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| 1 | +#ifndef DICE_TEMPLATELIBRARY_EXCHANGECHANNEL_HPP |
| 2 | +#define DICE_TEMPLATELIBRARY_EXCHANGECHANNEL_HPP |
| 3 | + |
| 4 | +#include <atomic> |
| 5 | +#include <cassert> |
| 6 | +#include <condition_variable> |
| 7 | +#include <cstddef> |
| 8 | +#include <iterator> |
| 9 | +#include <mutex> |
| 10 | +#include <optional> |
| 11 | +#include <type_traits> |
| 12 | +#include <utility> |
| 13 | + |
| 14 | +namespace dice::template_library { |
| 15 | + |
| 16 | + /** |
| 17 | + * A multi producer, multi consumer channel |
| 18 | + * that only retains the last sent value. |
| 19 | + */ |
| 20 | + template<typename T> |
| 21 | + struct exchange_channel { |
| 22 | + using value_type = T; |
| 23 | + using size_type = size_t; |
| 24 | + using difference_type = std::ptrdiff_t; |
| 25 | + using reference = value_type &; |
| 26 | + using const_reference = value_type const &; |
| 27 | + using pointer = T *; |
| 28 | + using const_pointer = T const *; |
| 29 | + |
| 30 | + private: |
| 31 | + std::mutex value_mutex_; ///< mutex for value_ |
| 32 | + std::optional<T> value_; ///< the last value that was sent |
| 33 | + |
| 34 | + std::atomic_flag closed_ = ATOMIC_FLAG_INIT; ///< true if this channel is closed |
| 35 | + std::condition_variable has_value_; ///< condvar for value_.has_value() |
| 36 | + |
| 37 | + public: |
| 38 | + exchange_channel() = default; |
| 39 | + |
| 40 | + // there is no way to safely implement these with concurrent access |
| 41 | + exchange_channel(exchange_channel const &other) = delete; |
| 42 | + exchange_channel(exchange_channel &&other) = delete; |
| 43 | + exchange_channel &operator=(exchange_channel const &other) = delete; |
| 44 | + exchange_channel &operator=(exchange_channel &&other) noexcept = delete; |
| 45 | + |
| 46 | + ~exchange_channel() noexcept = default; |
| 47 | + |
| 48 | + /** |
| 49 | + * Close the channel. |
| 50 | + * After calling close calls to push() will return false |
| 51 | + * and calls to try_pop will return std::nullopt once the already present elements are exhausted |
| 52 | + */ |
| 53 | + void close() noexcept { |
| 54 | + { |
| 55 | + // "Even if the shared variable is atomic, it must be modified while owning the mutex to correctly publish the modification to the waiting thread." |
| 56 | + // - https://en.cppreference.com/w/cpp/thread/condition_variable |
| 57 | + // |
| 58 | + // Here closed_ is the shared variable used by has_value_ in another thread (the one waiting in pop) |
| 59 | + std::lock_guard lock{value_mutex_}; |
| 60 | + closed_.test_and_set(std::memory_order_release); |
| 61 | + } |
| 62 | + has_value_.notify_all(); // notify pop() so that it does not get stuck |
| 63 | + } |
| 64 | + |
| 65 | + /** |
| 66 | + * @return true if this channel is closed |
| 67 | + */ |
| 68 | + [[nodiscard]] bool closed() const noexcept { |
| 69 | + return closed_.test(std::memory_order_acquire); |
| 70 | + } |
| 71 | + |
| 72 | + /** |
| 73 | + * Emplace an element into the channel, replaces the current element in the channel if there is one. |
| 74 | + * |
| 75 | + * @param args constructor args |
| 76 | + * @return true if emplacing the element succeeded because the channel is not yet closed |
| 77 | + */ |
| 78 | + template<typename... Args> |
| 79 | + bool emplace(Args &&...args) noexcept(std::is_nothrow_constructible_v<value_type, decltype(std::forward<Args>(args))...>) { |
