json/develop/detail/parsing/binary_writer.hpp

#pragma once

#include <algorithm> // reverse
#include <array> // array
#include <cstdint> // uint8_t, uint16_t, uint32_t, uint64_t
#include <cstring> // memcpy
#include <limits> // numeric_limits

#include "detail/parsing/binary_reader.hpp"
#include "detail/parsing/output_adapters.hpp"

namespace nlohmann
{
namespace detail
{
///////////////////
// binary writer //
///////////////////

/*!
@brief serialization to CBOR and MessagePack values
*/
template<typename BasicJsonType, typename CharType>
class binary_writer
{
  public:
    /*!
    @brief create a binary writer

    @param[in] adapter  output adapter to write to
    */
    explicit binary_writer(output_adapter_t<CharType> adapter) : oa(adapter)
    {
        assert(oa);
    }

    /*!
    @brief[in] j  JSON value to serialize
    */
    void write_cbor(const BasicJsonType& j)
    {
        switch (j.type())
        {
            case value_t::null:
            {
                oa->write_character(static_cast<CharType>(0xF6));
                break;
            }

            case value_t::boolean:
            {
                oa->write_character(j.m_value.boolean
                                    ? static_cast<CharType>(0xF5)
                                    : static_cast<CharType>(0xF4));
                break;
            }

            case value_t::number_integer:
            {
                if (j.m_value.number_integer >= 0)
                {
                    // CBOR does not differentiate between positive signed
                    // integers and unsigned integers. Therefore, we used the
                    // code from the value_t::number_unsigned case here.
                    if (j.m_value.number_integer <= 0x17)
                    {
                        write_number(static_cast<uint8_t>(j.m_value.number_integer));
                    }
                    else if (j.m_value.number_integer <= (std::numeric_limits<uint8_t>::max)())
                    {
                        oa->write_character(static_cast<CharType>(0x18));
                        write_number(static_cast<uint8_t>(j.m_value.number_integer));
                    }
                    else if (j.m_value.number_integer <= (std::numeric_limits<uint16_t>::max)())
                    {
                        oa->write_character(static_cast<CharType>(0x19));
                        write_number(static_cast<uint16_t>(j.m_value.number_integer));
                    }
                    else if (j.m_value.number_integer <= (std::numeric_limits<uint32_t>::max)())
                    {
                        oa->write_character(static_cast<CharType>(0x1A));
                        write_number(static_cast<uint32_t>(j.m_value.number_integer));
                    }
                    else
                    {
                        oa->write_character(static_cast<CharType>(0x1B));
                        write_number(static_cast<uint64_t>(j.m_value.number_integer));
                    }
                }
                else
                {
                    // The conversions below encode the sign in the first
                    // byte, and the value is converted to a positive number.
                    const auto positive_number = -1 - j.m_value.number_integer;
                    if (j.m_value.number_integer >= -24)
                    {
                        write_number(static_cast<uint8_t>(0x20 + positive_number));
                    }
                    else if (positive_number <= (std::numeric_limits<uint8_t>::max)())
                    {
                        oa->write_character(static_cast<CharType>(0x38));
                        write_number(static_cast<uint8_t>(positive_number));
                    }
                    else if (positive_number <= (std::numeric_limits<uint16_t>::max)())
                    {
                        oa->write_character(static_cast<CharType>(0x39));
                        write_number(static_cast<uint16_t>(positive_number));
                    }
                    else if (positive_number <= (std::numeric_limits<uint32_t>::max)())
                    {
                        oa->write_character(static_cast<CharType>(0x3A));
                        write_number(static_cast<uint32_t>(positive_number));
                    }
                    else
                    {
                        oa->write_character(static_cast<CharType>(0x3B));
                        write_number(static_cast<uint64_t>(positive_number));
                    }
                }
                break;
            }

