//
// Created by Stepan Usatiuk on 15.04.2023.
//

#ifndef SEMBACKUP_SERIALIZE_H
#define SEMBACKUP_SERIALIZE_H

#include <cstddef>
#include <memory>
#include <optional>
#include <type_traits>
#include <variant>

#include "Exception.h"
#include "stuff.hpp"

/// Serialization library
/**
 *  To serialize the objects in Repository, we have to handle a couple of cases:
 *  1. Serializing integers (object ids, etc...)
 *  2. Serializing enums (object types)
 *  3. Serializing char vectors and strings
 *  4. Serializing other STL containers (which also requires serializing pairs)
 *  5. Serializing custom structs (including the objects themselves)
 *
 *  With this library it is possible to do all of that.
 *
 */
namespace Serialize {
template<typename, typename = void, typename = void>
struct is_pair : std::false_type {};

template<typename P>
struct is_pair<P, std::void_t<decltype(std::declval<P>().first)>, std::void_t<decltype(std::declval<P>().second)>>
    : std::true_type {};

template<typename Args, template<typename...> class T>
struct is_template_of : std::false_type {};

template<typename... Args, template<typename...> class T>
struct is_template_of<T<Args...>, T> : std::true_type {};

template<typename, typename, typename = void>
struct has_emplace_back : std::false_type {};

template<typename T, typename V>
struct has_emplace_back<T, V, std::void_t<decltype(T().emplace_back(std::declval<V>()))>> : std::true_type {};

template<typename, typename = void, typename = void>
struct serializable : std::false_type {};

/// Checks if the object has the `serializable` type
/// In that case, its serialization will be delegated to its .serialize() parameter,
/// and deserialization to its T(char vector iterator in, const char vector iterator end) constructor,
/// similar to Serialize::deserialize
template<typename T>
struct serializable<T, std::void_t<decltype(T::serializable::value)>> : std::true_type {};

/// Deserializes object of type \p T starting from fist byte \p in, advances the iterator past the end of object
/// \tparam T   Type to deserialize
/// \param in   Iterator to the first byte of the object
/// \param end  End iterator of source container
/// \return     Deserialized value
template<typename T, typename C = std::vector<uint8_t>>
static std::optional<T> deserializeOpt(typename C::const_iterator& in, const typename C::const_iterator& end);

/// Deserializes object of type \p T starting from fist byte \p in, advances the iterator past the end of object
/// \tparam T   Type to deserialize
/// \param in   Iterator to the first byte of the object
/// \param end  End iterator of source container
/// \return     Deserialized value
template<typename T, typename C = std::vector<uint8_t>>
static T deserialize(typename C::const_iterator& in, const typename C::const_iterator& end);

/// Serializes object of type \p T into vector \p out
/// \tparam T   Type to serialize
/// \param what Constant reference to the serialized object
/// \param out  Reference to output vector
template<typename T, typename C = std::vector<uint8_t>>
static void serialize(const T& what, C& out);

/// Serializes the object of type \p T and returns the resulting vector
/// \tparam T   Type to serialize
/// \param o    Constant reference to the serialized object
/// \return     Serialized data
template<typename T, typename C = std::vector<uint8_t>>
static C serialize(const T& o);

/// Deserializes object of type \p T from input vector \p from
/// \tparam T   Type to deserialize
/// \param from Constant reference to the serialized object
/// \return     Deserialized value
template<typename T, typename C = std::vector<uint8_t>>
static std::optional<T> deserializeOpt(const C& from);

/// Deserializes object of type \p T from input vector \p from
/// \tparam T   Type to deserialize
/// \param from Constant reference to the serialized object
/// \return     Deserialized value
template<typename T, typename C = std::vector<uint8_t>>
static T deserialize(const C& from);

// Deserializes an std::variant instance, with readIdx serving as an index of the alternative on the wire
template<std::size_t I = 0, typename C, typename V>
std::optional<V> deserializeVar(size_t readIdx, typename C::const_iterator& in, const typename C::const_iterator& end)
    requires is_template_of<V, std::variant>::value
{
    if constexpr (I < std::variant_size_v<V>) {
        if (readIdx == I) {
            auto res = deserializeOpt<std::variant_alternative_t<I, V>>(in, end);

            if (res)
                return std::make_optional(V{std::move(*res)});

            return std::nullopt;
        }
        return deserializeVar<I + 1, C, V>(readIdx, in, end);
    } else {
        return std::nullopt;
    }
}

template<typename T, typename C>
std::optional<T> deserializeOpt(typename C::const_iterator& in, const typename C::const_iterator& end) {
    if constexpr (serializable<T>::value) {
        // If the object declares itself as serializable, call its constructor with in and end
        return T(in, end);
    } else if constexpr (std::is_same_v<T, std::monostate>) {
        // No-op
        return std::make_optional(std::monostate{});
    } else {
        if (in >= end)
            return std::nullopt;

