at the end to maintain cascading order.
    pub fn get_all_matching_rules<E, F>(
        &self,
        element: &E,
        rule_hash_target: &E,
        matching_rules_list: &mut ApplicableDeclarationList,
        context: &mut MatchingContext<E::Impl>,
        quirks_mode: QuirksMode,
        flags_setter: &mut F,
        cascade_level: CascadeLevel,
    )
    where
        E: TElement,
        F: FnMut(&E, ElementSelectorFlags),
    {
        if self.is_empty() {
            return
        }

        // At the end, we're going to sort the rules that we added, so remember where we began.
        let init_len = matching_rules_list.len();
        if let Some(id) = rule_hash_target.get_id() {
            if let Some(rules) = self.id_hash.get(&id, quirks_mode) {
                SelectorMap::get_matching_rules(element,
                                                rules,
                                                matching_rules_list,
                                                context,
                                                flags_setter,
                                                cascade_level)
            }
        }

        rule_hash_target.each_class(|class| {
            if let Some(rules) = self.class_hash.get(&class, quirks_mode) {
                SelectorMap::get_matching_rules(element,
                                                rules,
                                                matching_rules_list,
                                                context,
                                                flags_setter,
                                                cascade_level)
            }
        });

        if let Some(rules) = self.local_name_hash.get(rule_hash_target.get_local_name()) {
            SelectorMap::get_matching_rules(element,
                                            rules,
                                            matching_rules_list,
                                            context,
                                            flags_setter,
                                            cascade_level)
        }

        SelectorMap::get_matching_rules(element,
                                        &self.other,
                                        matching_rules_list,
                                        context,
                                        flags_setter,
                                        cascade_level);

        // Sort only the rules we just added.
        matching_rules_list[init_len..].sort_unstable_by_key(|block| (block.specificity, block.source_order()));
    }

    /// Adds rules in `rules` that match `element` to the `matching_rules` list.
    fn get_matching_rules<E, F>(
        element: &E,
        rules: &[Rule],
        matching_rules: &mut ApplicableDeclarationList,
        context: &mut MatchingContext<E::Impl>,
        flags_setter: &mut F,
        cascade_level: CascadeLevel,
    )
    where
        E: TElement,
        F: FnMut(&E, ElementSelectorFlags),
    {
        for rule in rules {
            if matches_selector(&rule.selector,
                                0,
                                Some(&rule.hashes),
                                element,
                                context,
                                flags_setter) {
                matching_rules.push(
                    rule.to_applicable_declaration_block(cascade_level));
            }
        }
    }
}

impl<T: SelectorMapEntry> SelectorMap<T> {
    /// Inserts into the correct hash, trying id, class, and localname.
    pub fn insert(
        &mut self,
        entry: T,
        quirks_mode: QuirksMode
    ) -> Result<(), FailedAllocationError> {
        self.count += 1;

        let vector = match find_bucket(entry.selector()) {
            Bucket::ID(id) => {
                self.id_hash.try_entry(id.clone(), quirks_mode)?
                    .or_insert_with(SmallVec::new)
            }
            Bucket::Class(class) => {
                self.class_hash.try_entry(class.clone(), quirks_mode)?
                    .or_insert_with(SmallVec::new)
            }
            Bucket::LocalName { name, lower_name } => {
                // If the local name in the selector isn't lowercase, insert it
                // into the rule hash twice. This means that, during lookup, we
                // can always find the rules based on the local name of the
                // element, regardless of whether it's an html element in an
                // html document (in which case we match against lower_name) or
                // not (in which case we match against name).
                //
                // In the case of a non-html-element-in-html-document with a
                // lowercase localname and a non-lowercase selector, the
                // rulehash lookup may produce superfluous selectors, but the
                // subsequent selector matching work will filter them out.
                if name != lower_name {
                    self.local_name_hash
                        .try_entry(lower_name.clone())?
                        .or_insert_with(SmallVec::new)
                        .try_push(entry.clone())?;
                }
                self.local_name_hash.try_entry(name.clone())?
                    .or_insert_with(SmallVec::new)
            }
            Bucket::Universal => {
                &mut self.other
            }
        };

        vector.try_push(entry)
    }

    /// Looks up entries by id, class, local name, and other (in order).
    ///
    /// Each entry is passed to the callback, which returns true to continue
    /// iterating entries, or false to terminate the lookup.
    ///
    /// Returns false if the callback ever returns false.
    ///
    /// FIXME(bholley) This overlaps with SelectorMap<Rule>::get_all_matching_rules,
    /// but that function is extremely hot and I'd rather not rearrange it.
    #[inline]
    pub fn lookup<'a, E, F>(&'a self, element: E, quirks_mode: QuirksMode, mut f: F) -> bool
    where
        E: TElement,
        F: FnMut(&'a T) -> bool
    {
        // Id.
        if let Some(id) = element.get_id() {
            if let Some(v) = self.id_hash.get(&id, quirks_mode) {
                for entry in v.iter() {
                    if !f(&entry) {
                        return false;
                    }
                }
            }
        }

        // Class.
        let mut done = false;
        element.each_class(|class| {
            if !done {
                if let Some(v) = self.class_hash.get(class, quirks_mode) {
                    for entry in v.iter() {
                        if !f(&entry) {
                            done = true;
                            return;
                        }
                    }
                }
            }
        });
        if done {
            return false;
        }