            case value_t::number_unsigned:
            {
                if (j.m_value.number_unsigned <= 0x17)
                {
                    write_number(static_cast<uint8_t>(j.m_value.number_unsigned));
                }
                else if (j.m_value.number_unsigned <= (std::numeric_limits<uint8_t>::max)())
                {
                    oa->write_character(static_cast<CharType>(0x18));
                    write_number(static_cast<uint8_t>(j.m_value.number_unsigned));
                }
                else if (j.m_value.number_unsigned <= (std::numeric_limits<uint16_t>::max)())
                {
                    oa->write_character(static_cast<CharType>(0x19));
                    write_number(static_cast<uint16_t>(j.m_value.number_unsigned));
                }
                else if (j.m_value.number_unsigned <= (std::numeric_limits<uint32_t>::max)())
                {
                    oa->write_character(static_cast<CharType>(0x1A));
                    write_number(static_cast<uint32_t>(j.m_value.number_unsigned));
                }
                else
                {
                    oa->write_character(static_cast<CharType>(0x1B));
                    write_number(static_cast<uint64_t>(j.m_value.number_unsigned));
                }
                break;
            }

            case value_t::number_float: // Double-Precision Float
            {
                oa->write_character(static_cast<CharType>(0xFB));
                write_number(j.m_value.number_float);
                break;
            }

            case value_t::string:
            {
                // step 1: write control byte and the string length
                const auto N = j.m_value.string->size();
                if (N <= 0x17)
                {
                    write_number(static_cast<uint8_t>(0x60 + N));
                }
                else if (N <= (std::numeric_limits<uint8_t>::max)())
                {
                    oa->write_character(static_cast<CharType>(0x78));
                    write_number(static_cast<uint8_t>(N));
                }
                else if (N <= (std::numeric_limits<uint16_t>::max)())
                {
                    oa->write_character(static_cast<CharType>(0x79));
                    write_number(static_cast<uint16_t>(N));
                }
                else if (N <= (std::numeric_limits<uint32_t>::max)())
                {
                    oa->write_character(static_cast<CharType>(0x7A));
                    write_number(static_cast<uint32_t>(N));
                }
                // LCOV_EXCL_START
                else if (N <= (std::numeric_limits<uint64_t>::max)())
                {
                    oa->write_character(static_cast<CharType>(0x7B));
                    write_number(static_cast<uint64_t>(N));
                }
                // LCOV_EXCL_STOP

                // step 2: write the string
                oa->write_characters(
                    reinterpret_cast<const CharType*>(j.m_value.string->c_str()),
                    j.m_value.string->size());
                break;
            }

            case value_t::array:
            {
                // step 1: write control byte and the array size
                const auto N = j.m_value.array->size();
                if (N <= 0x17)
                {
                    write_number(static_cast<uint8_t>(0x80 + N));
                }
                else if (N <= (std::numeric_limits<uint8_t>::max)())
                {
                    oa->write_character(static_cast<CharType>(0x98));
                    write_number(static_cast<uint8_t>(N));
                }
                else if (N <= (std::numeric_limits<uint16_t>::max)())
                {
                    oa->write_character(static_cast<CharType>(0x99));
                    write_number(static_cast<uint16_t>(N));
                }
                else if (N <= (std::numeric_limits<uint32_t>::max)())
                {
                    oa->write_character(static_cast<CharType>(0x9A));
                    write_number(static_cast<uint32_t>(N));
                }
                // LCOV_EXCL_START
                else if (N <= (std::numeric_limits<uint64_t>::max)())
                {
                    oa->write_character(static_cast<CharType>(0x9B));
                    write_number(static_cast<uint64_t>(N));
                }
                // LCOV_EXCL_STOP

