ah-generator.H

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/*
                          Aleph_w
 
  Data structures & Algorithms
  https://github.com/lrleon/Aleph-w
 
  This file is part of Aleph-w library
 
  Copyright (c) 2002-2026 Leandro Rabindranath Leon
 
  Permission is hereby granted, free of charge, to any person obtaining a copy
  of this software and associated documentation files (the "Software"), to deal
  in the Software without restriction, including without limitation the rights
  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  copies of the Software, and to permit persons to whom the Software is
  furnished to do so, subject to the following conditions:
 
  The above copyright notice and this permission notice shall be included in all
  copies or substantial portions of the Software.
 
  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  SOFTWARE.
*/
 
#ifndef AH_GENERATOR_H
#define AH_GENERATOR_H
 
#include <coroutine>
#include <exception>
#include <iterator>
#include <memory>
#include <type_traits>
#include <utility>
 
#include <ah-errors.H>
 
namespace Aleph
{
 
template <class T>
class Generator
{
public:
  struct promise_type
  {
    std::add_pointer_t<T> current_value = nullptr;
    std::exception_ptr pending_exception = nullptr;
 
    static std::suspend_always initial_suspend() noexcept { return {}; }
 
    static std::suspend_always final_suspend() noexcept { return {}; }
 
    Generator get_return_object() noexcept
    {
      return Generator{Handle::from_promise(*this)};
    }
 
    std::suspend_always yield_value(std::remove_reference_t<T> &value) noexcept
    {
      current_value = std::addressof(value);
      return {};
    }
 
    std::suspend_always yield_value(std::remove_reference_t<T> &&value) noexcept
    {
      current_value = std::addressof(value);
      return {};
    }
 
    static void return_void() noexcept {}
 
    void unhandled_exception() noexcept
    {
      pending_exception = std::current_exception();
    }
 
    template <class U>
    std::suspend_never await_transform(U &&) = delete;
  };
 
  using Handle = std::coroutine_handle<promise_type>;
 
  class iterator
  {
    Handle handle_{};
 
  public:
    using iterator_category = std::input_iterator_tag; 
    using value_type = std::remove_reference_t<T>;      
    using difference_type = std::ptrdiff_t;              
    using reference = T &;                               
    using pointer = std::add_pointer_t<T>;                
 
    iterator() noexcept = default;
 
    explicit iterator(Handle h) noexcept : handle_(h) {}
 
    iterator &operator++()
    {
      ah_domain_error_if(done())
        << "Generator::iterator::operator++(): cannot advance past end()";
      handle_.resume();
      if (handle_.promise().pending_exception)
        std::rethrow_exception(std::exchange(handle_.promise().pending_exception, nullptr));
      return *this;
    }
 
    void operator++(int) { ++*this; }
 
    reference operator*() const noexcept
    {
      return static_cast<reference>(*handle_.promise().current_value);
    }
 
    pointer operator->() const noexcept
    {
      return handle_.promise().current_value;
    }
 
    [[nodiscard]] bool done() const noexcept
    {
      return not handle_ or handle_.done();
    }
 
    friend bool operator==(const iterator &it, std::default_sentinel_t) noexcept
    {
      return it.done();
    }
  };
 
  Generator() noexcept = default;
 
  explicit Generator(Handle h) noexcept : handle_(h) {}
 
  Generator(const Generator &) = delete;
 
  Generator &operator=(const Generator &) = delete;
 
  Generator(Generator &&o) noexcept : handle_(std::exchange(o.handle_, {})) {}
 
  Generator &operator=(Generator &&o) noexcept
  {
    if (this != &o)
      {
        if (handle_)
          handle_.destroy();
        handle_ = std::exchange(o.handle_, {});
      }
    return *this;
  }
 
  ~Generator()
  {
    if (handle_)
      handle_.destroy();
  }
 
  iterator begin() &
  {
    if (handle_ and not handle_.done())
      {
        handle_.resume();
        if (handle_.promise().pending_exception)
          std::rethrow_exception(std::exchange(handle_.promise().pending_exception, nullptr));
      }
    return iterator{handle_};
  }
 
  static std::default_sentinel_t end() noexcept { return std::default_sentinel; }
 
private:
  Handle handle_{};
};
 
} // end namespace Aleph
 
#endif // AH_GENERATOR_H