        if constexpr (is_pair<T>::value) {
            // If the object is pair, deserialize the first and second element and return the pair
            using KT = typename std::remove_const<decltype(T::first)>::type;
            using VT = typename std::remove_const<decltype(T::second)>::type;
            auto K   = deserialize<KT>(in, end);
            auto V   = deserialize<VT>(in, end);
            return T(std::move(K), std::move(V));
        } else if constexpr (std::is_enum<T>::value) {
            // If the object is an enum, deserialize an int and cast it to the enum
            auto tmp = deserialize<uint32_t>(in, end);
            if (tmp >= 0 && tmp < static_cast<uint32_t>(T::END))
                return static_cast<T>(tmp);
            else
                return std::nullopt;
        } else if constexpr (sizeof(T) == 1) {
            // If it's a single byte, just copy it
            if (std::distance(in, end) < checked_cast<ssize_t>(sizeof(T)))
                return std::nullopt;
            return *(in++);
        } else if constexpr (std::is_integral<T>::value) {
            static_assert(sizeof(T) <= sizeof(uint64_t));
            uint64_t tmp;
            // If the object is a number, copy it byte-by-byte
            if (std::distance(in, end) < checked_cast<ssize_t>(sizeof(tmp)))
                return std::nullopt;

            std::copy(in, in + sizeof(tmp), reinterpret_cast<char*>(&tmp));
            in += sizeof(tmp);
            return static_cast<T>(be64toh(tmp));
        } else if constexpr (is_template_of<T, std::variant>::value) {
            auto index = deserializeOpt<uint64_t>(in, end);
            if (!index.has_value())
                return std::nullopt;
            return deserializeVar<0, C, T>(*index - 1, in, end);
        } else {
            // Otherwise we treat it as a container, in format of <number of elements>b<elements>e
            auto size = deserializeOpt<size_t>(in, end);
            if (!size)
                return std::nullopt;

            auto b = deserialize<char>(in, end);
            if (!b || b != 'b')
                return std::nullopt;

            T out;
            if constexpr (sizeof(typename T::value_type) == 1) {
                // Optimization for char vectors
                if (std::distance(in, end) < checked_cast<ssize_t>(*size))
                    return std::nullopt;
                out.insert(out.end(), in, in + checked_cast<ssize_t>(*size));
                in += checked_cast<ssize_t>(*size);
            } else
                for (size_t i = 0; i < *size; i++) {
                    using V = typename T::value_type;
                    V v     = deserialize<V>(in, end);
                    // Try either emplace_back or emplace if it doesn't exist
                    if constexpr (has_emplace_back<T, V>::value)
                        out.emplace_back(std::move(v));
                    else
                        out.emplace(std::move(v));
                }

            b = deserialize<char>(in, end);
            if (!b || b != 'e')
                return std::nullopt;

            return out;
        }
    }
}

template<typename T, typename C>
static T deserialize(typename C::const_iterator& in, const typename C::const_iterator& end) {
    std::optional<T> out = deserializeOpt<T>(in, end);
    if (!out)
        throw Exception("deserialize failed");
    return out.value();
}

template<typename T, typename C>
static T deserialize(const C& from) {
    std::optional<T> out = deserializeOpt<T>(from);
    if (!out)
        throw Exception("deserialize failed");
    return out.value();
}

template<typename... T, typename C>
void serialize(const std::variant<T...>& what, C& out) {
    serialize<uint64_t>(what.index() + 1, out);
    std::visit(
            [&out](auto&& arg) -> void {
                using I = std::decay_t<decltype(arg)>;
                serialize<I>(arg, out);
            },
            what);
}

template<typename T, typename C>
void serialize(const T& what, C& out) {
    if constexpr (serializable<T>::value) {
        // If the object declares itself as serializable, call its serialize method
        what.serialize(out);
    } else if constexpr (std::is_same_v<T, std::monostate>) {
        // No-op
    } else if constexpr (is_pair<T>::value) {
        // If the object is pair, serialize the first and second element
        serialize(what.first, out);
        serialize(what.second, out);
    } else if constexpr (std::is_enum<T>::value) {
        // If the object is an enum, cast it to an int and serialize that
        serialize(static_cast<uint32_t>(what), out);
    } else if constexpr (sizeof(T) == 1) {
        // If it's a single byte, just copy it
        out.push_back(static_cast<typename C::value_type>(what));
    } else if constexpr (std::is_integral<T>::value) {
        static_assert(sizeof(T) <= sizeof(uint64_t));
        // If the object is a number, copy it byte-by-byte
        uint64_t tmp = htobe64(static_cast<uint64_t>(what));
        static_assert(sizeof(tmp) == 8);
        out.insert(out.end(), (reinterpret_cast<const char*>(&tmp)),
                   (reinterpret_cast<const char*>(&tmp) + sizeof(tmp)));
    } else {
        // Otherwise we treat it as a container, in format of <number of elements>b<elements>e
        serialize(what.size(), out);
        serialize('b', out);
        if constexpr (sizeof(typename T::value_type) == 1) {
            // Optimization for char vectors
            out.insert(out.end(), what.begin(), what.end());
        } else
            for (auto const& i: what) {
                serialize(i, out);
            }
        serialize('e', out);
    }
}

template<typename T, typename C>
C serialize(const T& o) {
    C out;
    serialize(o, out);
    return out;
}

template<typename T, typename C>
std::optional<T> deserializeOpt(const C& from) {
    auto bgwr = from.begin();
    return deserialize<T>(bgwr, from.end());
}
} // namespace Serialize

#endif // SEMBACKUP_SERIALIZE_H