                // step 2: write each element
                for (const auto& el : *j.m_value.array)
                {
                    write_cbor(el);
                }
                break;
            }

            case value_t::object:
            {
                // step 1: write control byte and the object size
                const auto N = j.m_value.object->size();
                if (N <= 0x17)
                {
                    write_number(static_cast<uint8_t>(0xA0 + N));
                }
                else if (N <= (std::numeric_limits<uint8_t>::max)())
                {
                    oa->write_character(static_cast<CharType>(0xB8));
                    write_number(static_cast<uint8_t>(N));
                }
                else if (N <= (std::numeric_limits<uint16_t>::max)())
                {
                    oa->write_character(static_cast<CharType>(0xB9));
                    write_number(static_cast<uint16_t>(N));
                }
                else if (N <= (std::numeric_limits<uint32_t>::max)())
                {
                    oa->write_character(static_cast<CharType>(0xBA));
                    write_number(static_cast<uint32_t>(N));
                }
                // LCOV_EXCL_START
                else if (N <= (std::numeric_limits<uint64_t>::max)())
                {
                    oa->write_character(static_cast<CharType>(0xBB));
                    write_number(static_cast<uint64_t>(N));
                }
                // LCOV_EXCL_STOP

                // step 2: write each element
                for (const auto& el : *j.m_value.object)
                {
                    write_cbor(el.first);
                    write_cbor(el.second);
                }
                break;
            }

            default:
                break;
        }
    }

    /*!
    @brief[in] j  JSON value to serialize
    */
    void write_msgpack(const BasicJsonType& j)
    {
        switch (j.type())
        {
            case value_t::null: // nil
            {
                oa->write_character(static_cast<CharType>(0xC0));
                break;
            }

            case value_t::boolean: // true and false
            {
                oa->write_character(j.m_value.boolean
                                    ? static_cast<CharType>(0xC3)
                                    : static_cast<CharType>(0xC2));
                break;
            }

            case value_t::number_integer:
            {
                if (j.m_value.number_integer >= 0)
                {
                    // MessagePack does not differentiate between positive
                    // signed integers and unsigned integers. Therefore, we used
                    // the code from the value_t::number_unsigned case here.
                    if (j.m_value.number_unsigned < 128)
                    {
                        // positive fixnum
                        write_number(static_cast<uint8_t>(j.m_value.number_integer));
                    }
                    else if (j.m_value.number_unsigned <= (std::numeric_limits<uint8_t>::max)())
                    {
                        // uint 8
                        oa->write_character(static_cast<CharType>(0xCC));
                        write_number(static_cast<uint8_t>(j.m_value.number_integer));
                    }
                    else if (j.m_value.number_unsigned <= (std::numeric_limits<uint16_t>::max)())
                    {
                        // uint 16
                        oa->write_character(static_cast<CharType>(0xCD));
                        write_number(static_cast<uint16_t>(j.m_value.number_integer));
                    }
                    else if (j.m_value.number_unsigned <= (std::numeric_limits<uint32_t>::max)())
                    {
                        // uint 32
                        oa->write_character(static_cast<CharType>(0xCE));
                        write_number(static_cast<uint32_t>(j.m_value.number_integer));
                    }
                    else if (j.m_value.number_unsigned <= (std::numeric_limits<uint64_t>::max)())
                    {
                        // uint 64
                        oa->write_character(static_cast<CharType>(0xCF));
                        write_number(static_cast<uint64_t>(j.m_value.number_integer));
                    }
                }
                else
                {
                    if (j.m_value.number_integer >= -32)
                    {
                        // negative fixnum
                        write_number(static_cast<int8_t>(j.m_value.number_integer));
                    }
                    else if (j.m_value.number_integer >= (std::numeric_limits<int8_t>::min)() and
                             j.m_value.number_integer <= (std::numeric_limits<int8_t>::max)())
                    {
                        // int 8
                        oa->write_character(static_cast<CharType>(0xD0));
                        write_number(static_cast<int8_t>(j.m_value.number_integer));
                    }
                    else if (j.m_value.number_integer >= (std::numeric_limits<int16_t>::min)() and
                             j.m_value.number_integer <= (std::numeric_limits<int16_t>::max)())
                    {
                        // int 16
                        oa->write_character(static_cast<CharType>(0xD1));
                        write_number(static_cast<int16_t>(j.m_value.number_integer));
                    }
                    else if (j.m_value.number_integer >= (std::numeric_limits<int32_t>::min)() and
                             j.m_value.number_integer <= (std::numeric_limits<int32_t>::max)())
                    {
                        // int 32
                        oa->write_character(static_cast<CharType>(0xD2));
                        write_number(static_cast<int32_t>(j.m_value.number_integer));
                    }
                    else if (j.m_value.number_integer >= (std::numeric_limits<int64_t>::min)() and
                             j.m_value.number_integer <= (std::numeric_limits<int64_t>::max)())
                    {
                        // int 64
                        oa->write_character(static_cast<CharType>(0xD3));
                        write_number(static_cast<int64_t>(j.m_value.number_integer));
                    }
                }
                break;
            }

