mijin2/source/mijin/async/future.hpp

#pragma once

#if !defined(MIJIN_ASYNC_FUTURE_HPP_INCLUDED)
#define MIJIN_ASYNC_FUTURE_HPP_INCLUDED 1

#include <exception>
#include <memory>
#include <optional>
#include <tuple>
#include <type_traits>
#include "./signal.hpp"
#include "../container/optional.hpp"
#include "../debug/assert.hpp"
#include "../internal/common.hpp"

namespace mijin
{

//
// public defines
//

//
// public constants
//

//
// public types
//
template<typename TValue, template<typename> typename TAllocator = MIJIN_DEFAULT_ALLOCATOR, bool exceptions = false>
class Future;

// TODO: add support for mutexes and waiting for futures
namespace impl
{
template<typename TValue, bool exceptions>
struct FutureStorage
{
    Optional<TValue> value;

    [[nodiscard]]
    bool hasValue() const MIJIN_NOEXCEPT
    {
        return !value.empty();
    }

    void setValue(TValue value_) MIJIN_NOEXCEPT { value = std::move(value_); }
    [[nodiscard]] TValue& getValue() MIJIN_NOEXCEPT { return value.get(); }

};

template<>
struct FutureStorage<void, false>
{
    bool isSet = false;

    [[nodiscard]]
    bool hasValue() const MIJIN_NOEXCEPT
    {
        return isSet;
    }
    void setValue() MIJIN_NOEXCEPT
    {
        isSet = true;
    }
    void getValue() MIJIN_NOEXCEPT {}
};

#if MIJIN_ENABLE_EXCEPTIONS
template<typename TValue>
struct FutureStorage<TValue, true>
{
    Optional<TValue> value;
    std::exception_ptr exception;

    [[nodiscard]]
    bool hasValue() const MIJIN_NOEXCEPT
    {
        if (exception) {
            return true;
        }
        return !value.empty();
    }

    void setException(std::exception_ptr exc) MIJIN_NOEXCEPT
    {
        exception = std::move(exc);
    }
    void setValue(TValue value_) MIJIN_NOEXCEPT { value = std::move(value_); }
    [[nodiscard]] TValue& getValue()
    {
        if (exception) {
            std::rethrow_exception(exception);
        }
        return value.get();
    }

};

template<>
struct FutureStorage<void, true>
{
    bool isSet = false;
    std::exception_ptr exception;

    [[nodiscard]]
    bool hasValue() const MIJIN_NOEXCEPT
    {
        if (exception) {
            return true;
        }
        return isSet;
    }
    void setException(std::exception_ptr exc) MIJIN_NOEXCEPT
    {
        exception = std::move(exc);
    }
    void setValue() MIJIN_NOEXCEPT
    {
        isSet = true;
    }
    void getValue()
    {
        if (exception) {
            std::rethrow_exception(exception);
        }
    }
};
#endif
} // namespace impl

template<typename TValue, template<typename> typename TAllocator, bool exceptions>
class Future
{
private:
    impl::FutureStorage<TValue, exceptions> value_;
public:
    Future() = default;
    Future(const Future&) = delete;
    Future(Future&&) MIJIN_NOEXCEPT = default;
    explicit Future(TAllocator<void> allocator) : sigSet(std::move(allocator)) {}
public:
    Future& operator=(const Future&) = delete;
    Future& operator=(Future&&) MIJIN_NOEXCEPT = default;

    [[nodiscard]]
    constexpr explicit operator bool() const MIJIN_NOEXCEPT { return ready(); }

