json/include/nlohmann/detail/output/serializer.hpp

//     __ _____ _____ _____
//  __|  |   __|     |   | |  JSON for Modern C++
// |  |  |__   |  |  | | | |  version 3.12.0
// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
//
// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
// SPDX-License-Identifier: MIT

#pragma once

#include <algorithm> // remove, fill, find, none_of, min
#include <array> // array
#include <clocale> // localeconv, lconv
#include <cmath> // isfinite
#include <cstddef> // size_t, ptrdiff_t
#include <cstdint> // uint8_t
#include <cstdio> // snprintf
#include <cstring> // memcpy, memset
#include <iterator> // next
#include <limits> // numeric_limits
#include <string> // string, char_traits
#include <type_traits> // is_same
#include <vector> // vector

#include <nlohmann/detail/conversions/to_chars.hpp>
#include <nlohmann/detail/exceptions.hpp>
#include <nlohmann/detail/input/string_scan.hpp>
#include <nlohmann/detail/macro_scope.hpp>
#include <nlohmann/detail/meta/cpp_future.hpp>
#include <nlohmann/detail/output/output_adapters.hpp>
#include <nlohmann/detail/recursion_depth_limit.hpp>
#include <nlohmann/detail/string_concat.hpp>
#include <nlohmann/detail/string_utils.hpp>
#include <nlohmann/detail/value_t.hpp>

NLOHMANN_JSON_NAMESPACE_BEGIN
namespace detail
{

///////////////////
// serialization //
///////////////////

/// how to treat decoding errors
enum class error_handler_t
{
    strict,  ///< throw a type_error exception in case of invalid UTF-8
    replace, ///< replace invalid UTF-8 sequences with U+FFFD
    ignore   ///< ignore invalid UTF-8 sequences
};

template<typename BasicJsonType>
class serializer
{
    using string_t = typename BasicJsonType::string_t;
    using number_float_t = typename BasicJsonType::number_float_t;
    using number_integer_t = typename BasicJsonType::number_integer_t;
    using number_unsigned_t = typename BasicJsonType::number_unsigned_t;
    using binary_char_t = typename BasicJsonType::binary_t::value_type;

  public:
    /*!
    @param[in] s  output adapter to serialize to; not owned by the serializer,
                  so it must outlive it (it lives at the call site)
    @param[in] ichar  indentation character to use
    @param[in] pretty_print_  whether the output shall be pretty-printed
    @param[in] ensure_ascii_ If @a ensure_ascii_ is true, all non-ASCII
    characters in the output are escaped with `\\uXXXX` sequences, and the
    result consists of ASCII characters only.
    @param[in] indent_step_  the indent level
    @param[in] error_handler_  how to react on decoding errors

    None of @a pretty_print_, @a ensure_ascii_ and @a indent_step_ change over
    the life of the serializer, so they are captured once here instead of
    being threaded through every call to @ref dump, @ref dump_internal and
    @ref dump_iteratively.
    */
    serializer(output_adapter_protocol<char>& s, const char ichar,
               const bool pretty_print_ = false,
               const bool ensure_ascii_ = false,
               const std::size_t indent_step_ = 0,
               error_handler_t error_handler_ = error_handler_t::strict)
        : o(&s)
        , indent_char(ichar)
        , pretty_print(pretty_print_)
        , ensure_ascii(ensure_ascii_)
        , indent_step(indent_step_)
        , error_handler(error_handler_)
    {}

    // deleted because of pointer members
    serializer(const serializer&) = delete;
    serializer& operator=(const serializer&) = delete;
    serializer(serializer&&) = delete;
    serializer& operator=(serializer&&) = delete;
    ~serializer() = default;

    /*!
    @brief internal implementation of the serialization function

    This function is called by the public member function dump and organizes
    the serialization internally. The indentation level is propagated as
    additional parameter. Arrays and objects are serialized without recursion,
    however deeply they are nested.

    - strings and object keys are escaped using @ref dump_escaped
    - integer numbers are converted using a digit-pair lookup table (@ref dump_integer)
    - floating-point numbers are converted to a string using @ref dump_float, which
      uses `to_chars` for IEEE-754 types and `snprintf` otherwise
    - binary values are serialized as objects containing the subtype and the
      byte array

    @param[in] val               value to serialize
    @param[in] current_indent    the current indent level (only used internally)
    */
    void dump(const BasicJsonType& val,
              const std::size_t current_indent = 0)
    {
        dump_internal(val, current_indent);
        flush();
    }

  JSON_PRIVATE_UNLESS_TESTED:
    /*!
    @brief worker for @ref dump

    Identical in behavior to the historical @ref dump, but writes into the
    serializer's internal @ref write_buffer instead of issuing a virtual call
    per token. The public @ref dump wraps this and flushes the buffer once the
    top-level value has been serialized.

    Serializing a container descends into its elements, so a value nested deeply
    enough used to exhaust the call stack and terminate the process with no
    exception to catch. The descent is bounded here: once @ref recursion_depth_limit
    levels have been entered, @ref dump_iteratively writes out what is left
    without the call stack. A value nested less deeply than that - all but a
    vanishing minority - is written by exactly the code that always wrote it.

    @sa https://github.com/nlohmann/json/issues/5387
    */
    void dump_internal(const BasicJsonType& val,
                       const std::size_t current_indent = 0,
                       const std::size_t depth = 0)
    {
        switch (val.m_data.m_type)
        {
            case value_t::object:
            {
                if (JSON_HEDLEY_UNLIKELY(depth >= recursion_depth_limit()))
                {
                    dump_iteratively(val, current_indent);
                    return;
                }

                if (val.m_data.m_value.object->empty())
                {
                    put_literal("{}");
                    return;
                }

                if (pretty_print)
                {
                    put_literal("{\n");

                    // variable to hold indentation for recursive calls
                    const auto new_indent = next_indent(current_indent, indent_step);

                    // first n-1 elements
                    auto i = val.m_data.m_value.object->cbegin();
                    for (std::size_t cnt = 0; cnt < val.m_data.m_value.object->size() - 1; ++cnt, ++i)
                    {
                        put_indent(new_indent);
                        put_char('"');
                        dump_escaped(i->first);
                        put_literal("\": ");
                        dump_internal(i->second, new_indent, depth + 1);
                        put_literal(",\n");
                    }

