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#pragma once
#include <map>
#include <list>
#include <experimental/optional>
namespace nix {
/* A simple least-recently used cache. Not thread-safe. */
template<typename Key, typename Value>
class LRUCache
{
private:
size_t capacity;
// Stupid wrapper to get around circular dependency between Data
// and LRU.
struct LRUIterator;
using Data = std::map<Key, std::pair<LRUIterator, Value>>;
using LRU = std::list<typename Data::iterator>;
struct LRUIterator { typename LRU::iterator it; };
Data data;
LRU lru;
public:
LRUCache(size_t capacity) : capacity(capacity) { }
/* Insert or upsert an item in the cache. */
void upsert(const Key & key, const Value & value)
{
if (capacity == 0) return;
erase(key);
if (data.size() >= capacity) {
/* Retire the oldest item. */
auto oldest = lru.begin();
data.erase(*oldest);
lru.erase(oldest);
}
auto res = data.emplace(key, std::make_pair(LRUIterator(), value));
assert(res.second);
auto & i(res.first);
auto j = lru.insert(lru.end(), i);
i->second.first.it = j;
}
bool erase(const Key & key)
{
auto i = data.find(key);
if (i == data.end()) return false;
lru.erase(i->second.first.it);
data.erase(i);
return true;
}
/* Look up an item in the cache. If it exists, it becomes the most
recently used item. */
std::experimental::optional<Value> get(const Key & key)
{
auto i = data.find(key);
if (i == data.end()) return {};
/* Move this item to the back of the LRU list. */
lru.erase(i->second.first.it);
auto j = lru.insert(lru.end(), i);
i->second.first.it = j;
return i->second.second;
}
size_t size()
{
return data.size();
}
void clear()
{
data.clear();
lru.clear();
}
};
}
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