/*
    __ _____ _____ _____
 __|  |   __|     |   | |  JSON for Modern C++ (test suite)
|  |  |__   |  |  | | | |  version 3.8.0
|_____|_____|_____|_|___|  https://github.com/nlohmann/json

Licensed under the MIT License <http://opensource.org/licenses/MIT>.
SPDX-License-Identifier: MIT
Copyright (c) 2013-2019 Niels Lohmann <http://nlohmann.me>.

Permission is hereby  granted, free of charge, to any  person obtaining a copy
of this software and associated  documentation files (the "Software"), to deal
in the Software  without restriction, including without  limitation the rights
to  use, copy,  modify, merge,  publish, distribute,  sublicense, and/or  sell
copies  of  the Software,  and  to  permit persons  to  whom  the Software  is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE  IS PROVIDED "AS  IS", WITHOUT WARRANTY  OF ANY KIND,  EXPRESS OR
IMPLIED,  INCLUDING BUT  NOT  LIMITED TO  THE  WARRANTIES OF  MERCHANTABILITY,
FITNESS FOR  A PARTICULAR PURPOSE AND  NONINFRINGEMENT. IN NO EVENT  SHALL THE
AUTHORS  OR COPYRIGHT  HOLDERS  BE  LIABLE FOR  ANY  CLAIM,  DAMAGES OR  OTHER
LIABILITY, WHETHER IN AN ACTION OF  CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE  OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/

#include "doctest_compatibility.h"

// for some reason including this after the json header leads to linker errors with VS 2017...
#include <locale>

#define private public
#include <nlohmann/json.hpp>
using nlohmann::json;
#undef private

#include <fstream>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <test_data.hpp>

namespace
{
extern size_t calls;
size_t calls = 0;

void check_utf8dump(bool success_expected, int byte1, int byte2, int byte3, int byte4);

void check_utf8dump(bool success_expected, int byte1, int byte2 = -1, int byte3 = -1, int byte4 = -1)
{
    std::string json_string;

    CAPTURE(byte1)
    CAPTURE(byte2)
    CAPTURE(byte3)
    CAPTURE(byte4)

    json_string += std::string(1, static_cast<char>(byte1));

    if (byte2 != -1)
    {
        json_string += std::string(1, static_cast<char>(byte2));
    }

    if (byte3 != -1)
    {
        json_string += std::string(1, static_cast<char>(byte3));
    }

    if (byte4 != -1)
    {
        json_string += std::string(1, static_cast<char>(byte4));
    }

    CAPTURE(json_string)

    // store the string in a JSON value
    json j = json_string;
    json j2 = "abc" + json_string + "xyz";

    // dumping with ignore/replace must not throw in any case
    auto s_ignored = j.dump(-1, ' ', false, json::error_handler_t::ignore);
    auto s_ignored2 = j2.dump(-1, ' ', false, json::error_handler_t::ignore);
    auto s_ignored_ascii = j.dump(-1, ' ', true, json::error_handler_t::ignore);
    auto s_ignored2_ascii = j2.dump(-1, ' ', true, json::error_handler_t::ignore);
    auto s_replaced = j.dump(-1, ' ', false, json::error_handler_t::replace);
    auto s_replaced2 = j2.dump(-1, ' ', false, json::error_handler_t::replace);
    auto s_replaced_ascii = j.dump(-1, ' ', true, json::error_handler_t::replace);
    auto s_replaced2_ascii = j2.dump(-1, ' ', true, json::error_handler_t::replace);

    if (success_expected)
    {
        // strict mode must not throw if success is expected
        auto s_strict = j.dump();
        // all dumps should agree on the string
        CHECK(s_strict == s_ignored);
        CHECK(s_strict == s_replaced);
    }
    else
    {
        // strict mode must throw if success is not expected
        CHECK_THROWS_AS(j.dump(), json::type_error&);
        // ignore and replace must create different dumps
        CHECK(s_ignored != s_replaced);

        // check that replace string contains a replacement character
        CHECK(s_replaced.find("\xEF\xBF\xBD") != std::string::npos);
    }

    // check that prefix and suffix are preserved
    CHECK(s_ignored2.substr(1, 3) == "abc");
    CHECK(s_ignored2.substr(s_ignored2.size() - 4, 3) == "xyz");
    CHECK(s_ignored2_ascii.substr(1, 3) == "abc");
    CHECK(s_ignored2_ascii.substr(s_ignored2_ascii.size() - 4, 3) == "xyz");
    CHECK(s_replaced2.substr(1, 3) == "abc");
    CHECK(s_replaced2.substr(s_replaced2.size() - 4, 3) == "xyz");
    CHECK(s_replaced2_ascii.substr(1, 3) == "abc");
    CHECK(s_replaced2_ascii.substr(s_replaced2_ascii.size() - 4, 3) == "xyz");
}

void check_utf8string(bool success_expected, int byte1, int byte2, int byte3, int byte4);

// create and check a JSON string with up to four UTF-8 bytes
void check_utf8string(bool success_expected, int byte1, int byte2 = -1, int byte3 = -1, int byte4 = -1)
{
    if (++calls % 100000 == 0)
    {
        std::cout << calls << " of 8860608 UTF-8 strings checked" << std::endl;
    }

    std::string json_string = "\"";

    CAPTURE(byte1)
    json_string += std::string(1, static_cast<char>(byte1));

    if (byte2 != -1)
    {
        CAPTURE(byte2)
        json_string += std::string(1, static_cast<char>(byte2));
    }

    if (byte3 != -1)
    {
        CAPTURE(byte3)
        json_string += std::string(1, static_cast<char>(byte3));
    }

    if (byte4 != -1)
    {
        CAPTURE(byte4)
        json_string += std::string(1, static_cast<char>(byte4));
    }

    json_string += "\"";

    CAPTURE(json_string)

    json _;
    if (success_expected)
    {
        CHECK_NOTHROW(_ = json::parse(json_string));
    }
    else
    {
        CHECK_THROWS_AS(_ = json::parse(json_string), json::parse_error&);
    }
}
}

TEST_CASE("Unicode" * doctest::skip())
{
    SECTION("RFC 3629")
    {
        /*
        RFC 3629 describes in Sect. 4 the syntax of UTF-8 byte sequences as
        follows:

            A UTF-8 string is a sequence of octets representing a sequence of UCS
            characters.  An octet sequence is valid UTF-8 only if it matches the
            following syntax, which is derived from the rules for encoding UTF-8
            and is expressed in the ABNF of [RFC2234].

