Add filter_stream (#594)

Implement `filter_stream`.

Returns a stream that contains the values from the input stream that evaluate 
true on the predicate `filterFunc(val)`.

doc/api_reference.md

@@ -844,6 +844,11 @@ of `tag_invoke(tag_t<for_each>, your_stream_type, Func)`.
 Returns a stream that produces values that are the result of calling
 `func(value)` on each value produced by the input stream.
 
+### `filter_stream(Stream stream, FilterFunc filterFunc) -> Stream`
+
+Returns a stream that contains the values from the input stream that evaluate 
+true on the predicate `filterFunc(val)`.
+
 ### `via_stream(Scheduler scheduler, Stream stream) -> Stream`
 
 Returns a stream that calls the receiver methods on the specified scheduler's

include/unifex/filter_stream.hpp

@@ -0,0 +1,254 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * Licensed under the Apache License Version 2.0 with LLVM Exceptions
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *   https://llvm.org/LICENSE.txt
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#pragma once
+
+#include <unifex/bind_back.hpp>
+#include <unifex/manual_lifetime.hpp>
+#include <unifex/std_concepts.hpp>
+#include <unifex/stream_concepts.hpp>
+
+#include <exception>
+#include <functional>
+#include <type_traits>
+#include <utility>
+
+#include <unifex/detail/prologue.hpp>
+
+namespace unifex {
+namespace _filter {
+template <typename StreamSender, typename FilterFunc, typename Receiver>
+struct _op {
+  struct type;
+};
+template <typename StreamSender, typename FilterFunc, typename Receiver>
+using operation = typename _op<
+    remove_cvref_t<StreamSender>,
+    remove_cvref_t<FilterFunc>,
+    remove_cvref_t<Receiver>>::type;
+
+template <typename StreamSender, typename FilterFunc, typename Receiver>
+struct _next_receiver {
+  struct type;
+};
+template <typename StreamSender, typename FilterFunc, typename Receiver>
+using next_receiver = typename _next_receiver<
+    remove_cvref_t<StreamSender>,
+    remove_cvref_t<FilterFunc>,
+    remove_cvref_t<Receiver>>::type;
+
+template <typename StreamSender, typename FilterFunc, typename Receiver>
+struct _next_receiver<StreamSender, FilterFunc, Receiver>::type {
+  using next_receiver_t = next_receiver<StreamSender, FilterFunc, Receiver>;
+  operation<StreamSender, FilterFunc, Receiver>& op_;
+
+  template(typename CPO)                       //
+      (requires is_receiver_query_cpo_v<CPO>)  //
+      friend auto tag_invoke(CPO cpo, const type& r) noexcept(
+          is_nothrow_callable_v<CPO, const Receiver&>)
+          -> callable_result_t<CPO, const Receiver&> {
+    return std::move(cpo)(std::as_const(r.op_.receiver_));
+  }
+
+#if UNIFEX_ENABLE_CONTINUATION_VISITATIONS
+  template <typename Func>
+  friend void
+  tag_invoke(tag_t<visit_continuations>, const type& r, Func&& func) {
+    std::invoke(func, r.op_.receiver_);
+  }
+#endif
+
+  template <typename... Values>
+  void set_value(Values&&... values) && noexcept {
+    auto& op = op_;
+    UNIFEX_TRY {
+      const bool doFilter = !std::invoke(op.filter_, std::as_const(values)...);
+
+      if (doFilter) {
+        op.next_.destruct();
+        op.nextEngaged_ = false;
+        op.next_.construct_with([&] {
+          return unifex::connect(next(op.stream_), next_receiver_t{op});
+        });
+        op.nextEngaged_ = true;
+        unifex::start(op.next_.get());
+      } else {
+        unifex::set_value(
+            std::move(op.receiver_), std::forward<Values>(values)...);
+      }
+    }
+    UNIFEX_CATCH(...) {
+      unifex::set_error(std::move(op.receiver_), std::current_exception());
+    }
+  }
+
+  void set_done() && noexcept { unifex::set_done(std::move(op_.receiver_)); }
+
+  template <typename Error>
+  void set_error(Error&& e) && noexcept {
+    unifex::set_error(std::move(op_.receiver_), std::forward<Error>(e));
+  }
+};
+
+template <typename StreamSender, typename FilterFunc, typename Receiver>
+struct _op<StreamSender, FilterFunc, Receiver>::type {
+  StreamSender& stream_;
+  FilterFunc& filter_;
+  Receiver receiver_;
+
+  using next_receiver_t = next_receiver<StreamSender, FilterFunc, Receiver>;
+  using next_op =
+      manual_lifetime<next_operation_t<StreamSender, next_receiver_t>>;
+
+  next_op next_;
+  bool nextEngaged_{false};
+
+  template <typename Receiver2>
+  explicit type(
+      StreamSender& stream,
+      FilterFunc& filter,
+      Receiver2&& receiver) noexcept(std::
