ClickHouse/dbms/src/Common/SimpleCache.h

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#pragma once
#include <map>
#include <tuple>
#include <mutex>
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#include <ext/function_traits.h>
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/** The simplest cache for a free function.
* You can also pass a static class method or lambda without captures.
* The size is unlimited. Values are stored permanently and never evicted.
* Mutex is used for synchronization.
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* Suitable only for the simplest cases.
*
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* Usage
*
* SimpleCache<decltype(func), &func> func_cached;
* std::cerr << func_cached(args...);
*/
template <typename F, F* f>
class SimpleCache
{
private:
using Key = typename function_traits<F>::arguments_decay;
using Result = typename function_traits<F>::result;
std::map<Key, Result> cache;
std::mutex mutex;
public:
template <typename... Args>
Result operator() (Args &&... args)
{
{
std::lock_guard<std::mutex> lock(mutex);
Key key{std::forward<Args>(args)...};
auto it = cache.find(key);
if (cache.end() != it)
return it->second;
}
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/// The calculations themselves are not done under mutex.
Result res = f(std::forward<Args>(args)...);
{
std::lock_guard<std::mutex> lock(mutex);
cache.emplace(std::forward_as_tuple(args...), res);
}
return res;
}
void drop()
{
std::lock_guard<std::mutex> lock(mutex);
cache.clear();
}
};