servo/components/style/selector_matching.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/. */

use std::collections::hashmap::HashMap;
use std::hash::Hash;
use std::num::div_rem;
use sync::Arc;

use url::Url;

use servo_util::bloom::BloomFilter;
use servo_util::resource_files::read_resource_file;
use servo_util::smallvec::VecLike;
use servo_util::sort;
use string_cache::Atom;

use media_queries::{Device, Screen};
use node::{TElement, TNode};
use properties::{PropertyDeclaration, PropertyDeclarationBlock};
use selectors::*;
use stylesheets::{Stylesheet, iter_stylesheet_style_rules};

pub enum StylesheetOrigin {
    UserAgentOrigin,
    AuthorOrigin,
    UserOrigin,
}

/// The definition of whitespace per CSS Selectors Level 3 § 4.
pub static SELECTOR_WHITESPACE: &'static [char] = &[' ', '\t', '\n', '\r', '\x0C'];

/// Map node attributes to Rules whose last simple selector starts with them.
///
/// e.g.,
/// "p > img" would go into the set of Rules corresponding to the
/// element "img"
/// "a .foo .bar.baz" would go into the set of Rules corresponding to
/// the class "bar"
///
/// Because we match Rules right-to-left (i.e., moving up the tree
/// from a node), we need to compare the last simple selector in the
/// Rule with the node.
///
/// So, if a node 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 node's classes, ID,
/// element name, etc. will contain the Rules that actually match that
/// node.
struct SelectorMap {
    // TODO: Tune the initial capacity of the HashMap
    id_hash: HashMap<Atom, Vec<Rule>>,
    class_hash: HashMap<Atom, Vec<Rule>>,
    local_name_hash: HashMap<Atom, Vec<Rule>>,
    /// Same as local_name_hash, but keys are lower-cased.
    /// For HTML elements in HTML documents.
    lower_local_name_hash: HashMap<Atom, Vec<Rule>>,
    // For Rules that don't have ID, class, or element selectors.
    universal_rules: Vec<Rule>,
    /// Whether this hash is empty.
    empty: bool,
}

impl SelectorMap {
    fn new() -> SelectorMap {
        SelectorMap {
            id_hash: HashMap::new(),
            class_hash: HashMap::new(),
            local_name_hash: HashMap::new(),
            lower_local_name_hash: HashMap::new(),
            universal_rules: vec!(),
            empty: true,
        }
    }

    /// Append to `rule_list` all Rules in `self` that match node.
    ///
    /// Extract matching rules as per node's ID, classes, tag name, etc..
    /// Sort the Rules at the end to maintain cascading order.
    fn get_all_matching_rules<'a,
                              E:TElement<'a>,
                              N:TNode<'a, E>,
                              V:VecLike<DeclarationBlock>>(
                              &self,
                              node: &N,
                              parent_bf: &Option<BloomFilter>,
                              matching_rules_list: &mut V,
                              shareable: &mut bool) {
        if self.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.vec_len();
        let element = node.as_element();
        match element.get_id() {
            Some(id) => {
                SelectorMap::get_matching_rules_from_hash(node,
                                                          parent_bf,
                                                          &self.id_hash,
                                                          &id,
                                                          matching_rules_list,
                                                          shareable)
            }
            None => {}
        }

        match element.get_attr(&ns!(""), "class") {
            Some(ref class_attr) => {
                // FIXME: Store classes pre-split as atoms to make the loop below faster.
                for class in class_attr.split(SELECTOR_WHITESPACE) {
                    SelectorMap::get_matching_rules_from_hash(node,
                                                              parent_bf,
                                                              &self.class_hash,
                                                              &Atom::from_slice(class),
                                                              matching_rules_list,
                                                              shareable);
                }
            }
            None => {}
        }

        let local_name_hash = if node.is_html_element_in_html_document() {
            &self.lower_local_name_hash
        } else {
            &self.local_name_hash
        };
        SelectorMap::get_matching_rules_from_hash(node,
                                                  parent_bf,
                                                  local_name_hash,
                                                  element.get_local_name(),
                                                  matching_rules_list,
                                                  shareable);

        SelectorMap::get_matching_rules(node,
                                        parent_bf,
                                        self.universal_rules.as_slice(),
                                        matching_rules_list,
                                        shareable);