                    // last element
                    JSON_ASSERT(i != val.m_data.m_value.object->cend());
                    JSON_ASSERT(std::next(i) == val.m_data.m_value.object->cend());
                    put_indent(new_indent);
                    put_char('"');
                    dump_escaped(i->first);
                    put_literal("\": ");
                    dump_internal(i->second, new_indent, depth + 1);

                    put_char('\n');
                    put_indent(current_indent);
                    put_char('}');
                }
                else
                {
                    put_char('{');

                    // first n-1 elements
                    auto i = val.m_data.m_value.object->cbegin();
                    for (std::size_t cnt = 0; cnt < val.m_data.m_value.object->size() - 1; ++cnt, ++i)
                    {
                        put_char('"');
                        dump_escaped(i->first);
                        put_literal("\":");
                        dump_internal(i->second, current_indent, depth + 1);
                        put_char(',');
                    }

                    // last element
                    JSON_ASSERT(i != val.m_data.m_value.object->cend());
                    JSON_ASSERT(std::next(i) == val.m_data.m_value.object->cend());
                    put_char('"');
                    dump_escaped(i->first);
                    put_literal("\":");
                    dump_internal(i->second, current_indent, depth + 1);

                    put_char('}');
                }

                return;
            }

            case value_t::array:
            {
                if (JSON_HEDLEY_UNLIKELY(depth >= recursion_depth_limit()))
                {
                    dump_iteratively(val, current_indent);
                    return;
                }

                if (val.m_data.m_value.array->empty())
                {
                    put_literal("[]");
                    return;
                }

                if (pretty_print)
                {
                    put_literal("[\n");

                    // variable to hold indentation for recursive calls
                    const auto new_indent = next_indent(current_indent, indent_step);

                    // first n-1 elements
                    for (auto i = val.m_data.m_value.array->cbegin();
                            i != val.m_data.m_value.array->cend() - 1; ++i)
                    {
                        put_indent(new_indent);
                        dump_internal(*i, new_indent, depth + 1);
                        put_literal(",\n");
                    }

                    // last element
                    JSON_ASSERT(!val.m_data.m_value.array->empty());
                    put_indent(new_indent);
                    dump_internal(val.m_data.m_value.array->back(), new_indent, depth + 1);

                    put_char('\n');
                    put_indent(current_indent);
                    put_char(']');
                }
                else
                {
                    put_char('[');

                    // first n-1 elements
                    for (auto i = val.m_data.m_value.array->cbegin();
                            i != val.m_data.m_value.array->cend() - 1; ++i)
                    {
                        dump_internal(*i, current_indent, depth + 1);
                        put_char(',');
                    }

                    // last element
                    JSON_ASSERT(!val.m_data.m_value.array->empty());
                    dump_internal(val.m_data.m_value.array->back(), current_indent, depth + 1);

                    put_char(']');
                }

                return;
            }

            case value_t::string:
            case value_t::binary:
            case value_t::boolean:
            case value_t::number_integer:
            case value_t::number_unsigned:
            case value_t::number_float:
            case value_t::discarded:
            case value_t::null:
            default:
                dump_scalar(val, current_indent);
                return;
        }
    }

  private:
    /*!
    @brief write out @a val and everything below it without the call stack

    Emits the same bytes as @ref dump_internal, keeping the containers it has
    entered on an explicit stack instead of descending into them. Only reached
    for values nested deeper than @ref recursion_depth_limit, which is why it is not
    written for speed: walking every value this way measured up to 20% slower on
    object-heavy documents than letting the compiler drive the descent.
    */
    void dump_iteratively(const BasicJsonType& val,
                          const std::size_t current_indent = 0)
    {
        // Scalars, empty containers and binary values are written by dump_value
        // alone, so nothing is allocated for them: only a container with
        // elements is ever pushed.
        std::vector<dump_frame> stack;

        dump_value(val, current_indent, stack);

        while (!stack.empty())
        {
            dump_frame& frame = stack.back();

            if (frame.value->m_data.m_type == value_t::object)
            {
                const auto* object = frame.value->m_data.m_value.object;

                if (frame.object_it == object->cend())
                {
                    if (pretty_print)
                    {
                        put_char('\n');
                        put_indent(frame.current_indent);
                    }

                    put_char('}');
                    stack.pop_back();
                    continue;
                }

                // the separator goes in front of every element but the first,
                // which puts exactly one between each pair and none at the end
                if (frame.object_it != object->cbegin())
                {
                    if (pretty_print)
                    {
                        put_literal(",\n");
                    }
                    else
                    {
                        put_char(',');
                    }
                }

                if (pretty_print)
                {
                    put_indent(frame.child_indent);
                }

                put_char('"');
                dump_escaped(frame.object_it->first);

                if (pretty_print)
                {
                    put_literal("\": ");
                }
                else
                {
                    put_literal("\":");
                }

                const BasicJsonType& element = frame.object_it->second;
                ++frame.object_it;

                // read everything needed from the frame before this: entering a
                // container pushes another one and can move them all
                const std::size_t element_indent = frame.child_indent;
                dump_value(element, element_indent, stack);
            }
            else
            {
                const auto* array = frame.value->m_data.m_value.array;

                if (frame.array_it == array->cend())
                {
                    if (pretty_print)
                    {
                        put_char('\n');
                        put_indent(frame.current_indent);
                    }

                    put_char(']');
                    stack.pop_back();
                    continue;
                }

                if (frame.array_it != array->cbegin())
                {
                    if (pretty_print)
                    {
                        put_literal(",\n");
                    }
                    else
                    {
                        put_char(',');
                    }
                }

                if (pretty_print)
                {
                    put_indent(frame.child_indent);
                }

                const BasicJsonType& element = *frame.array_it;
                ++frame.array_it;

                // see above
                const std::size_t element_indent = frame.child_indent;
                dump_value(element, element_indent, stack);
            }
        }
    }

  private:
    /// @brief a container that has been opened but not closed yet
    struct dump_frame
    {
        dump_frame(const BasicJsonType* value_, const std::size_t current_indent_,
                   const std::size_t child_indent_) noexcept
            : value(value_)
            , current_indent(current_indent_)
            , child_indent(child_indent_)
        {}