            UTF8-octets = *( UTF8-char )
            UTF8-char   = UTF8-1 / UTF8-2 / UTF8-3 / UTF8-4
            UTF8-1      = %x00-7F
            UTF8-2      = %xC2-DF UTF8-tail
            UTF8-3      = %xE0 %xA0-BF UTF8-tail / %xE1-EC 2( UTF8-tail ) /
                          %xED %x80-9F UTF8-tail / %xEE-EF 2( UTF8-tail )
            UTF8-4      = %xF0 %x90-BF 2( UTF8-tail ) / %xF1-F3 3( UTF8-tail ) /
                          %xF4 %x80-8F 2( UTF8-tail )
            UTF8-tail   = %x80-BF
        */

        SECTION("ill-formed first byte")
        {
            for (int byte1 = 0x80; byte1 <= 0xC1; ++byte1)
            {
                check_utf8string(false, byte1);
                check_utf8dump(false, byte1);
            }

            for (int byte1 = 0xF5; byte1 <= 0xFF; ++byte1)
            {
                check_utf8string(false, byte1);
                check_utf8dump(false, byte1);
            }
        }

        SECTION("UTF8-1 (x00-x7F)")
        {
            SECTION("well-formed")
            {
                for (int byte1 = 0x00; byte1 <= 0x7F; ++byte1)
                {
                    // unescaped control characters are parse errors in JSON
                    if (0x00 <= byte1 and byte1 <= 0x1F)
                    {
                        check_utf8string(false, byte1);
                        continue;
                    }

                    // a single quote is a parse error in JSON
                    if (byte1 == 0x22)
                    {
                        check_utf8string(false, byte1);
                        continue;
                    }

                    // a single backslash is a parse error in JSON
                    if (byte1 == 0x5C)
                    {
                        check_utf8string(false, byte1);
                        continue;
                    }

                    // all other characters are OK
                    check_utf8string(true, byte1);
                    check_utf8dump(true, byte1);
                }
            }
        }

        SECTION("UTF8-2 (xC2-xDF UTF8-tail)")
        {
            SECTION("well-formed")
            {
                for (int byte1 = 0xC2; byte1 <= 0xDF; ++byte1)
                {
                    for (int byte2 = 0x80; byte2 <= 0xBF; ++byte2)
                    {
                        check_utf8string(true, byte1, byte2);
                        check_utf8dump(true, byte1, byte2);
                    }
                }
            }

            SECTION("ill-formed: missing second byte")
            {
                for (int byte1 = 0xC2; byte1 <= 0xDF; ++byte1)
                {
                    check_utf8string(false, byte1);
                    check_utf8dump(false, byte1);
                }
            }

            SECTION("ill-formed: wrong second byte")
            {
                for (int byte1 = 0xC2; byte1 <= 0xDF; ++byte1)
                {
                    for (int byte2 = 0x00; byte2 <= 0xFF; ++byte2)
                    {
                        // skip correct second byte
                        if (0x80 <= byte2 and byte2 <= 0xBF)
                        {
                            continue;
                        }

                        check_utf8string(false, byte1, byte2);
                        check_utf8dump(false, byte1, byte2);
                    }
                }
            }
        }

        SECTION("UTF8-3 (xE0 xA0-BF UTF8-tail)")
        {
            SECTION("well-formed")
            {
                for (int byte1 = 0xE0; byte1 <= 0xE0; ++byte1)
                {
                    for (int byte2 = 0xA0; byte2 <= 0xBF; ++byte2)
                    {
                        for (int byte3 = 0x80; byte3 <= 0xBF; ++byte3)
                        {
                            check_utf8string(true, byte1, byte2, byte3);
                            check_utf8dump(true, byte1, byte2, byte3);
                        }
                    }
                }
            }

            SECTION("ill-formed: missing second byte")
            {
                for (int byte1 = 0xE0; byte1 <= 0xE0; ++byte1)
                {
                    check_utf8string(false, byte1);
                    check_utf8dump(false, byte1);
                }
            }

            SECTION("ill-formed: missing third byte")
            {
                for (int byte1 = 0xE0; byte1 <= 0xE0; ++byte1)
                {
                    for (int byte2 = 0xA0; byte2 <= 0xBF; ++byte2)
                    {
                        check_utf8string(false, byte1, byte2);
                        check_utf8dump(false, byte1, byte2);
                    }
                }
            }

            SECTION("ill-formed: wrong second byte")
            {
                for (int byte1 = 0xE0; byte1 <= 0xE0; ++byte1)
                {
                    for (int byte2 = 0x00; byte2 <= 0xFF; ++byte2)
                    {
                        // skip correct second byte
                        if (0xA0 <= byte2 and byte2 <= 0xBF)
                        {
                            continue;
                        }

                        for (int byte3 = 0x80; byte3 <= 0xBF; ++byte3)
                        {
                            check_utf8string(false, byte1, byte2, byte3);
                            check_utf8dump(false, byte1, byte2, byte3);
                        }
                    }
                }
            }

            SECTION("ill-formed: wrong third byte")
            {
                for (int byte1 = 0xE0; byte1 <= 0xE0; ++byte1)
                {
                    for (int byte2 = 0xA0; byte2 <= 0xBF; ++byte2)
                    {
                        for (int byte3 = 0x00; byte3 <= 0xFF; ++byte3)
                        {
                            // skip correct third byte
                            if (0x80 <= byte3 and byte3 <= 0xBF)
                            {
                                continue;
                            }

                            check_utf8string(false, byte1, byte2, byte3);
                            check_utf8dump(false, byte1, byte2, byte3);
                        }
                    }
                }
            }
        }

        SECTION("UTF8-3 (xE1-xEC UTF8-tail UTF8-tail)")
        {
            SECTION("well-formed")
            {
                for (int byte1 = 0xE1; byte1 <= 0xEC; ++byte1)
                {
                    for (int byte2 = 0x80; byte2 <= 0xBF; ++byte2)
                    {
                        for (int byte3 = 0x80; byte3 <= 0xBF; ++byte3)
                        {
                            check_utf8string(true, byte1, byte2, byte3);
                            check_utf8dump(true, byte1, byte2, byte3);
                        }
                    }
                }
            }

            SECTION("ill-formed: missing second byte")
            {
                for (int byte1 = 0xE1; byte1 <= 0xEC; ++byte1)
                {
                    check_utf8string(false, byte1);
                    check_utf8dump(false, byte1);
                }
            }

            SECTION("ill-formed: missing third byte")
            {
                for (int byte1 = 0xE1; byte1 <= 0xEC; ++byte1)
                {
                    for (int byte2 = 0x80; byte2 <= 0xBF; ++byte2)
                    {
                        check_utf8string(false, byte1, byte2);
                        check_utf8dump(false, byte1, byte2);
                    }
                }
            }

            SECTION("ill-formed: wrong second byte")
            {
                for (int byte1 = 0xE1; byte1 <= 0xEC; ++byte1)
                {
                    for (int byte2 = 0x00; byte2 <= 0xFF; ++byte2)
                    {
                        // skip correct second byte
                        if (0x80 <= byte2 and byte2 <= 0xBF)
                        {
                            continue;
                        }

                        for (int byte3 = 0x80; byte3 <= 0xBF; ++byte3)
                        {
                            check_utf8string(false, byte1, byte2, byte3);
                            check_utf8dump(false, byte1, byte2, byte3);
                        }
                    }
                }
            }

            SECTION("ill-formed: wrong third byte")
            {
                for (int byte1 = 0xE1; byte1 <= 0xEC; ++byte1)
                {
                    for (int byte2 = 0x80; byte2 <= 0xBF; ++byte2)
                    {
                        for (int byte3 = 0x00; byte3 <= 0xFF; ++byte3)
                        {
                            // skip correct third byte
                            if (0x80 <= byte3 and byte3 <= 0xBF)
                            {
                                continue;
                            }

                            check_utf8string(false, byte1, byte2, byte3);
                            check_utf8dump(false, byte1, byte2, byte3);
                        }
                    }
                }
            }
        }

        SECTION("UTF8-3 (xED x80-9F UTF8-tail)")
        {
            SECTION("well-formed")
            {
                for (int byte1 = 0xED; byte1 <= 0xED; ++byte1)
                {
                    for (int byte2 = 0x80; byte2 <= 0x9F; ++byte2)
                    {
                        for (int byte3 = 0x80; byte3 <= 0xBF; ++byte3)
                        {
                            check_utf8string(true, byte1, byte2, byte3);
                            check_utf8dump(true, byte1, byte2, byte3);
                        }
                    }
                }
            }

            SECTION("ill-formed: missing second byte")
            {
                for (int byte1 = 0xED; byte1 <= 0xED; ++byte1)
                {
                    check_utf8string(false, byte1);
                    check_utf8dump(false, byte1);
                }
            }