        // Sort only the rules we just added.
        sort::quicksort_by(matching_rules_list.vec_slice_from_mut(init_len), compare);

        fn compare(a: &DeclarationBlock, b: &DeclarationBlock) -> Ordering {
            (a.specificity, a.source_order).cmp(&(b.specificity, b.source_order))
        }
    }

    fn get_matching_rules_from_hash<'a,
                                    E:TElement<'a>,
                                    N:TNode<'a, E>,
                                    V:VecLike<DeclarationBlock>>(
                                    node: &N,
                                    parent_bf: &Option<BloomFilter>,
                                    hash: &HashMap<Atom, Vec<Rule>>,
                                    key: &Atom,
                                    matching_rules: &mut V,
                                    shareable: &mut bool) {
        match hash.find(key) {
            Some(rules) => {
                SelectorMap::get_matching_rules(node, parent_bf, rules.as_slice(), matching_rules, shareable)
            }
            None => {}
        }
    }

    /// Adds rules in `rules` that match `node` to the `matching_rules` list.
    fn get_matching_rules<'a,
                          E:TElement<'a>,
                          N:TNode<'a, E>,
                          V:VecLike<DeclarationBlock>>(
                          node: &N,
                          parent_bf: &Option<BloomFilter>,
                          rules: &[Rule],
                          matching_rules: &mut V,
                          shareable: &mut bool) {
        for rule in rules.iter() {
            if matches_compound_selector(&*rule.selector, node, parent_bf, shareable) {
                matching_rules.vec_push(rule.declarations.clone());
            }
        }
    }

    /// Insert rule into the correct hash.
    /// Order in which to try: id_hash, class_hash, local_name_hash, universal_rules.
    fn insert(&mut self, rule: Rule) {
        self.empty = false;

        match SelectorMap::get_id_name(&rule) {
            Some(id_name) => {
                self.id_hash.find_push(id_name, rule);
                return;
            }
            None => {}
        }
        match SelectorMap::get_class_name(&rule) {
            Some(class_name) => {
                self.class_hash.find_push(class_name, rule);
                return;
            }
            None => {}
        }

        match SelectorMap::get_local_name(&rule) {
            Some(LocalName { name, lower_name }) => {
                self.local_name_hash.find_push(name, rule.clone());
                self.lower_local_name_hash.find_push(lower_name, rule);
                return;
            }
            None => {}
        }

        self.universal_rules.push(rule);
    }

    /// Retrieve the first ID name in Rule, or None otherwise.
    fn get_id_name(rule: &Rule) -> Option<Atom> {
        let simple_selector_sequence = &rule.selector.simple_selectors;
        for ss in simple_selector_sequence.iter() {
            match *ss {
                // TODO(pradeep): Implement case-sensitivity based on the document type and quirks
                // mode.
                IDSelector(ref id) => return Some(id.clone()),
                _ => {}
            }
        }
        return None
    }

    /// Retrieve the FIRST class name in Rule, or None otherwise.
    fn get_class_name(rule: &Rule) -> Option<Atom> {
        let simple_selector_sequence = &rule.selector.simple_selectors;
        for ss in simple_selector_sequence.iter() {
            match *ss {
                // TODO(pradeep): Implement case-sensitivity based on the document type and quirks
                // mode.
                ClassSelector(ref class) => return Some(class.clone()),
                _ => {}
            }
        }
        return None
    }

    /// Retrieve the name if it is a type selector, or None otherwise.
    fn get_local_name(rule: &Rule) -> Option<LocalName> {
        let simple_selector_sequence = &rule.selector.simple_selectors;
        for ss in simple_selector_sequence.iter() {
            match *ss {
                LocalNameSelector(ref name) => {
                    return Some(name.clone())
                }
                _ => {}
            }
        }
        return None
    }
}

