cppcoro/include/cppcoro/generator.hpp

///////////////////////////////////////////////////////////////////////////////
// Copyright (c) Lewis Baker
// Licenced under MIT license. See LICENSE.txt for details.
///////////////////////////////////////////////////////////////////////////////
#ifndef CPPCORO_GENERATOR_HPP_INCLUDED
#define CPPCORO_GENERATOR_HPP_INCLUDED

#include <experimental/coroutine>
#include <type_traits>
#include <utility>

namespace cppcoro
{
	template<typename T>
	class generator;

	namespace detail
	{
		template<typename T>
		class generator_promise
		{
		public:

			using value_type = std::remove_reference_t<T>;
			using reference_type = std::conditional_t<std::is_reference_v<T>, T, T&>;
			using pointer_type = value_type*;

			generator_promise() = default;

			generator<T> get_return_object() noexcept;

			constexpr std::experimental::suspend_always initial_suspend() const { return {}; }
			constexpr std::experimental::suspend_always final_suspend() const { return {}; }

			template<
				typename U,
				typename = std::enable_if_t<std::is_same<U, T>::value>>
			std::experimental::suspend_always yield_value(U& value) noexcept
			{
				m_value = std::addressof(value);
				return {};
			}

			std::experimental::suspend_always yield_value(T&& value) noexcept
			{
				m_value = std::addressof(value);
				return {};
			}

			void unhandled_exception()
			{
				std::rethrow_exception(std::current_exception());
			}

			void return_void()
			{
			}

			reference_type value() const noexcept
			{
				return *m_value;
			}

			// Don't allow any use of 'co_await' inside the generator coroutine.
			template<typename U>
			std::experimental::suspend_never await_transform(U&& value) = delete;

		private:

			pointer_type m_value;

		};

		template<typename T>
		class generator_iterator
		{
			using coroutine_handle = std::experimental::coroutine_handle<generator_promise<T>>;

		public:

			using iterator_category = std::input_iterator_tag;
			// What type should we use for counting elements of a potentially infinite sequence?
			using difference_type = std::size_t;
			using value_type = std::remove_reference_t<T>;
			using reference = value_type&;
			using pointer = value_type*;

			explicit generator_iterator(std::nullptr_t) noexcept
				: m_coroutine(nullptr)
			{}

			explicit generator_iterator(coroutine_handle coroutine) noexcept
				: m_coroutine(coroutine)
			{}

			bool operator==(const generator_iterator& other) const noexcept
			{
				return m_coroutine == other.m_coroutine;
			}

			bool operator!=(const generator_iterator& other) const noexcept
			{
				return !(*this == other);
			}

			generator_iterator& operator++()
			{
				m_coroutine.resume();
				if (m_coroutine.done())
				{
					m_coroutine = nullptr;
				}

				return *this;
			}

			// Don't support post-increment as that would require taking a
			// copy of the old value into the returned iterator as there
			// are no guarantees it's still going to be valid after the
			// increment is executed.
			generator_iterator operator++(int) = delete;

			reference operator*() const noexcept
			{
				return m_coroutine.promise().value();
			}

			pointer operator->() const noexcept
			{
				return std::addressof(operator*());
			}

		private:

			coroutine_handle m_coroutine;
		};
	}

	template<typename T>
	class generator
	{
	public:

		using promise_type = detail::generator_promise<T>;
		using iterator = detail::generator_iterator<T>;

		generator() noexcept
			: m_coroutine(nullptr)
		{}

		generator(generator&& other) noexcept
			: m_coroutine(other.m_coroutine)
		{
			other.m_coroutine = nullptr;
		}

		generator(const generator& other) = delete;

		~generator()
		{
			if (m_coroutine)
			{
				m_coroutine.destroy();
			}
		}

		generator& operator=(generator other) noexcept
		{
			swap(other);
			return *this;
		}

		iterator begin()
		{
			if (m_coroutine)
			{
				m_coroutine.resume();
				if (!m_coroutine.done())
				{
					return iterator{ m_coroutine };
				}
			}

			return iterator{ nullptr };
		}

		iterator end() noexcept
		{
			return iterator{ nullptr };
		}

		void swap(generator& other) noexcept
		{
			std::swap(m_coroutine, other.m_coroutine);
		}

	private:

		friend class detail::generator_promise<T>;

		explicit generator(std::experimental::coroutine_handle<promise_type> coroutine) noexcept
			: m_coroutine(coroutine)
		{}

		std::experimental::coroutine_handle<promise_type> m_coroutine;

	};

	template<typename T>
	void swap(generator<T>& a, generator<T>& b)
	{
		a.swap(b);
	}

	namespace detail
	{
		template<typename T>
		generator<T> generator_promise<T>::get_return_object() noexcept
		{
			using coroutine_handle = std::experimental::coroutine_handle<generator_promise<T>>;
			return generator<T>{ coroutine_handle::from_promise(*this) };
		}
	}

	template<typename FUNC, typename T>
	generator<std::result_of_t<FUNC&&(T&)>> fmap(FUNC func, generator<T> source)
	{
		for (auto& value : source)
		{
			co_yield std::invoke(func, value);
		}
	}
}

#endif