/* 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 attr::{ParsedAttrSelectorOperation, AttrSelectorOperation, NamespaceConstraint};
use bloom::{BLOOM_HASH_MASK, BloomFilter};
use nth_index_cache::NthIndexCacheInner;
use parser::{AncestorHashes, Combinator, Component, LocalName};
use parser::{Selector, SelectorImpl, SelectorIter, SelectorList};
use std::borrow::Borrow;
use tree::Element;

pub use context::*;

// 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: usize = 4096;

bitflags! {
    /// Set of flags that are set on either the element or its parent (depending
    /// on the flag) if the element could potentially match a selector.
    pub struct ElementSelectorFlags: usize {
        /// When a child is added or removed from the parent, all the children
        /// must be restyled, because they may match :nth-last-child,
        /// :last-of-type, :nth-last-of-type, or :only-of-type.
        const HAS_SLOW_SELECTOR = 1 << 0;

        /// When a child is added or removed from the parent, any later
        /// children must be restyled, because they may match :nth-child,
        /// :first-of-type, or :nth-of-type.
        const HAS_SLOW_SELECTOR_LATER_SIBLINGS = 1 << 1;

        /// When a child is added or removed from the parent, the first and
        /// last children must be restyled, because they may match :first-child,
        /// :last-child, or :only-child.
        const HAS_EDGE_CHILD_SELECTOR = 1 << 2;

        /// The element has an empty selector, so when a child is appended we
        /// might need to restyle the parent completely.
        const HAS_EMPTY_SELECTOR = 1 << 3;
    }
}

impl ElementSelectorFlags {
    /// Returns the subset of flags that apply to the element.
    pub fn for_self(self) -> ElementSelectorFlags {
        self & (ElementSelectorFlags::HAS_EMPTY_SELECTOR)
    }

    /// Returns the subset of flags that apply to the parent.
    pub fn for_parent(self) -> ElementSelectorFlags {
        self & (ElementSelectorFlags::HAS_SLOW_SELECTOR |
                ElementSelectorFlags::HAS_SLOW_SELECTOR_LATER_SIBLINGS |
                ElementSelectorFlags::HAS_EDGE_CHILD_SELECTOR)
    }
}

/// Holds per-compound-selector data.
struct LocalMatchingContext<'a, 'b: 'a, Impl: SelectorImpl> {
    shared: &'a mut MatchingContext<'b, Impl>,
    matches_hover_and_active_quirk: bool,
    visited_handling: VisitedHandlingMode,
}

#[inline(always)]
pub fn matches_selector_list<E>(
    selector_list: &SelectorList<E::Impl>,
    element: &E,
    context: &mut MatchingContext<E::Impl>,
) -> bool
where
    E: Element
{
    // This is pretty much any(..) but manually inlined because the compiler
    // refuses to do so from querySelector / querySelectorAll.
    for selector in &selector_list.0 {
        let matches = matches_selector(
            selector,
            0,
            None,
            element,
            context,
            &mut |_, _| {},
        );

        if matches {
            return true;
        }
    }

    false
}

#[inline(always)]
fn may_match(hashes: &AncestorHashes, bf: &BloomFilter) -> bool {
    // Check the first three hashes. Note that we can check for zero before
    // masking off the high bits, since if any of the first three hashes is
    // zero the fourth will be as well. We also take care to avoid the
    // special-case complexity of the fourth hash until we actually reach it,
    // because we usually don't.
    //
    // To be clear: this is all extremely hot.
    for i in 0..3 {
        let packed = hashes.packed_hashes[i];
        if packed == 0 {
            // No more hashes left - unable to fast-reject.
            return true;
        }

        if !bf.might_contain_hash(packed & BLOOM_HASH_MASK) {
            // Hooray! We fast-rejected on this hash.
            return false;
        }
    }

    // Now do the slighty-more-complex work of synthesizing the fourth hash,
    // and check it against the filter if it exists.
    let fourth = hashes.fourth_hash();
    fourth == 0 || bf.might_contain_hash(fourth)
}