// The bloom filter for descendant CSS selectors will have a <1% false
// positive rate until it has this many selectors in it, then it will
// rapidly increase.
pub static RECOMMENDED_SELECTOR_BLOOM_FILTER_SIZE: uint = 4096;

pub struct Stylist {
    element_map: PerPseudoElementSelectorMap,
    before_map: PerPseudoElementSelectorMap,
    after_map: PerPseudoElementSelectorMap,
    rules_source_order: uint,
}

impl Stylist {
    #[inline]
    pub fn new() -> Stylist {
        let mut stylist = Stylist {
            element_map: PerPseudoElementSelectorMap::new(),
            before_map: PerPseudoElementSelectorMap::new(),
            after_map: PerPseudoElementSelectorMap::new(),
            rules_source_order: 0u,
        };
        let ua_stylesheet = Stylesheet::from_bytes(
            read_resource_file(["user-agent.css"]).unwrap().as_slice(),
            Url::parse("chrome:///user-agent.css").unwrap(),
            None,
            None);
        stylist.add_stylesheet(ua_stylesheet, UserAgentOrigin);
        stylist
    }

    pub fn add_stylesheet(&mut self, stylesheet: Stylesheet, origin: StylesheetOrigin) {
        let (mut element_map, mut before_map, mut after_map) = match origin {
            UserAgentOrigin => (
                &mut self.element_map.user_agent,
                &mut self.before_map.user_agent,
                &mut self.after_map.user_agent,
            ),
            AuthorOrigin => (
                &mut self.element_map.author,
                &mut self.before_map.author,
                &mut self.after_map.author,
            ),
            UserOrigin => (
                &mut self.element_map.user,
                &mut self.before_map.user,
                &mut self.after_map.user,
            ),
        };
        let mut rules_source_order = self.rules_source_order;

        // Take apart the StyleRule into individual Rules and insert
        // them into the SelectorMap of that priority.
        macro_rules! append(
            ($style_rule: ident, $priority: ident) => {
                if $style_rule.declarations.$priority.len() > 0 {
                    for selector in $style_rule.selectors.iter() {
                        let map = match selector.pseudo_element {
                            None => &mut element_map,
                            Some(Before) => &mut before_map,
                            Some(After) => &mut after_map,
                        };
                        map.$priority.insert(Rule {
                                selector: selector.compound_selectors.clone(),
                                declarations: DeclarationBlock {
                                    specificity: selector.specificity,
                                    declarations: $style_rule.declarations.$priority.clone(),
                                    source_order: rules_source_order,
                                },
                        });
                    }
                }
            };
        );

        let device = &Device { media_type: Screen };  // TODO, use Print when printing
        iter_stylesheet_style_rules(&stylesheet, device, |style_rule| {
            append!(style_rule, normal);
            append!(style_rule, important);
            rules_source_order += 1;
        });
        self.rules_source_order = rules_source_order;
    }

    /// Returns the applicable CSS declarations for the given element. This corresponds to
    /// `ElementRuleCollector` in WebKit.
    ///
    /// The returned boolean indicates whether the style is *shareable*; that is, whether the
    /// matched selectors are simple enough to allow the matching logic to be reduced to the logic
    /// in `css::matching::PrivateMatchMethods::candidate_element_allows_for_style_sharing`.
    pub fn push_applicable_declarations<'a,
                                        E:TElement<'a>,
                                        N:TNode<'a, E>,
                                        V:VecLike<DeclarationBlock>>(
                                        &self,
                                        element: &N,
                                        parent_bf: &Option<BloomFilter>,
                                        style_attribute: Option<&PropertyDeclarationBlock>,
                                        pseudo_element: Option<PseudoElement>,
                                        applicable_declarations: &mut V)
                                        -> bool {
        assert!(element.is_element());
        assert!(style_attribute.is_none() || pseudo_element.is_none(),
                "Style attributes do not apply to pseudo-elements");

        let map = match pseudo_element {
            None => &self.element_map,
            Some(Before) => &self.before_map,
            Some(After) => &self.after_map,
        };

        let mut shareable = true;

        // Step 1: Normal rules.
        map.user_agent.normal.get_all_matching_rules(element,
                                                     parent_bf,
                                                     applicable_declarations,
                                                     &mut shareable);
        map.user.normal.get_all_matching_rules(element, parent_bf, applicable_declarations, &mut shareable);
        map.author.normal.get_all_matching_rules(element, parent_bf, applicable_declarations, &mut shareable);

        // Step 2: Normal style attributes.
        style_attribute.map(|sa| {
            shareable = false;
            applicable_declarations.vec_push(DeclarationBlock::from_declarations(sa.normal.clone()))
        });

        // Step 3: Author-supplied `!important` rules.
        map.author.important.get_all_matching_rules(element,
                                                    parent_bf,
                                                    applicable_declarations,
                                                    &mut shareable);