+                                         is_nothrow_constructible_v<
+                                             Receiver,
+                                             Receiver2>&&
+                                             is_nothrow_connectable_v<
+                                                 next_sender_t<StreamSender>,
+                                                 Receiver&>)
+    : stream_(stream)
+    , filter_(filter)
+    , receiver_(std::forward<Receiver2>(receiver)) {
+    next_.construct_with(
+        [&] { return unifex::connect(next(stream_), next_receiver_t{*this}); });
+    nextEngaged_ = true;
+  }
+
+  ~type() {
+    if (nextEngaged_) {
+      next_.destruct();
+    }
+  }
+
+  void start() noexcept { unifex::start(next_.get()); }
+};
+
+template <typename StreamSender, typename FilterFunc>
+struct _sender {
+  struct type;
+};
+template <typename StreamSender, typename FilterFunc>
+using sender =
+    typename _sender<remove_cvref_t<StreamSender>, remove_cvref_t<FilterFunc>>::
+        type;
+
+template <typename StreamSender, typename FilterFunc>
+struct _sender<StreamSender, FilterFunc>::type {
+  using sender = type;
+  StreamSender& stream_;
+  FilterFunc& filter_;
+
+  // value and error types just point to the ones from the stream sender
+  template <
+      template <typename...>
+      class Variant,
+      template <typename...>
+      class Tuple>
+  using value_types =
+      sender_value_types_t<next_sender_t<StreamSender>, Variant, Tuple>;
+
+  template <template <typename...> class Variant>
+  using error_types = typename concat_type_lists_unique_t<
+      sender_error_types_t<next_sender_t<StreamSender>, type_list>,
+      type_list<std::exception_ptr>>::template apply<Variant>;
+
+  static constexpr bool sends_done =
+      sender_traits<next_sender_t<StreamSender>>::sends_done;
+
+  template <typename Receiver>
+  using operation_t = operation<StreamSender, FilterFunc, Receiver>;
+  template <typename Receiver>
+  using next_receiver_t = next_receiver<StreamSender, FilterFunc, Receiver>;
+
+  template(typename Self, typename Receiver)  //
+      (requires same_as<remove_cvref_t<Self>, type> AND receiver<Receiver> AND
+           sender_to<
+               next_sender_t<StreamSender>,
+               next_receiver_t<Receiver>>)  //
+      friend operation_t<Receiver> tag_invoke(
+          tag_t<connect>, Self&& self, Receiver&& receiver) {
+    return operation_t<Receiver>{
+        self.stream_,
+        std::move(self).filter_,
+        std::forward<Receiver>(receiver)};
+  }
+};
+}  // namespace _filter
+
+namespace _filter_stream {
+
+template <typename StreamSender, typename FilterFunc>
+struct _filter_stream {
+  struct type;
+};
+
+template <typename StreamSender, typename FilterFunc>
+struct _filter_stream<StreamSender, FilterFunc>::type {
+  UNIFEX_NO_UNIQUE_ADDRESS StreamSender stream_;
+  UNIFEX_NO_UNIQUE_ADDRESS FilterFunc filter_;
+
+  friend auto tag_invoke(tag_t<next>, type& s) noexcept(
+      std::is_nothrow_constructible_v<
+          _filter::sender<StreamSender, FilterFunc>,
+          StreamSender&,
+          FilterFunc&>) {
+    return _filter::sender<StreamSender, FilterFunc>{s.stream_, s.filter_};
+  }
+  friend auto
+  tag_invoke(tag_t<cleanup>, type& s) noexcept(noexcept(cleanup(s.stream_))) {
+    return cleanup(s.stream_);
+  }
+};
+
+template <typename StreamSender, typename FilterFunc>
+using filter_stream = typename _filter_stream<
+    remove_cvref_t<StreamSender>,
+    remove_cvref_t<FilterFunc>>::type;
+
+struct _fn {
+  template <typename StreamSender, typename FilterFunc>
+  auto operator()(StreamSender&& stream, FilterFunc&& filterFunc) const
+      noexcept(std::is_nothrow_constructible_v<
+               filter_stream<StreamSender, FilterFunc>,
+               StreamSender,
+               FilterFunc>) -> filter_stream<StreamSender, FilterFunc> {
+    return filter_stream<StreamSender, FilterFunc>{
+        std::forward<StreamSender>(stream),
+        std::forward<FilterFunc>(filterFunc)};
+  }
+
+  template <typename FilterFunc>
+  constexpr auto operator()(FilterFunc&& filterFunc) const
+      noexcept(is_nothrow_callable_v<tag_t<bind_back>, _fn, FilterFunc>)
+          -> bind_back_result_t<_fn, FilterFunc> {
+    return bind_back(*this, std::forward<FilterFunc>(filterFunc));
+  }
+};
+
+}  // namespace _filter_stream
+
+inline constexpr _filter_stream::_fn filter_stream{};
+
+}  // namespace unifex
+
+#include <unifex/detail/epilogue.hpp>