/* Coroutines for OctaSTD.
 *
 * This file is part of OctaSTD. See COPYING.md for further information.
 */

#ifndef OSTD_COROUTINE_HH
#define OSTD_COROUTINE_HH

#include <signal.h>

#include <memory>
#include <exception>
#include <stdexcept>
#include <utility>
#include <tuple>
#include <type_traits>
#include <optional>

#include "ostd/types.hh"
#include "ostd/range.hh"

#include "ostd/internal/context.hh"

namespace ostd {

constexpr size_t COROUTINE_DEFAULT_STACK_SIZE = SIGSTKSZ;

struct coroutine_error: std::runtime_error {
    using std::runtime_error::runtime_error;
};

template<typename T>
struct coroutine;

template<typename T>
struct coroutine_range;

namespace detail {
    /* like reference_wrapper but for any value */
    template<typename T>
    struct arg_wrapper {
        arg_wrapper() = default;
        arg_wrapper(T arg): p_arg(std::move(arg)) {}

        void operator=(T arg) {
            p_arg = std::move(arg);
        }
        operator T &&() {
            return std::move(p_arg);
        }

        void swap(arg_wrapper &other) {
            using std::swap;
            swap(p_arg, other.p_arg);
        }

    private:
        T p_arg = T{};
    };

    template<typename T>
    struct arg_wrapper<T &&> {
        arg_wrapper() = default;
        arg_wrapper(T &&arg): p_arg(&arg) {}

        void operator=(T &&arg) {
            p_arg = &arg;
        }
        operator T &&() {
            return *p_arg;
        }

        void swap(arg_wrapper &other) {
            using std::swap;
            swap(p_arg, other.p_arg);
        }

    private:
        T *p_arg = nullptr;
    };

    template<typename T>
    struct arg_wrapper<T &> {
        arg_wrapper() = default;
        arg_wrapper(T &arg): p_arg(&arg) {}

        void operator=(T &arg) {
            p_arg = &arg;
        }
        operator T &() {
            return *p_arg;
        }

        void swap(arg_wrapper &other) {
            using std::swap;
            swap(p_arg, other.p_arg);
        }

    private:
        T *p_arg = nullptr;
    };

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

    template<typename ...A>
    struct coro_types {
        using yield_type = std::tuple<A...>;
    };

    template<typename A>
    struct coro_types<A> {
        using yield_type = A;
    };

    template<typename A, typename B>
    struct coro_types<A, B> {
        using yield_type = std::pair<A, B>;
    };

    template<typename ...A>
    using coro_args = typename coro_types<A...>::yield_type;

    template<typename ...A, size_t ...I>
    inline coro_args<A...> yield_ret(
        std::tuple<arg_wrapper<A>...> &args, std::index_sequence<I...>
    ) {
        if constexpr(sizeof...(A) == 1) {
            return std::forward<A...>(std::get<0>(args));
        } else if constexpr(sizeof...(A) == 2) {
            return std::make_pair(std::forward<A>(std::get<I>(args))...);
        } else {
            return std::move(args);
        }
    }

    /* default case, yield returns args and takes a value */
    template<typename R, typename ...A>
    struct coro_base: coroutine_context {
    protected:
        struct yielder {
            yielder(coro_base<R, A...> &coro): p_coro(coro) {}

            coro_args<A...> operator()(R &&ret) {
                p_coro.p_result = std::forward<R>(ret);
                p_coro.yield_jump();
                return yield_ret(
                    p_coro.p_args, std::make_index_sequence<sizeof...(A)>{}
                );
            }
        private:
            coro_base<R, A...> &p_coro;
        };

        template<typename F, size_t ...I>
        void call_helper(F &func, std::index_sequence<I...>) {
            p_result = std::forward<R>(
                func(yielder{*this}, std::forward<A>(std::get<I>(p_args))...)
            );
        }

        R call(A ...args) {
            p_args = std::make_tuple(arg_wrapper<A>(std::forward<A>(args))...);
            coroutine_context::call();
            return std::forward<R>(p_result);
        }

        void swap(coro_base &other) {
            using std::swap;
            swap(p_args, other.p_args);
            swap(p_result, other.p_result);
            coroutine_context::swap(other);
        }

        std::tuple<arg_wrapper<A>...> p_args;
        arg_wrapper<R> p_result;
    };

    /* yield takes a value but doesn't return any args */
    template<typename R>
    struct coro_base<R>: coroutine_context {
        coroutine_range<R> iter();

    protected:
        struct yielder {
            yielder(coro_base<R> &coro): p_coro(coro) {}

            void operator()(R &&ret) {
                p_coro.p_result = std::forward<R>(ret);
                p_coro.yield_jump();
            }
        private:
            coro_base<R> &p_coro;
        };

        template<typename F, size_t ...I>
        void call_helper(F &func, std::index_sequence<I...>) {
            p_result = std::forward<R>(func(yielder{*this}));
        }

        R call() {
            coroutine_context::call();
            return std::forward<R>(this->p_result);
        }

        void swap(coro_base &other) {
            using std::swap;
            swap(p_result, other.p_result);
            coroutine_context::swap(other);
        }

        arg_wrapper<R> p_result;
    };