            case value_t::number_unsigned:
            {
                if (j.m_value.number_unsigned < 128)
                {
                    // positive fixnum
                    write_number(static_cast<uint8_t>(j.m_value.number_integer));
                }
                else if (j.m_value.number_unsigned <= (std::numeric_limits<uint8_t>::max)())
                {
                    // uint 8
                    oa->write_character(static_cast<CharType>(0xCC));
                    write_number(static_cast<uint8_t>(j.m_value.number_integer));
                }
                else if (j.m_value.number_unsigned <= (std::numeric_limits<uint16_t>::max)())
                {
                    // uint 16
                    oa->write_character(static_cast<CharType>(0xCD));
                    write_number(static_cast<uint16_t>(j.m_value.number_integer));
                }
                else if (j.m_value.number_unsigned <= (std::numeric_limits<uint32_t>::max)())
                {
                    // uint 32
                    oa->write_character(static_cast<CharType>(0xCE));
                    write_number(static_cast<uint32_t>(j.m_value.number_integer));
                }
                else if (j.m_value.number_unsigned <= (std::numeric_limits<uint64_t>::max)())
                {
                    // uint 64
                    oa->write_character(static_cast<CharType>(0xCF));
                    write_number(static_cast<uint64_t>(j.m_value.number_integer));
                }
                break;
            }

            case value_t::number_float: // float 64
            {
                oa->write_character(static_cast<CharType>(0xCB));
                write_number(j.m_value.number_float);
                break;
            }

            case value_t::string:
            {
                // step 1: write control byte and the string length
                const auto N = j.m_value.string->size();
                if (N <= 31)
                {
                    // fixstr
                    write_number(static_cast<uint8_t>(0xA0 | N));
                }
                else if (N <= (std::numeric_limits<uint8_t>::max)())
                {
                    // str 8
                    oa->write_character(static_cast<CharType>(0xD9));
                    write_number(static_cast<uint8_t>(N));
                }
                else if (N <= (std::numeric_limits<uint16_t>::max)())
                {
                    // str 16
                    oa->write_character(static_cast<CharType>(0xDA));
                    write_number(static_cast<uint16_t>(N));
                }
                else if (N <= (std::numeric_limits<uint32_t>::max)())
                {
                    // str 32
                    oa->write_character(static_cast<CharType>(0xDB));
                    write_number(static_cast<uint32_t>(N));
                }

                // step 2: write the string
                oa->write_characters(
                    reinterpret_cast<const CharType*>(j.m_value.string->c_str()),
                    j.m_value.string->size());
                break;
            }

            case value_t::array:
            {
                // step 1: write control byte and the array size
                const auto N = j.m_value.array->size();
                if (N <= 15)
                {
                    // fixarray
                    write_number(static_cast<uint8_t>(0x90 | N));
                }
                else if (N <= (std::numeric_limits<uint16_t>::max)())
                {
                    // array 16
                    oa->write_character(static_cast<CharType>(0xDC));
                    write_number(static_cast<uint16_t>(N));
                }
                else if (N <= (std::numeric_limits<uint32_t>::max)())
                {
                    // array 32
                    oa->write_character(static_cast<CharType>(0xDD));
                    write_number(static_cast<uint32_t>(N));
                }