| 80 | + if (closed_.test(std::memory_order_acquire)) [[unlikely]] { |
| 81 | + return false; |
| 82 | + } |
| 83 | + |
| 84 | + { |
| 85 | + std::unique_lock lock{value_mutex_}; |
| 86 | + if (closed_.test(std::memory_order_relaxed)) [[unlikely]] { |
| 87 | + // relaxed is enough because we hold the lock |
| 88 | + return false; |
| 89 | + } |
| 90 | + |
| 91 | + value_. emplace(std::forward<Args>(args)...); |
| 92 | + } |
| 93 | + |
| 94 | + has_value_.notify_one(); |
| 95 | + return true; |
| 96 | + } |
| 97 | + |
| 98 | + /** |
| 99 | + * Push a single element into the channel, replaces the current element in the channel if there is one. |
| 100 | + * |
| 101 | + * @param value the element to push |
| 102 | + * @return true if pushing the element succeeded because the channel is not yet closed |
| 103 | + */ |
| 104 | + bool push(value_type const &value) noexcept(std::is_nothrow_copy_constructible_v<value_type>) { |
| 105 | + return emplace(value); |
| 106 | + } |
| 107 | + |
| 108 | + /** |
| 109 | + * Push a single element into the channel, replaces the current element in the channel if there is one. |
| 110 | + * |
| 111 | + * @param value the element to push |
| 112 | + * @return true if pushing the element succeeded because the channel is not yet closed |
| 113 | + */ |
| 114 | + bool push(value_type &&value) noexcept(std::is_nothrow_move_constructible_v<value_type>) { |
| 115 | + return emplace(std::move(value)); |
| 116 | + } |
| 117 | + |
| 118 | + /** |
| 119 | + * Try to get a (previously pushed) element from the channel. |
| 120 | + * If there is no element available, blocks until there is one available or the channel is closed. |
| 121 | + * |
| 122 | + * @return std::nullopt if the channel was closed, an element otherwise |
| 123 | + */ |
| 124 | + [[nodiscard]] std::optional<value_type> pop() noexcept(std::is_nothrow_move_constructible_v<value_type>) { |
| 125 | + std::unique_lock lock{value_mutex_}; |
| 126 | + has_value_.wait(lock, [this]() noexcept { |
| 127 | + return value_.has_value() || closed_.test(std::memory_order_relaxed); |
| 128 | + }); |
| 129 | + |
| 130 | + if (!value_.has_value()) [[unlikely]] { |
| 131 | + // implies closed_ == true |
| 132 | + return std::nullopt; |
| 133 | + } |
| 134 | + |
| 135 | + return *std::exchange(value_, std::nullopt); |
| 136 | + } |
| 137 | + |
| 138 | + /** |
| 139 | + * Try to get a (previously pushed) element from the channel. |
| 140 | + * Unlike pop(), if there is no element available, returns std::nullopt immediatly. |
| 141 | + */ |
| 142 | + [[nodiscard]] std::optional<value_type> try_pop() noexcept(std::is_nothrow_move_constructible_v<value_type>) { |
| 143 | + std::unique_lock lock{value_mutex_}; |
| 144 | + if (!value_.has_value()) { |
| 145 | + return std::nullopt; |
| 146 | + } |
| 147 | + |
| 148 | + return *std::exchange(value_, std::nullopt); |
| 149 | + } |
| 150 | + |
| 151 | + struct iterator { |
| 152 | + using channel_type = exchange_channel; |
| 153 | + using value_type = T; |
| 154 | + using difference_type = std::ptrdiff_t; |
| 155 | + using reference = T &; |
| 156 | + using const_reference = T const &; |
| 157 | + using pointer = typename exchange_channel::pointer; |
| 158 | + using const_pointer = typename exchange_channel::const_pointer; |
| 159 | + using iterator_category = std::input_iterator_tag; |