        /// the object or array being serialized
        const BasicJsonType* value;
        /// the element to serialize next; which of the two is live follows from
        /// the type of @a value. They are kept side by side rather than in a
        /// union, which would need its special members written out by hand, see
        /// detail/iterators/internal_iterator.hpp
        typename BasicJsonType::object_t::const_iterator object_it{};
        typename BasicJsonType::array_t::const_iterator array_it{};
        /// the indentation of the container itself, used by its closing bracket
        std::size_t current_indent;
        /// the indentation of the container's elements
        std::size_t child_indent;
    };

    /*!
    @brief serialize the value @a val, but not the elements of a container

    An object or array with elements is opened and pushed onto @a stack for
    @ref dump_iteratively to walk; everything else - including a binary value,
    which looks like an object but has no elements to descend into - is written
    out in full by @ref dump_scalar.
    */
    void dump_value(const BasicJsonType& val,
                    const std::size_t current_indent,
                    std::vector<dump_frame>& stack)
    {
        switch (val.m_data.m_type)
        {
            case value_t::object:
            {
                if (val.m_data.m_value.object->empty())
                {
                    put_literal("{}");
                    return;
                }

                std::size_t child_indent = current_indent;

                if (pretty_print)
                {
                    put_literal("{\n");
                    child_indent = next_indent(current_indent, indent_step);
                }
                else
                {
                    put_char('{');
                }

                stack.emplace_back(&val, current_indent, child_indent);
                stack.back().object_it = val.m_data.m_value.object->cbegin();
                return;
            }

            case value_t::array:
            {
                if (val.m_data.m_value.array->empty())
                {
                    put_literal("[]");
                    return;
                }

                std::size_t child_indent = current_indent;

                if (pretty_print)
                {
                    put_literal("[\n");
                    child_indent = next_indent(current_indent, indent_step);
                }
                else
                {
                    put_char('[');
                }

                stack.emplace_back(&val, current_indent, child_indent);
                stack.back().array_it = val.m_data.m_value.array->cbegin();
                return;
            }

            case value_t::string:
            case value_t::binary:
            case value_t::boolean:
            case value_t::number_integer:
            case value_t::number_unsigned:
            case value_t::number_float:
            case value_t::discarded:
            case value_t::null:
            default:
                dump_scalar(val, current_indent);
                return;
        }
    }

    /*!
    @brief serialize the value @a val, which is neither an object nor an array

    Shared by @ref dump_internal and @ref dump_value, so that a value is written
    the same way however deeply it is nested. A binary value is written out here
    in full: it looks like an object, but has no elements to descend into.

    @param[in] val             value to serialize; not an object or array
    @param[in] current_indent  the indentation of @a val, used for a
                               pretty-printed binary value
    */
    void dump_scalar(const BasicJsonType& val, const std::size_t current_indent)
    {
        switch (val.m_data.m_type)
        {
            case value_t::string:
            {
                put_char('"');
                dump_escaped(*val.m_data.m_value.string);
                put_char('"');
                return;
            }

            case value_t::binary:
            {
                if (pretty_print)
                {
                    put_literal("{\n");

                    // variable to hold indentation for the bytes
                    const auto new_indent = next_indent(current_indent, indent_step);

                    put_indent(new_indent);

                    put_literal("\"bytes\": [");

                    if (!val.m_data.m_value.binary->empty())
                    {
                        for (auto i = val.m_data.m_value.binary->cbegin();
                                i != val.m_data.m_value.binary->cend() - 1; ++i)
                        {
                            dump_byte(*i);
                            put_literal(", ");
                        }
                        dump_byte(val.m_data.m_value.binary->back());
                    }

                    put_literal("],\n");
                    put_indent(new_indent);

                    put_literal("\"subtype\": ");
                    if (val.m_data.m_value.binary->has_subtype())
                    {
                        dump_integer(val.m_data.m_value.binary->subtype());
                    }
                    else
                    {
                        put_literal("null");
                    }
                    put_char('\n');
                    put_indent(current_indent);
                    put_char('}');
                }
                else
                {
                    put_literal("{\"bytes\":[");

                    if (!val.m_data.m_value.binary->empty())
                    {
                        for (auto i = val.m_data.m_value.binary->cbegin();
                                i != val.m_data.m_value.binary->cend() - 1; ++i)
                        {
                            dump_byte(*i);
                            put_char(',');
                        }
                        dump_byte(val.m_data.m_value.binary->back());
                    }

                    put_literal("],\"subtype\":");
                    if (val.m_data.m_value.binary->has_subtype())
                    {
                        dump_integer(val.m_data.m_value.binary->subtype());
                        put_char('}');
                    }
                    else
                    {
                        put_literal("null}");
                    }
                }
                return;
            }

            case value_t::boolean:
            {
                if (val.m_data.m_value.boolean)
                {
                    put_literal("true");
                }
                else
                {
                    put_literal("false");
                }
                return;
            }

            case value_t::number_integer:
            {
                dump_integer(val.m_data.m_value.number_integer);
                return;
            }

            case value_t::number_unsigned:
            {
                dump_integer(val.m_data.m_value.number_unsigned);
                return;
            }

            case value_t::number_float:
            {
                dump_float(val.m_data.m_value.number_float);
                return;
            }

            case value_t::discarded:
            {
                put_literal("<discarded>");
                return;
            }

            case value_t::null:
            {
                put_literal("null");
                return;
            }

            case value_t::object: // LCOV_EXCL_LINE
            case value_t::array:  // LCOV_EXCL_LINE
            default:            // LCOV_EXCL_LINE
                JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert) LCOV_EXCL_LINE
        }
    }


    /*!
    @brief the indentation level to use for the children of the current value

    A very large @a indent_step can wrap the unsigned accumulation on deep
    nesting, which would silently truncate the indentation. Far harder to reach
    now that the accumulator is a std::size_t, but still reachable where that is
    32 bits wide.
    */
    static std::size_t next_indent(const std::size_t current_indent, const std::size_t indent_step)
    {
        const std::size_t new_indent = current_indent + indent_step;
        JSON_ASSERT(new_indent >= current_indent);
        return new_indent;
    }

  JSON_PRIVATE_UNLESS_TESTED:
    /*!
    @brief dump escaped string

    Escape a string by replacing certain special characters by a sequence of an
    escape character (backslash) and another character and other control
    characters by a sequence of "\u" followed by a four-digit hex
    representation. The escaped string is appended to @ref write_buffer.

