revng-revng/include/revng/Support/Generator.h

#pragma once

//
// This file is distributed under the MIT License. See LICENSE.md for details.
//

// This files has been from the cppcoro project by Lewis Baker, which is
// licensed under MIT license.

#include <exception>
#include <experimental/coroutine>
#include <functional>
#include <iterator>
#include <type_traits>
#include <utility>

namespace cppcoro {
template<typename T>
class generator;

namespace detail {
template<typename T>
class generator_promise {
public:
  using value_type = std::remove_reference_t<T>;
  using reference_type = std::conditional_t<std::is_reference_v<T>, T, T &>;
  using pointer_type = value_type *;

  generator_promise() = default;

  generator<T> get_return_object() noexcept;

  constexpr std::experimental::suspend_always initial_suspend() const noexcept {
    return {};
  }
  constexpr std::experimental::suspend_always final_suspend() const noexcept {
    return {};
  }

  template<typename U = T,
           std::enable_if_t<!std::is_rvalue_reference<U>::value, int> = 0>
  std::experimental::suspend_always
  yield_value(std::remove_reference_t<T> &value) noexcept {
    m_value = std::addressof(value);
    return {};
  }

  std::experimental::suspend_always
  yield_value(std::remove_reference_t<T> &&value) noexcept {
    m_value = std::addressof(value);
    return {};
  }

  void unhandled_exception() { m_exception = std::current_exception(); }

  void return_void() {}

  reference_type value() const noexcept {
    return static_cast<reference_type>(*m_value);
  }

  // Don't allow any use of 'co_await' inside the generator coroutine.
  template<typename U>
  std::experimental::suspend_never await_transform(U &&value) = delete;

  void rethrow_if_exception() {
    if (m_exception) {
      std::rethrow_exception(m_exception);
    }
  }

private:
  pointer_type m_value;
  std::exception_ptr m_exception;
};

struct generator_sentinel {};

template<typename T>
class generator_iterator {
  using promise = generator_promise<T>;
  using coroutine_handle = std::experimental::coroutine_handle<promise>;

public:
  using iterator_category = std::input_iterator_tag;
  // What type should we use for counting elements of a potentially infinite
  // sequence?
  using difference_type = std::ptrdiff_t;
  using value_type = typename generator_promise<T>::value_type;
  using reference = typename generator_promise<T>::reference_type;
  using pointer = typename generator_promise<T>::pointer_type;

  // Iterator needs to be default-constructible to satisfy the Range concept.
  generator_iterator() noexcept : m_coroutine(nullptr) {}

  explicit generator_iterator(coroutine_handle coroutine) noexcept :
    m_coroutine(coroutine) {}

  friend bool
  operator==(const generator_iterator &it, generator_sentinel) noexcept {
    return !it.m_coroutine || it.m_coroutine.done();
  }

  friend bool
  operator!=(const generator_iterator &it, generator_sentinel s) noexcept {
    return !(it == s);
  }

  friend bool
  operator==(generator_sentinel s, const generator_iterator &it) noexcept {
    return (it == s);
  }

  friend bool
  operator!=(generator_sentinel s, const generator_iterator &it) noexcept {
    return it != s;
  }

  generator_iterator &operator++() {
    m_coroutine.resume();
    if (m_coroutine.done()) {
      m_coroutine.promise().rethrow_if_exception();
    }

    return *this;
  }

  // Need to provide post-increment operator to implement the 'Range' concept.
  void operator++(int) { (void) operator++(); }

  reference operator*() const noexcept { return m_coroutine.promise().value(); }

  pointer operator->() const noexcept { return std::addressof(operator*()); }

private:
  coroutine_handle m_coroutine;
};
} // namespace detail

template<typename T>
class [[nodiscard]] generator {
public:
  using promise_type = detail::generator_promise<T>;
  using iterator = detail::generator_iterator<T>;

  generator() noexcept : m_coroutine(nullptr) {}

  generator(generator && other) noexcept : m_coroutine(other.m_coroutine) {
    other.m_coroutine = nullptr;
  }

  generator(const generator &other) = delete;

  ~generator() {
    if (m_coroutine) {
      m_coroutine.destroy();
    }
  }

  generator &operator=(generator other) noexcept {
    swap(other);
    return *this;
  }

  iterator begin() {
    if (m_coroutine) {
      m_coroutine.resume();
      if (m_coroutine.done()) {
        m_coroutine.promise().rethrow_if_exception();
      }
    }

    return iterator{ m_coroutine };
  }

  detail::generator_sentinel end() noexcept {
    return detail::generator_sentinel{};
  }

  void swap(generator & other) noexcept {
    std::swap(m_coroutine, other.m_coroutine);
  }

private:
  friend class detail::generator_promise<T>;

  using coroutine_handle = std::experimental::coroutine_handle<promise_type>;

  explicit generator(coroutine_handle coroutine) noexcept :
    m_coroutine(coroutine) {}

  std::experimental::coroutine_handle<promise_type> m_coroutine;
};

template<typename T>
void swap(generator<T> &a, generator<T> &b) {
  a.swap(b);
}

namespace detail {
template<typename T>
generator<T> generator_promise<T>::get_return_object() noexcept {
  using promise = generator_promise<T>;
  using coroutine_handle = std::experimental::coroutine_handle<promise>;
  return generator<T>{ coroutine_handle::from_promise(*this) };
}
} // namespace detail

} // namespace cppcoro