From 151959c1d683134551c8812aee957158c4586439 Mon Sep 17 00:00:00 2001
From: Corentin B <32176761+CorentinBT@users.noreply.github.com>
Date: Sun, 18 Feb 2024 20:27:52 +0100
Subject: [PATCH] Add on_error_resume_next operator (#528)

---
 src/benchmarks/benchmarks.cpp                 |  24 +++
 src/rpp/rpp/operators.hpp                     |   9 +
 .../rpp/operators/on_error_resume_next.hpp    | 109 ++++++++++++
 src/tests/rpp/test_on_error_resume_next.cpp   | 156 ++++++++++++++++++
 4 files changed, 298 insertions(+)
 create mode 100644 src/rpp/rpp/operators/on_error_resume_next.hpp
 create mode 100644 src/tests/rpp/test_on_error_resume_next.cpp

diff --git a/src/benchmarks/benchmarks.cpp b/src/benchmarks/benchmarks.cpp
index af3ca7490..55b0f4f62 100644
--- a/src/benchmarks/benchmarks.cpp
+++ b/src/benchmarks/benchmarks.cpp
@@ -581,6 +581,30 @@ int main(int argc, char* argv[]) // NOLINT(bugprone-exception-escape)
         }
     } // BENCHMARK("Aggregating Operators")
 
+    BENCHMARK("Error Handling Operators")
+    {
+        SECTION("create(on_next(1), on_error())+on_error_resume_next(immediate_just(2)))+subscribe")
+        {
+            TEST_RPP([&]() {
+                rpp::source::create<int>([&](auto&& observer) {
+                    observer.on_next(1);
+                    observer.on_error(std::make_exception_ptr(std::runtime_error{""}));
+                })
+                    | rpp::operators::on_error_resume_next([](const std::exception_ptr&) { return rpp::immediate_just(2); })
+                    | rpp::operators::subscribe([](int v) { ankerl::nanobench::doNotOptimizeAway(v); });
+            });
+
+            TEST_RXCPP([&]() {
+                rxcpp::observable<>::create<int>([&](auto&& observer) {
+                    observer.on_next(1);
+                    observer.on_error(std::make_exception_ptr(std::runtime_error{""}));
+                })
+                    | rxcpp::operators::on_error_resume_next([](const std::exception_ptr&) { return rxcpp::immediate_just(2); })
+                    | rxcpp::operators::subscribe<int>([](int v) { ankerl::nanobench::doNotOptimizeAway(v); });
+            });
+        }
+    } // BENCHMARK("Error Handling Operators")
+
     BENCHMARK("Subjects")
     {
         SECTION("publish_subject with 1 observer - on_next")
diff --git a/src/rpp/rpp/operators.hpp b/src/rpp/rpp/operators.hpp
index 3ec4ec6ba..d95860c95 100644
--- a/src/rpp/rpp/operators.hpp
+++ b/src/rpp/rpp/operators.hpp
@@ -117,3 +117,12 @@
 
 #include <rpp/operators/concat.hpp>
 #include <rpp/operators/reduce.hpp>
+
+/**
+ * @defgroup error_handling_operators Error Handling Operators
+ * @brief Operators that help to recover from error notifications from an Observable
+ * @see https://reactivex.io/documentation/operators.html#error
+ * @ingroup operators
+ */
+
+#include <rpp/operators/on_error_resume_next.hpp>
diff --git a/src/rpp/rpp/operators/on_error_resume_next.hpp b/src/rpp/rpp/operators/on_error_resume_next.hpp
new file mode 100644
index 000000000..f6449d6c9
--- /dev/null
+++ b/src/rpp/rpp/operators/on_error_resume_next.hpp
@@ -0,0 +1,109 @@
+//                  ReactivePlusPlus library
+//
+//          Copyright Aleksey Loginov 2023 - present.
+// Distributed under the Boost Software License, Version 1.0.