    @param[in] s  the string to escape

    Complexity: Linear in the length of string @a s.
    */
    void dump_escaped(const string_t& s)
    {
        // dispatch once here rather than test the flag inside the loop: it does
        // not change while a string is written, and folding it lets each of the
        // two scanners be inlined into a loop of its own
        if (ensure_ascii)
        {
            dump_escaped_impl<true>(s);
        }
        else
        {
            dump_escaped_impl<false>(s);
        }
    }

    /*!
    @brief worker for @ref dump_escaped

    @a ensure_ascii is a template parameter here so that the branch on it is
    resolved once, outside the loop; see @ref dump_escaped.
    */
    template<bool EnsureAscii>
    void dump_escaped_impl(const string_t& s)
    {
        std::uint32_t codepoint{};
        std::uint8_t state = UTF8_ACCEPT;
        std::size_t bytes = 0;  // number of bytes written to string_buffer

        // number of bytes written at the point of the last valid byte
        std::size_t bytes_after_last_accept = 0;
        std::size_t undumped_chars = 0;

        for (std::size_t i = 0; i < s.size(); ++i)
        {
            // Fast path: at a character boundary (state == UTF8_ACCEPT),
            // bulk-copy the longest run of bytes that need no escaping using a
            // SWAR scanner shared with the lexer's contiguous path. The scanner
            // stops exactly at the first byte dump_escaped would handle
            // individually, so that byte is left to the byte-at-a-time path
            // below, keeping escaping output and error diagnostics unchanged.
            //
            // - EnsureAscii == false: string_bulk_run() copies ordinary bytes
            //   and complete well-formed UTF-8, stopping at a quote, backslash,
            //   control character (< 0x20), or ill-formed/truncated sequence.
            // - EnsureAscii == true: only printable ASCII may be copied
            //   verbatim; find_ascii_copyable_run() additionally stops at 0x7F
            //   and every non-ASCII byte (>= 0x80), which must be \u-escaped.
            if (state == UTF8_ACCEPT)
            {
                const auto* const data = reinterpret_cast<const unsigned char*>(s.data());
                // A run can only be non-empty when the very first byte is one
                // the scanner may copy, so test that single byte before paying
                // for the scan. Without it, text whose characters all have to be
                // escaped - CJK under ensure_ascii, where every byte is >= 0x80 -
                // runs the scanner once per character only to be told zero.
                std::size_t run = 0;
                if (!EnsureAscii)
                {
                    run = string_bulk_run(data + i, s.size() - i);
                }
                else if (is_ascii_copyable(data[i]))
                {
                    run = find_ascii_copyable_run(data + i, s.size() - i);
                }
                if (run != 0)
                {
                    // emit any bytes still pending in string_buffer first to
                    // preserve output order, then write the run directly
                    if (bytes != 0)
                    {
                        put_buffer(string_buffer, bytes);
                        bytes = 0;
                    }
                    put_string(s, i, i + run);
                    bytes_after_last_accept = 0;
                    undumped_chars = 0;
                    i += run;
                    if (i >= s.size())
                    {
                        break;
                    }
                }
            }

            const auto byte = static_cast<std::uint8_t>(s[i]);

            switch (decode(state, codepoint, byte))
            {
                case UTF8_ACCEPT:  // decode found a new code point
                {
                    switch (codepoint)
                    {
                        case 0x08: // backspace
                        {
                            string_buffer[bytes++] = '\\';
                            string_buffer[bytes++] = 'b';
                            break;
                        }

                        case 0x09: // horizontal tab
                        {
                            string_buffer[bytes++] = '\\';
                            string_buffer[bytes++] = 't';
                            break;
                        }

                        case 0x0A: // newline
                        {
                            string_buffer[bytes++] = '\\';
                            string_buffer[bytes++] = 'n';
                            break;
                        }

                        case 0x0C: // formfeed
                        {
                            string_buffer[bytes++] = '\\';
                            string_buffer[bytes++] = 'f';
                            break;
                        }

                        case 0x0D: // carriage return
                        {
                            string_buffer[bytes++] = '\\';
                            string_buffer[bytes++] = 'r';
                            break;
                        }

                        case 0x22: // quotation mark
                        {
                            string_buffer[bytes++] = '\\';
                            string_buffer[bytes++] = '"';
                            break;
                        }

                        case 0x5C: // reverse solidus
                        {
                            string_buffer[bytes++] = '\\';
                            string_buffer[bytes++] = '\\';
                            break;
                        }

                        default:
                        {
                            // escape control characters (0x00..0x1F) or, if
                            // EnsureAscii parameter is used, non-ASCII characters
                            if ((codepoint <= 0x1F) || (EnsureAscii && (codepoint >= 0x7F)))
                            {
                                if (codepoint <= 0xFFFF)
                                {
                                    write_u_escape(bytes, static_cast<std::uint16_t>(codepoint));
                                }
                                else
                                {
                                    write_u_escape(bytes, static_cast<std::uint16_t>(0xD7C0u + (codepoint >> 10u)));
                                    write_u_escape(bytes, static_cast<std::uint16_t>(0xDC00u + (codepoint & 0x3FFu)));
                                }
                            }
                            else
                            {
                                // copy byte to buffer (all previous bytes
                                // been copied have in default case above)
                                string_buffer[bytes++] = s[i];
                            }
                            break;
                        }
                    }

                    // write buffer and reset index; there must be 13 bytes
                    // left, as this is the maximal number of bytes to be
                    // written ("\uxxxx\uxxxx\0") for one code point
                    if (string_buffer.size() - bytes < 13)
                    {
                        put_buffer(string_buffer, bytes);
                        bytes = 0;
                    }

                    // remember the byte position of this accept
                    bytes_after_last_accept = bytes;
                    undumped_chars = 0;
                    break;
                }

                case UTF8_REJECT:  // decode found invalid UTF-8 byte
                {
                    switch (error_handler)
                    {
                        case error_handler_t::strict:
                        {
                            JSON_THROW(type_error::create(316, concat("invalid UTF-8 byte at index ", std::to_string(i), ": 0x", detail::hex_byte(byte)), nullptr));
                        }

