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Telegram/lib_rpl/rpl/combine_previous.h

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+// This file is part of Desktop App Toolkit,
+// a set of libraries for developing nice desktop applications.
+//
+// For license and copyright information please follow this link:
+// https://github.com/desktop-app/legal/blob/master/LEGAL
+//
+#pragma once
+
+#include <rpl/producer.h>
+#include "base/optional.h"
+
+namespace rpl {
+namespace details {
+
+class combine_previous_helper {
+public:
+	template <typename Value, typename Error, typename Generator>
+	auto operator()(
+			producer<Value, Error, Generator> &&initial) const {
+		return make_producer<std::tuple<Value, Value>, Error>([
+			initial = std::move(initial)
+		](const auto &consumer) mutable {
+			auto previous = consumer.template make_state<
+				std::optional<Value>
+			>();
+			return std::move(initial).start(
+				[consumer, previous](auto &&value) {
+					if (auto &exists = *previous) {
+						auto &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, typename Generator>
+	auto operator()(producer<Value, Error, Generator> &&initial) {
+		return make_producer<std::tuple<Value, Value>, Error>([
+			initial = std::move(initial),
+			value = Value(std::move(_value))
+		](const auto &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