tests/src/unit-serialization.cpp

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

#include "doctest_compatibility.h"

#include <nlohmann/json.hpp>
using nlohmann::json;

#include <array>
#include <sstream>
#include <iomanip>

TEST_CASE("serialization")
{
    SECTION("operator<<")
    {
        SECTION("no given width")
        {
            std::stringstream ss;
            const json j = {"foo", 1, 2, 3, false, {{"one", 1}}};
            ss << j;
            CHECK(ss.str() == "[\"foo\",1,2,3,false,{\"one\":1}]");
        }

        SECTION("given width")
        {
            std::stringstream ss;
            const json j = {"foo", 1, 2, 3, false, {{"one", 1}}};
            ss << std::setw(4) << j;
            CHECK(ss.str() ==
                  "[\n    \"foo\",\n    1,\n    2,\n    3,\n    false,\n    {\n        \"one\": 1\n    }\n]");
        }

        SECTION("given fill")
        {
            std::stringstream ss;
            const json j = {"foo", 1, 2, 3, false, {{"one", 1}}};
            ss << std::setw(1) << std::setfill('\t') << j;
            CHECK(ss.str() ==
                  "[\n\t\"foo\",\n\t1,\n\t2,\n\t3,\n\tfalse,\n\t{\n\t\t\"one\": 1\n\t}\n]");
        }
    }

    SECTION("operator>>")
    {
        SECTION("no given width")
        {
            std::stringstream ss;
            const json j = {"foo", 1, 2, 3, false, {{"one", 1}}};
            j >> ss;
            CHECK(ss.str() == "[\"foo\",1,2,3,false,{\"one\":1}]");
        }

        SECTION("given width")
        {
            std::stringstream ss;
            const json j = {"foo", 1, 2, 3, false, {{"one", 1}}};
            ss.width(4);
            j >> ss;
            CHECK(ss.str() ==
                  "[\n    \"foo\",\n    1,\n    2,\n    3,\n    false,\n    {\n        \"one\": 1\n    }\n]");
        }

        SECTION("given fill")
        {
            std::stringstream ss;
            const json j = {"foo", 1, 2, 3, false, {{"one", 1}}};
            ss.width(1);
            ss.fill('\t');
            j >> ss;
            CHECK(ss.str() ==
                  "[\n\t\"foo\",\n\t1,\n\t2,\n\t3,\n\tfalse,\n\t{\n\t\t\"one\": 1\n\t}\n]");
        }
    }

    SECTION("dump")
    {
        SECTION("invalid character")
        {
            const json j = "ä\xA9ü";

            CHECK_THROWS_WITH_AS(j.dump(), "[json.exception.type_error.316] invalid UTF-8 byte at index 2: 0xA9", json::type_error&);
            CHECK_THROWS_WITH_AS(j.dump(1, ' ', false, json::error_handler_t::strict), "[json.exception.type_error.316] invalid UTF-8 byte at index 2: 0xA9", json::type_error&);
            CHECK(j.dump(-1, ' ', false, json::error_handler_t::ignore) == "\"äü\"");
            CHECK(j.dump(-1, ' ', false, json::error_handler_t::replace) == "\"ä\xEF\xBF\xBDü\"");
            CHECK(j.dump(-1, ' ', true, json::error_handler_t::replace) == "\"\\u00e4\\ufffd\\u00fc\"");
        }

        SECTION("ending with incomplete character")
        {
            const json j = "123\xC2";

            CHECK_THROWS_WITH_AS(j.dump(), "[json.exception.type_error.316] incomplete UTF-8 string; last byte: 0xC2", json::type_error&);
            CHECK_THROWS_AS(j.dump(1, ' ', false, json::error_handler_t::strict), json::type_error&);
            CHECK(j.dump(-1, ' ', false, json::error_handler_t::ignore) == "\"123\"");
            CHECK(j.dump(-1, ' ', false, json::error_handler_t::replace) == "\"123\xEF\xBF\xBD\"");
            CHECK(j.dump(-1, ' ', true, json::error_handler_t::replace) == "\"123\\ufffd\"");
        }

        SECTION("unexpected character")
        {
            const json j = "123\xF1\xB0\x34\x35\x36";