/// 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 element for "d1", this selector matching raises
/// NotMatchedGlobally since even if "d2" is moved to more upper element, the
/// candidates for "d1" becomes less than before and d1 .
///
/// The next example is siblings. When the selector "b1 + b2 ~ d1 a" is
/// provided and we cannot *find* an appropriate brother element 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".
#[derive(Clone, Copy, Eq, PartialEq)]
enum SelectorMatchingResult {
    Matched,
    NotMatchedAndRestartFromClosestLaterSibling,
    NotMatchedAndRestartFromClosestDescendant,
    NotMatchedGlobally,
}

/// Matches a selector, fast-rejecting against a bloom filter.
///
/// We accept an offset to allow consumers to represent and match against
/// partial selectors (indexed from the right). We use this API design, rather
/// than having the callers pass a SelectorIter, because creating a SelectorIter
/// requires dereferencing the selector to get the length, which adds an
/// unncessary cache miss for cases when we can fast-reject with AncestorHashes
/// (which the caller can store inline with the selector pointer).
#[inline(always)]
pub fn matches_selector<E, F>(
    selector: &Selector<E::Impl>,
    offset: usize,
    hashes: Option<&AncestorHashes>,
    element: &E,
    context: &mut MatchingContext<E::Impl>,
    flags_setter: &mut F,
) -> bool
where
    E: Element,
    F: FnMut(&E, ElementSelectorFlags),
{
    // Use the bloom filter to fast-reject.
    if let Some(hashes) = hashes {
        if let Some(filter) = context.bloom_filter {
            if !may_match(hashes, filter) {
                return false;
            }
        }
    }

    matches_complex_selector(selector.iter_from(offset), element, context, flags_setter)
}

/// Whether a compound selector matched, and whether it was the rightmost
/// selector inside the complex selector.
pub enum CompoundSelectorMatchingResult {
    /// The selector was fully matched.
    FullyMatched,
    /// The compound selector matched, and the next combinator offset is
    /// `next_combinator_offset`.
    Matched { next_combinator_offset: usize, },
    /// The selector didn't match.
    NotMatched,
}

/// Matches a compound selector belonging to `selector`, starting at offset
/// `from_offset`, matching left to right.
///
/// Requires that `from_offset` points to a `Combinator`.
///
/// NOTE(emilio): This doesn't allow to match in the leftmost sequence of the
/// complex selector, but it happens to be the case we don't need it.
pub fn matches_compound_selector<E>(
    selector: &Selector<E::Impl>,
    mut from_offset: usize,
    context: &mut MatchingContext<E::Impl>,
    element: &E,
) -> CompoundSelectorMatchingResult
where
    E: Element
{
    if cfg!(debug_assertions) && from_offset != 0 {
        selector.combinator_at_parse_order(from_offset - 1); // This asserts.
    }

    let visited_handling = context.visited_handling;
    let mut local_context = LocalMatchingContext {
        shared: context,
        visited_handling,
        matches_hover_and_active_quirk: false,
    };

    for component in selector.iter_raw_parse_order_from(from_offset) {
        if matches!(*component, Component::Combinator(..)) {
            debug_assert_ne!(from_offset, 0, "Selector started with a combinator?");
            return CompoundSelectorMatchingResult::Matched {
                next_combinator_offset: from_offset,
            }
        }

        if !matches_simple_selector(
            component,
            element,
            &mut local_context,
            &mut |_, _| {}) {
            return CompoundSelectorMatchingResult::NotMatched;
        }

        from_offset += 1;
    }

    CompoundSelectorMatchingResult::FullyMatched
}

/// Matches a complex selector.
#[inline(always)]
pub fn matches_complex_selector<E, F>(
    mut iter: SelectorIter<E::Impl>,
    element: &E,
    context: &mut MatchingContext<E::Impl>,
    flags_setter: &mut F,
) -> bool
where
    E: Element,
    F: FnMut(&E, ElementSelectorFlags),
{
    // If this is the special pseudo-element mode, consume the ::pseudo-element
    // before proceeding, since the caller has already handled that part.
    if context.matching_mode == MatchingMode::ForStatelessPseudoElement &&
        context.nesting_level == 0 {
        // Consume the pseudo.
        match *iter.next().unwrap() {
            Component::PseudoElement(ref pseudo) => {
                if let Some(ref f) = context.pseudo_element_matching_fn {
                    if !f(pseudo) {
                        return false;
                    }
                }
            }
            _ => {
                debug_assert!(false,
                              "Used MatchingMode::ForStatelessPseudoElement \
                               in a non-pseudo selector");
            }
        }