                        case error_handler_t::ignore:
                        case error_handler_t::replace:
                        {
                            // in case we saw this character the first time, we
                            // would like to read it again, because the byte
                            // may be OK for itself, but just not OK for the
                            // previous sequence
                            if (undumped_chars > 0)
                            {
                                --i;
                            }

                            // reset length buffer to the last accepted index;
                            // thus removing/ignoring the invalid characters
                            bytes = bytes_after_last_accept;

                            if (error_handler == error_handler_t::replace)
                            {
                                // add a replacement character
                                if (EnsureAscii)
                                {
                                    string_buffer[bytes++] = '\\';
                                    string_buffer[bytes++] = 'u';
                                    string_buffer[bytes++] = 'f';
                                    string_buffer[bytes++] = 'f';
                                    string_buffer[bytes++] = 'f';
                                    string_buffer[bytes++] = 'd';
                                }
                                else
                                {
                                    string_buffer[bytes++] = '\xEF';
                                    string_buffer[bytes++] = '\xBF';
                                    string_buffer[bytes++] = '\xBD';
                                }

                                // write buffer and reset index; there must be 13 bytes
                                // left, as this is the maximal number of bytes to be
                                // written ("\uxxxx\uxxxx\0") for one code point
                                if (string_buffer.size() - bytes < 13)
                                {
                                    put_buffer(string_buffer, bytes);
                                    bytes = 0;
                                }

                                bytes_after_last_accept = bytes;
                            }

                            undumped_chars = 0;

                            // continue processing the string
                            state = UTF8_ACCEPT;
                            break;
                        }

                        default:            // LCOV_EXCL_LINE
                            JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert) LCOV_EXCL_LINE
                    }
                    break;
                }

                default:  // decode found yet incomplete multibyte code point
                {
                    if (!EnsureAscii)
                    {
                        // code point will not be escaped - copy byte to buffer
                        string_buffer[bytes++] = s[i];
                    }
                    ++undumped_chars;
                    break;
                }
            }
        }

        // we finished processing the string
        if (JSON_HEDLEY_LIKELY(state == UTF8_ACCEPT))
        {
            // write buffer
            if (bytes > 0)
            {
                put_buffer(string_buffer, bytes);
            }
        }
        else
        {
            // we finish reading, but do not accept: string was incomplete
            switch (error_handler)
            {
                case error_handler_t::strict:
                {
                    JSON_THROW(type_error::create(316, concat("incomplete UTF-8 string; last byte: 0x", detail::hex_byte(static_cast<std::uint8_t>(s[s.size() - 1]))), nullptr));
                }

                case error_handler_t::ignore:
                {
                    // write all accepted bytes
                    put_buffer(string_buffer, bytes_after_last_accept);
                    break;
                }

                case error_handler_t::replace:
                {
                    // write all accepted bytes
                    put_buffer(string_buffer, bytes_after_last_accept);
                    // add a replacement character
                    if (EnsureAscii)
                    {
                        put_literal("\\ufffd");
                    }
                    else
                    {
                        put_literal("\xEF\xBF\xBD");
                    }
                    break;
                }

                default:            // LCOV_EXCL_LINE
                    JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert) LCOV_EXCL_LINE
            }
        }
    }

  private:
    /*!
    @brief append a single character to the write buffer

    Structural characters ('{', '"', ',', ...) previously went straight to the
    output adapter, one virtual call each. Buffering them and flushing in bulk
    turns those many indirect calls into a single memcpy plus an occasional
    flush, which dominates the cost of serializing object/array-heavy values.
    */
    void put_char(char c)
    {
        if (JSON_HEDLEY_UNLIKELY(write_buffer_pos == write_buffer.size()))
        {
            flush();
        }
        write_buffer[write_buffer_pos++] = c;
    }

    /*!
    @brief append @a indent indentation characters to the write buffer

    Writes the indentation straight into the buffer instead of copying it out of
    a pre-grown indentation string, so no auxiliary string has to be sized,
    resized, or kept in sync with the deepest nesting level reached.

    An indentation wider than the buffer is emitted by filling the buffer with
    the indentation character once and flushing that same content repeatedly:
    flushing does not disturb what the buffer holds, so re-filling it between
    flushes would be redundant work.
    */
    void put_indent(std::size_t indent)
    {
        // closing braces at the outermost level ask for no indentation at all
        if (indent == 0)
        {
            return;
        }

        const std::size_t capacity = write_buffer.size();

        // fill whatever room is left in the buffer; this is the whole job
        // whenever the indentation is narrower than the buffer, which is the
        // case for every sane indent_step
        const std::size_t head = (std::min)(indent, capacity - write_buffer_pos);
        std::memset(write_buffer.data() + write_buffer_pos, indent_char, head);
        write_buffer_pos += head;
        indent -= head;

        if (JSON_HEDLEY_LIKELY(indent == 0))
        {
            return;
        }

        // the buffer is full and the remainder spans whole buffer-fulls: flush
        // what is pending, then fill the buffer with the indentation character
        // exactly once and hand the same bytes to the adapter as often as needed
        flush();
        std::memset(write_buffer.data(), indent_char, capacity);

        while (indent >= capacity)
        {
            write_buffer_pos = capacity;
            flush();
            indent -= capacity;
        }

        // the buffer still holds indentation characters throughout, so the tail
        // only has to be claimed, not written again
        write_buffer_pos = indent;
    }

    /*!
    @brief append a string literal to the write buffer

    The length comes from the array bound rather than a hand-written count, so
    it cannot drift out of sync with the literal. A literal always fits into the
    buffer (checked at compile time), so unlike @ref put_string this needs no
    write-through path for oversized runs.
    */
    template<std::size_t N>
    void put_literal(const char (&s)[N]) // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
    {
        static_assert(N >= 2, "put_literal expects a non-empty string literal");
        // the array bound counts the terminating NUL, which is not written
        constexpr std::size_t length = N - 1;
        static_assert(length < write_buffer_size, "string literal must fit into the write buffer");

        if (JSON_HEDLEY_UNLIKELY(write_buffer_pos + length > write_buffer.size()))
        {
            flush();
        }
        std::memcpy(write_buffer.data() + write_buffer_pos, s, length);
        write_buffer_pos += length;
    }

