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 //
 // coroutine.hpp
 // ~~~~~~~~~~~~~
 //
 // Copyright (c) 2003-2023 Christopher M. Kohlhoff (chris at kohlhoff dot com)
 //
 // Distributed under the Boost Software License, Version 1.0. (See accompanying
 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 //
 
 #ifndef BOOST_ASIO_COROUTINE_HPP
 #define BOOST_ASIO_COROUTINE_HPP
 
 namespace boost {
 namespace asio {
 namespace detail {
 
 class coroutine_ref;
 
 } // namespace detail
 
 /// Provides support for implementing stackless coroutines.
 /**
  * The @c coroutine class may be used to implement stackless coroutines. The
  * class itself is used to store the current state of the coroutine.
  *
  * Coroutines are copy-constructible and assignable, and the space overhead is
  * a single int. They can be used as a base class:
  *
  * @code class session : coroutine
  * {
  *   ...
  * }; @endcode
  *
  * or as a data member:
  *
  * @code class session
  * {
  *   ...
  *   coroutine coro_;
  * }; @endcode
  *
  * or even bound in as a function argument using lambdas or @c bind(). The
  * important thing is that as the application maintains a copy of the object
  * for as long as the coroutine must be kept alive.
  *
  * @par Pseudo-keywords
  *
  * A coroutine is used in conjunction with certain "pseudo-keywords", which
  * are implemented as macros. These macros are defined by a header file:
  *
  * @code #include <boost/asio/yield.hpp>@endcode
  *
  * and may conversely be undefined as follows:
  *
  * @code #include <boost/asio/unyield.hpp>@endcode
  *
  * <b>reenter</b>
  *
  * The @c reenter macro is used to define the body of a coroutine. It takes a
  * single argument: a pointer or reference to a coroutine object. For example,
  * if the base class is a coroutine object you may write:
  *
  * @code reenter (this)
  * {
  *   ... coroutine body ...
  * } @endcode
  *
  * and if a data member or other variable you can write:
  *
  * @code reenter (coro_)
  * {
  *   ... coroutine body ...
  * } @endcode
  *
  * When @c reenter is executed at runtime, control jumps to the location of the
  * last @c yield or @c fork.
  *
  * The coroutine body may also be a single statement, such as:
  *
  * @code reenter (this) for (;;)
  * {
  *   ...
  * } @endcode
  *
  * @b Limitation: The @c reenter macro is implemented using a switch. This
  * means that you must take care when using local variables within the
  * coroutine body. The local variable is not allowed in a position where
  * reentering the coroutine could bypass the variable definition.
  *
  * <b>yield <em>statement</em></b>
  *
  * This form of the @c yield keyword is often used with asynchronous operations:
  *
  * @code yield socket_->async_read_some(buffer(*buffer_), *this); @endcode
  *
  * This divides into four logical steps:
  *
  * @li @c yield saves the current state of the coroutine.
  * @li The statement initiates the asynchronous operation.
  * @li The resume point is defined immediately following the statement.
  * @li Control is transferred to the end of the coroutine body.
  *
  * When the asynchronous operation completes, the function object is invoked
  * and @c reenter causes control to transfer to the resume point. It is
  * important to remember to carry the coroutine state forward with the
  * asynchronous operation. In the above snippet, the current class is a
  * function object object with a coroutine object as base class or data member.
  *
  * The statement may also be a compound statement, and this permits us to
  * define local variables with limited scope:
  *
  * @code yield
  * {
  *   mutable_buffers_1 b = buffer(*buffer_);
  *   socket_->async_read_some(b, *this);
  * } @endcode
  *
  * <b>yield return <em>expression</em> ;</b>
  *
  * This form of @c yield is often used in generators or coroutine-based parsers.
  * For example, the function object:
  *
  * @code struct interleave : coroutine
  * {
  *   istream& is1;
  *   istream& is2;
  *   char operator()(char c)
  *   {
  *     reenter (this) for (;;)
  *     {
  *       yield return is1.get();
  *       yield return is2.get();
  *     }
  *   }
  * }; @endcode
  *
  * defines a trivial coroutine that interleaves the characters from two input
  * streams.
  *
  * This type of @c yield divides into three logical steps:
  *
  * @li @c yield saves the current state of the coroutine.
  * @li The resume point is defined immediately following the semicolon.
  * @li The value of the expression is returned from the function.
  *
  * <b>yield ;</b>
  *
  * This form of @c yield is equivalent to the following steps:
  *
  * @li @c yield saves the current state of the coroutine.
  * @li The resume point is defined immediately following the semicolon.
  * @li Control is transferred to the end of the coroutine body.
  *
  * This form might be applied when coroutines are used for cooperative
  * threading and scheduling is explicitly managed. For example:
  *
  * @code struct task : coroutine
  * {
  *   ...
  *   void operator()()
  *   {
  *     reenter (this)
  *     {
  *       while (... not finished ...)
  *       {
  *         ... do something ...
  *         yield;
  *         ... do some more ...
  *         yield;
  *       }
  *     }
  *   }
  *   ...
  * };
  * ...
  * task t1, t2;
  * for (;;)
  * {
  *   t1();
  *   t2();
  * } @endcode
  *
  * <b>yield break ;</b>
  *
  * The final form of @c yield is used to explicitly terminate the coroutine.
  * This form is comprised of two steps:
  *
  * @li @c yield sets the coroutine state to indicate termination.
  * @li Control is transferred to the end of the coroutine body.
  *
  * Once terminated, calls to is_complete() return true and the coroutine cannot
  * be reentered.
  *
  * Note that a coroutine may also be implicitly terminated if the coroutine
  * body is exited without a yield, e.g. by return, throw or by running to the
  * end of the body.
  *
  * <b>fork <em>statement</em></b>
  *
  * The @c fork pseudo-keyword is used when "forking" a coroutine, i.e. splitting
  * it into two (or more) copies. One use of @c fork is in a server, where a new
  * coroutine is created to handle each client connection:
  * 
  * @code reenter (this)
  * {
  *   do
  *   {
  *     socket_.reset(new tcp::socket(my_context_));
  *     yield acceptor->async_accept(*socket_, *this);
  *     fork server(*this)();
  *   } while (is_parent());
  *   ... client-specific handling follows ...
  * } @endcode
  * 
  * The logical steps involved in a @c fork are:
  * 
  * @li @c fork saves the current state of the coroutine.
  * @li The statement creates a copy of the coroutine and either executes it
  *     immediately or schedules it for later execution.
  * @li The resume point is defined immediately following the semicolon.
  * @li For the "parent", control immediately continues from the next line.
  *
  * The functions is_parent() and is_child() can be used to differentiate
  * between parent and child. You would use these functions to alter subsequent
  * control flow.
  *
  * Note that @c fork doesn't do the actual forking by itself. It is the
  * application's responsibility to create a clone of the coroutine and call it.
  * The clone can be called immediately, as above, or scheduled for delayed
  * execution using something like boost::asio::post().
  *
  * @par Alternate macro names
  *
  * If preferred, an application can use macro names that follow a more typical
  * naming convention, rather than the pseudo-keywords. These are:
  *
  * @li @c BOOST_ASIO_CORO_REENTER instead of @c reenter
  * @li @c BOOST_ASIO_CORO_YIELD instead of @c yield
  * @li @c BOOST_ASIO_CORO_FORK instead of @c fork
  */
 class coroutine
 {
 public:
   /// Constructs a coroutine in its initial state.
   coroutine() : value_(0) {}
 