        // The only other parser-allowed Component in this sequence is a state
        // class. We just don't match in that case.
        if let Some(s) = iter.next() {
            debug_assert!(matches!(*s, Component::NonTSPseudoClass(..)),
                          "Someone messed up pseudo-element parsing");
            return false;
        }

        // Advance to the non-pseudo-element part of the selector, but let the
        // context note that .
        if iter.next_sequence().is_none() {
            return true;
        }
    }

    let visited_handling = context.visited_handling;
    let result = matches_complex_selector_internal(
        iter,
        element,
        context,
        visited_handling,
        flags_setter,
        Rightmost::Yes,
    );

    match result {
        SelectorMatchingResult::Matched => true,
        _ => false
    }
}

#[inline]
fn matches_hover_and_active_quirk<Impl: SelectorImpl>(
    selector_iter: &SelectorIter<Impl>,
    context: &MatchingContext<Impl>,
    rightmost: Rightmost,
) -> bool {
    if context.quirks_mode() != QuirksMode::Quirks {
        return false;
    }

    if context.nesting_level != 0 {
        return false;
    }

    // This compound selector had a pseudo-element to the right that we
    // intentionally skipped.
    if matches!(rightmost, Rightmost::Yes) &&
        context.matching_mode == MatchingMode::ForStatelessPseudoElement {
        return false;
    }

    selector_iter.clone().all(|simple| {
        match *simple {
            Component::LocalName(_) |
            Component::AttributeInNoNamespaceExists { .. } |
            Component::AttributeInNoNamespace { .. } |
            Component::AttributeOther(_) |
            Component::ID(_) |
            Component::Class(_) |
            Component::PseudoElement(_) |
            Component::Negation(_) |
            Component::FirstChild |
            Component::LastChild |
            Component::OnlyChild |
            Component::Empty |
            Component::NthChild(_, _) |
            Component::NthLastChild(_, _) |
            Component::NthOfType(_, _) |
            Component::NthLastOfType(_, _) |
            Component::FirstOfType |
            Component::LastOfType |
            Component::OnlyOfType => false,
            Component::NonTSPseudoClass(ref pseudo_class) => {
                Impl::is_active_or_hover(pseudo_class)
            },
            _ => true,
        }
    })
}

enum Rightmost {
    Yes,
    No,
}

#[inline(always)]
fn next_element_for_combinator<E>(
    element: &E,
    combinator: Combinator,
) -> Option<E>
where
    E: Element,
{
    match combinator {
        Combinator::NextSibling |
        Combinator::LaterSibling => {
            element.prev_sibling_element()
        }
        Combinator::Child |
        Combinator::Descendant => {
            if element.blocks_ancestor_combinators() {
                return None;
            }

            element.parent_element()
        }
        Combinator::SlotAssignment => {
            element.assigned_slot()
        }
        Combinator::PseudoElement => {
            element.pseudo_element_originating_element()
        }
    }
}

fn matches_complex_selector_internal<E, F>(
    mut selector_iter: SelectorIter<E::Impl>,
    element: &E,
    context: &mut MatchingContext<E::Impl>,
    visited_handling: VisitedHandlingMode,
    flags_setter: &mut F,
    rightmost: Rightmost,
) -> SelectorMatchingResult
where
    E: Element,
    F: FnMut(&E, ElementSelectorFlags),
{
    debug!("Matching complex selector {:?} for {:?}", selector_iter, element);

    let matches_all_simple_selectors = {
        let matches_hover_and_active_quirk =
            matches_hover_and_active_quirk(&selector_iter, context, rightmost);
        let mut local_context =
            LocalMatchingContext {
                shared: context,
                visited_handling,
                matches_hover_and_active_quirk,
            };
        selector_iter.all(|simple| {
            matches_simple_selector(
                simple,
                element,
                &mut local_context,
                flags_setter,
            )
        })
    };