            CHECK_THROWS_WITH_AS(j.dump(), "[json.exception.type_error.316] invalid UTF-8 byte at index 5: 0x34", json::type_error&);
            CHECK_THROWS_AS(j.dump(1, ' ', false, json::error_handler_t::strict), json::type_error&);
            CHECK(j.dump(-1, ' ', false, json::error_handler_t::ignore) == "\"123456\"");
            CHECK(j.dump(-1, ' ', false, json::error_handler_t::replace) == "\"123\xEF\xBF\xBD\x34\x35\x36\"");
            CHECK(j.dump(-1, ' ', true, json::error_handler_t::replace) == "\"123\\ufffd456\"");
        }

        SECTION("U+FFFD Substitution of Maximal Subparts")
        {
            // Some tests (mostly) from
            // https://www.unicode.org/versions/Unicode11.0.0/ch03.pdf
            // Section 3.9 -- U+FFFD Substitution of Maximal Subparts

            auto test = [&](std::string const & input, std::string const & expected)
            {
                const json j = input;
                CHECK(j.dump(-1, ' ', true, json::error_handler_t::replace) == "\"" + expected + "\"");
            };

            test("\xC2", "\\ufffd");
            test("\xC2\x41\x42", "\\ufffd" "\x41" "\x42");
            test("\xC2\xF4", "\\ufffd" "\\ufffd");

            test("\xF0\x80\x80\x41", "\\ufffd" "\\ufffd" "\\ufffd" "\x41");
            test("\xF1\x80\x80\x41", "\\ufffd" "\x41");
            test("\xF2\x80\x80\x41", "\\ufffd" "\x41");
            test("\xF3\x80\x80\x41", "\\ufffd" "\x41");
            test("\xF4\x80\x80\x41", "\\ufffd" "\x41");
            test("\xF5\x80\x80\x41", "\\ufffd" "\\ufffd" "\\ufffd" "\x41");

            test("\xF0\x90\x80\x41", "\\ufffd" "\x41");
            test("\xF1\x90\x80\x41", "\\ufffd" "\x41");
            test("\xF2\x90\x80\x41", "\\ufffd" "\x41");
            test("\xF3\x90\x80\x41", "\\ufffd" "\x41");
            test("\xF4\x90\x80\x41", "\\ufffd" "\\ufffd" "\\ufffd" "\x41");
            test("\xF5\x90\x80\x41", "\\ufffd" "\\ufffd" "\\ufffd" "\x41");

            test("\xC0\xAF\xE0\x80\xBF\xF0\x81\x82\x41", "\\ufffd" "\\ufffd" "\\ufffd" "\\ufffd" "\\ufffd" "\\ufffd" "\\ufffd" "\\ufffd" "\x41");
            test("\xED\xA0\x80\xED\xBF\xBF\xED\xAF\x41", "\\ufffd" "\\ufffd" "\\ufffd" "\\ufffd" "\\ufffd" "\\ufffd" "\\ufffd" "\\ufffd" "\x41");
            test("\xF4\x91\x92\x93\xFF\x41\x80\xBF\x42", "\\ufffd" "\\ufffd" "\\ufffd" "\\ufffd" "\\ufffd" "\x41" "\\ufffd""\\ufffd" "\x42");
            test("\xE1\x80\xE2\xF0\x91\x92\xF1\xBF\x41", "\\ufffd" "\\ufffd" "\\ufffd" "\\ufffd" "\x41");
        }
    }

    SECTION("to_string")
    {
        auto test = [&](std::string const & input, std::string const & expected)
        {
            using std::to_string;
            const json j = input;
            CHECK(to_string(j) == "\"" + expected + "\"");
        };

        test(R"({"x":5,"y":6})", R"({\"x\":5,\"y\":6})");
        test("{\"x\":[10,null,null,null]}", R"({\"x\":[10,null,null,null]})");
        test("test", "test");
        test("[3,\"false\",false]", R"([3,\"false\",false])");
    }
}

TEST_CASE_TEMPLATE("serialization for extreme integer values", T, int32_t, uint32_t, int64_t, uint64_t) // NOLINT(readability-math-missing-parentheses, bugprone-throwing-static-initialization)
{
    SECTION("minimum")
    {
        constexpr auto minimum = (std::numeric_limits<T>::min)();
        const json j = minimum;
        CHECK(j.dump() == std::to_string(minimum));
    }

