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selector_filter.h
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selector_filter.h
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/*
* Copyright (C) 1999 Lars Knoll (knoll@kde.org)
* (C) 2004-2005 Allan Sandfeld Jensen (kde@carewolf.com)
* Copyright (C) 2006, 2007 Nicholas Shanks (webkit@nickshanks.com)
* Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights
* reserved.
* Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
* Copyright (C) 2007, 2008 Eric Seidel <eric@webkit.org>
* Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved.
* (http://www.torchmobile.com/)
* Copyright (c) 2011, Code Aurora Forum. All rights reserved.
* Copyright (C) Research In Motion Limited 2011. All rights reserved.
* Copyright (C) 2012 Google Inc. All rights reserved.
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Library General Public
* License as published by the Free Software Foundation; either
* version 2 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Library General Public License for more details.
*
* You should have received a copy of the GNU Library General Public License
* along with this library; see the file COPYING.LIB. If not, write to
* the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
* Boston, MA 02110-1301, USA.
*/
#ifndef THIRD_PARTY_BLINK_RENDERER_CORE_CSS_SELECTOR_FILTER_H_
#define THIRD_PARTY_BLINK_RENDERER_CORE_CSS_SELECTOR_FILTER_H_
#include <memory>
#include "third_party/blink/renderer/core/core_export.h"
#include "third_party/blink/renderer/core/dom/element.h"
#include "third_party/blink/renderer/platform/wtf/bloom_filter.h"
#include "third_party/blink/renderer/platform/wtf/vector.h"
namespace blink {
class CSSSelector;
// SelectorFilter is a bloom filter for rapidly discarding style rules that
// have ancestor requirements. When we traverse the DOM, we call PushParent()
// for each parent, which inserts a number of relevant properties for that
// parent (e.g. ID, tag name, attributes etc.) into the filter. (We also call
// PopParent() when exiting a node, which is possible because the filter is
// a counting filter.) Then, when we want to match a style rule with at least
// one such ancestor attribute, we can very cheaply check whether an ancestor
// exists in the filter (with some false positives, but that's fine).
// For instance, assume this tree:
//
// <div id="a" data-foo="bar">
// <div class="cls">
// <div id="b">
//
// When we get to computing style for the innermost element, the bloom filter
// will contain hashes corresponding to <div> (twice), [data-foo], #a and .cls.
// If we then have a rule saying e.g. “article #b”, we can look up <article> in
// the bloom filter and get a negative result (save for false positives),
// proving that the rule definitely does not apply, discarding it right away.
// However, a rule like “.cls[data-foo] #b” would pass the filter, as there are
// indeed hashes for both .cls and [data-foo] in the filter. Thus, any rule
// passing the filter must still be subjected to match checking as usual.
//
// For performance reasons, we compute the ancestor hash values for each style
// rule ahead-of-time. We stop after at most four hashes to avoid allocating
// memory dynamically, but elements (represented by ParentStackFrame) cannot
// have such a limit, or we would risk false negatives, causing us to miss
// applicable style rules in matching.
//
// For practical web pages as of 2022, we've seen SelectorFilter discard 60-70%
// of rules in early processing, which makes the 4 kB of RAM/cache it uses
// worthwhile.
class CORE_EXPORT SelectorFilter {
DISALLOW_NEW();
public:
class ParentStackFrame {
DISALLOW_NEW();
public:
ParentStackFrame() : element(nullptr) {}
explicit ParentStackFrame(Element& element) : element(&element) {}
void Trace(Visitor*) const;
Member<Element> element;
Vector<unsigned, 4> identifier_hashes;
};
SelectorFilter() = default;
SelectorFilter(const SelectorFilter&) = delete;
SelectorFilter& operator=(const SelectorFilter&) = delete;
void PushParent(Element& parent);
void PopParent(Element& parent);
bool ParentStackIsConsistent(const ContainerNode* parent_node) const {
return !parent_stack_.IsEmpty() &&
parent_stack_.back().element == parent_node;
}
template <unsigned maximumIdentifierCount>
inline bool FastRejectSelector(const unsigned* identifier_hashes) const;
static void CollectIdentifierHashes(const CSSSelector&,
unsigned* identifier_hashes,
unsigned maximum_identifier_count);
void Trace(Visitor*) const;
private:
void PushParentStackFrame(Element& parent);
void PopParentStackFrame();
HeapVector<ParentStackFrame> parent_stack_;
// With 100 unique strings in the filter, 2^12 slot table has false positive
// rate of ~0.2%.
using IdentifierFilter = CountingBloomFilter<12>;
std::unique_ptr<IdentifierFilter> ancestor_identifier_filter_;
};
template <unsigned maximumIdentifierCount>
inline bool SelectorFilter::FastRejectSelector(
const unsigned* identifier_hashes) const {
DCHECK(ancestor_identifier_filter_);
for (unsigned n = 0; n < maximumIdentifierCount && identifier_hashes[n];
++n) {
if (!ancestor_identifier_filter_->MayContain(identifier_hashes[n]))
return true;
}
return false;
}
} // namespace blink
WTF_ALLOW_INIT_WITH_MEM_FUNCTIONS(blink::SelectorFilter::ParentStackFrame)
#endif // THIRD_PARTY_BLINK_RENDERER_CORE_CSS_SELECTOR_FILTER_H_