    let combinator = selector_iter.next_sequence();
    if combinator.map_or(false, |c| c.is_sibling()) {
        flags_setter(element, ElementSelectorFlags::HAS_SLOW_SELECTOR_LATER_SIBLINGS);
    }

    if !matches_all_simple_selectors {
        return SelectorMatchingResult::NotMatchedAndRestartFromClosestLaterSibling;
    }

    let combinator = match combinator {
        None => return SelectorMatchingResult::Matched,
        Some(c) => c,
    };

    let candidate_not_found = match combinator {
        Combinator::NextSibling |
        Combinator::LaterSibling => {
            SelectorMatchingResult::NotMatchedAndRestartFromClosestDescendant
        }
        Combinator::Child |
        Combinator::Descendant |
        Combinator::SlotAssignment |
        Combinator::PseudoElement => {
            SelectorMatchingResult::NotMatchedGlobally
        }
    };

    let mut next_element = next_element_for_combinator(element, combinator);

    // Stop matching :visited as soon as we find a link, or a combinator for
    // something that isn't an ancestor.
    let mut visited_handling =
        if element.is_link() || combinator.is_sibling() {
            VisitedHandlingMode::AllLinksUnvisited
        } else {
            visited_handling
        };

    loop {
        let element = match next_element {
            None => return candidate_not_found,
            Some(next_element) => next_element,
        };

        let result = matches_complex_selector_internal(
            selector_iter.clone(),
            &element,
            context,
            visited_handling,
            flags_setter,
            Rightmost::No,
        );

        match (result, combinator) {
            // Return the status immediately.
            (SelectorMatchingResult::Matched, _) |
            (SelectorMatchingResult::NotMatchedGlobally, _) |
            (_, Combinator::NextSibling) => {
                return result;
            }

            // Upgrade the failure status to
            // NotMatchedAndRestartFromClosestDescendant.
            (_, Combinator::PseudoElement) |
            (_, Combinator::Child) => {
                return SelectorMatchingResult::NotMatchedAndRestartFromClosestDescendant;
            }

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

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

        visited_handling =
            if element.is_link() || combinator.is_sibling() {
                VisitedHandlingMode::AllLinksUnvisited
            } else {
                visited_handling
            };