| 160 | + |
| 161 | + private: |
| 162 | + exchange_channel *chan_; |
| 163 | + mutable std::optional<value_type> buf_; ///< this has to be mutable for this iterator to fullfill std::input_iterator |
| 164 | + |
| 165 | + void advance() noexcept(std::is_nothrow_move_constructible_v<value_type>) { |
| 166 | + buf_ = chan_->pop(); |
| 167 | + } |
| 168 | + |
| 169 | + public: |
| 170 | + explicit iterator(channel_type *chan) noexcept(std::is_nothrow_move_constructible_v<value_type>) |
| 171 | + : chan_{chan} { |
| 172 | + advance(); |
| 173 | + } |
| 174 | + |
| 175 | + iterator(iterator const &other) noexcept(std::is_nothrow_copy_constructible_v<value_type>) |
| 176 | + : chan_{other.chan_}, |
| 177 | + buf_{other.buf_} { |
| 178 | + } |
| 179 | + |
| 180 | + iterator &operator=(iterator const &other) noexcept(std::is_nothrow_copy_assignable_v<value_type>) { |
| 181 | + if (this == &other) { |
| 182 | + return *this; |
| 183 | + } |
| 184 | + |
| 185 | + chan_ = other.chan_; |
| 186 | + buf_ = other.buf_; |
| 187 | + return *this; |
| 188 | + } |
| 189 | + |
| 190 | + iterator(iterator &&other) noexcept(std::is_nothrow_move_constructible_v<value_type>) |
| 191 | + : chan_{other.chan_}, |
| 192 | + buf_{std::move(other.buf_)} { |
| 193 | + } |
| 194 | + |
| 195 | + iterator &operator=(iterator &&other) noexcept(std::is_nothrow_swappable_v<value_type>) { |
| 196 | + assert(this != &other); |
| 197 | + std::swap(chan_, other.chan_); |
| 198 | + std::swap(buf_, other.buf_); |
| 199 | + return *this; |
| 200 | + } |
| 201 | + |
| 202 | + /// intentionally returns a reference instead of a const_reference to be able to extract it |
| 203 | + reference operator*() noexcept { |
| 204 | + return *buf_; |
| 205 | + } |
| 206 | + |
| 207 | + reference operator*() const noexcept { |
| 208 | + return *buf_; |
| 209 | + } |
| 210 | + |
| 211 | + /// intentionally returns a pointer instead of a const_pointer to be able to extract it |
| 212 | + pointer operator->() noexcept { |
| 213 | + return &*buf_; |
| 214 | + } |
| 215 | + |
| 216 | + pointer operator->() const noexcept { |
| 217 | + return &*buf_; |
| 218 | + } |
| 219 | + |
| 220 | + iterator &operator++() noexcept(std::is_nothrow_move_constructible_v<value_type>) { |
| 221 | + advance(); |
| 222 | + return *this; |
| 223 | + } |
| 224 | + |
| 225 | + void operator++(int) noexcept(std::is_nothrow_move_constructible_v<value_type>) { |
| 226 | + advance(); |
| 227 | + } |
| 228 | + |
| 229 | + bool operator==(std::default_sentinel_t) const noexcept { |
| 230 | + return !buf_.has_value(); |
| 231 | + } |
| 232 | + }; |
| 233 | + |
| 234 | + using sentinel = std::default_sentinel_t; |
| 235 | + |
| 236 | + /** |
| 237 | + * @return an iterator over all present and future elements of this channel |
| 238 | + * @note iterator == end() is true once the channel is closed |
| 239 | + */ |
| 240 | + [[nodiscard]] iterator begin() noexcept(std::is_nothrow_move_constructible_v<value_type>) { |
| 241 | + return iterator{this}; |
| 242 | + } |
| 243 | + |
| 244 | + [[nodiscard]] sentinel end() const noexcept { |
| 245 | + return std::default_sentinel; |
| 246 | + } |
| 247 | + }; |
| 248 | + |
| 249 | +} // namespace dice::template_library |
| 250 | + |
| 251 | +#endif // DICE_TEMPLATELIBRARY_EXCHANGECHANNEL_HPP |
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