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// Copyright (c) 2013, Facebook, Inc. All rights reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
//
#pragma once
#include <atomic>
#include "util/mutexlock.h"
namespace ROCKSDB_NAMESPACE {
// LRU element definition
//
// Any object that needs to be part of the LRU algorithm should extend this
// class
template <class T>
struct LRUElement {
explicit LRUElement() : next_(nullptr), prev_(nullptr), refs_(0) {}
virtual ~LRUElement() { assert(!refs_); }
T* next_;
T* prev_;
std::atomic<size_t> refs_;
};
// LRU implementation
//
// In place LRU implementation. There is no copy or allocation involved when
// inserting or removing an element. This makes the data structure slim
template <class T>
class LRUList {
public:
virtual ~LRUList() {
MutexLock _(&lock_);
assert(!head_);
assert(!tail_);
}
// Push element into the LRU at the cold end
inline void Push(T* const t) {
assert(t);
assert(!t->next_);
assert(!t->prev_);
MutexLock _(&lock_);
assert((!head_ && !tail_) || (head_ && tail_));
assert(!head_ || !head_->prev_);
assert(!tail_ || !tail_->next_);
t->next_ = head_;
if (head_) {
head_->prev_ = t;
}
head_ = t;
if (!tail_) {
tail_ = t;
}
}
// Unlink the element from the LRU
inline void Unlink(T* const t) {
MutexLock _(&lock_);
UnlinkImpl(t);
}
// Evict an element from the LRU
inline T* Pop() {
MutexLock _(&lock_);
assert(tail_ && head_);
assert(!tail_->next_);
assert(!head_->prev_);
T* t = head_;
while (t && t->refs_) {
t = t->next_;
}
if (!t) {
// nothing can be evicted
return nullptr;
}
assert(!t->refs_);
// unlike the element
UnlinkImpl(t);
return t;
}
// Move the element from the front of the list to the back of the list
inline void Touch(T* const t) {
MutexLock _(&lock_);
UnlinkImpl(t);
PushBackImpl(t);
}
// Check if the LRU is empty
inline bool IsEmpty() const {
MutexLock _(&lock_);
return !head_ && !tail_;
}
private:
// Unlink an element from the LRU
void UnlinkImpl(T* const t) {
assert(t);
lock_.AssertHeld();
assert(head_ && tail_);
assert(t->prev_ || head_ == t);
assert(t->next_ || tail_ == t);
if (t->prev_) {
t->prev_->next_ = t->next_;
}
if (t->next_) {
t->next_->prev_ = t->prev_;
}
if (tail_ == t) {
tail_ = tail_->prev_;
}
if (head_ == t) {
head_ = head_->next_;
}
t->next_ = t->prev_ = nullptr;
}
// Insert an element at the hot end
inline void PushBack(T* const t) {
MutexLock _(&lock_);
PushBackImpl(t);
}
inline void PushBackImpl(T* const t) {
assert(t);
assert(!t->next_);
assert(!t->prev_);
lock_.AssertHeld();
assert((!head_ && !tail_) || (head_ && tail_));
assert(!head_ || !head_->prev_);
assert(!tail_ || !tail_->next_);
t->prev_ = tail_;
if (tail_) {
tail_->next_ = t;
}
tail_ = t;
if (!head_) {
head_ = tail_;
}
}
mutable port::Mutex lock_; // synchronization primitive
T* head_ = nullptr; // front (cold)
T* tail_ = nullptr; // back (hot)
};
} // namespace ROCKSDB_NAMESPACE
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