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// Copyright 2023 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef V8_BASE_INTRUSIVE_SET_H_
#define V8_BASE_INTRUSIVE_SET_H_
#include <iterator>
#include <limits>
#include <type_traits>
#include "src/base/logging.h"
#include "src/base/optional.h"
namespace v8::base {
class IntrusiveSetIndex {
private:
template <class T, class GetIntrusiveSetIndex, class Container>
friend class IntrusiveSet;
static constexpr size_t kNotInSet = std::numeric_limits<size_t>::max();
size_t value = kNotInSet;
};
// A set of pointer-like values (`T`) that point to memory containing the
// position inside of the set (`IntrusiveSetIndex`), to allow for O(1) insertion
// and removal without using a hash table. This set is intrusive in the sense
// that elements need to know their position inside of the set by storing an
// `IntrusiveSetIndex` somewhere. In particular, all copies of a `T` value
// should point to the same `IntrusiveSetIndex` instance. `GetIntrusiveSetIndex`
// has to be a functor that produces `IntrusiveSetIndex&` given a `T`. The
// reference has to remain valid and refer to the same memory location while the
// element is in the set and until we finish iterating over the data structure
// if the element is removed during iteration.
//
// Add(T): amortized O(1)
// Contain(T): O(1)
// Remove(T): O(1)
template <class T, class GetIntrusiveSetIndex, class Container>
class IntrusiveSet {
public:
// This is not needed for soundness, but rather serves as a hint that `T`
// should be a lightweight pointer-like value.
static_assert(std::is_trivially_copyable_v<T>);
explicit IntrusiveSet(Container container,
GetIntrusiveSetIndex index_functor = {})
: elements_(std::move(container)), index_functor_(index_functor) {
static_assert(std::is_same_v<decltype(index_functor(std::declval<T>())),
IntrusiveSetIndex&>);
}
bool Contains(T x) const { return Index(x) != IntrusiveSetIndex::kNotInSet; }
// Adding elements while iterating is allowed.
void Add(T x) {
DCHECK(!Contains(x));
Index(x) = elements_.size();
elements_.push_back(x);
}
// Removing while iterating is allowed under very specific circumstances. See
// comment on `IntrusiveSet::iterator`.
void Remove(T x) {
DCHECK(Contains(x));
size_t& index = Index(x);
DCHECK_EQ(x, elements_[index]);
Index(elements_.back()) = index;
elements_[index] = elements_.back();
index = IntrusiveSetIndex::kNotInSet;
elements_.pop_back();
}
// Since C++17, it is possible to have a sentinel end-iterator that is not an
// iterator itself.
class end_iterator {};
// This iterator supports insertion (newly inserted elements will be visited
// as part of the iteration) and removal of the current element while
// iterating. Removing previously visited elements is undefined behavior.
// ATTENTION! The memory the removed element points to needs to remain alive
// until the end of the iteration.
class iterator {
public:
explicit iterator(const IntrusiveSet& set) : set_(set) {}
T operator*() {
T result = set_.elements_[index_];
last_index_location_ = &set_.Index(result);
return result;
}
iterator& operator++() {
// This iterator requires `operator*` being used before `operator++`.
DCHECK_NOT_NULL(last_index_location_);
if (index_ < set_.elements_.size() &&
last_index_location_ == &set_.Index(set_.elements_[index_])) {
index_++;
}
return *this;
}
bool operator!=(end_iterator) const {
return index_ < set_.elements_.size();
}
private:
const IntrusiveSet& set_;
size_t index_ = 0;
// If the current element is removed, another element is swapped in to the
// same position. We notice this by remembering the index memory location of
// the last retrieved element.
const size_t* last_index_location_ = nullptr;
};
// These iterators are only intended for range-based for loops.
iterator begin() const { return iterator{*this}; }
end_iterator end() const { return end_iterator{}; }
private:
Container elements_;
GetIntrusiveSetIndex index_functor_;
size_t& Index(T x) const { return index_functor_(x).value; }
};
} // namespace v8::base
#endif // V8_BASE_INTRUSIVE_SET_H_
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