                // step 2: write each element
                for (const auto& el : *j.m_value.array)
                {
                    write_msgpack(el);
                }
                break;
            }

            case value_t::object:
            {
                // step 1: write control byte and the object size
                const auto N = j.m_value.object->size();
                if (N <= 15)
                {
                    // fixmap
                    write_number(static_cast<uint8_t>(0x80 | (N & 0xF)));
                }
                else if (N <= (std::numeric_limits<uint16_t>::max)())
                {
                    // map 16
                    oa->write_character(static_cast<CharType>(0xDE));
                    write_number(static_cast<uint16_t>(N));
                }
                else if (N <= (std::numeric_limits<uint32_t>::max)())
                {
                    // map 32
                    oa->write_character(static_cast<CharType>(0xDF));
                    write_number(static_cast<uint32_t>(N));
                }

                // step 2: write each element
                for (const auto& el : *j.m_value.object)
                {
                    write_msgpack(el.first);
                    write_msgpack(el.second);
                }
                break;
            }

            default:
                break;
        }
    }

    /*!
    @param[in] j  JSON value to serialize
    @param[in] use_count   whether to use '#' prefixes (optimized format)
    @param[in] use_type    whether to use '$' prefixes (optimized format)
    @param[in] add_prefix  whether prefixes need to be used for this value
    */
    void write_ubjson(const BasicJsonType& j, const bool use_count,
                      const bool use_type, const bool add_prefix = true)
    {
        switch (j.type())
        {
            case value_t::null:
            {
                if (add_prefix)
                {
                    oa->write_character(static_cast<CharType>('Z'));
                }
                break;
            }

            case value_t::boolean:
            {
                if (add_prefix)
                    oa->write_character(j.m_value.boolean
                                        ? static_cast<CharType>('T')
                                        : static_cast<CharType>('F'));
                break;
            }

            case value_t::number_integer:
            {
                write_number_with_ubjson_prefix(j.m_value.number_integer, add_prefix);
                break;
            }

            case value_t::number_unsigned:
            {
                write_number_with_ubjson_prefix(j.m_value.number_unsigned, add_prefix);
                break;
            }

            case value_t::number_float:
            {
                write_number_with_ubjson_prefix(j.m_value.number_float, add_prefix);
                break;
            }

            case value_t::string:
            {
                if (add_prefix)
                {
                    oa->write_character(static_cast<CharType>('S'));
                }
                write_number_with_ubjson_prefix(j.m_value.string->size(), true);
                oa->write_characters(
                    reinterpret_cast<const CharType*>(j.m_value.string->c_str()),
                    j.m_value.string->size());
                break;
            }

            case value_t::array:
            {
                if (add_prefix)
                {
                    oa->write_character(static_cast<CharType>('['));
                }

                bool prefix_required = true;
                if (use_type and not j.m_value.array->empty())
                {
                    assert(use_count);
                    const char first_prefix = ubjson_prefix(j.front());
                    const bool same_prefix = std::all_of(j.begin() + 1, j.end(),
                                                         [this, first_prefix](const BasicJsonType & v)
                    {
                        return ubjson_prefix(v) == first_prefix;
                    });

                    if (same_prefix)
                    {
                        prefix_required = false;
                        oa->write_character(static_cast<CharType>('$'));
                        oa->write_character(static_cast<CharType>(first_prefix));
                    }
                }

                if (use_count)
                {
                    oa->write_character(static_cast<CharType>('#'));
                    write_number_with_ubjson_prefix(j.m_value.array->size(), true);
                }

                for (const auto& el : *j.m_value.array)
                {
                    write_ubjson(el, use_count, use_type, prefix_required);
                }

                if (not use_count)
                {
                    oa->write_character(static_cast<CharType>(']'));
                }

                break;
            }

            case value_t::object:
            {
                if (add_prefix)
                {
                    oa->write_character(static_cast<CharType>('{'));
                }

