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