kotatogram-desktop/Telegram/SourceFiles/rpl/combine_previous.h

/*
This file is part of Telegram Desktop,
the official desktop version of Telegram messaging app, see https://telegram.org

Telegram Desktop is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

It is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.

In addition, as a special exception, the copyright holders give permission
to link the code of portions of this program with the OpenSSL library.

Full license: https://github.com/telegramdesktop/tdesktop/blob/master/LICENSE
Copyright (c) 2014-2017 John Preston, https://desktop.telegram.org
*/
#pragma once

#include <rpl/producer.h>
#include "base/optional.h"

namespace rpl {
namespace details {

class combine_previous_helper {
public:
	template <typename Value, typename Error>
	rpl::producer<std::tuple<Value, Value>, Error> operator()(
			rpl::producer<Value, Error> &&initial) const {
		using consumer_type = consumer<
			std::tuple<Value, Value>,
			Error>;
		return [initial = std::move(initial)](
				const consumer_type &consumer) mutable {
			auto previous = consumer.template make_state<
				base::optional<Value>
			>();
			return std::move(initial).start(
				[consumer, previous](auto &&value) {
					if (auto exists = *previous) {
						&existing = *exists;
						auto next = std::make_tuple(
							std::move(existing),
							value);
						consumer.put_next(std::move(next));
						existing = std::forward<decltype(value)>(
							value);
					} else {
						*previous = std::forward<decltype(value)>(
							value);
					}
				}, [consumer](auto &&error) {
					consumer.put_error_forward(std::forward<decltype(error)>(error));
				}, [consumer] {
					consumer.put_done();
				});
		};
	}

};

template <typename DefaultValue>
class combine_previous_with_default_helper {
public:
	template <typename OtherValue>
	combine_previous_with_default_helper(OtherValue &&value)
		: _value(std::forward<OtherValue>(value)) {
	}

	template <typename Value, typename Error>
	rpl::producer<std::tuple<Value, Value>, Error> operator()(
			rpl::producer<Value, Error> &&initial) {
		using consumer_type = consumer<
			std::tuple<Value, Value>,
			Error>;
		return [
			initial = std::move(initial),
			value = Value(std::move(_value))
		](const consumer_type &consumer) mutable {
			auto previous = consumer.template make_state<Value>(
				std::move(value));
			return std::move(initial).start(
				[consumer, previous](auto &&value) {
					auto &existing = *previous;
					auto next = std::make_tuple(
						std::move(existing),
						value);
					consumer.put_next(std::move(next));
					existing = std::forward<decltype(value)>(value);
				}, [consumer](auto &&error) {
					consumer.put_error_forward(std::forward<decltype(error)>(error));
				}, [consumer] {
					consumer.put_done();
				});
		};
	}

private:
	DefaultValue _value;

};

template <typename DefaultValue>
combine_previous_with_default_helper<std::decay_t<DefaultValue>>
combine_previous_with_default(DefaultValue &&value) {
	return { std::forward<DefaultValue>(value) };
}

} // namespace details

inline auto combine_previous()
-> details::combine_previous_helper {
	return details::combine_previous_helper();
}

template <typename DefaultValue>
inline auto combine_previous(DefaultValue &&value) {
	return details::combine_previous_with_default(
		std::forward<DefaultValue>(value));
}

} // namespace rpl