    [[nodiscard]]
    constexpr bool operator!() const MIJIN_NOEXCEPT { return !ready(); }
public: // access
    [[nodiscard]]
    constexpr decltype(auto) get() MIJIN_NOEXCEPT_IF(!exceptions)
    {
        MIJIN_ASSERT(ready(), "Attempting to get from future that is not ready.");
        if constexpr(std::is_same_v<TValue, void>)
        {
            value_.getValue(); // in case of exceptions
            return;
        }
        else {
            return value_.getValue();
        }
    }
    [[nodiscard]]
    constexpr decltype(auto) get() const MIJIN_NOEXCEPT_IF(!exceptions)
    {
        MIJIN_ASSERT(ready(), "Attempting to get from future that is not ready.");
        if constexpr(std::is_same_v<TValue, void>)
        {
            value_.getValue(); // in case of exceptions
            return;
        }
        else {
            return value_.getValue();
        }
    }
    [[nodiscard]]
    constexpr bool ready() const MIJIN_NOEXCEPT
    {
        return value_.hasValue();
    }
public: // modification
    template<typename TArg> requires (!std::is_same_v<TValue, void>)
    constexpr void set(TArg&& value) MIJIN_NOEXCEPT
    {
        MIJIN_ASSERT(!ready(), "Trying to set a future twice!");
        value_.setValue(std::move(value));
        sigSet.emit();
    }
    constexpr void set() MIJIN_NOEXCEPT requires (std::is_same_v<TValue, void>)
    {
        MIJIN_ASSERT(!ready(), "Trying to set a future twice!");
        if constexpr (std::is_same_v<TValue, void>)
        {
            value_.setValue();
            sigSet.emit();
        }
        else {
            // would love to make this a compile-time error :/
            MIJIN_ERROR("Attempting to call set(void) on future with value.");
        }
    }
    constexpr void setException(std::exception_ptr exception) requires (exceptions)
    {
        MIJIN_ASSERT(!ready(), "Trying to set a future twice!");
        if constexpr (exceptions)
        {
            value_.setException(std::move(exception));
        }
    }
public: // signals
    BaseSignal<TAllocator> sigSet;
};

template<typename TValue = void, template<typename> typename TAllocator = MIJIN_DEFAULT_ALLOCATOR, bool exceptions = false>
using FuturePtr = std::shared_ptr<Future<TValue, TAllocator, exceptions>>;

template<typename TValue = void, template<typename> typename TAllocator = MIJIN_DEFAULT_ALLOCATOR>
using ExceptFuture = Future<TValue, TAllocator, true>;

template<typename TValue = void, template<typename> typename TAllocator = MIJIN_DEFAULT_ALLOCATOR>
using ExceptFuturePtr = std::shared_ptr<Future<TValue, TAllocator, true>>;

template<typename T>
struct is_future : std::false_type {};

template<typename TValue, template<typename> typename TAllocator, bool exceptions>
struct is_future<Future<TValue, TAllocator, exceptions>> : std::true_type {};

template<typename T>
inline constexpr bool is_future_t = is_future<T>::value;

template<typename T>
concept FutureType = is_future<T>::value;

//
// public functions
//

namespace impl
{
template<typename... TResult>
struct MultiFutureHelper
{
    template<std::size_t... indices>
    static bool allReady(const std::tuple<FuturePtr<TResult>...>& futures, std::index_sequence<indices...>) MIJIN_NOEXCEPT
    {
        return (std::get<indices>(futures)->ready() && ...);
    }

    template<std::size_t... indices>
    static std::tuple<std::remove_reference_t<TResult>...> getAll(const std::tuple<FuturePtr<TResult>...>& futures, std::index_sequence<indices...>) MIJIN_NOEXCEPT
    {
        return std::make_tuple(std::move(std::get<indices>(futures)->get())...);
    }
};
}

template<typename T, template<typename> typename TAllocator = MIJIN_DEFAULT_ALLOCATOR>
constexpr FuturePtr<T> makeSharedFuture(TAllocator<Future<T>> allocator = {}) MIJIN_NOEXCEPT
{
    return std::allocate_shared<Future<T>>(std::move(allocator));
}

template<typename... TResult>
constexpr bool allReady(const std::tuple<FuturePtr<TResult>...>& futures) MIJIN_NOEXCEPT
{
    return impl::MultiFutureHelper<TResult...>::allReady(futures, std::index_sequence_for<TResult...>());
}

template<typename... TResult>
constexpr std::tuple<std::remove_reference_t<TResult>...> getAll(const std::tuple<FuturePtr<TResult>...>& futures) MIJIN_NOEXCEPT
{
    return impl::MultiFutureHelper<TResult...>::getAll(futures, std::index_sequence_for<TResult...>());
}
} // namespace mijin

#endif // !defined(MIJIN_ASYNC_FUTURE_HPP_INCLUDED)