        // Step 4: `!important` style attributes.
        style_attribute.map(|sa| {
            shareable = false;
            applicable_declarations.vec_push(DeclarationBlock::from_declarations(sa.important.clone()))
        });

        // Step 5: User and UA `!important` rules.
        map.user.important.get_all_matching_rules(element,
                                                  parent_bf,
                                                  applicable_declarations,
                                                  &mut shareable);
        map.user_agent.important.get_all_matching_rules(element,
                                                        parent_bf,
                                                        applicable_declarations,
                                                        &mut shareable);

        shareable
    }
}

struct PerOriginSelectorMap {
    normal: SelectorMap,
    important: SelectorMap,
}

impl PerOriginSelectorMap {
    #[inline]
    fn new() -> PerOriginSelectorMap {
        PerOriginSelectorMap {
            normal: SelectorMap::new(),
            important: SelectorMap::new(),
        }
    }
}

struct PerPseudoElementSelectorMap {
    user_agent: PerOriginSelectorMap,
    author: PerOriginSelectorMap,
    user: PerOriginSelectorMap,
}

impl PerPseudoElementSelectorMap {
    #[inline]
    fn new() -> PerPseudoElementSelectorMap {
        PerPseudoElementSelectorMap {
            user_agent: PerOriginSelectorMap::new(),
            author: PerOriginSelectorMap::new(),
            user: PerOriginSelectorMap::new(),
        }
    }
}

#[deriving(Clone)]
struct Rule {
    // This is an Arc because Rule will essentially be cloned for every node
    // that it matches. Selector contains an owned vector (through
    // CompoundSelector) and we want to avoid the allocation.
    selector: Arc<CompoundSelector>,
    declarations: DeclarationBlock,
}

/// A property declaration together with its precedence among rules of equal specificity so that
/// we can sort them.
#[deriving(Clone)]
pub struct DeclarationBlock {
    pub declarations: Arc<Vec<PropertyDeclaration>>,
    source_order: uint,
    specificity: u32,
}

impl DeclarationBlock {
    #[inline]
    pub fn from_declarations(declarations: Arc<Vec<PropertyDeclaration>>) -> DeclarationBlock {
        DeclarationBlock {
            declarations: declarations,
            source_order: 0,
            specificity: 0,
        }
    }
}

pub fn matches<'a, E:TElement<'a>, N:TNode<'a, E>>(selector_list: &SelectorList, element: &N, parent_bf: &Option<BloomFilter>) -> bool {
    get_selector_list_selectors(selector_list).iter().any(|selector|
        selector.pseudo_element.is_none() &&
        matches_compound_selector(&*selector.compound_selectors, element, parent_bf, &mut false))
}

/// Determines whether the given element matches the given single or compound selector.
///
/// NB: If you add support for any new kinds of selectors to this routine, be sure to set
/// `shareable` to false unless you are willing to update the style sharing logic. Otherwise things
/// will almost certainly break as nodes will start mistakenly sharing styles. (See the code in
/// `main/css/matching.rs`.)
fn matches_compound_selector<'a,
                             E:TElement<'a>,
                             N:TNode<'a, E>>(
                             selector: &CompoundSelector,
                             element: &N,
                             parent_bf: &Option<BloomFilter>,
                             shareable: &mut bool)
                             -> bool {
    match matches_compound_selector_internal(selector, element, parent_bf, shareable) {
        Matched => true,
        _ => false
    }
}

