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#ifndef MO2_THREAD_UTILS_H
#define MO2_THREAD_UTILS_H
#include <mutex>
#include <thread>
namespace MOShared {
/**
* Class that can be used to perform thread-safe memoization.
*
* Each instance hold a flag indicating if the current value is up-to-date
* or not. This flag can be reset using `invalidate()`. When the value is queried,
* the flag is checked, and if it is not up-to-date, the given callback is used
* to compute the value.
*
* The computation and update of the value is locked to avoid concurrent modifications.
*
* @tparam T Type of value ot memoized.
* @tparam Fn Type of the callback.
*/
template <class T, class Fn>
struct MemoizedLocked {
MemoizedLocked(Fn callback, T value = {}) :
m_Fn{ callback }, m_Value{ std::move(value) } { }
template <class... Args>
T& value(Args&&... args) const {
if (m_NeedUpdating) {
std::scoped_lock lock(m_Mutex);
if (m_NeedUpdating) {
m_Value = std::invoke(m_Fn, std::forward<Args>(args)... );
m_NeedUpdating.store(false);
}
}
return m_Value;
}
void invalidate() const {
m_NeedUpdating.store(true);
}
private:
mutable std::mutex m_Mutex;
mutable std::atomic<bool> m_NeedUpdating{ true };
Fn m_Fn;
mutable T m_Value;
};
/**
* @brief Apply the given callable to each element between the two given iterators
* in a parallel way.
*
* The callable should be independent, or properly synchronized, and the source of
* the range should not change during this call.
*
* @param start Beginning of the range.
* @param end End of the range.
* @param callable Callable to apply to every element of the range. See std::invoke
* requirements. Must be copiable.
* @param nThreads Number of threads to use.
*
*/
template <class It, class Callable>
void parallelMap(It begin, It end, Callable callable, std::size_t nThreads)
{
std::mutex m;
std::vector<std::thread> threads(nThreads);
// Create the thread:
// - The mutex is only used to fetch/increment the iterator.
// - The callable is copied in each thread to avoid conflicts.
for (auto &thread: threads) {
thread = std::thread([&m, &begin, end, callable]() {
while (true) {
decltype(begin) it;
{
std::scoped_lock lock(m);
if (begin == end) {
break;
}
it = begin++;
}
if (it != end) {
std::invoke(callable, *it);
}
}
});
}
// Join everything:
for (auto& t : threads) {
t.join();
}
}
}
#endif
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