    /*!
    @brief append the characters of @a str in [@a start, @a end)

    The only way to append a run of characters: @a str carries its own bound,
    so the range can be checked against it, which a bare pointer plus a count
    could not do. Runs that do not fit the buffer are written straight through
    the output adapter (after flushing what is pending), so large string and
    number payloads are not copied an extra time.
    */
    template<typename StringType>
    void put_string(const StringType& str, std::size_t start, std::size_t end)
    {
        JSON_ASSERT(start <= end);
        JSON_ASSERT(end <= str.size());

        const char* const s = str.data() + start;
        const std::size_t length = end - start;

        if (JSON_HEDLEY_UNLIKELY(length >= write_buffer.size()))
        {
            flush();
            o->write_characters(s, length);
            return;
        }
        if (JSON_HEDLEY_UNLIKELY(write_buffer_pos + length > write_buffer.size()))
        {
            flush();
        }
        std::memcpy(write_buffer.data() + write_buffer_pos, s, length);
        write_buffer_pos += length;
    }

    /*!
    @brief append the first @a length characters of a fixed-size buffer
    */
    template<std::size_t N>
    void put_buffer(const std::array<char, N>& buffer, std::size_t length)
    {
        put_string(buffer, 0, length);
    }

  JSON_PRIVATE_UNLESS_TESTED:
    /*!
    @brief flush the write buffer to the output adapter

    Writing zero characters is a well-defined no-op for every output adapter, so
    the buffered length is passed through unconditionally (no empty-guard branch
    to leave uncovered).

    @note dump_escaped() and dump_integer()/dump_float() write into the internal
    write buffer; callers that invoke them directly (rather than through the
    public dump()) must call flush() before inspecting the output.
    */
    void flush()
    {
        o->write_characters(write_buffer.data(), write_buffer_pos);
        write_buffer_pos = 0;
    }

  private:
    /*!
    @brief count digits

    Count the number of decimal (base 10) digits for an input unsigned integer.

    @param[in] x  unsigned integer number to count its digits
    @return    number of decimal digits
    */
    unsigned int count_digits(number_unsigned_t x) noexcept
    {
        unsigned int n_digits = 1;
        for (;;)
        {
            if (x < 10)
            {
                return n_digits;
            }
            if (x < 100)
            {
                return n_digits + 1;
            }
            if (x < 1000)
            {
                return n_digits + 2;
            }
            if (x < 10000)
            {
                return n_digits + 3;
            }
            x = x / 10000u;
            n_digits += 4;
        }
    }

    /*!
     * @brief write a lowercase "\\uXXXX" escape sequence into @a string_buffer
     *
     * Branch-free replacement for `snprintf(buf, 7, "\\u%04x", codeunit)` in the
     * string escaping hot path. It writes exactly six characters ('\\', 'u' and
     * four hex digits) at position @a pos of @a string_buffer via a nibble
     * lookup table, avoiding the format-string parsing and locale machinery of
     * `snprintf`. Advances @a pos by the number of bytes written (6).
     *
     * @param[in] pos       position in @a string_buffer to write at; there must
     *                      be at least 6 bytes of headroom
     * @param[in] codeunit  16-bit value to encode
     */
    void write_u_escape(std::size_t& pos, std::uint16_t codeunit) noexcept
    {
        JSON_ASSERT(string_buffer.size() - pos >= 6);
        constexpr const char* nibble_to_hex = "0123456789abcdef";
        string_buffer[pos + 0] = '\\';
        string_buffer[pos + 1] = 'u';
        string_buffer[pos + 2] = nibble_to_hex[(codeunit >> 12u) & 0x0Fu];
        string_buffer[pos + 3] = nibble_to_hex[(codeunit >> 8u) & 0x0Fu];
        string_buffer[pos + 4] = nibble_to_hex[(codeunit >> 4u) & 0x0Fu];
        string_buffer[pos + 5] = nibble_to_hex[codeunit & 0x0Fu];
        pos += 6;
    }

    /*!
    @brief convert a single element of a binary value to its byte value

    The elements of a binary value are dumped as the numbers 0..255, regardless
    of the value type of the configured BinaryType: that type may be signed
    (`char`), unsigned (`std::uint8_t`), or not an integer at all
    (`std::byte`), none of which @ref dump_integer can handle uniformly.
    */
    static std::uint8_t to_byte_value(binary_char_t x) noexcept
    {
        return static_cast<std::uint8_t>(x);
    }

    // templates to avoid warnings about useless casts
    template <typename NumberType, enable_if_t<std::is_signed<NumberType>::value, int> = 0>
    bool is_negative_number(NumberType x)
    {
        return x < 0;
    }

    template < typename NumberType, enable_if_t <std::is_unsigned<NumberType>::value, int > = 0 >
    bool is_negative_number(NumberType /*unused*/)
    {
        return false;
    }

    /*!
    @brief write the decimal representation of the byte @a value

    A binary value's bytes are always in [0, 255], so writing one needs neither
    the digit counting nor the 64-bit arithmetic that @ref dump_integer does for
    an arbitrary number, and the three digits it takes at most are written
    straight into the write buffer.

    Any byte type that is not a plain unsigned byte is converted to its
    @ref to_byte_value "byte value" and left to @ref dump_integer, so a signed
    or non-integral BinaryType::value_type (`char`, `std::byte`, ...) still
    dumps as 0..255.
    */
    template<typename ByteType>
    void dump_byte(const ByteType value)
    {
        dump_byte(value, std::integral_constant < bool,
                  std::is_unsigned<ByteType>::value && sizeof(ByteType) == 1
                  && !std::is_same<ByteType, bool>::value > {});
    }

    template<typename ByteType>
    void dump_byte(const ByteType value, std::false_type /*is_plain_byte*/)
    {
        dump_integer(to_byte_value(value));
    }

    template<typename ByteType>
    void dump_byte(const ByteType value, std::true_type /*is_plain_byte*/)
    {
        if (JSON_HEDLEY_UNLIKELY(write_buffer_pos + 3 > write_buffer.size()))
        {
            flush();
        }