        next_element = next_element_for_combinator(&element, combinator);
    }
}

/// Determines whether the given element matches the given single selector.
#[inline]
fn matches_simple_selector<E, F>(
    selector: &Component<E::Impl>,
    element: &E,
    context: &mut LocalMatchingContext<E::Impl>,
    flags_setter: &mut F,
) -> bool
where
    E: Element,
    F: FnMut(&E, ElementSelectorFlags),
{
    match *selector {
        Component::Combinator(_) => unreachable!(),
        Component::Slotted(ref selector) => {
            context.shared.nesting_level += 1;
            let result =
                element.assigned_slot().is_some() &&
                matches_complex_selector(
                    selector.iter(),
                    element,
                    context.shared,
                    flags_setter,
                );
            context.shared.nesting_level -= 1;
            result
        }
        Component::PseudoElement(ref pseudo) => {
            element.match_pseudo_element(pseudo, context.shared)
        }
        Component::LocalName(LocalName { ref name, ref lower_name }) => {
            let is_html = element.is_html_element_in_html_document();
            element.get_local_name() == select_name(is_html, name, lower_name).borrow()
        }
        Component::ExplicitUniversalType |
        Component::ExplicitAnyNamespace => {
            true
        }
        Component::Namespace(_, ref url) |
        Component::DefaultNamespace(ref url) => {
            element.get_namespace() == url.borrow()
        }
        Component::ExplicitNoNamespace => {
            let ns = ::parser::namespace_empty_string::<E::Impl>();
            element.get_namespace() == ns.borrow()
        }
        Component::ID(ref id) => {
            element.has_id(id, context.shared.classes_and_ids_case_sensitivity())
        }
        Component::Class(ref class) => {
            element.has_class(class, context.shared.classes_and_ids_case_sensitivity())
        }
        Component::AttributeInNoNamespaceExists { ref local_name, ref local_name_lower } => {
            let is_html = element.is_html_element_in_html_document();
            element.attr_matches(
                &NamespaceConstraint::Specific(&::parser::namespace_empty_string::<E::Impl>()),
                select_name(is_html, local_name, local_name_lower),
                &AttrSelectorOperation::Exists
            )
        }
        Component::AttributeInNoNamespace {
            ref local_name,
            ref local_name_lower,
            ref value,
            operator,
            case_sensitivity,
            never_matches,
        } => {
            if never_matches {
                return false
            }
            let is_html = element.is_html_element_in_html_document();
            element.attr_matches(
                &NamespaceConstraint::Specific(&::parser::namespace_empty_string::<E::Impl>()),
                select_name(is_html, local_name, local_name_lower),
                &AttrSelectorOperation::WithValue {
                    operator: operator,
                    case_sensitivity: case_sensitivity.to_unconditional(is_html),
                    expected_value: value,
                }
            )
        }
        Component::AttributeOther(ref attr_sel) => {
            if attr_sel.never_matches {
                return false
            }
            let is_html = element.is_html_element_in_html_document();
            element.attr_matches(
                &attr_sel.namespace(),
                select_name(is_html, &attr_sel.local_name, &attr_sel.local_name_lower),
                &match attr_sel.operation {
                    ParsedAttrSelectorOperation::Exists => AttrSelectorOperation::Exists,
                    ParsedAttrSelectorOperation::WithValue {
                        operator,
                        case_sensitivity,
                        ref expected_value,
                    } => {
                        AttrSelectorOperation::WithValue {
                            operator: operator,
                            case_sensitivity: case_sensitivity.to_unconditional(is_html),
                            expected_value: expected_value,
                        }
                    }
                }
            )
        }
        Component::NonTSPseudoClass(ref pc) => {
            if context.matches_hover_and_active_quirk &&
               context.shared.nesting_level == 0 &&
               E::Impl::is_active_or_hover(pc) &&
               !element.is_link() {
                return false;
            }

            element.match_non_ts_pseudo_class(
                pc,
                &mut context.shared,
                context.visited_handling,
                flags_setter
            )
        }
        Component::FirstChild => {
            matches_first_child(element, flags_setter)
        }
        Component::LastChild => {
            matches_last_child(element, flags_setter)
        }
        Component::OnlyChild => {
            matches_first_child(element, flags_setter) &&
            matches_last_child(element, flags_setter)
        }
        Component::Root => {
            element.is_root()
        }
        Component::Empty => {
            flags_setter(element, ElementSelectorFlags::HAS_EMPTY_SELECTOR);
            element.is_empty()
        }
        Component::Scope => {
            match context.shared.scope_element {
                Some(ref scope_element) => element.opaque() == *scope_element,
                None => element.is_root(),
            }
        }
        Component::NthChild(a, b) => {
            matches_generic_nth_child(element, context, a, b, false, false, flags_setter)
        }
        Component::NthLastChild(a, b) => {
            matches_generic_nth_child(element, context, a, b, false, true, flags_setter)
        }
        Component::NthOfType(a, b) => {
            matches_generic_nth_child(element, context, a, b, true, false, flags_setter)
        }
        Component::NthLastOfType(a, b) => {
            matches_generic_nth_child(element, context, a, b, true, true, flags_setter)
        }
        Component::FirstOfType => {
            matches_generic_nth_child(element, context, 0, 1, true, false, flags_setter)
        }
        Component::LastOfType => {
            matches_generic_nth_child(element, context, 0, 1, true, true, flags_setter)
        }
        Component::OnlyOfType => {
            matches_generic_nth_child(element, context, 0, 1, true, false, flags_setter) &&
            matches_generic_nth_child(element, context, 0, 1, true, true, flags_setter)
        }
        Component::Negation(ref negated) => {
            context.shared.nesting_level += 1;
            let result = !negated.iter().all(|ss| {
                matches_simple_selector(
                    ss,
                    element,
                    context,
                    flags_setter,
                )
            });
            context.shared.nesting_level -= 1;
            result
        }
    }
}

#[inline(always)]
fn select_name<'a, T>(is_html: bool, local_name: &'a T, local_name_lower: &'a T) -> &'a T {
    if is_html {
        local_name_lower
    } else {
        local_name
    }
}