/// A result of selector matching, includes 3 failure types,
///
///     NotMatchedAndRestartFromClosestLaterSibling
///     NotMatchedAndRestartFromClosestDescendant
///     NotMatchedGlobally
///
/// When NotMatchedGlobally appears, stop selector matching completely since
/// the succeeding selectors never matches.
/// It is raised when
///     Child combinator cannot find the candidate element.
///     Descendant combinator cannot find the candidate element.
///
/// When NotMatchedAndRestartFromClosestDescendant appears, the selector
/// matching does backtracking and restarts from the closest Descendant
/// combinator.
/// It is raised when
///     NextSibling combinator cannot find the candidate element.
///     LaterSibling combinator cannot find the candidate element.
///     Child combinator doesn't match on the found element.
///
/// When NotMatchedAndRestartFromClosestLaterSibling appears, the selector
/// matching does backtracking and restarts from the closest LaterSibling
/// combinator.
/// It is raised when
///     NextSibling combinator doesn't match on the found element.
///
/// For example, when the selector "d1 d2 a" is provided and we cannot *find*
/// an appropriate ancestor node for "d1", this selector matching raises
/// NotMatchedGlobally since even if "d2" is moved to more upper node, the
/// candidates for "d1" becomes less than before and d1 .
///
/// The next example is siblings. When the selector "b1 + b2 ~ d1 a" is
/// providied and we cannot *find* an appropriate brother node for b1,
/// the selector matching raises NotMatchedAndRestartFromClosestDescendant.
/// The selectors ("b1 + b2 ~") doesn't match and matching restart from "d1".
///
/// The additional example is child and sibling. When the selector
/// "b1 + c1 > b2 ~ d1 a" is provided and the selector "b1" doesn't match on
/// the element, this "b1" raises NotMatchedAndRestartFromClosestLaterSibling.
/// However since the selector "c1" raises
/// NotMatchedAndRestartFromClosestDescendant. So the selector
/// "b1 + c1 > b2 ~ " doesn't match and restart matching from "d1".
enum SelectorMatchingResult {
    Matched,
    NotMatchedAndRestartFromClosestLaterSibling,
    NotMatchedAndRestartFromClosestDescendant,
    NotMatchedGlobally,
}

/// Quickly figures out whether or not the compound selector is worth doing more
/// work on. If the simple selectors don't match, or there's a child selector
/// that does not appear in the bloom parent bloom filter, we can exit early.
fn can_fast_reject<'a, E: TElement<'a>, N: TNode<'a, E>>(
  mut selector: &CompoundSelector,
  element: &N,
  parent_bf: &Option<BloomFilter>,
  shareable: &mut bool) -> Option<SelectorMatchingResult> {
    if !selector.simple_selectors.iter().all(|simple_selector| {
      matches_simple_selector(simple_selector, element, shareable) }) {
        return Some(NotMatchedAndRestartFromClosestLaterSibling);
    }

    let bf: &BloomFilter =
        match *parent_bf {
            None => return None,
            Some(ref bf) => bf,
        };

    // See if the bloom filter can exclude any of the descendant selectors, and
    // reject if we can.
    loop {
         match selector.next {
             None => break,
             Some((ref cs, Descendant)) => selector = &**cs,
             Some((ref cs, _)) => {
                 selector = &**cs;
                 continue;
             }
         };

        for ss in selector.simple_selectors.iter() {
            match *ss {
                LocalNameSelector(LocalName { ref name, ref lower_name })  => {
                    if bf.definitely_excludes(name)
                    && bf.definitely_excludes(lower_name) {
                        return Some(NotMatchedGlobally);
                    }
                },
                NamespaceSelector(ref namespace) => {
                    if bf.definitely_excludes(namespace) {
                        return Some(NotMatchedGlobally);
                    }
                },
                IDSelector(ref id) => {
                    if bf.definitely_excludes(id) {
                        return Some(NotMatchedGlobally);
                    }
                },
                ClassSelector(ref class) => {
                    if bf.definitely_excludes(&class.as_slice()) {
                        return Some(NotMatchedGlobally);
                    }
                },
                _ => {},
            }
        }

    }

    // Can't fast reject.
    return None;
}

fn matches_compound_selector_internal<'a,
                                      E:TElement<'a>,
                                      N:TNode<'a, E>>(
                                      selector: &CompoundSelector,
                                      element: &N,
                                      parent_bf: &Option<BloomFilter>,
                                      shareable: &mut bool)
                                      -> SelectorMatchingResult {
    match can_fast_reject(selector, element, parent_bf, shareable) {
        None => {},
        Some(result) => return result,
    };

    match selector.next {
        None => Matched,
        Some((ref next_selector, combinator)) => {
            let (siblings, candidate_not_found) = match combinator {
                Child => (false, NotMatchedGlobally),
                Descendant => (false, NotMatchedGlobally),
                NextSibling => (true, NotMatchedAndRestartFromClosestDescendant),
                LaterSibling => (true, NotMatchedAndRestartFromClosestDescendant),
            };
            let mut node = (*element).clone();
            loop {
                let next_node = if siblings {
                    node.prev_sibling()
                } else {
                    node.parent_node()
                };
                match next_node {
                    None => return candidate_not_found,
                    Some(next_node) => node = next_node,
                }
                if node.is_element() {
                    let result = matches_compound_selector_internal(&**next_selector,
                                                                    &node,
                                                                    parent_bf,
                                                                    shareable);
                    match (result, combinator) {
                        // Return the status immediately.
                        (Matched, _) => return result,
                        (NotMatchedGlobally, _) => return result,