        const auto byte = static_cast<unsigned>(value);
        // Accumulate the offset in a local and store it back once. Writing
        // through write_buffer[] is a char write, which may alias any object,
        // so with the member updated in place the compiler has to reload and
        // store it around every digit - measured 2.4x slower on a dump of a
        // multi-megabyte binary value.
        std::size_t pos = write_buffer_pos;

        if (byte >= 100)
        {
            write_buffer[pos++] = static_cast<char>('0' + (byte / 100));
            write_buffer[pos++] = static_cast<char>('0' + ((byte / 10) % 10));
        }
        else if (byte >= 10)
        {
            write_buffer[pos++] = static_cast<char>('0' + (byte / 10));
        }

        write_buffer[pos++] = static_cast<char>('0' + (byte % 10));

        write_buffer_pos = pos;
    }

    /*!
    @brief dump an integer

    Dump a given integer, appending it to @ref write_buffer. Works internally with
    @a number_buffer.

    @param[in] x  integer number (signed or unsigned) to dump
    @tparam NumberType either @a number_integer_t or @a number_unsigned_t
    */
    template < typename NumberType, detail::enable_if_t <
                   std::is_integral<NumberType>::value ||
                   std::is_same<NumberType, number_unsigned_t>::value ||
                   std::is_same<NumberType, number_integer_t>::value,
                   int > = 0 >
    void dump_integer(NumberType x)
    {
        static constexpr std::array<std::array<char, 2>, 100> digits_to_99
        {
            {
                {{'0', '0'}}, {{'0', '1'}}, {{'0', '2'}}, {{'0', '3'}}, {{'0', '4'}}, {{'0', '5'}}, {{'0', '6'}}, {{'0', '7'}}, {{'0', '8'}}, {{'0', '9'}},
                {{'1', '0'}}, {{'1', '1'}}, {{'1', '2'}}, {{'1', '3'}}, {{'1', '4'}}, {{'1', '5'}}, {{'1', '6'}}, {{'1', '7'}}, {{'1', '8'}}, {{'1', '9'}},
                {{'2', '0'}}, {{'2', '1'}}, {{'2', '2'}}, {{'2', '3'}}, {{'2', '4'}}, {{'2', '5'}}, {{'2', '6'}}, {{'2', '7'}}, {{'2', '8'}}, {{'2', '9'}},
                {{'3', '0'}}, {{'3', '1'}}, {{'3', '2'}}, {{'3', '3'}}, {{'3', '4'}}, {{'3', '5'}}, {{'3', '6'}}, {{'3', '7'}}, {{'3', '8'}}, {{'3', '9'}},
                {{'4', '0'}}, {{'4', '1'}}, {{'4', '2'}}, {{'4', '3'}}, {{'4', '4'}}, {{'4', '5'}}, {{'4', '6'}}, {{'4', '7'}}, {{'4', '8'}}, {{'4', '9'}},
                {{'5', '0'}}, {{'5', '1'}}, {{'5', '2'}}, {{'5', '3'}}, {{'5', '4'}}, {{'5', '5'}}, {{'5', '6'}}, {{'5', '7'}}, {{'5', '8'}}, {{'5', '9'}},
                {{'6', '0'}}, {{'6', '1'}}, {{'6', '2'}}, {{'6', '3'}}, {{'6', '4'}}, {{'6', '5'}}, {{'6', '6'}}, {{'6', '7'}}, {{'6', '8'}}, {{'6', '9'}},
                {{'7', '0'}}, {{'7', '1'}}, {{'7', '2'}}, {{'7', '3'}}, {{'7', '4'}}, {{'7', '5'}}, {{'7', '6'}}, {{'7', '7'}}, {{'7', '8'}}, {{'7', '9'}},
                {{'8', '0'}}, {{'8', '1'}}, {{'8', '2'}}, {{'8', '3'}}, {{'8', '4'}}, {{'8', '5'}}, {{'8', '6'}}, {{'8', '7'}}, {{'8', '8'}}, {{'8', '9'}},
                {{'9', '0'}}, {{'9', '1'}}, {{'9', '2'}}, {{'9', '3'}}, {{'9', '4'}}, {{'9', '5'}}, {{'9', '6'}}, {{'9', '7'}}, {{'9', '8'}}, {{'9', '9'}},
            }
        };

        // special case for "0"
        if (x == 0)
        {
            put_char('0');
            return;
        }

        // use a pointer to fill the buffer
        auto buffer_ptr = number_buffer.begin(); // NOLINT(llvm-qualified-auto,readability-qualified-auto)

        number_unsigned_t abs_value;

        unsigned int n_chars{};

        if (is_negative_number(x))
        {
            *buffer_ptr = '-';
            abs_value = remove_sign(static_cast<number_integer_t>(x));

            // account one more byte for the minus sign
            n_chars = 1 + count_digits(abs_value);
        }
        else
        {
            abs_value = static_cast<number_unsigned_t>(x);
            n_chars = count_digits(abs_value);
        }

        // spare 1 byte for '\0'
        JSON_ASSERT(n_chars < number_buffer.size() - 1);

        // jump to the end to generate the string from backward,
        // so we later avoid reversing the result
        buffer_ptr += static_cast<typename decltype(number_buffer)::difference_type>(n_chars);

        // Fast int2ascii implementation inspired by "Fastware" talk by Andrei Alexandrescu
        // See: https://www.youtube.com/watch?v=o4-CwDo2zpg
        while (abs_value >= 100)
        {
            const auto digits_index = static_cast<unsigned>((abs_value % 100));
            abs_value /= 100;
            *(--buffer_ptr) = digits_to_99[digits_index][1];
            *(--buffer_ptr) = digits_to_99[digits_index][0];
        }

        if (abs_value >= 10)
        {
            const auto digits_index = static_cast<unsigned>(abs_value);
            *(--buffer_ptr) = digits_to_99[digits_index][1];
            *(--buffer_ptr) = digits_to_99[digits_index][0];
        }
        else
        {
            *(--buffer_ptr) = static_cast<char>('0' + abs_value);
        }

        put_buffer(number_buffer, n_chars);
    }

    /*!
    @brief dump a floating-point number

    Dump a given floating-point number, appending it to @ref write_buffer. Works internally
    with @a number_buffer.