        // Local name.
        if let Some(v) = self.local_name_hash.get(element.get_local_name()) {
            for entry in v.iter() {
                if !f(&entry) {
                    return false;
                }
            }
        }

        // Other.
        for entry in self.other.iter() {
            if !f(&entry) {
                return false;
            }
        }

        true
    }

    /// Performs a normal lookup, and also looks up entries for the passed-in
    /// id and classes.
    ///
    /// Each entry is passed to the callback, which returns true to continue
    /// iterating entries, or false to terminate the lookup.
    ///
    /// Returns false if the callback ever returns false.
    #[inline]
    pub fn lookup_with_additional<'a, E, F>(
        &'a self,
        element: E,
        quirks_mode: QuirksMode,
        additional_id: Option<&Atom>,
        additional_classes: &[Atom],
        mut f: F,
    ) -> bool
    where
        E: TElement,
        F: FnMut(&'a T) -> bool
    {
        // Do the normal lookup.
        if !self.lookup(element, quirks_mode, |entry| f(entry)) {
            return false;
        }

        // Check the additional id.
        if let Some(id) = additional_id {
            if let Some(v) = self.id_hash.get(id, quirks_mode) {
                for entry in v.iter() {
                    if !f(&entry) {
                        return false;
                    }
                }
            }
        }

        // Check the additional classes.
        for class in additional_classes {
            if let Some(v) = self.class_hash.get(class, quirks_mode) {
                for entry in v.iter() {
                    if !f(&entry) {
                        return false;
                    }
                }
            }
        }

        true
    }
}

enum Bucket<'a> {
    ID(&'a Atom),
    Class(&'a Atom),
    LocalName { name: &'a LocalName, lower_name: &'a LocalName, },
    Universal,
}

fn specific_bucket_for<'a>(
    component: &'a Component<SelectorImpl>
) -> Bucket<'a> {
    match *component {
        Component::ID(ref id) => Bucket::ID(id),
        Component::Class(ref class) => Bucket::Class(class),
        Component::LocalName(ref selector) => {
            Bucket::LocalName {
                name: &selector.name,
                lower_name: &selector.lower_name,
            }
        }
        _ => Bucket::Universal
    }
}

/// Searches a compound selector from left to right, and returns the appropriate
/// bucket for it.
#[inline(always)]
fn find_bucket<'a>(mut iter: SelectorIter<'a, SelectorImpl>) -> Bucket<'a> {
    let mut current_bucket = Bucket::Universal;

    loop {
        // We basically want to find the most specific bucket,
        // where:
        //
        //   id > class > local name > universal.
        //
        for ss in &mut iter {
            let new_bucket = specific_bucket_for(ss);
            match new_bucket {
                Bucket::ID(..) => return new_bucket,
                Bucket::Class(..) => {
                    current_bucket = new_bucket;
                }
                Bucket::LocalName { .. } => {
                    if matches!(current_bucket, Bucket::Universal) {
                        current_bucket = new_bucket;
                    }
                }
                Bucket::Universal => {},
            }
        }

        // Effectively, pseudo-elements are ignored, given only state
        // pseudo-classes may appear before them.
        if iter.next_sequence() != Some(Combinator::PseudoElement) {
            break;
        }
    }

    return current_bucket
}

/// Wrapper for PrecomputedHashMap that does ASCII-case-insensitive lookup in quirks mode.
#[derive(Debug, MallocSizeOf)]
pub struct MaybeCaseInsensitiveHashMap<K: PrecomputedHash + Hash + Eq, V: 'static>(PrecomputedHashMap<K, V>);

// FIXME(Manishearth) the 'static bound can be removed when
// our HashMap fork (hashglobe) is able to use NonZero,
// or when stdlib gets fallible collections
impl<V: 'static> MaybeCaseInsensitiveHashMap<Atom, V> {
    /// Empty map
    pub fn new() -> Self {
        MaybeCaseInsensitiveHashMap(PrecomputedHashMap::default())
    }

    /// HashMap::entry
    pub fn entry(&mut self, mut key: Atom, quirks_mode: QuirksMode) -> hash_map::Entry<Atom, V> {
        if quirks_mode == QuirksMode::Quirks {
            key = key.to_ascii_lowercase()
        }
        self.0.entry(key)
    }

    /// HashMap::try_entry
    pub fn try_entry(
        &mut self,
        mut key: Atom,
        quirks_mode: QuirksMode
    ) -> Result<hash_map::Entry<Atom, V>, FailedAllocationError> {
        if quirks_mode == QuirksMode::Quirks {
            key = key.to_ascii_lowercase()
        }
        self.0.try_entry(key)
    }

    /// HashMap::iter
    pub fn iter(&self) -> hash_map::Iter<Atom, V> {
        self.0.iter()
    }

    /// HashMap::clear
    pub fn clear(&mut self) {
        self.0.clear()
    }

    /// HashMap::get
    pub fn get(&self, key: &Atom, quirks_mode: QuirksMode) -> Option<&V> {
        if quirks_mode == QuirksMode::Quirks {
            self.0.get(&key.to_ascii_lowercase())
        } else {
            self.0.get(key)
        }
    }
}