            SECTION("ill-formed: missing third byte")
            {
                for (int byte1 = 0xED; byte1 <= 0xED; ++byte1)
                {
                    for (int byte2 = 0x80; byte2 <= 0x9F; ++byte2)
                    {
                        check_utf8string(false, byte1, byte2);
                        check_utf8dump(false, byte1, byte2);
                    }
                }
            }

            SECTION("ill-formed: wrong second byte")
            {
                for (int byte1 = 0xED; byte1 <= 0xED; ++byte1)
                {
                    for (int byte2 = 0x00; byte2 <= 0xFF; ++byte2)
                    {
                        // skip correct second byte
                        if (0x80 <= byte2 and byte2 <= 0x9F)
                        {
                            continue;
                        }

                        for (int byte3 = 0x80; byte3 <= 0xBF; ++byte3)
                        {
                            check_utf8string(false, byte1, byte2, byte3);
                            check_utf8dump(false, byte1, byte2, byte3);
                        }
                    }
                }
            }

            SECTION("ill-formed: wrong third byte")
            {
                for (int byte1 = 0xED; byte1 <= 0xED; ++byte1)
                {
                    for (int byte2 = 0x80; byte2 <= 0x9F; ++byte2)
                    {
                        for (int byte3 = 0x00; byte3 <= 0xFF; ++byte3)
                        {
                            // skip correct third byte
                            if (0x80 <= byte3 and byte3 <= 0xBF)
                            {
                                continue;
                            }

                            check_utf8string(false, byte1, byte2, byte3);
                            check_utf8dump(false, byte1, byte2, byte3);
                        }
                    }
                }
            }
        }

        SECTION("UTF8-3 (xEE-xEF UTF8-tail UTF8-tail)")
        {
            SECTION("well-formed")
            {
                for (int byte1 = 0xEE; byte1 <= 0xEF; ++byte1)
                {
                    for (int byte2 = 0x80; byte2 <= 0xBF; ++byte2)
                    {
                        for (int byte3 = 0x80; byte3 <= 0xBF; ++byte3)
                        {
                            check_utf8string(true, byte1, byte2, byte3);
                            check_utf8dump(true, byte1, byte2, byte3);
                        }
                    }
                }
            }

            SECTION("ill-formed: missing second byte")
            {
                for (int byte1 = 0xEE; byte1 <= 0xEF; ++byte1)
                {
                    check_utf8string(false, byte1);
                    check_utf8dump(false, byte1);
                }
            }

            SECTION("ill-formed: missing third byte")
            {
                for (int byte1 = 0xEE; byte1 <= 0xEF; ++byte1)
                {
                    for (int byte2 = 0x80; byte2 <= 0xBF; ++byte2)
                    {
                        check_utf8string(false, byte1, byte2);
                        check_utf8dump(false, byte1, byte2);
                    }
                }
            }

            SECTION("ill-formed: wrong second byte")
            {
                for (int byte1 = 0xEE; byte1 <= 0xEF; ++byte1)
                {
                    for (int byte2 = 0x00; byte2 <= 0xFF; ++byte2)
                    {
                        // skip correct second byte
                        if (0x80 <= byte2 and byte2 <= 0xBF)
                        {
                            continue;
                        }

                        for (int byte3 = 0x80; byte3 <= 0xBF; ++byte3)
                        {
                            check_utf8string(false, byte1, byte2, byte3);
                            check_utf8dump(false, byte1, byte2, byte3);
                        }
                    }
                }
            }

            SECTION("ill-formed: wrong third byte")
            {
                for (int byte1 = 0xEE; byte1 <= 0xEF; ++byte1)
                {
                    for (int byte2 = 0x80; byte2 <= 0xBF; ++byte2)
                    {
                        for (int byte3 = 0x00; byte3 <= 0xFF; ++byte3)
                        {
                            // skip correct third byte
                            if (0x80 <= byte3 and byte3 <= 0xBF)
                            {
                                continue;
                            }

                            check_utf8string(false, byte1, byte2, byte3);
                            check_utf8dump(false, byte1, byte2, byte3);
                        }
                    }
                }
            }
        }

        SECTION("UTF8-4 (xF0 x90-BF UTF8-tail UTF8-tail)")
        {
            SECTION("well-formed")
            {
                for (int byte1 = 0xF0; byte1 <= 0xF0; ++byte1)
                {
                    for (int byte2 = 0x90; byte2 <= 0xBF; ++byte2)
                    {
                        for (int byte3 = 0x80; byte3 <= 0xBF; ++byte3)
                        {
                            for (int byte4 = 0x80; byte4 <= 0xBF; ++byte4)
                            {
                                check_utf8string(true, byte1, byte2, byte3, byte4);
                                check_utf8dump(true, byte1, byte2, byte3, byte4);
                            }
                        }
                    }
                }
            }

            SECTION("ill-formed: missing second byte")
            {
                for (int byte1 = 0xF0; byte1 <= 0xF0; ++byte1)
                {
                    check_utf8string(false, byte1);
                    check_utf8dump(false, byte1);
                }
            }

            SECTION("ill-formed: missing third byte")
            {
                for (int byte1 = 0xF0; byte1 <= 0xF0; ++byte1)
                {
                    for (int byte2 = 0x90; byte2 <= 0xBF; ++byte2)
                    {
                        check_utf8string(false, byte1, byte2);
                        check_utf8dump(false, byte1, byte2);
                    }
                }
            }

            SECTION("ill-formed: missing fourth byte")
            {
                for (int byte1 = 0xF0; byte1 <= 0xF0; ++byte1)
                {
                    for (int byte2 = 0x90; byte2 <= 0xBF; ++byte2)
                    {
                        for (int byte3 = 0x80; byte3 <= 0xBF; ++byte3)
                        {
                            check_utf8string(false, byte1, byte2, byte3);
                            check_utf8dump(false, byte1, byte2, byte3);
                        }
                    }
                }
            }

            SECTION("ill-formed: wrong second byte")
            {
                for (int byte1 = 0xF0; byte1 <= 0xF0; ++byte1)
                {
                    for (int byte2 = 0x00; byte2 <= 0xFF; ++byte2)
                    {
                        // skip correct second byte
                        if (0x90 <= byte2 and byte2 <= 0xBF)
                        {
                            continue;
                        }

                        for (int byte3 = 0x80; byte3 <= 0xBF; ++byte3)
                        {
                            for (int byte4 = 0x80; byte4 <= 0xBF; ++byte4)
                            {
                                check_utf8string(false, byte1, byte2, byte3, byte4);
                                check_utf8dump(false, byte1, byte2, byte3, byte4);
                            }
                        }
                    }
                }
            }

            SECTION("ill-formed: wrong third byte")
            {
                for (int byte1 = 0xF0; byte1 <= 0xF0; ++byte1)
                {
                    for (int byte2 = 0x90; byte2 <= 0xBF; ++byte2)
                    {
                        for (int byte3 = 0x00; byte3 <= 0xFF; ++byte3)
                        {
                            // skip correct third byte
                            if (0x80 <= byte3 and byte3 <= 0xBF)
                            {
                                continue;
                            }

                            for (int byte4 = 0x80; byte4 <= 0xBF; ++byte4)
                            {
                                check_utf8string(false, byte1, byte2, byte3, byte4);
                                check_utf8dump(false, byte1, byte2, byte3, byte4);
                            }
                        }
                    }
                }
            }

