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// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*-
// vim: ts=8 sw=2 smarttab
#pragma once
#include <memory>
#include <optional>
#include <boost/smart_ptr/local_shared_ptr.hpp>
#include <boost/smart_ptr/weak_ptr.hpp>
#include "simple_lru.h"
/// SharedLRU does its best to cache objects. It not only tracks the objects
/// in its LRU cache with strong references, it also tracks objects with
/// weak_ptr even if the cache does not hold any strong references to them. so
/// that it can return the objects after they are evicted, as long as they've
/// ever been cached and have not been destroyed yet.
template<class K, class V>
class SharedLRU {
using shared_ptr_t = boost::local_shared_ptr<V>;
using weak_ptr_t = boost::weak_ptr<V>;
using value_type = std::pair<K, shared_ptr_t>;
// weak_refs is already ordered, and we don't use accessors like
// LRUCache::lower_bound(), so unordered LRUCache would suffice.
SimpleLRU<K, shared_ptr_t, false> cache;
std::map<K, std::pair<weak_ptr_t, V*>> weak_refs;
struct Deleter {
SharedLRU<K,V>* cache;
const K key;
void operator()(V* ptr) {
cache->_erase_weak(key);
delete ptr;
}
};
void _erase_weak(const K& key) {
weak_refs.erase(key);
}
public:
SharedLRU(size_t max_size = 20)
: cache{max_size}
{}
~SharedLRU() {
cache.clear();
// initially, we were assuming that no pointer obtained from SharedLRU
// can outlive the lru itself. However, since going with the interruption
// concept for handling shutdowns, this is no longer valid.
weak_refs.clear();
}
/**
* Returns a reference to the given key, and perform an insertion if such
* key does not already exist
*/
shared_ptr_t operator[](const K& key);
/**
* Returns true iff there are no live references left to anything that has been
* in the cache.
*/
bool empty() const {
return weak_refs.empty();
}
size_t size() const {
return cache.size();
}
size_t capacity() const {
return cache.capacity();
}
/***
* Inserts a key if not present, or bumps it to the front of the LRU if
* it is, and then gives you a reference to the value. If the key already
* existed, you are responsible for deleting the new value you tried to
* insert.
*
* @param key The key to insert
* @param value The value that goes with the key
* @param existed Set to true if the value was already in the
* map, false otherwise
* @return A reference to the map's value for the given key
*/
shared_ptr_t insert(const K& key, std::unique_ptr<V> value);
// clear all strong reference from the lru.
void clear() {
cache.clear();
}
shared_ptr_t find(const K& key);
// return the last element that is not greater than key
shared_ptr_t lower_bound(const K& key);
// return the first element that is greater than key
std::optional<value_type> upper_bound(const K& key);
void erase(const K& key) {
cache.erase(key);
_erase_weak(key);
}
};
template<class K, class V>
typename SharedLRU<K,V>::shared_ptr_t
SharedLRU<K,V>::insert(const K& key, std::unique_ptr<V> value)
{
shared_ptr_t val;
if (auto found = weak_refs.find(key); found != weak_refs.end()) {
val = found->second.first.lock();
}
if (!val) {
val.reset(value.release(), Deleter{this, key});
weak_refs.emplace(key, std::make_pair(val, val.get()));
}
cache.insert(key, val);
return val;
}
template<class K, class V>
typename SharedLRU<K,V>::shared_ptr_t
SharedLRU<K,V>::operator[](const K& key)
{
if (auto found = cache.find(key); found) {
return *found;
}
shared_ptr_t val;
if (auto found = weak_refs.find(key); found != weak_refs.end()) {
val = found->second.first.lock();
}
if (!val) {
val.reset(new V{}, Deleter{this, key});
weak_refs.emplace(key, std::make_pair(val, val.get()));
}
cache.insert(key, val);
return val;
}
template<class K, class V>
typename SharedLRU<K,V>::shared_ptr_t
SharedLRU<K,V>::find(const K& key)
{
if (auto found = cache.find(key); found) {
return *found;
}
shared_ptr_t val;
if (auto found = weak_refs.find(key); found != weak_refs.end()) {
val = found->second.first.lock();
}
if (val) {
cache.insert(key, val);
}
return val;
}
template<class K, class V>
typename SharedLRU<K,V>::shared_ptr_t
SharedLRU<K,V>::lower_bound(const K& key)
{
if (weak_refs.empty()) {
return {};
}
auto found = weak_refs.lower_bound(key);
if (found == weak_refs.end()) {
--found;
}
if (auto val = found->second.first.lock(); val) {
cache.insert(key, val);
return val;
} else {
return {};
}
}
template<class K, class V>
std::optional<typename SharedLRU<K,V>::value_type>
SharedLRU<K,V>::upper_bound(const K& key)
{
for (auto found = weak_refs.upper_bound(key);
found != weak_refs.end();
++found) {
if (auto val = found->second.first.lock(); val) {
return std::make_pair(found->first, val);
}
}
return std::nullopt;
}
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