                        // Upgrade the failure status to
                        // NotMatchedAndRestartFromClosestDescendant.
                        (_, Child) => return NotMatchedAndRestartFromClosestDescendant,

                        // Return the status directly.
                        (_, NextSibling) => return result,

                        // If the failure status is NotMatchedAndRestartFromClosestDescendant
                        // and combinator is LaterSibling, give up this LaterSibling matching
                        // and restart from the closest descendant combinator.
                        (NotMatchedAndRestartFromClosestDescendant, LaterSibling) => return result,

                        // The Descendant combinator and the status is
                        // NotMatchedAndRestartFromClosestLaterSibling or
                        // NotMatchedAndRestartFromClosestDescendant,
                        // or the LaterSibling combinator and the status is
                        // NotMatchedAndRestartFromClosestDescendant
                        // can continue to matching on the next candidate element.
                        _ => {},
                    }
                }
            }
        }
    }
}

/// Determines whether the given element matches the given single selector.
///
/// NB: If you add support for any new kinds of selectors to this routine, be sure to set
/// `shareable` to false unless you are willing to update the style sharing logic. Otherwise things
/// will almost certainly break as nodes will start mistakenly sharing styles. (See the code in
/// `main/css/matching.rs`.)
#[inline]
pub fn matches_simple_selector<'a, E:TElement<'a>,
                           N:TNode<'a, E>>(
                           selector: &SimpleSelector,
                           element: &N,
                           shareable: &mut bool)
                           -> bool {
    match *selector {
        LocalNameSelector(LocalName { ref name, ref lower_name }) => {
            let name = if element.is_html_element_in_html_document() { lower_name } else { name };
            let element = element.as_element();
            element.get_local_name() == name
        }

        NamespaceSelector(ref namespace) => {
            *shareable = false;
            let element = element.as_element();
            element.get_namespace() == namespace
        }
        // TODO: case-sensitivity depends on the document type and quirks mode
        // TODO: cache and intern IDs on elements.
        IDSelector(ref id) => {
            *shareable = false;
            let element = element.as_element();
            element.get_id().map_or(false, |attr| {
                attr == *id
            })
        }
        // TODO: cache and intern class names on elements.
        ClassSelector(ref class) => {
            let element = element.as_element();
            element.has_class(class.as_slice())
        }

        AttrExists(ref attr) => {
            *shareable = false;
            element.match_attr(attr, |_| true)
        }
        AttrEqual(ref attr, ref value) => {
            if value.as_slice() != "DIR" {
                // FIXME(pcwalton): Remove once we start actually supporting RTL text. This is in
                // here because the UA style otherwise disables all style sharing completely.
                *shareable = false
            }
            element.match_attr(attr, |attr_value| {
                attr_value == value.as_slice()
            })
        }
        AttrIncludes(ref attr, ref value) => {
            *shareable = false;
            element.match_attr(attr, |attr_value| {
                attr_value.split(SELECTOR_WHITESPACE).any(|v| v == value.as_slice())
            })
        }
        AttrDashMatch(ref attr, ref value, ref dashing_value) => {
            *shareable = false;
            element.match_attr(attr, |attr_value| {
                attr_value == value.as_slice() ||
                attr_value.starts_with(dashing_value.as_slice())
            })
        }
        AttrPrefixMatch(ref attr, ref value) => {
            *shareable = false;
            element.match_attr(attr, |attr_value| {
                attr_value.starts_with(value.as_slice())
            })
        }
        AttrSubstringMatch(ref attr, ref value) => {
            *shareable = false;
            element.match_attr(attr, |attr_value| {
                attr_value.contains(value.as_slice())
            })
        }
        AttrSuffixMatch(ref attr, ref value) => {
            *shareable = false;
            element.match_attr(attr, |attr_value| {
                attr_value.ends_with(value.as_slice())
            })
        }