            SECTION("ill-formed: wrong fourth byte")
            {
                for (int byte1 = 0xF0; byte1 <= 0xF0; ++byte1)
                {
                    for (int byte2 = 0x90; byte2 <= 0xBF; ++byte2)
                    {
                        for (int byte3 = 0x80; byte3 <= 0xBF; ++byte3)
                        {
                            for (int byte4 = 0x00; byte4 <= 0xFF; ++byte4)
                            {
                                // skip fourth second byte
                                if (0x80 <= byte3 and byte3 <= 0xBF)
                                {
                                    continue;
                                }

                                check_utf8string(false, byte1, byte2, byte3, byte4);
                                check_utf8dump(false, byte1, byte2, byte3, byte4);
                            }
                        }
                    }
                }
            }
        }

        SECTION("UTF8-4 (xF1-F3 UTF8-tail UTF8-tail UTF8-tail)")
        {
            SECTION("well-formed")
            {
                for (int byte1 = 0xF1; byte1 <= 0xF3; ++byte1)
                {
                    for (int byte2 = 0x80; byte2 <= 0xBF; ++byte2)
                    {
                        for (int byte3 = 0x80; byte3 <= 0xBF; ++byte3)
                        {
                            for (int byte4 = 0x80; byte4 <= 0xBF; ++byte4)
                            {
                                check_utf8string(true, byte1, byte2, byte3, byte4);
                                check_utf8dump(true, byte1, byte2, byte3, byte4);
                            }
                        }
                    }
                }
            }

            SECTION("ill-formed: missing second byte")
            {
                for (int byte1 = 0xF1; byte1 <= 0xF3; ++byte1)
                {
                    check_utf8string(false, byte1);
                    check_utf8dump(false, byte1);
                }
            }

            SECTION("ill-formed: missing third byte")
            {
                for (int byte1 = 0xF1; byte1 <= 0xF3; ++byte1)
                {
                    for (int byte2 = 0x80; byte2 <= 0xBF; ++byte2)
                    {
                        check_utf8string(false, byte1, byte2);
                        check_utf8dump(false, byte1, byte2);
                    }
                }
            }

            SECTION("ill-formed: missing fourth byte")
            {
                for (int byte1 = 0xF1; byte1 <= 0xF3; ++byte1)
                {
                    for (int byte2 = 0x80; byte2 <= 0xBF; ++byte2)
                    {
                        for (int byte3 = 0x80; byte3 <= 0xBF; ++byte3)
                        {
                            check_utf8string(false, byte1, byte2, byte3);
                            check_utf8dump(false, byte1, byte2, byte3);
                        }
                    }
                }
            }

            SECTION("ill-formed: wrong second byte")
            {
                for (int byte1 = 0xF1; byte1 <= 0xF3; ++byte1)
                {
                    for (int byte2 = 0x00; byte2 <= 0xFF; ++byte2)
                    {
                        // skip correct second byte
                        if (0x80 <= byte2 and byte2 <= 0xBF)
                        {
                            continue;
                        }

                        for (int byte3 = 0x80; byte3 <= 0xBF; ++byte3)
                        {
                            for (int byte4 = 0x80; byte4 <= 0xBF; ++byte4)
                            {
                                check_utf8string(false, byte1, byte2, byte3, byte4);
                                check_utf8dump(false, byte1, byte2, byte3, byte4);
                            }
                        }
                    }
                }
            }

            SECTION("ill-formed: wrong third byte")
            {
                for (int byte1 = 0xF1; byte1 <= 0xF3; ++byte1)
                {
                    for (int byte2 = 0x80; byte2 <= 0xBF; ++byte2)
                    {
                        for (int byte3 = 0x00; byte3 <= 0xFF; ++byte3)
                        {
                            // skip correct third byte
                            if (0x80 <= byte3 and byte3 <= 0xBF)
                            {
                                continue;
                            }

                            for (int byte4 = 0x80; byte4 <= 0xBF; ++byte4)
                            {
                                check_utf8string(false, byte1, byte2, byte3, byte4);
                                check_utf8dump(false, byte1, byte2, byte3, byte4);
                            }
                        }
                    }
                }
            }

            SECTION("ill-formed: wrong fourth byte")
            {
                for (int byte1 = 0xF1; byte1 <= 0xF3; ++byte1)
                {
                    for (int byte2 = 0x80; byte2 <= 0xBF; ++byte2)
                    {
                        for (int byte3 = 0x80; byte3 <= 0xBF; ++byte3)
                        {
                            for (int byte4 = 0x00; byte4 <= 0xFF; ++byte4)
                            {
                                // skip correct fourth byte
                                if (0x80 <= byte3 and byte3 <= 0xBF)
                                {
                                    continue;
                                }

                                check_utf8string(false, byte1, byte2, byte3, byte4);
                                check_utf8dump(false, byte1, byte2, byte3, byte4);
                            }
                        }
                    }
                }
            }
        }

        SECTION("UTF8-4 (xF4 x80-8F UTF8-tail UTF8-tail)")
        {
            SECTION("well-formed")
            {
                for (int byte1 = 0xF4; byte1 <= 0xF4; ++byte1)
                {
                    for (int byte2 = 0x80; byte2 <= 0x8F; ++byte2)
                    {
                        for (int byte3 = 0x80; byte3 <= 0xBF; ++byte3)
                        {
                            for (int byte4 = 0x80; byte4 <= 0xBF; ++byte4)
                            {
                                check_utf8string(true, byte1, byte2, byte3, byte4);
                                check_utf8dump(true, byte1, byte2, byte3, byte4);
                            }
                        }
                    }
                }
            }

            SECTION("ill-formed: missing second byte")
            {
                for (int byte1 = 0xF4; byte1 <= 0xF4; ++byte1)
                {
                    check_utf8string(false, byte1);
                    check_utf8dump(false, byte1);
                }
            }

            SECTION("ill-formed: missing third byte")
            {
                for (int byte1 = 0xF4; byte1 <= 0xF4; ++byte1)
                {
                    for (int byte2 = 0x80; byte2 <= 0x8F; ++byte2)
                    {
                        check_utf8string(false, byte1, byte2);
                        check_utf8dump(false, byte1, byte2);
                    }
                }
            }

            SECTION("ill-formed: missing fourth byte")
            {
                for (int byte1 = 0xF4; byte1 <= 0xF4; ++byte1)
                {
                    for (int byte2 = 0x80; byte2 <= 0x8F; ++byte2)
                    {
                        for (int byte3 = 0x80; byte3 <= 0xBF; ++byte3)
                        {
                            check_utf8string(false, byte1, byte2, byte3);
                            check_utf8dump(false, byte1, byte2, byte3);
                        }
                    }
                }
            }

            SECTION("ill-formed: wrong second byte")
            {
                for (int byte1 = 0xF4; byte1 <= 0xF4; ++byte1)
                {
                    for (int byte2 = 0x00; byte2 <= 0xFF; ++byte2)
                    {
                        // skip correct second byte
                        if (0x80 <= byte2 and byte2 <= 0x8F)
                        {
                            continue;
                        }

                        for (int byte3 = 0x80; byte3 <= 0xBF; ++byte3)
                        {
                            for (int byte4 = 0x80; byte4 <= 0xBF; ++byte4)
                            {
                                check_utf8string(false, byte1, byte2, byte3, byte4);
                                check_utf8dump(false, byte1, byte2, byte3, byte4);
                            }
                        }
                    }
                }
            }

            SECTION("ill-formed: wrong third byte")
            {
                for (int byte1 = 0xF4; byte1 <= 0xF4; ++byte1)
                {
                    for (int byte2 = 0x80; byte2 <= 0x8F; ++byte2)
                    {
                        for (int byte3 = 0x00; byte3 <= 0xFF; ++byte3)
                        {
                            // skip correct third byte
                            if (0x80 <= byte3 and byte3 <= 0xBF)
                            {
                                continue;
                            }