                bool prefix_required = true;
                if (use_type and not j.m_value.object->empty())
                {
                    assert(use_count);
                    const char first_prefix = ubjson_prefix(j.front());
                    const bool same_prefix = std::all_of(j.begin(), j.end(),
                                                         [this, first_prefix](const BasicJsonType & v)
                    {
                        return ubjson_prefix(v) == first_prefix;
                    });

                    if (same_prefix)
                    {
                        prefix_required = false;
                        oa->write_character(static_cast<CharType>('$'));
                        oa->write_character(static_cast<CharType>(first_prefix));
                    }
                }

                if (use_count)
                {
                    oa->write_character(static_cast<CharType>('#'));
                    write_number_with_ubjson_prefix(j.m_value.object->size(), true);
                }

                for (const auto& el : *j.m_value.object)
                {
                    write_number_with_ubjson_prefix(el.first.size(), true);
                    oa->write_characters(
                        reinterpret_cast<const CharType*>(el.first.c_str()),
                        el.first.size());
                    write_ubjson(el.second, use_count, use_type, prefix_required);
                }

                if (not use_count)
                {
                    oa->write_character(static_cast<CharType>('}'));
                }

                break;
            }

            default:
                break;
        }
    }

  private:
    /*
    @brief write a number to output input

    @param[in] n number of type @a NumberType
    @tparam NumberType the type of the number

    @note This function needs to respect the system's endianess, because bytes
          in CBOR and MessagePack are stored in network order (big endian) and
          therefore need reordering on little endian systems.
    */
    template<typename NumberType> void write_number(NumberType n)
    {
        // step 1: write number to array of length NumberType
        std::array<CharType, sizeof(NumberType)> vec;
        std::memcpy(vec.data(), &n, sizeof(NumberType));

        // step 2: write array to output (with possible reordering)
        if (is_little_endian)
        {
            // reverse byte order prior to conversion if necessary
            std::reverse(vec.begin(), vec.end());
        }

        oa->write_characters(vec.data(), sizeof(NumberType));
    }

    template<typename NumberType>
    void write_number_with_ubjson_prefix(const NumberType n,
                                         const bool add_prefix)
    {
        if (std::is_floating_point<NumberType>::value)
        {
            if (add_prefix)
            {
                oa->write_character(static_cast<CharType>('D'));  // float64
            }
            write_number(n);
        }
        else if (std::is_unsigned<NumberType>::value)
        {
            if (n <= (std::numeric_limits<int8_t>::max)())
            {
                if (add_prefix)
                {
                    oa->write_character(static_cast<CharType>('i'));  // uint8
                }
                write_number(static_cast<uint8_t>(n));
            }
            else if (n <= (std::numeric_limits<uint8_t>::max)())
            {
                if (add_prefix)
                {
                    oa->write_character(static_cast<CharType>('U'));  // uint8
                }
                write_number(static_cast<uint8_t>(n));
            }
            else if (n <= (std::numeric_limits<int16_t>::max)())
            {
                if (add_prefix)
                {
                    oa->write_character(static_cast<CharType>('I'));  // int16
                }
                write_number(static_cast<int16_t>(n));
            }
            else if (n <= (std::numeric_limits<int32_t>::max)())
            {
                if (add_prefix)
                {
                    oa->write_character(static_cast<CharType>('l'));  // int32
                }
                write_number(static_cast<int32_t>(n));
            }
            else if (n <= (std::numeric_limits<int64_t>::max)())
            {
                if (add_prefix)
                {
                    oa->write_character(static_cast<CharType>('L'));  // int64
                }
                write_number(static_cast<int64_t>(n));
            }
            else
            {
                JSON_THROW(out_of_range::create(407, "number overflow serializing " + std::to_string(n)));
            }
        }
        else
        {
            if ((std::numeric_limits<int8_t>::min)() <= n and n <= (std::numeric_limits<int8_t>::max)())
            {
                if (add_prefix)
                {
                    oa->write_character(static_cast<CharType>('i'));  // int8
                }
                write_number(static_cast<int8_t>(n));
            }
            else if ((std::numeric_limits<uint8_t>::min)() <= n and n <= (std::numeric_limits<uint8_t>::max)())
            {
                if (add_prefix)
                {
                    oa->write_character(static_cast<CharType>('U'));  // uint8
                }
                write_number(static_cast<uint8_t>(n));
            }
            else if ((std::numeric_limits<int16_t>::min)() <= n and n <= (std::numeric_limits<int16_t>::max)())
            {
                if (add_prefix)
                {
                    oa->write_character(static_cast<CharType>('I'));  // int16
                }
                write_number(static_cast<int16_t>(n));
            }
            else if ((std::numeric_limits<int32_t>::min)() <= n and n <= (std::numeric_limits<int32_t>::max)())
            {
                if (add_prefix)
                {
                    oa->write_character(static_cast<CharType>('l'));  // int32
                }
                write_number(static_cast<int32_t>(n));
            }
            else if ((std::numeric_limits<int64_t>::min)() <= n and n <= (std::numeric_limits<int64_t>::max)())
            {
                if (add_prefix)
                {
                    oa->write_character(static_cast<CharType>('L'));  // int64
                }
                write_number(static_cast<int64_t>(n));
            }
            // LCOV_EXCL_START
            else
            {
                JSON_THROW(out_of_range::create(407, "number overflow serializing " + std::to_string(n)));
            }
            // LCOV_EXCL_STOP
        }
    }