        AnyLink => {
            *shareable = false;
            let element = element.as_element();
            element.get_link().is_some()
        }
        Link => {
            *shareable = false;
            let elem = element.as_element();
            match elem.get_link() {
                Some(url) => !url_is_visited(url),
                None => false,
            }
        }
        Visited => {
            *shareable = false;
            let elem = element.as_element();
            match elem.get_link() {
                Some(url) => url_is_visited(url),
                None => false,
            }
        }

        Hover => {
            *shareable = false;
            let elem = element.as_element();
            elem.get_hover_state()
        },
        // http://www.whatwg.org/html/#selector-disabled
        Disabled => {
            *shareable = false;
            let elem = element.as_element();
            elem.get_disabled_state()
        },
        // http://www.whatwg.org/html/#selector-enabled
        Enabled => {
            *shareable = false;
            let elem = element.as_element();
            elem.get_enabled_state()
        },
        FirstChild => {
            *shareable = false;
            matches_first_child(element)
        }
        LastChild => {
            *shareable = false;
            matches_last_child(element)
        }
        OnlyChild => {
            *shareable = false;
            matches_first_child(element) && matches_last_child(element)
        }

        Root => {
            *shareable = false;
            matches_root(element)
        }

        NthChild(a, b) => {
            *shareable = false;
            matches_generic_nth_child(element, a, b, false, false)
        }
        NthLastChild(a, b) => {
            *shareable = false;
            matches_generic_nth_child(element, a, b, false, true)
        }
        NthOfType(a, b) => {
            *shareable = false;
            matches_generic_nth_child(element, a, b, true, false)
        }
        NthLastOfType(a, b) => {
            *shareable = false;
            matches_generic_nth_child(element, a, b, true, true)
        }

        FirstOfType => {
            *shareable = false;
            matches_generic_nth_child(element, 0, 1, true, false)
        }
        LastOfType => {
            *shareable = false;
            matches_generic_nth_child(element, 0, 1, true, true)
        }
        OnlyOfType => {
            *shareable = false;
            matches_generic_nth_child(element, 0, 1, true, false) &&
                matches_generic_nth_child(element, 0, 1, true, true)
        }

        Negation(ref negated) => {
            *shareable = false;
            !negated.iter().all(|s| matches_simple_selector(s, element, shareable))
        },
    }
}

fn url_is_visited(_url: &str) -> bool {
    // FIXME: implement this.
    // This function will probably need to take a "session"
    // or something containing browsing history as an additional parameter.
    false
}

#[inline]
fn matches_generic_nth_child<'a,
                             E:TElement<'a>,
                             N:TNode<'a, E>>(
                             element: &N,
                             a: i32,
                             b: i32,
                             is_of_type: bool,
                             is_from_end: bool)
                             -> bool {
    let mut node = element.clone();
    // fail if we can't find a parent or if the node is the root element
    // of the document (Cf. Selectors Level 3)
    match node.parent_node() {
        Some(parent) => if parent.is_document() {
            return false;
        },
        None => return false
    };

    let mut index = 1;
    loop {
        if is_from_end {
            match node.next_sibling() {
                None => break,
                Some(next_sibling) => node = next_sibling
            }
        } else {
            match node.prev_sibling() {
                None => break,
                Some(prev_sibling) => node = prev_sibling
            }
        }

        if node.is_element() {
            if is_of_type {
                let element = element.as_element();
                let node = node.as_element();
                if element.get_local_name() == node.get_local_name() &&
                    element.get_namespace() == node.get_namespace() {
                    index += 1;
                }
            } else {
              index += 1;
            }
        }

    }

    if a == 0 {
        return b == index;
    }

    let (n, r) = div_rem(index - b, a);
    n >= 0 && r == 0
}

#[inline]
fn matches_root<'a, E:TElement<'a>,N:TNode<'a, E>>(element: &N) -> bool {
    match element.parent_node() {
        Some(parent) => parent.is_document(),
        None => false
    }
}

#[inline]
fn matches_first_child<'a, E:TElement<'a>,N:TNode<'a, E>>(element: &N) -> bool {
    let mut node = element.clone();
    loop {
        match node.prev_sibling() {
            Some(prev_sibling) => {
                node = prev_sibling;
                if node.is_element() {
                    return false
                }
            },
            None => match node.parent_node() {
                // Selectors level 3 says :first-child does not match the
                // root of the document; Warning, level 4 says, for the time
                // being, the contrary...
                Some(parent) => return !parent.is_document(),
                None => return false
            }
        }
    }
}