   /// Returns true if the coroutine is the child of a fork.
   bool is_child() const { return value_ < 0; }
 
   /// Returns true if the coroutine is the parent of a fork.
   bool is_parent() const { return !is_child(); }
 
   /// Returns true if the coroutine has reached its terminal state.
   bool is_complete() const { return value_ == -1; }
 
 private:
   friend class detail::coroutine_ref;
   int value_;
 };
 
 
 namespace detail {
 
 class coroutine_ref
 {
 public:
   coroutine_ref(coroutine& c) : value_(c.value_), modified_(false) {}
   coroutine_ref(coroutine* c) : value_(c->value_), modified_(false) {}
   ~coroutine_ref() { if (!modified_) value_ = -1; }
   operator int() const { return value_; }
   int& operator=(int v) { modified_ = true; return value_ = v; }
 private:
   void operator=(const coroutine_ref&);
   int& value_;
   bool modified_;
 };
 
 } // namespace detail
 } // namespace asio
 } // namespace boost
 
 #define BOOST_ASIO_CORO_REENTER(c) \
   switch (::boost::asio::detail::coroutine_ref _coro_value = c) \
     case -1: if (_coro_value) \
     { \
       goto terminate_coroutine; \
       terminate_coroutine: \
       _coro_value = -1; \
       goto bail_out_of_coroutine; \
       bail_out_of_coroutine: \
       break; \
     } \
     else /* fall-through */ case 0:
 
 #define BOOST_ASIO_CORO_YIELD_IMPL(n) \
   for (_coro_value = (n);;) \
     if (_coro_value == 0) \
     { \
       case (n): ; \
       break; \
     } \
     else \
       switch (_coro_value ? 0 : 1) \
         for (;;) \
           /* fall-through */ case -1: if (_coro_value) \
             goto terminate_coroutine; \
           else for (;;) \
             /* fall-through */ case 1: if (_coro_value) \
               goto bail_out_of_coroutine; \
             else /* fall-through */ case 0:
 
 #define BOOST_ASIO_CORO_FORK_IMPL(n) \
   for (_coro_value = -(n);; _coro_value = (n)) \
     if (_coro_value == (n)) \
     { \
       case -(n): ; \
       break; \
     } \
     else
 
 #if defined(_MSC_VER)
 # define BOOST_ASIO_CORO_YIELD BOOST_ASIO_CORO_YIELD_IMPL(__COUNTER__ + 1)
 # define BOOST_ASIO_CORO_FORK BOOST_ASIO_CORO_FORK_IMPL(__COUNTER__ + 1)
 #else // defined(_MSC_VER)
 # define BOOST_ASIO_CORO_YIELD BOOST_ASIO_CORO_YIELD_IMPL(__LINE__)
 # define BOOST_ASIO_CORO_FORK BOOST_ASIO_CORO_FORK_IMPL(__LINE__)
 #endif // defined(_MSC_VER)
 
 #endif // BOOST_ASIO_COROUTINE_HPP

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