                            for (int byte4 = 0x80; byte4 <= 0xBF; ++byte4)
                            {
                                check_utf8string(false, byte1, byte2, byte3, byte4);
                                check_utf8dump(false, byte1, byte2, byte3, byte4);
                            }
                        }
                    }
                }
            }

            SECTION("ill-formed: wrong fourth byte")
            {
                for (int byte1 = 0xF4; byte1 <= 0xF4; ++byte1)
                {
                    for (int byte2 = 0x80; byte2 <= 0x8F; ++byte2)
                    {
                        for (int byte3 = 0x80; byte3 <= 0xBF; ++byte3)
                        {
                            for (int byte4 = 0x00; byte4 <= 0xFF; ++byte4)
                            {
                                // skip correct fourth byte
                                if (0x80 <= byte3 and byte3 <= 0xBF)
                                {
                                    continue;
                                }

                                check_utf8string(false, byte1, byte2, byte3, byte4);
                                check_utf8dump(false, byte1, byte2, byte3, byte4);
                            }
                        }
                    }
                }
            }
        }
    }

    SECTION("\\uxxxx sequences")
    {
        // create an escaped string from a code point
        const auto codepoint_to_unicode = [](std::size_t cp)
        {
            // code points are represented as a six-character sequence: a
            // reverse solidus, followed by the lowercase letter u, followed
            // by four hexadecimal digits that encode the character's code
            // point
            std::stringstream ss;
            ss << "\\u" << std::setw(4) << std::setfill('0') << std::hex << cp;
            return ss.str();
        };

        SECTION("correct sequences")
        {
            // generate all UTF-8 code points; in total, 1112064 code points are
            // generated: 0x1FFFFF code points - 2048 invalid values between
            // 0xD800 and 0xDFFF.
            for (std::size_t cp = 0; cp <= 0x10FFFFu; ++cp)
            {
                // string to store the code point as in \uxxxx format
                std::string json_text = "\"";

                // decide whether to use one or two \uxxxx sequences
                if (cp < 0x10000u)
                {
                    // The Unicode standard permanently reserves these code point
                    // values for UTF-16 encoding of the high and low surrogates, and
                    // they will never be assigned a character, so there should be no
                    // reason to encode them. The official Unicode standard says that
                    // no UTF forms, including UTF-16, can encode these code points.
                    if (cp >= 0xD800u and cp <= 0xDFFFu)
                    {
                        // if we would not skip these code points, we would get a
                        // "missing low surrogate" exception
                        continue;
                    }

                    // code points in the Basic Multilingual Plane can be
                    // represented with one \uxxxx sequence
                    json_text += codepoint_to_unicode(cp);
                }
                else
                {
                    // To escape an extended character that is not in the Basic
                    // Multilingual Plane, the character is represented as a
                    // 12-character sequence, encoding the UTF-16 surrogate pair
                    const auto codepoint1 = 0xd800u + (((cp - 0x10000u) >> 10) & 0x3ffu);
                    const auto codepoint2 = 0xdc00u + ((cp - 0x10000u) & 0x3ffu);
                    json_text += codepoint_to_unicode(codepoint1) + codepoint_to_unicode(codepoint2);
                }

                json_text += "\"";
                CAPTURE(json_text)
                json _;
                CHECK_NOTHROW(_ = json::parse(json_text));
            }
        }

        SECTION("incorrect sequences")
        {
            SECTION("incorrect surrogate values")
            {
                json _;

                CHECK_THROWS_AS(_ = json::parse("\"\\uDC00\\uDC00\""), json::parse_error&);
                CHECK_THROWS_WITH(_ = json::parse("\"\\uDC00\\uDC00\""),
                                  "[json.exception.parse_error.101] parse error at line 1, column 7: syntax error while parsing value - invalid string: surrogate U+DC00..U+DFFF must follow U+D800..U+DBFF; last read: '\"\\uDC00'");

                CHECK_THROWS_AS(_ = json::parse("\"\\uD7FF\\uDC00\""), json::parse_error&);
                CHECK_THROWS_WITH(_ = json::parse("\"\\uD7FF\\uDC00\""),
                                  "[json.exception.parse_error.101] parse error at line 1, column 13: syntax error while parsing value - invalid string: surrogate U+DC00..U+DFFF must follow U+D800..U+DBFF; last read: '\"\\uD7FF\\uDC00'");

                CHECK_THROWS_AS(_ = json::parse("\"\\uD800]\""), json::parse_error&);
                CHECK_THROWS_WITH(_ = json::parse("\"\\uD800]\""),
                                  "[json.exception.parse_error.101] parse error at line 1, column 8: syntax error while parsing value - invalid string: surrogate U+D800..U+DBFF must be followed by U+DC00..U+DFFF; last read: '\"\\uD800]'");

                CHECK_THROWS_AS(_ = json::parse("\"\\uD800\\v\""), json::parse_error&);
                CHECK_THROWS_WITH(_ = json::parse("\"\\uD800\\v\""),
                                  "[json.exception.parse_error.101] parse error at line 1, column 9: syntax error while parsing value - invalid string: surrogate U+D800..U+DBFF must be followed by U+DC00..U+DFFF; last read: '\"\\uD800\\v'");

                CHECK_THROWS_AS(_ = json::parse("\"\\uD800\\u123\""), json::parse_error&);
                CHECK_THROWS_WITH(_ = json::parse("\"\\uD800\\u123\""),
                                  "[json.exception.parse_error.101] parse error at line 1, column 13: syntax error while parsing value - invalid string: '\\u' must be followed by 4 hex digits; last read: '\"\\uD800\\u123\"'");

                CHECK_THROWS_AS(_ = json::parse("\"\\uD800\\uDBFF\""), json::parse_error&);
                CHECK_THROWS_WITH(_ = json::parse("\"\\uD800\\uDBFF\""),
                                  "[json.exception.parse_error.101] parse error at line 1, column 13: syntax error while parsing value - invalid string: surrogate U+D800..U+DBFF must be followed by U+DC00..U+DFFF; last read: '\"\\uD800\\uDBFF'");

                CHECK_THROWS_AS(_ = json::parse("\"\\uD800\\uE000\""), json::parse_error&);
                CHECK_THROWS_WITH(_ = json::parse("\"\\uD800\\uE000\""),
                                  "[json.exception.parse_error.101] parse error at line 1, column 13: syntax error while parsing value - invalid string: surrogate U+D800..U+DBFF must be followed by U+DC00..U+DFFF; last read: '\"\\uD800\\uE000'");
            }
        }

#if 0
        SECTION("incorrect sequences")
        {
            SECTION("high surrogate without low surrogate")
            {
                // D800..DBFF are high surrogates and must be followed by low
                // surrogates DC00..DFFF; here, nothing follows
                for (std::size_t cp = 0xD800u; cp <= 0xDBFFu; ++cp)
                {
                    std::string json_text = "\"" + codepoint_to_unicode(cp) + "\"";
                    CAPTURE(json_text)
                    CHECK_THROWS_AS(json::parse(json_text), json::parse_error&);
                }
            }

            SECTION("high surrogate with wrong low surrogate")
            {
                // D800..DBFF are high surrogates and must be followed by low
                // surrogates DC00..DFFF; here a different sequence follows
                for (std::size_t cp1 = 0xD800u; cp1 <= 0xDBFFu; ++cp1)
                {
                    for (std::size_t cp2 = 0x0000u; cp2 <= 0xFFFFu; ++cp2)
                    {
                        if (0xDC00u <= cp2 and cp2 <= 0xDFFFu)
                        {
                            continue;
                        }

