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diff --git a/include/jsoncons_ext/jsonpath/flatten.hpp b/include/jsoncons_ext/jsonpath/flatten.hpp
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+++ b/include/jsoncons_ext/jsonpath/flatten.hpp
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+// Copyright 2021 Daniel Parker
+// Distributed under the Boost license, Version 1.0.
+// (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+// See https://github.com/danielaparker/jsoncons for latest version
+
+#ifndef JSONCONS_JSONPATH_FLATTEN_HPP
+#define JSONCONS_JSONPATH_FLATTEN_HPP
+
+#include <string>
+#include <vector>
+#include <memory>
+#include <type_traits> // std::is_const
+#include <limits> // std::numeric_limits
+#include <utility> // std::move
+#include <algorithm> // std::copy
+#include <iterator> // std::back_inserter
+#include <jsoncons_ext/jsonpath/jsonpath.hpp>
+
+namespace jsoncons { namespace jsonpath {
+
+    template <class CharT, class Sink>
+    std::size_t escape_string(const CharT* s, std::size_t length,
+                              Sink& sink)
+    {
+        std::size_t count = 0;
+        const CharT* begin = s;
+        const CharT* end = s + length;
+        for (const CharT* it = begin; it != end; ++it)
+        {
+            CharT c = *it;
+            switch (c)
+            {
+                case '\\':
+                    sink.push_back('\\');
+                    sink.push_back('\\');
+                    count += 2;
+                    break;
+                case '\'':
+                    sink.push_back('\\');
+                    sink.push_back('\'');
+                    count += 2;
+                    break;
+                case '\b':
+                    sink.push_back('\\');
+                    sink.push_back('b');
+                    count += 2;
+                    break;
+                case '\f':
+                    sink.push_back('\\');
+                    sink.push_back('f');
+                    count += 2;
+                    break;
+                case '\n':
+                    sink.push_back('\\');
+                    sink.push_back('n');
+                    count += 2;
+                    break;
+                case '\r':
+                    sink.push_back('\\');
+                    sink.push_back('r');
+                    count += 2;
+                    break;
+                case '\t':
+                    sink.push_back('\\');
+                    sink.push_back('t');
+                    count += 2;
+                    break;
+                default:
+                    sink.push_back(c);
+                    ++count;
+                    break;
+            }
+        }
+        return count;
+    }
+
+    template<class Json>
+    void flatten_(const std::basic_string<typename Json::char_type>& parent_key,
+                  const Json& parent_value,
+                  Json& result)
+    {
+        using char_type = typename Json::char_type;
+        using string_type = std::basic_string<char_type>;
+
+        switch (parent_value.type())
+        {
+            case json_type::array_value:
+            {
+                if (parent_value.empty())
+                {
+                    result.try_emplace(parent_key, parent_value);
+                }
+                else
+                {
+                    for (std::size_t i = 0; i < parent_value.size(); ++i)
+                    {
+                        string_type key(parent_key);
+                        key.push_back('[');
+                        jsoncons::detail::from_integer(i,key);
+                        key.push_back(']');
+                        flatten_(key, parent_value.at(i), result);
+                    }
+                }
+                break;
+            }
+
+            case json_type::object_value:
+            {
+                if (parent_value.empty())
+                {
+                    result.try_emplace(parent_key, Json());
+                }
+                else
+                {
+                    for (const auto& item : parent_value.object_range())
+                    {
+                        string_type key(parent_key);
+                        key.push_back('[');
+                        key.push_back('\'');
+                        escape_string(item.key().data(), item.key().length(), key);
+                        key.push_back('\'');
+                        key.push_back(']');
+                        flatten_(key, item.value(), result);
+                    }
+                }
+                break;
+            }
+
+            default:
+            {
+                result[parent_key] = parent_value;
+                break;
+            }
+        }
+    }
+
+    template<class Json>
+    Json flatten(const Json& value)
+    {
+        Json result;
+        std::basic_string<typename Json::char_type> parent_key = {'$'};
+        flatten_(parent_key, value, result);
+        return result;
+    }
+
+    enum class unflatten_state 
+    {
+        start,
+        expect_lbracket,
+        lbracket,
+        single_quoted_name_state,
+        double_quoted_name_state,
+        index_state,
+        expect_rbracket,
+        double_quoted_string_escape_char,
+        single_quoted_string_escape_char
+    };
+
+    template<class Json>
+    Json unflatten(const Json& value)
+    {
+        using char_type = typename Json::char_type;
+        using string_type = std::basic_string<char_type>;
+
+        if (JSONCONS_UNLIKELY(!value.is_object()))
+        {
+            JSONCONS_THROW(jsonpath_error(jsonpath_errc::argument_to_unflatten_invalid));
+        }
+
+        Json result;
+
+        for (const auto& item : value.object_range())
+        {
+            Json* part = &result;
+            string_type buffer;
+            unflatten_state state = unflatten_state::start;
+
+            auto it = item.key().begin();