+//    (See accompanying file LICENSE_1_0.txt or copy at
+//          https://www.boost.org/LICENSE_1_0.txt)
+//
+// Project home: https://github.com/victimsnino/ReactivePlusPlus
+//
+
+#pragma once
+
+#include <rpp/operators/fwd.hpp>
+
+#include <rpp/defs.hpp>
+#include <rpp/operators/details/strategy.hpp>
+
+namespace rpp::operators::details
+{
+    template<rpp::constraint::observer TObserver, rpp::constraint::decayed_type Selector>
+    struct on_error_resume_next_observer_strategy
+    {
+        using preferred_disposable_strategy = rpp::details::observers::none_disposable_strategy;
+
+        RPP_NO_UNIQUE_ADDRESS mutable TObserver observer;
+        RPP_NO_UNIQUE_ADDRESS Selector          selector;
+        // Manually control disposable to ensure observer is not used after move in on_error emission
+        mutable rpp::composite_disposable_wrapper disposable = composite_disposable_wrapper::make();
+
+        RPP_CALL_DURING_CONSTRUCTION(
+            {
+                observer.set_upstream(disposable);
+            });
+
+        template<typename T>
+        void on_next(T&& v) const
+        {
+            observer.on_next(std::forward<T>(v));
+        }
+
+        void on_error(const std::exception_ptr& err) const
+        {
+            disposable.dispose();
+            selector(err).subscribe(std::move(observer));
+        }
+
+        void on_completed() const
+        {
+            disposable.dispose();
+            observer.on_completed();
+        }
+
+        void set_upstream(const disposable_wrapper& d)
+        {
+            disposable.add(d);
+        }
+
+        bool is_disposed() const { return disposable.is_disposed(); }
+    };
+
+    template<rpp::constraint::decayed_type Selector>
+    struct on_error_resume_next_t : lift_operator<on_error_resume_next_t<Selector>, Selector>
+    {
+        template<rpp::constraint::decayed_type T>
+        struct operator_traits
+        {
+            using selector_observable_result_type =
+                rpp::utils::extract_observable_type_t<std::invoke_result_t<Selector, std::exception_ptr>>;
+
+            static_assert(
+                rpp::constraint::decayed_same_as<selector_observable_result_type, T>,
+                "Selector observable result type is not the same as T");
+
+            using result_type = T;
+
+            template<rpp::constraint::observer_of_type<result_type> TObserver>
+            using observer_strategy = on_error_resume_next_observer_strategy<TObserver, Selector>;
+        };
+
+        template<rpp::details::observables::constraint::disposable_strategy Prev>
+        using updated_disposable_strategy = rpp::details::observables::atomic_dynamic_disposable_strategy_selector<1>;
+    };
+} // namespace rpp::operators::details
+
+namespace rpp::operators
+{
+    /**
+     * @brief If an error occurs, take the result from the Selector and subscribe to that instead.
+     *
+     * @marble on_error_resume_next
+       {
+           source observable                                  : +-1-x
+           operator "on_error_resume_next: () => obs(-2-3-|)" : +-1-2-3-|
+       }
+     *
+     * @param selector callable taking a std::exception_ptr and returning observable to continue on
+     *
+     * @warning #include <rpp/operators/on_error_resume_next.hpp>
+     *
+     * @ingroup error_handling_operators
+     * @see https://reactivex.io/documentation/operators/catch.html
+     */
+    template<typename Selector>
+        requires rpp::constraint::observable<std::invoke_result_t<Selector, std::exception_ptr>>
+    auto on_error_resume_next(Selector&& selector)
+    {
+        return details::on_error_resume_next_t<std::decay_t<Selector>>{std::forward<Selector>(selector)};
+    }
+} // namespace rpp::operators
\ No newline at end of file
diff --git a/src/tests/rpp/test_on_error_resume_next.cpp b/src/tests/rpp/test_on_error_resume_next.cpp
new file mode 100644
index 000000000..c30719665
--- /dev/null
+++ b/src/tests/rpp/test_on_error_resume_next.cpp
@@ -0,0 +1,156 @@
+//                  ReactivePlusPlus library
+//
+//          Copyright Aleksey Loginov 2023 - present.
+// Distributed under the Boost Software License, Version 1.0.
+//    (See accompanying file LICENSE_1_0.txt or copy at
+//          https://www.boost.org/LICENSE_1_0.txt)
+//
+// Project home: https://github.com/victimsnino/ReactivePlusPlus
+//
+
+#include <snitch/snitch.hpp>
+
+#include <rpp/operators/on_error_resume_next.hpp>
+#include <rpp/schedulers/immediate.hpp>
+#include <rpp/sources/create.hpp>
+#include <rpp/sources/empty.hpp>
+#include <rpp/sources/just.hpp>
+
+#include "disposable_observable.hpp"
+#include "mock_observer.hpp"
+
+TEMPLATE_TEST_CASE("on_error_resume_next switches observable on error", "", rpp::memory_model::use_stack, rpp::memory_model::use_shared)
+{
+    auto mock = mock_observer_strategy<int>();
+    SECTION("observable without error emission")
+    {
+        auto obs = rpp::source::just<TestType>(rpp::schedulers::immediate{}, 1, 2, 3);
+        SECTION("subscribe")
+        {
+            obs | rpp::operators::on_error_resume_next([](const std::exception_ptr&) { return rpp::source::empty<int>(); })
+                | rpp::ops::subscribe(mock);