                        std::string json_text = "\"" + codepoint_to_unicode(cp1) + codepoint_to_unicode(cp2) + "\"";
                        CAPTURE(json_text)
                        CHECK_THROWS_AS(json::parse(json_text), json::parse_error&);
                    }
                }
            }

            SECTION("low surrogate without high surrogate")
            {
                // low surrogates DC00..DFFF must follow high surrogates; here,
                // they occur alone
                for (std::size_t cp = 0xDC00u; cp <= 0xDFFFu; ++cp)
                {
                    std::string json_text = "\"" + codepoint_to_unicode(cp) + "\"";
                    CAPTURE(json_text)
                    CHECK_THROWS_AS(json::parse(json_text), json::parse_error&);
                }
            }

        }
#endif
    }

    SECTION("read all unicode characters")
    {
        // read a file with all unicode characters stored as single-character
        // strings in a JSON array
        std::ifstream f(TEST_DATA_DIRECTORY "/json_nlohmann_tests/all_unicode.json");
        json j;
        CHECK_NOTHROW(f >> j);

        // the array has 1112064 + 1 elements (a terminating "null" value)
        // Note: 1112064 = 0x1FFFFF code points - 2048 invalid values between
        // 0xD800 and 0xDFFF.
        CHECK(j.size() == 1112065);

        SECTION("check JSON Pointers")
        {
            for (auto s : j)
            {
                // skip non-string JSON values
                if (not s.is_string())
                {
                    continue;
                }

                std::string ptr = s;

                // tilde must be followed by 0 or 1
                if (ptr == "~")
                {
                    ptr += "0";
                }

                // JSON Pointers must begin with "/"
                ptr = "/" + ptr;

                CHECK_NOTHROW(json::json_pointer("/" + ptr));

                // check escape/unescape roundtrip
                auto escaped = json::json_pointer::escape(ptr);
                json::json_pointer::unescape(escaped);
                CHECK(escaped == ptr);
            }
        }
    }

    SECTION("ignore byte-order-mark")
    {
        SECTION("in a stream")
        {
            // read a file with a UTF-8 BOM
            std::ifstream f(TEST_DATA_DIRECTORY "/json_nlohmann_tests/bom.json");
            json j;
            CHECK_NOTHROW(f >> j);
        }

        SECTION("with an iterator")
        {
            std::string i = "\xef\xbb\xbf{\n   \"foo\": true\n}";
            CHECK_NOTHROW(json::parse(i.begin(), i.end()));
        }
    }

    SECTION("error for incomplete/wrong BOM")
    {
        json _;
        CHECK_THROWS_AS(_ = json::parse("\xef\xbb"), json::parse_error&);
        CHECK_THROWS_AS(_ = json::parse("\xef\xbb\xbb"), json::parse_error&);
    }
}

namespace
{
void roundtrip(bool success_expected, const std::string& s);

void roundtrip(bool success_expected, const std::string& s)
{
    CAPTURE(s)
    json _;

    // create JSON string value
    json j = s;
    // create JSON text
    std::string ps = std::string("\"") + s + "\"";

    if (success_expected)
    {
        // serialization succeeds
        CHECK_NOTHROW(j.dump());

        // exclude parse test for U+0000
        if (s[0] != '\0')
        {
            // parsing JSON text succeeds
            CHECK_NOTHROW(_ = json::parse(ps));
        }

        // roundtrip succeeds
        CHECK_NOTHROW(_ = json::parse(j.dump()));

        // after roundtrip, the same string is stored
        json jr = json::parse(j.dump());
        CHECK(jr.get<std::string>() == s);
    }
    else
    {
        // serialization fails
        CHECK_THROWS_AS(j.dump(), json::type_error&);

        // parsing JSON text fails
        CHECK_THROWS_AS(_ = json::parse(ps), json::parse_error&);
    }
}
}

TEST_CASE("Markus Kuhn's UTF-8 decoder capability and stress test")
{
    // Markus Kuhn <http://www.cl.cam.ac.uk/~mgk25/> - 2015-08-28 - CC BY 4.0
    // http://www.cl.cam.ac.uk/~mgk25/ucs/examples/UTF-8-test.txt

    SECTION("1  Some correct UTF-8 text")
    {
        roundtrip(true, "κόσμε");
    }

    SECTION("2  Boundary condition test cases")
    {
        SECTION("2.1  First possible sequence of a certain length")
        {
            // 2.1.1  1 byte  (U-00000000)
            roundtrip(true, std::string("\0", 1));
            // 2.1.2  2 bytes (U-00000080)
            roundtrip(true, "\xc2\x80");
            // 2.1.3  3 bytes (U-00000800)
            roundtrip(true, "\xe0\xa0\x80");
            // 2.1.4  4 bytes (U-00010000)
            roundtrip(true, "\xf0\x90\x80\x80");

            // 2.1.5  5 bytes (U-00200000)
            roundtrip(false, "\xF8\x88\x80\x80\x80");
            // 2.1.6  6 bytes (U-04000000)
            roundtrip(false, "\xFC\x84\x80\x80\x80\x80");
        }

        SECTION("2.2  Last possible sequence of a certain length")
        {
            // 2.2.1  1 byte  (U-0000007F)
            roundtrip(true, "\x7f");
            // 2.2.2  2 bytes (U-000007FF)
            roundtrip(true, "\xdf\xbf");
            // 2.2.3  3 bytes (U-0000FFFF)
            roundtrip(true, "\xef\xbf\xbf");

            // 2.2.4  4 bytes (U-001FFFFF)
            roundtrip(false, "\xF7\xBF\xBF\xBF");
            // 2.2.5  5 bytes (U-03FFFFFF)
            roundtrip(false, "\xFB\xBF\xBF\xBF\xBF");
            // 2.2.6  6 bytes (U-7FFFFFFF)
            roundtrip(false, "\xFD\xBF\xBF\xBF\xBF\xBF");
        }

        SECTION("2.3  Other boundary conditions")
        {
            // 2.3.1  U-0000D7FF = ed 9f bf
            roundtrip(true, "\xed\x9f\xbf");
            // 2.3.2  U-0000E000 = ee 80 80
            roundtrip(true, "\xee\x80\x80");
            // 2.3.3  U-0000FFFD = ef bf bd
            roundtrip(true, "\xef\xbf\xbd");
            // 2.3.4  U-0010FFFF = f4 8f bf bf
            roundtrip(true, "\xf4\x8f\xbf\xbf");

            // 2.3.5  U-00110000 = f4 90 80 80
            roundtrip(false, "\xf4\x90\x80\x80");
        }
    }

    SECTION("3  Malformed sequences")
    {
        SECTION("3.1  Unexpected continuation bytes")
        {
            // Each unexpected continuation byte should be separately signalled as a
            // malformed sequence of its own.