+            auto last = item.key().end();
+
+            for (; it != last; ++it)
+            {
+                switch (state)
+                {
+                    case unflatten_state::start:
+                    {
+                        switch (*it)
+                        {
+                            case '$':
+                                state = unflatten_state::expect_lbracket;
+                                break;
+                            default:
+                                break;
+                        }
+                        break;
+                    }
+                    case unflatten_state::expect_lbracket:
+                    {
+                        switch (*it)
+                        {
+                            case '[':
+                                state = unflatten_state::lbracket;
+                                break;
+                            default:
+                                JSONCONS_THROW(jsonpath_error(jsonpath_errc::invalid_flattened_key));
+                                break;
+                        }
+                        break;
+                    }
+                    case unflatten_state::lbracket:
+                    {
+                        switch (*it)
+                        {
+                            case '\'':
+                                state = unflatten_state::single_quoted_name_state;
+                                break;
+                            case '\"':
+                                state = unflatten_state::double_quoted_name_state;
+                                break;
+                            case '0':case '1':case '2':case '3':case '4':case '5':case '6':case '7':case '8':case '9':
+                                buffer.push_back(*it);
+                                state = unflatten_state::index_state;
+                                break;
+                            default:
+                                JSONCONS_THROW(jsonpath_error(jsonpath_errc::invalid_flattened_key));
+                                break;
+                        }
+                        break;
+                    }
+                    case unflatten_state::single_quoted_name_state:
+                    {
+                        switch (*it)
+                        {
+                            case '\'':
+                                if (it != last-2)
+                                {
+                                    auto res = part->try_emplace(buffer,Json());
+                                    part = &(res.first->value());
+                                }
+                                else
+                                {
+                                    auto res = part->try_emplace(buffer,item.value());
+                                    part = &(res.first->value());
+                                }
+                                buffer.clear();
+                                state = unflatten_state::expect_rbracket;
+                                break;
+                            case '\\':
+                                state = unflatten_state::single_quoted_string_escape_char;
+                                break;
+                            default:
+                                buffer.push_back(*it);
+                                break;
+                        }
+                        break;
+                    }
+                    case unflatten_state::double_quoted_name_state:
+                    {
+                        switch (*it)
+                        {
+                            case '\"':
+                                if (it != last-2)
+                                {
+                                    auto res = part->try_emplace(buffer,Json());
+                                    part = &(res.first->value());
+                                }
+                                else
+                                {
+                                    auto res = part->try_emplace(buffer,item.value());
+                                    part = &(res.first->value());
+                                }
+                                buffer.clear();
+                                state = unflatten_state::expect_rbracket;
+                                break;
+                            case '\\':
+                                state = unflatten_state::double_quoted_string_escape_char;
+                                break;
+                            default:
+                                buffer.push_back(*it);
+                                break;
+                        }
+                        break;
+                    }
+                    case unflatten_state::double_quoted_string_escape_char:
+                        switch (*it)
+                        {
+                            case '\"':
+                                buffer.push_back('\"');
+                                state = unflatten_state::double_quoted_name_state;
+                                break;
+                            case '\\': 
+                                buffer.push_back('\\');
+                                state = unflatten_state::double_quoted_name_state;
+                                break;
+                            case '/':
+                                buffer.push_back('/');
+                                state = unflatten_state::double_quoted_name_state;
+                                break;
+                            case 'b':
+                                buffer.push_back('\b');
+                                state = unflatten_state::double_quoted_name_state;
+                                break;
+                            case 'f':
+                                buffer.push_back('\f');
+                                state = unflatten_state::double_quoted_name_state;