#[inline]
fn matches_generic_nth_child<E, F>(
    element: &E,
    context: &mut LocalMatchingContext<E::Impl>,
    a: i32,
    b: i32,
    is_of_type: bool,
    is_from_end: bool,
    flags_setter: &mut F,
) -> bool
where
    E: Element,
    F: FnMut(&E, ElementSelectorFlags),
{
    if element.ignores_nth_child_selectors() {
        return false;
    }

    flags_setter(element, if is_from_end {
        ElementSelectorFlags::HAS_SLOW_SELECTOR
    } else {
        ElementSelectorFlags::HAS_SLOW_SELECTOR_LATER_SIBLINGS
    });

    // Grab a reference to the appropriate cache.
    let mut cache = context.shared.nth_index_cache.as_mut().map(|c| {
        c.get(is_of_type, is_from_end)
    });

    // Lookup or compute the index.
    let index = if let Some(i) = cache.as_mut().and_then(|c| c.lookup(element.opaque())) {
        i
    } else {
        let i = nth_child_index(element, is_of_type, is_from_end, cache.as_mut().map(|s| &mut **s));
        cache.as_mut().map(|c| c.insert(element.opaque(), i));
        i
    };
    debug_assert_eq!(index, nth_child_index(element, is_of_type, is_from_end, None), "invalid cache");

    // Is there a non-negative integer n such that An+B=index?
    match index.checked_sub(b) {
        None => false,
        Some(an) => match an.checked_div(a) {
            Some(n) => n >= 0 && a * n == an,
            None /* a == 0 */ => an == 0,
        },
    }
}

#[inline]
fn same_type<E: Element>(a: &E, b: &E) -> bool {
    a.get_local_name() == b.get_local_name() &&
    a.get_namespace() == b.get_namespace()
}

#[inline]
fn nth_child_index<E>(
    element: &E,
    is_of_type: bool,
    is_from_end: bool,
    mut cache: Option<&mut NthIndexCacheInner>,
) -> i32
where
    E: Element,
{
    // The traversal mostly processes siblings left to right. So when we walk
    // siblings to the right when computing NthLast/NthLastOfType we're unlikely
    // to get cache hits along the way. As such, we take the hit of walking the
    // siblings to the left checking the cache in the is_from_end case (this
    // matches what Gecko does). The indices-from-the-left is handled during the
    // regular look further below.
    if let Some(ref mut c) = cache {
        if is_from_end && !c.is_empty() {
            let mut index: i32 = 1;
            let mut curr = element.clone();
            while let Some(e) = curr.prev_sibling_element() {
                curr = e;
                if !is_of_type || same_type(element, &curr) {
                    if let Some(i) = c.lookup(curr.opaque()) {
                        return i - index;
                    }
                    index += 1;
                }
            }
        }
    }

    let mut index: i32 = 1;
    let mut curr = element.clone();
    let next = |e: E| if is_from_end { e.next_sibling_element() } else { e.prev_sibling_element() };
    while let Some(e) = next(curr) {
        curr = e;
        if !is_of_type || same_type(element, &curr) {
            // If we're computing indices from the left, check each element in the
            // cache. We handle the indices-from-the-right case at the top of this
            // function.
            if !is_from_end {
                if let Some(i) = cache.as_mut().and_then(|c| c.lookup(curr.opaque())) {
                    return i + index
                }
            }
            index += 1;
        }
    }

    index
}

#[inline]
fn matches_first_child<E, F>(element: &E, flags_setter: &mut F) -> bool
where
    E: Element,
    F: FnMut(&E, ElementSelectorFlags),
{
    flags_setter(element, ElementSelectorFlags::HAS_EDGE_CHILD_SELECTOR);
    element.prev_sibling_element().is_none()
}

#[inline]
fn matches_last_child<E, F>(element: &E, flags_setter: &mut F) -> bool
where
    E: Element,
    F: FnMut(&E, ElementSelectorFlags),
{
    flags_setter(element, ElementSelectorFlags::HAS_EDGE_CHILD_SELECTOR);
    element.next_sibling_element().is_none()
}