+            SECTION("observer obtains values from observable")
+            {
+                CHECK(mock.get_received_values() == std::vector{1, 2, 3});
+                CHECK(mock.get_total_on_next_count() == 3);
+                CHECK(mock.get_on_error_count() == 0);
+                CHECK(mock.get_on_completed_count() == 1);
+            }
+        }
+    }
+
+    SECTION("observable with one error emission")
+    {
+        auto obs = rpp::source::create<int>([](const auto& sub) {
+            sub.on_next(1);
+            sub.on_next(2);
+            sub.on_error(std::make_exception_ptr(std::runtime_error{""}));
+        });
+        SECTION("subscribe")
+        {
+            obs | rpp::operators::on_error_resume_next([](const std::exception_ptr&) {
+                return rpp::source::just<TestType>(rpp::schedulers::immediate{}, 3);
+            })
+                | rpp::ops::subscribe(mock);
+            SECTION("observer obtains values from both outer and inner observable")
+            {
+                CHECK(mock.get_received_values() == std::vector{1, 2, 3});
+                CHECK(mock.get_total_on_next_count() == 3);
+                CHECK(mock.get_on_error_count() == 0);
+                CHECK(mock.get_on_completed_count() == 1);
+            }
+        }
+    }
+
+    SECTION("observable with two error emissions")
+    {
+        auto obs = rpp::source::create<int>([](const auto& sub) {
+            sub.on_next(1);
+            sub.on_next(2);
+            sub.on_error(std::make_exception_ptr(std::runtime_error{""}));
+            sub.on_error(std::make_exception_ptr(std::runtime_error{""}));
+        });
+        SECTION("subscribe")
+        {
+            obs | rpp::operators::on_error_resume_next([](const std::exception_ptr&) {
+                return rpp::source::just<TestType>(rpp::schedulers::immediate{}, 3);
+            })
+                | rpp::ops::subscribe(mock);
+            SECTION("observer only receives values from first inner observable")
+            {
+                CHECK(mock.get_received_values() == std::vector{1, 2, 3});
+                CHECK(mock.get_total_on_next_count() == 3);
+                CHECK(mock.get_on_error_count() == 0);
+                CHECK(mock.get_on_completed_count() == 1);
+            }
+        }
+    }
+
+    SECTION("inner observable different emission type")
+    {
+        auto obs = rpp::source::create<int>([](const auto& sub) {
+            sub.on_next(1);
+            sub.on_next(2);
+            sub.on_error(std::make_exception_ptr(std::runtime_error{""}));
+            sub.on_error(std::make_exception_ptr(std::runtime_error{""}));
+        });
+        SECTION("subscribe")
+        {
+            obs | rpp::operators::on_error_resume_next([](const std::exception_ptr&) {
+                return rpp::source::just<TestType>(rpp::schedulers::immediate{}, 3);
+            })
+                | rpp::ops::subscribe(mock);
+            SECTION("observer only receives values from first inner observable")
+            {
+                CHECK(mock.get_received_values() == std::vector{1, 2, 3});
+                CHECK(mock.get_total_on_next_count() == 3);
+                CHECK(mock.get_on_error_count() == 0);
+                CHECK(mock.get_on_completed_count() == 1);
+            }
+        }
+    }
+
+    SECTION("nested on_error_resume_next operators")
+    {
+        auto obs = rpp::source::create<int>([](const auto& sub) {
+            sub.on_next(1);
+            sub.on_error(std::make_exception_ptr(std::runtime_error{""}));
+        });
+        SECTION("subscribe")
+        {
+            obs | rpp::operators::on_error_resume_next([](const std::exception_ptr&) {
+                return rpp::source::create<int>([](const auto& sub) {
+                           sub.on_next(2);
+                           sub.on_error(std::make_exception_ptr(std::runtime_error{""}));
+                       })
+                     | rpp::operators::on_error_resume_next([](const std::exception_ptr&) {
+                           return rpp::source::create<int>([](const auto& sub) {
+                               sub.on_next(3);
+                               sub.on_completed();
+                           });
+                       });
+            })
+                | rpp::ops::subscribe(mock);
+            SECTION("observer receives values without any errors")
+            {
+                CHECK(mock.get_received_values() == std::vector{1, 2, 3});
+                CHECK(mock.get_total_on_next_count() == 3);
+                CHECK(mock.get_on_error_count() == 0);
+                CHECK(mock.get_on_completed_count() == 1);
+            }
+        }
+    }
+}
+
+TEST_CASE("on_error_resume_next satisfies disposable contracts")
+{
+    auto observable_disposable = rpp::composite_disposable_wrapper::make();
+    {
+        auto observable = observable_with_disposable<int>(observable_disposable);
+
+        test_operator_with_disposable<int>(
+            rpp::ops::on_error_resume_next([](const std::exception_ptr&) { return rpp::source::empty<int>(); }));
+    }
+
+    CHECK(observable_disposable.is_disposed() || observable_disposable.lock().use_count() == 2);
+}
\ No newline at end of file