            // 3.1.1  First continuation byte 0x80
            roundtrip(false, "\x80");
            // 3.1.2  Last  continuation byte 0xbf
            roundtrip(false, "\xbf");

            // 3.1.3  2 continuation bytes
            roundtrip(false, "\x80\xbf");
            // 3.1.4  3 continuation bytes
            roundtrip(false, "\x80\xbf\x80");
            // 3.1.5  4 continuation bytes
            roundtrip(false, "\x80\xbf\x80\xbf");
            // 3.1.6  5 continuation bytes
            roundtrip(false, "\x80\xbf\x80\xbf\x80");
            // 3.1.7  6 continuation bytes
            roundtrip(false, "\x80\xbf\x80\xbf\x80\xbf");
            // 3.1.8  7 continuation bytes
            roundtrip(false, "\x80\xbf\x80\xbf\x80\xbf\x80");

            // 3.1.9  Sequence of all 64 possible continuation bytes (0x80-0xbf)
            roundtrip(false, "\x80\x81\x82\x83\x84\x85\x86\x87\x88\x89\x8a\x8b\x8c\x8d\x8e\x8f\x90\x91\x92\x93\x94\x95\x96\x97\x98\x99\x9a\x9b\x9c\x9d\x9e\x9f\xa0\xa1\xa2\xa3\xa4\xa5\xa6\xa7\xa8\xa9\xaa\xab\xac\xad\xae\xaf\xb0\xb1\xb2\xb3\xb4\xb5\xb6\xb7\xb8\xb9\xba\xbb\xbc\xbd\xbe\xbf");
        }

        SECTION("3.2  Lonely start characters")
        {
            // 3.2.1  All 32 first bytes of 2-byte sequences (0xc0-0xdf)
            roundtrip(false, "\xc0 \xc1 \xc2 \xc3 \xc4 \xc5 \xc6 \xc7 \xc8 \xc9 \xca \xcb \xcc \xcd \xce \xcf \xd0 \xd1 \xd2 \xd3 \xd4 \xd5 \xd6 \xd7 \xd8 \xd9 \xda \xdb \xdc \xdd \xde \xdf");
            // 3.2.2  All 16 first bytes of 3-byte sequences (0xe0-0xef)
            roundtrip(false, "\xe0 \xe1 \xe2 \xe3 \xe4 \xe5 \xe6 \xe7 \xe8 \xe9 \xea \xeb \xec \xed \xee \xef");
            // 3.2.3  All 8 first bytes of 4-byte sequences (0xf0-0xf7)
            roundtrip(false, "\xf0 \xf1 \xf2 \xf3 \xf4 \xf5 \xf6 \xf7");
            // 3.2.4  All 4 first bytes of 5-byte sequences (0xf8-0xfb)
            roundtrip(false, "\xf8 \xf9 \xfa \xfb");
            // 3.2.5  All 2 first bytes of 6-byte sequences (0xfc-0xfd)
            roundtrip(false, "\xfc \xfd");
        }

        SECTION("3.3  Sequences with last continuation byte missing")
        {
            // All bytes of an incomplete sequence should be signalled as a single
            // malformed sequence, i.e., you should see only a single replacement
            // character in each of the next 10 tests. (Characters as in section 2)

            // 3.3.1  2-byte sequence with last byte missing (U+0000)
            roundtrip(false, "\xc0");
            // 3.3.2  3-byte sequence with last byte missing (U+0000)
            roundtrip(false, "\xe0\x80");
            // 3.3.3  4-byte sequence with last byte missing (U+0000)
            roundtrip(false, "\xf0\x80\x80");
            // 3.3.4  5-byte sequence with last byte missing (U+0000)
            roundtrip(false, "\xf8\x80\x80\x80");
            // 3.3.5  6-byte sequence with last byte missing (U+0000)
            roundtrip(false, "\xfc\x80\x80\x80\x80");
            // 3.3.6  2-byte sequence with last byte missing (U-000007FF)
            roundtrip(false, "\xdf");
            // 3.3.7  3-byte sequence with last byte missing (U-0000FFFF)
            roundtrip(false, "\xef\xbf");
            // 3.3.8  4-byte sequence with last byte missing (U-001FFFFF)
            roundtrip(false, "\xf7\xbf\xbf");
            // 3.3.9  5-byte sequence with last byte missing (U-03FFFFFF)
            roundtrip(false, "\xfb\xbf\xbf\xbf");
            // 3.3.10 6-byte sequence with last byte missing (U-7FFFFFFF)
            roundtrip(false, "\xfd\xbf\xbf\xbf\xbf");
        }

        SECTION("3.4  Concatenation of incomplete sequences")
        {
            // All the 10 sequences of 3.3 concatenated, you should see 10 malformed
            // sequences being signalled:
            roundtrip(false, "\xc0\xe0\x80\xf0\x80\x80\xf8\x80\x80\x80\xfc\x80\x80\x80\x80\xdf\xef\xbf\xf7\xbf\xbf\xfb\xbf\xbf\xbf\xfd\xbf\xbf\xbf\xbf");
        }

        SECTION("3.5  Impossible bytes")
        {
            // The following two bytes cannot appear in a correct UTF-8 string

            // 3.5.1  fe
            roundtrip(false, "\xfe");
            // 3.5.2  ff
            roundtrip(false, "\xff");
            // 3.5.3  fe fe ff ff
            roundtrip(false, "\xfe\xfe\xff\xff");
        }
    }

    SECTION("4  Overlong sequences")
    {
        // The following sequences are not malformed according to the letter of
        // the Unicode 2.0 standard. However, they are longer then necessary and
        // a correct UTF-8 encoder is not allowed to produce them. A "safe UTF-8
        // decoder" should reject them just like malformed sequences for two
        // reasons: (1) It helps to debug applications if overlong sequences are
        // not treated as valid representations of characters, because this helps
        // to spot problems more quickly. (2) Overlong sequences provide
        // alternative representations of characters, that could maliciously be
        // used to bypass filters that check only for ASCII characters. For
        // instance, a 2-byte encoded line feed (LF) would not be caught by a
        // line counter that counts only 0x0a bytes, but it would still be
        // processed as a line feed by an unsafe UTF-8 decoder later in the
        // pipeline. From a security point of view, ASCII compatibility of UTF-8
        // sequences means also, that ASCII characters are *only* allowed to be
        // represented by ASCII bytes in the range 0x00-0x7f. To ensure this
        // aspect of ASCII compatibility, use only "safe UTF-8 decoders" that
        // reject overlong UTF-8 sequences for which a shorter encoding exists.

        SECTION("4.1  Examples of an overlong ASCII character")
        {
            // With a safe UTF-8 decoder, all of the following five overlong
            // representations of the ASCII character slash ("/") should be rejected
            // like a malformed UTF-8 sequence, for instance by substituting it with
            // a replacement character. If you see a slash below, you do not have a
            // safe UTF-8 decoder!

            // 4.1.1 U+002F = c0 af
            roundtrip(false, "\xc0\xaf");
            // 4.1.2 U+002F = e0 80 af
            roundtrip(false, "\xe0\x80\xaf");
            // 4.1.3 U+002F = f0 80 80 af
            roundtrip(false, "\xf0\x80\x80\xaf");
            // 4.1.4 U+002F = f8 80 80 80 af
            roundtrip(false, "\xf8\x80\x80\x80\xaf");
            // 4.1.5 U+002F = fc 80 80 80 80 af
            roundtrip(false, "\xfc\x80\x80\x80\x80\xaf");
        }

        SECTION("4.2  Maximum overlong sequences")
        {
            // Below you see the highest Unicode value that is still resulting in an
            // overlong sequence if represented with the given number of bytes. This
            // is a boundary test for safe UTF-8 decoders. All five characters should
            // be rejected like malformed UTF-8 sequences.

            // 4.2.1  U-0000007F = c1 bf
            roundtrip(false, "\xc1\xbf");
            // 4.2.2  U-000007FF = e0 9f bf
            roundtrip(false, "\xe0\x9f\xbf");
            // 4.2.3  U-0000FFFF = f0 8f bf bf
            roundtrip(false, "\xf0\x8f\xbf\xbf");
            // 4.2.4  U-001FFFFF = f8 87 bf bf bf
            roundtrip(false, "\xf8\x87\xbf\xbf\xbf");
            // 4.2.5  U-03FFFFFF = fc 83 bf bf bf bf
            roundtrip(false, "\xfc\x83\xbf\xbf\xbf\xbf");
        }

        SECTION("4.3  Overlong representation of the NUL character")
        {
            // The following five sequences should also be rejected like malformed
            // UTF-8 sequences and should not be treated like the ASCII NUL
            // character.

