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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
* 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/. */
#ifndef js_Fifo_h
#define js_Fifo_h
#include "mozilla/Move.h"
#include "js/Utility.h"
#include "js/Vector.h"
namespace js {
// A first-in-first-out queue container type. Fifo calls constructors and
// destructors of all elements added so non-PODs may be used safely. |Fifo|
// stores the first |MinInlineCapacity| elements in-place before resorting to
// dynamic allocation.
//
// T requirements:
// - Either movable or copyable.
// MinInlineCapacity requirements:
// - Must be even.
// AllocPolicy:
// - see "Allocation policies" in AllocPolicy.h
template <typename T,
size_t MinInlineCapacity = 0,
class AllocPolicy = TempAllocPolicy>
class Fifo
{
static_assert(MinInlineCapacity % 2 == 0, "MinInlineCapacity must be even!");
protected:
// An element A is "younger" than an element B if B was inserted into the
// |Fifo| before A was.
//
// Invariant 1: Every element within |front_| is older than every element
// within |rear_|.
// Invariant 2: Entries within |front_| are sorted from younger to older.
// Invariant 3: Entries within |rear_| are sorted from older to younger.
// Invariant 4: If the |Fifo| is not empty, then |front_| is not empty.
Vector<T, MinInlineCapacity / 2, AllocPolicy> front_;
Vector<T, MinInlineCapacity / 2, AllocPolicy> rear_;
private:
// Maintain invariants after adding or removing entries.
bool fixup() {
if (!front_.empty())
return true;
if (!front_.reserve(rear_.length()))
return false;
while (!rear_.empty()) {
front_.infallibleAppend(mozilla::Move(rear_.back()));
rear_.popBack();
}
return true;
}
public:
explicit Fifo(AllocPolicy alloc = AllocPolicy())
: front_(alloc)
, rear_(alloc)
{ }
Fifo(Fifo&& rhs)
: front_(mozilla::Move(rhs.front_))
, rear_(mozilla::Move(rhs.rear_))
{ }
Fifo& operator=(Fifo&& rhs) {
MOZ_ASSERT(&rhs != this, "self-move disallowed");
this->~Fifo();
new (this) Fifo(mozilla::Move(rhs));
return *this;
}
Fifo(const Fifo&) = delete;
Fifo& operator=(const Fifo&) = delete;
size_t length() const {
MOZ_ASSERT_IF(rear_.length() > 0, front_.length() > 0); // Invariant 4.
return front_.length() + rear_.length();
}
bool empty() const {
MOZ_ASSERT_IF(rear_.length() > 0, front_.length() > 0); // Invariant 4.
return front_.empty();
}
// Push an element to the back of the queue. This method can take either a
// |const T&| or a |T&&|.
template <typename U>
MOZ_MUST_USE bool pushBack(U&& u) {
if (!rear_.append(mozilla::Forward<U>(u)))
return false;
if (!fixup()) {
rear_.popBack();
return false;
}
return true;
}
// Construct a T in-place at the back of the queue.
template <typename... Args>
MOZ_MUST_USE bool emplaceBack(Args&&... args) {
if (!rear_.emplaceBack(mozilla::Forward<Args>(args)...))
return false;
if (!fixup()) {
rear_.popBack();
return false;
}
return true;
}
// Access the element at the front of the queue.
T& front() {
MOZ_ASSERT(!empty());
return front_.back();
}
const T& front() const {
MOZ_ASSERT(!empty());
return front_.back();
}
// Remove the front element from the queue.
MOZ_MUST_USE bool popFront() {
MOZ_ASSERT(!empty());
T t(mozilla::Move(front()));
front_.popBack();
if (!fixup()) {
// Attempt to remain in a valid state by reinserting the element
// back at the front. If we can't remain in a valid state in the
// face of OOMs, crash.
AutoEnterOOMUnsafeRegion oomUnsafe;
if (!front_.append(mozilla::Move(t)))
oomUnsafe.crash("js::Fifo::popFront");
return false;
}
return true;
}
// Clear all elements from the queue.
void clear() {
front_.clear();
rear_.clear();
}
};
} // namespace js
#endif /* js_Fifo_h */
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