#[inline]
fn matches_last_child<'a, E:TElement<'a>,N:TNode<'a, E>>(element: &N) -> bool {
    let mut node = element.clone();
    loop {
        match node.next_sibling() {
            Some(next_sibling) => {
                node = next_sibling;
                if node.is_element() {
                    return false
                }
            },
            None => match node.parent_node() {
                // Selectors level 3 says :last-child does not match the
                // root of the document; Warning, level 4 says, for the time
                // being, the contrary...
                Some(parent) => return !parent.is_document(),
                None => return false
            }
        }
    }
}


trait FindPush<K, V> {
    fn find_push(&mut self, key: K, value: V);
}

impl<K: Eq + Hash, V> FindPush<K, V> for HashMap<K, Vec<V>> {
    fn find_push(&mut self, key: K, value: V) {
        match self.find_mut(&key) {
            Some(vec) => {
                vec.push(value);
                return
            }
            None => {}
        }
        self.insert(key, vec![value]);
    }
}


#[cfg(test)]
mod tests {
    use sync::Arc;
    use super::{DeclarationBlock, Rule, SelectorMap};
    use selectors::LocalName;
    use string_cache::Atom;

    /// Helper method to get some Rules from selector strings.
    /// Each sublist of the result contains the Rules for one StyleRule.
    fn get_mock_rules(css_selectors: &[&str]) -> Vec<Vec<Rule>> {
        use namespaces::NamespaceMap;
        use selectors::parse_selector_list;
        use cssparser::tokenize;

        let namespaces = NamespaceMap::new();
        css_selectors.iter().enumerate().map(|(i, selectors)| {
            parse_selector_list(tokenize(*selectors).map(|(c, _)| c), &namespaces)
            .unwrap().into_iter().map(|s| {
                Rule {
                    selector: s.compound_selectors.clone(),
                    declarations: DeclarationBlock {
                        specificity: s.specificity,
                        declarations: Arc::new(vec!()),
                        source_order: i,
                    }
                }
            }).collect()
        }).collect()
    }

    #[test]
    fn test_rule_ordering_same_specificity(){
        let rules_list = get_mock_rules(["a.intro", "img.sidebar"]);
        let a = &rules_list[0][0].declarations;
        let b = &rules_list[1][0].declarations;
        assert!((a.specificity, a.source_order).cmp(&(b.specificity, b.source_order)) == Less,
                "The rule that comes later should win.");
    }

    #[test]
    fn test_get_id_name(){
        let rules_list = get_mock_rules([".intro", "#top"]);
        assert_eq!(SelectorMap::get_id_name(&rules_list[0][0]), None);
        assert_eq!(SelectorMap::get_id_name(&rules_list[1][0]), Some(Atom::from_slice("top")));
    }

    #[test]
    fn test_get_class_name(){
        let rules_list = get_mock_rules([".intro.foo", "#top"]);
        assert_eq!(SelectorMap::get_class_name(&rules_list[0][0]), Some(Atom::from_slice("intro")));
        assert_eq!(SelectorMap::get_class_name(&rules_list[1][0]), None);
    }

    #[test]
    fn test_get_local_name(){
        let rules_list = get_mock_rules(["img.foo", "#top", "IMG", "ImG"]);
        let check = |i, names: Option<(&str, &str)>| {
            assert!(SelectorMap::get_local_name(&rules_list[i][0])
                    == names.map(|(name, lower_name)| LocalName {
                            name: Atom::from_slice(name),
                            lower_name: Atom::from_slice(lower_name) }))
        };
        check(0, Some(("img", "img")));
        check(1, None);
        check(2, Some(("IMG", "img")));
        check(3, Some(("ImG", "img")));
    }

    #[test]
    fn test_insert(){
        let rules_list = get_mock_rules([".intro.foo", "#top"]);
        let mut selector_map = SelectorMap::new();
        selector_map.insert(rules_list[1][0].clone());
        assert_eq!(1, selector_map.id_hash.find(&Atom::from_slice("top")).unwrap()[0].declarations.source_order);
        selector_map.insert(rules_list[0][0].clone());
        assert_eq!(0, selector_map.class_hash.find(&Atom::from_slice("intro")).unwrap()[0].declarations.source_order);
        assert!(selector_map.class_hash.find(&Atom::from_slice("foo")).is_none());
    }
}