///
// generator - Single-header, ranges-compatible generator type built
// on C++20 coroutines
// Written in 2021 by Sy Brand (tartanllama@gmail.com, @TartanLlama)
//
// Documentation available at https://tl.tartanllama.xyz/
//
// To the extent possible under law, the author(s) have dedicated all
// copyright and related and neighboring rights to this software to the
// public domain worldwide. This software is distributed without any warranty.
//
// You should have received a copy of the CC0 Public Domain Dedication
// along with this software. If not, see
// <http://creativecommons.org/publicdomain/zero/1.0/>.
///

#ifndef TL_GENERATOR_HPP
#define TL_GENERATOR_HPP

#define TL_GENERATOR_VERSION_MAJOR 0
#define TL_GENERATOR_VERSION_MINOR 3
#define TL_GENERATOR_VERSION_PATCH 0

#include <coroutine>
#include <exception>
#include <utility>
#include <type_traits>
#include <ranges>

namespace tl {
   template <class T>
   class generator {
      struct promise {
         using value_type = std::remove_reference_t<T>;
         using reference_type = value_type&;
         using pointer_type = value_type*;

         promise() = default;

         generator get_return_object() {
            return generator(std::coroutine_handle<promise>::from_promise(*this));
         }

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

         void return_void() const noexcept { return; }

         void unhandled_exception() noexcept {
            exception_ = std::current_exception();
         }

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

         std::suspend_always yield_value(reference_type v) noexcept {
            value_ = std::addressof(v);
            return {};
         }

         std::exception_ptr exception_;
         pointer_type value_;
      };

   public:
      using promise_type = promise;
      class sentinel {};

      class iterator {
         using handle_type = std::coroutine_handle<promise_type>;

      public:
         using value_type = typename promise_type::value_type;
         using reference_type = typename promise_type::reference_type;
         using pointer_type = typename promise_type::pointer_type;
         using difference_type = std::ptrdiff_t;

         iterator() = default;
         ~iterator() {
            if (handle_) handle_.destroy();
         }

         //Non-copyable because coroutine handles point to a unique resource
         iterator(iterator const&) = delete;
         iterator(iterator&& rhs) noexcept : handle_(std::exchange(rhs.handle_, nullptr)) {}
         iterator& operator=(iterator const&) = delete;
         iterator& operator=(iterator&& rhs) noexcept {
            handle_ = std::exchange(rhs.handle_, nullptr);
            return *this;
         }

         friend bool operator==(iterator const& it, sentinel) noexcept {
            return (!it.handle_ || it.handle_.done());
         }

         iterator& operator++() {
            handle_.resume();
            if (handle_.done()) {
               handle_.promise().rethrow_if_exception();
            }
            return *this;
         }

         void operator++(int) {
            (void)this->operator++();
         }

         reference_type operator*() const
            noexcept(noexcept(std::is_nothrow_copy_constructible_v<reference_type>)){
            return *handle_.promise().value_;
         }

      private:
         friend class generator;
         iterator(handle_type handle) : handle_(handle) {}

         handle_type handle_;
      };

      using handle_type = std::coroutine_handle<promise_type>;

      generator() noexcept = default;
      ~generator() {
         if (handle_) handle_.destroy();
      }

      generator(generator const&) = delete;
      generator(generator&& rhs) noexcept : handle_(std::exchange(rhs.handle_, nullptr)) {}
      generator& operator=(generator const&) = delete;
      generator& operator=(generator&& rhs) noexcept {
         swap(rhs);
         return *this;
      }

      iterator begin() {
         handle_.resume();
         if (handle_.done()) {
            handle_.promise().rethrow_if_exception();
         }
         return {std::exchange(handle_, nullptr)};
      }

      sentinel end() const noexcept {
         return {};
      }

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

   private:
      friend class iterator;
      explicit generator(handle_type handle) noexcept : handle_(handle) {}

      handle_type handle_ = nullptr;
   };
}

template<class T>
inline constexpr bool std::ranges::enable_view<tl::generator<T>> = true;

#endif