+                                break;
+                            case 'n':
+                                buffer.push_back('\n');
+                                state = unflatten_state::double_quoted_name_state;
+                                break;
+                            case 'r':
+                                buffer.push_back('\r');
+                                state = unflatten_state::double_quoted_name_state;
+                                break;
+                            case 't':
+                                buffer.push_back('\t');
+                                state = unflatten_state::double_quoted_name_state;
+                                break;
+                            default:
+                                break;
+                        }
+                        break;
+                    case unflatten_state::single_quoted_string_escape_char:
+                        switch (*it)
+                        {
+                            case '\'':
+                                buffer.push_back('\'');
+                                state = unflatten_state::single_quoted_name_state;
+                                break;
+                            case '\\': 
+                                buffer.push_back('\\');
+                                state = unflatten_state::double_quoted_name_state;
+                                break;
+                            case '/':
+                                buffer.push_back('/');
+                                state = unflatten_state::double_quoted_name_state;
+                                break;
+                            case 'b':
+                                buffer.push_back('\b');
+                                state = unflatten_state::double_quoted_name_state;
+                                break;
+                            case 'f':
+                                buffer.push_back('\f');
+                                state = unflatten_state::double_quoted_name_state;
+                                break;
+                            case 'n':
+                                buffer.push_back('\n');
+                                state = unflatten_state::double_quoted_name_state;
+                                break;
+                            case 'r':
+                                buffer.push_back('\r');
+                                state = unflatten_state::double_quoted_name_state;
+                                break;
+                            case 't':
+                                buffer.push_back('\t');
+                                state = unflatten_state::double_quoted_name_state;
+                                break;
+                            default:
+                                break;
+                        }
+                        break;
+                    case unflatten_state::index_state:
+                    {
+                        switch (*it)
+                        {
+                            case ']':
+                            {
+                                std::size_t n{0};
+                                auto r = jsoncons::detail::to_integer(buffer.data(), buffer.size(), n);
+                                if (r)
+                                {
+                                    if (!part->is_array())
+                                    {
+                                        *part = Json(json_array_arg);
+                                    }
+                                    if (it != last-1)
+                                    {
+                                        if (n+1 > part->size())
+                                        {
+                                            Json& ref = part->emplace_back();
+                                            part = std::addressof(ref);
+                                        }
+                                        else
+                                        {
+                                            part = &part->at(n);
+                                        }
+                                    }
+                                    else
+                                    {
+                                        Json& ref = part->emplace_back(item.value());
+                                        part = std::addressof(ref);
+                                    }
+                                }
+                                buffer.clear();
+                                state = unflatten_state::expect_lbracket;
+                                break;
+                            }
+                            case '0':case '1':case '2':case '3':case '4':case '5':case '6':case '7':case '8':case '9':
+                                buffer.push_back(*it);
+                                break;
+                            default:
+                                JSONCONS_THROW(jsonpath_error(jsonpath_errc::invalid_flattened_key));
+                                break;
+                        }
+                        break;
+                    }
+                    case unflatten_state::expect_rbracket:
+                    {
+                        switch (*it)
+                        {
+                            case ']':
+                                state = unflatten_state::expect_lbracket;
+                                break;
+                            default:
+                                JSONCONS_THROW(jsonpath_error(jsonpath_errc::invalid_flattened_key));
+                                break;
+                        }
+                        break;
+                    }
+                    default:
+                        JSONCONS_UNREACHABLE();
+                        break;
+                }
+            }
+        }
+
+        return result;
+    }
+}}
+
+#endif