    /*!
    @brief determine the type prefix of container values

    @note This function does not need to be 100% accurate when it comes to
          integer limits. In case a number exceeds the limits of int64_t,
          this will be detected by a later call to function
          write_number_with_ubjson_prefix. Therefore, we return 'L' for any
          value that does not fit the previous limits.
    */
    char ubjson_prefix(const BasicJsonType& j) const noexcept
    {
        switch (j.type())
        {
            case value_t::null:
                return 'Z';

            case value_t::boolean:
                return j.m_value.boolean ? 'T' : 'F';

            case value_t::number_integer:
            {
                if ((std::numeric_limits<int8_t>::min)() <= j.m_value.number_integer and j.m_value.number_integer <= (std::numeric_limits<int8_t>::max)())
                {
                    return 'i';
                }
                else if ((std::numeric_limits<uint8_t>::min)() <= j.m_value.number_integer and j.m_value.number_integer <= (std::numeric_limits<uint8_t>::max)())
                {
                    return 'U';
                }
                else if ((std::numeric_limits<int16_t>::min)() <= j.m_value.number_integer and j.m_value.number_integer <= (std::numeric_limits<int16_t>::max)())
                {
                    return 'I';
                }
                else if ((std::numeric_limits<int32_t>::min)() <= j.m_value.number_integer and j.m_value.number_integer <= (std::numeric_limits<int32_t>::max)())
                {
                    return 'l';
                }
                else  // no check and assume int64_t (see note above)
                {
                    return 'L';
                }
            }

            case value_t::number_unsigned:
            {
                if (j.m_value.number_unsigned <= (std::numeric_limits<int8_t>::max)())
                {
                    return 'i';
                }
                else if (j.m_value.number_unsigned <= (std::numeric_limits<uint8_t>::max)())
                {
                    return 'U';
                }
                else if (j.m_value.number_unsigned <= (std::numeric_limits<int16_t>::max)())
                {
                    return 'I';
                }
                else if (j.m_value.number_unsigned <= (std::numeric_limits<int32_t>::max)())
                {
                    return 'l';
                }
                else  // no check and assume int64_t (see note above)
                {
                    return 'L';
                }
            }

            case value_t::number_float:
                return 'D';

            case value_t::string:
                return 'S';

            case value_t::array:
                return '[';

            case value_t::object:
                return '{';

            default:  // discarded values
                return 'N';
        }
    }

  private:
    /// whether we can assume little endianess
    const bool is_little_endian = binary_reader<BasicJsonType>::little_endianess();

    /// the output
    output_adapter_t<CharType> oa = nullptr;
};
}
}