    @param[in] x  floating-point number to dump
    */
    void dump_float(number_float_t x)
    {
        // NaN / inf
        if (!std::isfinite(x))
        {
            put_literal("null");
            return;
        }

        // If number_float_t is an IEEE-754 single or double precision number,
        // use the Grisu2 algorithm to produce short numbers which are
        // guaranteed to round-trip, using strtof and strtod, resp.
        //
        // NB: The test below works if <long double> == <double>.
        static constexpr bool is_ieee_single_or_double
            = (std::numeric_limits<number_float_t>::is_iec559 && std::numeric_limits<number_float_t>::digits == 24 && std::numeric_limits<number_float_t>::max_exponent == 128) ||
              (std::numeric_limits<number_float_t>::is_iec559 && std::numeric_limits<number_float_t>::digits == 53 && std::numeric_limits<number_float_t>::max_exponent == 1024);

        dump_float(x, std::integral_constant<bool, is_ieee_single_or_double>());
    }

    void dump_float(number_float_t x, std::true_type /*is_ieee_single_or_double*/)
    {
        auto* begin = number_buffer.data();
        auto* end = ::nlohmann::detail::to_chars(begin, begin + number_buffer.size(), x);

        put_buffer(number_buffer, static_cast<std::size_t>(end - begin));
    }

    JSON_HEDLEY_NON_NULL(1)
    static int snprintf_float(char* buf, std::size_t size, int d, double x)
    {
        // NOLINTNEXTLINE(cppcoreguidelines-pro-type-vararg,hicpp-vararg)
        return (std::snprintf)(buf, size, "%.*g", d, x);
    }

    JSON_HEDLEY_NON_NULL(1)
    static int snprintf_float(char* buf, std::size_t size, int d, long double x)
    {
        // NOLINTNEXTLINE(cppcoreguidelines-pro-type-vararg,hicpp-vararg)
        return (std::snprintf)(buf, size, "%.*Lg", d, x);
    }

    void dump_float(number_float_t x, std::false_type /*is_ieee_single_or_double*/)
    {
        // get the number of digits for a float -> text -> float round-trip
        static constexpr auto d = std::numeric_limits<number_float_t>::max_digits10;

        // the actual conversion
        std::ptrdiff_t len = snprintf_float(number_buffer.data(), number_buffer.size(), d, x);

        // negative value indicates an error
        JSON_ASSERT(len > 0);
        // check if the buffer was large enough
        JSON_ASSERT(static_cast<std::size_t>(len) < number_buffer.size());

        // look up the locale's thousands separator and decimal point now,
        // matching what snprintf_float() just used (see lexer::get_decimal_point())
        const auto* loc = std::localeconv();
        JSON_ASSERT(loc != nullptr);
        const char thousands_sep = (loc->thousands_sep == nullptr) ? '\0' : *loc->thousands_sep;
        const char decimal_point = (loc->decimal_point == nullptr) ? '\0' : *loc->decimal_point;

        // erase thousands separators
        if (thousands_sep != '\0')
        {
            // NOLINTNEXTLINE(readability-qualified-auto,llvm-qualified-auto): std::remove returns an iterator, see https://github.com/nlohmann/json/issues/3081
            const auto end = std::remove(number_buffer.begin(), number_buffer.begin() + len, thousands_sep);
            std::fill(end, number_buffer.end(), '\0');
            JSON_ASSERT((end - number_buffer.begin()) <= len);
            len = (end - number_buffer.begin());
        }

        // convert decimal point to '.'
        if (decimal_point != '\0' && decimal_point != '.')
        {
            // NOLINTNEXTLINE(readability-qualified-auto,llvm-qualified-auto): std::find returns an iterator, see https://github.com/nlohmann/json/issues/3081
            const auto dec_pos = std::find(number_buffer.begin(), number_buffer.end(), decimal_point);
            if (dec_pos != number_buffer.end())
            {
                *dec_pos = '.';
            }
        }

        put_buffer(number_buffer, static_cast<std::size_t>(len));

        // determine if we need to append ".0"
        const bool value_is_int_like =
            std::none_of(number_buffer.begin(), number_buffer.begin() + len + 1,
                         [](char c)
        {
            return c == '.' || c == 'e';
        });

        if (value_is_int_like)
        {
            put_literal(".0");
        }
    }

    /*
     * Overload to make the compiler happy while it is instantiating
     * dump_integer for number_unsigned_t.
     * Must never be called.
     */
    number_unsigned_t remove_sign(number_unsigned_t x)
    {
        JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert) LCOV_EXCL_LINE
        return x; // LCOV_EXCL_LINE
    }

    /*
     * Helper function for dump_integer
     *
     * This function takes a negative signed integer and returns its absolute
     * value as an unsigned integer. The plus/minus shuffling is necessary as we
     * cannot directly remove the sign of an arbitrary signed integer as the
     * absolute values of INT_MIN and INT_MAX are usually not the same. See
     * #1708 for details.
     */
    number_unsigned_t remove_sign(number_integer_t x) noexcept
    {
        JSON_ASSERT(x < 0);
        return static_cast<number_unsigned_t>(-(x + 1)) + 1;
    }

  private:
    /// the output of the serializer (non-owning; the adapter lives at the call site)
    output_adapter_protocol<char>* o = nullptr;

    /// a (hopefully) large enough character buffer
    std::array<char, 64> number_buffer{{}};

    /// string buffer
    std::array<char, 512> string_buffer{{}};

    /// the indentation character
    const char indent_char;

    /// whether to pretty-print the output
    const bool pretty_print;

    /// whether to escape non-ASCII characters with \\uXXXX sequences
    const bool ensure_ascii;

    /// the indent level
    const std::size_t indent_step;

    /// error_handler how to react on decoding errors
    const error_handler_t error_handler;

    /// buffer collecting output before it is flushed to the output adapter, so
    /// that the many small structural writes become few bulk writes
    static constexpr std::size_t write_buffer_size = 1024;
    std::array<char, write_buffer_size> write_buffer{{}};
    /// number of valid bytes currently held in @ref write_buffer
    std::size_t write_buffer_pos = 0;
};

}  // namespace detail
NLOHMANN_JSON_NAMESPACE_END