    /* yield doesn't take a value and returns args */
    template<typename ...A>
    struct coro_base<void, A...>: coroutine_context {
    protected:
        struct yielder {
            yielder(coro_base<void, A...> &coro): p_coro(coro) {}

            coro_args<A...> operator()() {
                p_coro.yield_jump();
                return yield_ret(
                    p_coro.p_args, std::make_index_sequence<sizeof...(A)>{}
                );
            }
        private:
            coro_base<void, A...> &p_coro;
        };

        template<typename F, size_t ...I>
        void call_helper(F &func, std::index_sequence<I...>) {
            func(yielder{*this}, std::forward<A>(std::get<I>(p_args))...);
        }

        void call(A ...args) {
            p_args = std::make_tuple(arg_wrapper<A>(std::forward<A>(args))...);
            coroutine_context::call();
        }

        void swap(coro_base &other) {
            using std::swap;
            swap(p_args, other.p_args);
            coroutine_context::swap(other);
        }

        std::tuple<arg_wrapper<A>...> p_args;
    };

    /* yield doesn't take a value or return any args */
    template<>
    struct coro_base<void>: coroutine_context {
    protected:
        struct yielder {
            yielder(coro_base<void> &coro): p_coro(coro) {}

            void operator()() {
                p_coro.yield_jump();
            }
        private:
            coro_base<void> &p_coro;
        };

        template<typename F, size_t ...I>
        void call_helper(F &func, std::index_sequence<I...>) {
            func(yielder{*this});
        }

        void call() {
            coroutine_context::call();
        }

        void swap(coro_base &other) {
            coroutine_context::swap(other);
        }
    };
} /* namespace detail */

template<typename R, typename ...A>
struct coroutine<R(A...)>: detail::coro_base<R, A...> {
private:
    using base_t = detail::coro_base<R, A...>;

public:
    using yield_type = typename detail::coro_base<R, A...>::yielder;

    /* we have no way to assign a function anyway... */
    coroutine() = delete;

    template<typename F>
    coroutine(F func, size_t ss = COROUTINE_DEFAULT_STACK_SIZE):
        base_t(), p_func(std::move(func))
    {
        /* that way there is no context creation/stack allocation */
        if (!p_func) {
            return;
        }
        this->make_context(ss, &context_call);
    }

    coroutine(coroutine const &) = delete;
    coroutine(coroutine &&c):
        detail::coro_base<R, A...>(std::move(c)), p_func(std::move(c.p_func))
    {
        c.p_func = nullptr;
    }

    coroutine &operator=(coroutine const &) = delete;
    coroutine &operator=(coroutine &&c) {
        base_t::operator=(std::move(c));
        p_func = std::move(c.p_func);
        c.p_func = nullptr;
    }

    ~coroutine() {
        if (!p_func) {
            /* the stack has already unwound by a normal return */
            return;
        }
        this->unwind();
    }

    explicit operator bool() const {
        return bool(p_func);
    }

    R resume(A ...args) {
        if (!p_func) {
            throw coroutine_error{"dead coroutine"};
        }
        return this->call(std::forward<A>(args)...);
    }

    R operator()(A ...args) {
        return resume(std::forward<A>(args)...);
    }

    void swap(coroutine &other) {
        std::swap(p_func, other.p_func);
        base_t::swap(other);
    }

private:
    /* the main entry point of the coroutine */
    static void context_call(detail::transfer_t t) {
        auto &self = *(static_cast<coroutine *>(t.data));
        self.p_orig = t.ctx;
        try {
            self.call_helper(self.p_func, std::index_sequence_for<A...>{});
        } catch (detail::coroutine_context::forced_unwind v) {
            /* forced_unwind is unique */
            self.p_orig = v.ctx;
        } catch (...) {
            /* some other exception, will be rethrown later */
            self.p_except = std::current_exception();
        }
        /* the func has fully finished here, so mark dead, stack
         * will be freed by the coroutine's destructor later
         */
        self.p_func = nullptr;
        /* perform a last switch back to original context */
        self.yield_jump();
    }

    std::function<R(yield_type, A...)> p_func;
};

template<typename R, typename ...A>
inline void swap(coroutine<R(A...)> &a, coroutine<R(A...)> &b) {
    a.swap(b);
}

template<typename T>
struct coroutine_range: input_range<coroutine_range<T>> {
    using range_category  = input_range_tag;
    using value_type      = T;
    using reference       = T &;
    using size_type       = size_t;
    using difference_type = stream_off_t;

    coroutine_range() = delete;
    coroutine_range(coroutine<T()> &c): p_coro(&c) {
        pop_front();
    }
    coroutine_range(coroutine_range const &r):
        p_coro(r.p_coro), p_item(r.p_item) {}

    bool empty() const {
        return !p_item;
    }

    void pop_front() {
        if (*p_coro) {
            p_item = (*p_coro)();
        } else {
            p_item = std::nullopt;
        }
    }

    reference front() const {
        return p_item.value();
    }

    bool equals_front(coroutine_range const &g) const {
        return p_coro == g.p_coro;
    }
private:
    coroutine<T()> *p_coro;
    mutable std::optional<T> p_item;
};

namespace detail {
    template<typename R>
    coroutine_range<R> coro_base<R>::iter() {
        return coroutine_range<R>{static_cast<coroutine<R()> &>(*this)};
    }
}

} /* namespace ostd */

#endif