            // 4.3.1  U+0000 = c0 80
            roundtrip(false, "\xc0\x80");
            // 4.3.2  U+0000 = e0 80 80
            roundtrip(false, "\xe0\x80\x80");
            // 4.3.3  U+0000 = f0 80 80 80
            roundtrip(false, "\xf0\x80\x80\x80");
            // 4.3.4  U+0000 = f8 80 80 80 80
            roundtrip(false, "\xf8\x80\x80\x80\x80");
            // 4.3.5  U+0000 = fc 80 80 80 80 80
            roundtrip(false, "\xfc\x80\x80\x80\x80\x80");
        }
    }

    SECTION("5  Illegal code positions")
    {
        // The following UTF-8 sequences should be rejected like malformed
        // sequences, because they never represent valid ISO 10646 characters and
        // a UTF-8 decoder that accepts them might introduce security problems
        // comparable to overlong UTF-8 sequences.

        SECTION("5.1 Single UTF-16 surrogates")
        {
            // 5.1.1  U+D800 = ed a0 80
            roundtrip(false, "\xed\xa0\x80");
            // 5.1.2  U+DB7F = ed ad bf
            roundtrip(false, "\xed\xad\xbf");
            // 5.1.3  U+DB80 = ed ae 80
            roundtrip(false, "\xed\xae\x80");
            // 5.1.4  U+DBFF = ed af bf
            roundtrip(false, "\xed\xaf\xbf");
            // 5.1.5  U+DC00 = ed b0 80
            roundtrip(false, "\xed\xb0\x80");
            // 5.1.6  U+DF80 = ed be 80
            roundtrip(false, "\xed\xbe\x80");
            // 5.1.7  U+DFFF = ed bf bf
            roundtrip(false, "\xed\xbf\xbf");
        }

        SECTION("5.2 Paired UTF-16 surrogates")
        {
            // 5.2.1  U+D800 U+DC00 = ed a0 80 ed b0 80
            roundtrip(false, "\xed\xa0\x80\xed\xb0\x80");
            // 5.2.2  U+D800 U+DFFF = ed a0 80 ed bf bf
            roundtrip(false, "\xed\xa0\x80\xed\xbf\xbf");
            // 5.2.3  U+DB7F U+DC00 = ed ad bf ed b0 80
            roundtrip(false, "\xed\xad\xbf\xed\xb0\x80");
            // 5.2.4  U+DB7F U+DFFF = ed ad bf ed bf bf
            roundtrip(false, "\xed\xad\xbf\xed\xbf\xbf");
            // 5.2.5  U+DB80 U+DC00 = ed ae 80 ed b0 80
            roundtrip(false, "\xed\xae\x80\xed\xb0\x80");
            // 5.2.6  U+DB80 U+DFFF = ed ae 80 ed bf bf
            roundtrip(false, "\xed\xae\x80\xed\xbf\xbf");
            // 5.2.7  U+DBFF U+DC00 = ed af bf ed b0 80
            roundtrip(false, "\xed\xaf\xbf\xed\xb0\x80");
            // 5.2.8  U+DBFF U+DFFF = ed af bf ed bf bf
            roundtrip(false, "\xed\xaf\xbf\xed\xbf\xbf");
        }

        SECTION("5.3 Noncharacter code positions")
        {
            // The following "noncharacters" are "reserved for internal use" by
            // applications, and according to older versions of the Unicode Standard
            // "should never be interchanged". Unicode Corrigendum #9 dropped the
            // latter restriction. Nevertheless, their presence in incoming UTF-8 data
            // can remain a potential security risk, depending on what use is made of
            // these codes subsequently. Examples of such internal use:
            //
            //  - Some file APIs with 16-bit characters may use the integer value -1
            //    = U+FFFF to signal an end-of-file (EOF) or error condition.
            //
            //  - In some UTF-16 receivers, code point U+FFFE might trigger a
            //    byte-swap operation (to convert between UTF-16LE and UTF-16BE).
            //
            // With such internal use of noncharacters, it may be desirable and safer
            // to block those code points in UTF-8 decoders, as they should never
            // occur legitimately in incoming UTF-8 data, and could trigger unsafe
            // behaviour in subsequent processing.

            // Particularly problematic noncharacters in 16-bit applications:

            // 5.3.1  U+FFFE = ef bf be
            roundtrip(true, "\xef\xbf\xbe");
            // 5.3.2  U+FFFF = ef bf bf
            roundtrip(true, "\xef\xbf\xbf");

            // 5.3.3  U+FDD0 .. U+FDEF
            roundtrip(true, "\xEF\xB7\x90");
            roundtrip(true, "\xEF\xB7\x91");
            roundtrip(true, "\xEF\xB7\x92");
            roundtrip(true, "\xEF\xB7\x93");
            roundtrip(true, "\xEF\xB7\x94");
            roundtrip(true, "\xEF\xB7\x95");
            roundtrip(true, "\xEF\xB7\x96");
            roundtrip(true, "\xEF\xB7\x97");
            roundtrip(true, "\xEF\xB7\x98");
            roundtrip(true, "\xEF\xB7\x99");
            roundtrip(true, "\xEF\xB7\x9A");
            roundtrip(true, "\xEF\xB7\x9B");
            roundtrip(true, "\xEF\xB7\x9C");
            roundtrip(true, "\xEF\xB7\x9D");
            roundtrip(true, "\xEF\xB7\x9E");
            roundtrip(true, "\xEF\xB7\x9F");
            roundtrip(true, "\xEF\xB7\xA0");
            roundtrip(true, "\xEF\xB7\xA1");
            roundtrip(true, "\xEF\xB7\xA2");
            roundtrip(true, "\xEF\xB7\xA3");
            roundtrip(true, "\xEF\xB7\xA4");
            roundtrip(true, "\xEF\xB7\xA5");
            roundtrip(true, "\xEF\xB7\xA6");
            roundtrip(true, "\xEF\xB7\xA7");
            roundtrip(true, "\xEF\xB7\xA8");
            roundtrip(true, "\xEF\xB7\xA9");
            roundtrip(true, "\xEF\xB7\xAA");
            roundtrip(true, "\xEF\xB7\xAB");
            roundtrip(true, "\xEF\xB7\xAC");
            roundtrip(true, "\xEF\xB7\xAD");
            roundtrip(true, "\xEF\xB7\xAE");
            roundtrip(true, "\xEF\xB7\xAF");

            // 5.3.4  U+nFFFE U+nFFFF (for n = 1..10)
            roundtrip(true, "\xF0\x9F\xBF\xBF");
            roundtrip(true, "\xF0\xAF\xBF\xBF");
            roundtrip(true, "\xF0\xBF\xBF\xBF");
            roundtrip(true, "\xF1\x8F\xBF\xBF");
            roundtrip(true, "\xF1\x9F\xBF\xBF");
            roundtrip(true, "\xF1\xAF\xBF\xBF");
            roundtrip(true, "\xF1\xBF\xBF\xBF");
            roundtrip(true, "\xF2\x8F\xBF\xBF");
            roundtrip(true, "\xF2\x9F\xBF\xBF");
            roundtrip(true, "\xF2\xAF\xBF\xBF");
        }
    }
}