    SECTION("maximum")
    {
        constexpr auto maximum = (std::numeric_limits<T>::max)();
        const json j = maximum;
        CHECK(j.dump() == std::to_string(maximum));
    }
}

TEST_CASE("dump with binary values")
{
    auto binary = json::binary({1, 2, 3, 4});
    auto binary_empty = json::binary({});
    auto binary_with_subtype = json::binary({1, 2, 3, 4}, 128);
    auto binary_empty_with_subtype = json::binary({}, 128);

    const json object = {{"key", binary}};
    const json object_empty = {{"key", binary_empty}};
    const json object_with_subtype = {{"key", binary_with_subtype}};
    const json object_empty_with_subtype = {{"key", binary_empty_with_subtype}};

    const json array = {"value", 1, binary};
    const json array_empty = {"value", 1, binary_empty};
    const json array_with_subtype = {"value", 1, binary_with_subtype};
    const json array_empty_with_subtype = {"value", 1, binary_empty_with_subtype};

    SECTION("normal")
    {
        CHECK(binary.dump() == "{\"bytes\":[1,2,3,4],\"subtype\":null}");
        CHECK(binary_empty.dump() == "{\"bytes\":[],\"subtype\":null}");
        CHECK(binary_with_subtype.dump() == "{\"bytes\":[1,2,3,4],\"subtype\":128}");
        CHECK(binary_empty_with_subtype.dump() == "{\"bytes\":[],\"subtype\":128}");

        CHECK(object.dump() == "{\"key\":{\"bytes\":[1,2,3,4],\"subtype\":null}}");
        CHECK(object_empty.dump() == "{\"key\":{\"bytes\":[],\"subtype\":null}}");
        CHECK(object_with_subtype.dump() == "{\"key\":{\"bytes\":[1,2,3,4],\"subtype\":128}}");
        CHECK(object_empty_with_subtype.dump() == "{\"key\":{\"bytes\":[],\"subtype\":128}}");

        CHECK(array.dump() == "[\"value\",1,{\"bytes\":[1,2,3,4],\"subtype\":null}]");
        CHECK(array_empty.dump() == "[\"value\",1,{\"bytes\":[],\"subtype\":null}]");
        CHECK(array_with_subtype.dump() == "[\"value\",1,{\"bytes\":[1,2,3,4],\"subtype\":128}]");
        CHECK(array_empty_with_subtype.dump() == "[\"value\",1,{\"bytes\":[],\"subtype\":128}]");
    }

    SECTION("pretty-printed")
    {
        CHECK(binary.dump(4) == "{\n"
              "    \"bytes\": [1, 2, 3, 4],\n"
              "    \"subtype\": null\n"
              "}");
        CHECK(binary_empty.dump(4) == "{\n"
              "    \"bytes\": [],\n"
              "    \"subtype\": null\n"
              "}");
        CHECK(binary_with_subtype.dump(4) == "{\n"
              "    \"bytes\": [1, 2, 3, 4],\n"
              "    \"subtype\": 128\n"
              "}");
        CHECK(binary_empty_with_subtype.dump(4) == "{\n"
              "    \"bytes\": [],\n"
              "    \"subtype\": 128\n"
              "}");

        CHECK(object.dump(4) == "{\n"
              "    \"key\": {\n"
              "        \"bytes\": [1, 2, 3, 4],\n"
              "        \"subtype\": null\n"
              "    }\n"
              "}");
        CHECK(object_empty.dump(4) == "{\n"
              "    \"key\": {\n"
              "        \"bytes\": [],\n"
              "        \"subtype\": null\n"
              "    }\n"
              "}");
        CHECK(object_with_subtype.dump(4) == "{\n"
              "    \"key\": {\n"
              "        \"bytes\": [1, 2, 3, 4],\n"
              "        \"subtype\": 128\n"
              "    }\n"
              "}");
        CHECK(object_empty_with_subtype.dump(4) == "{\n"
              "    \"key\": {\n"
              "        \"bytes\": [],\n"
              "        \"subtype\": 128\n"
              "    }\n"
              "}");

        CHECK(array.dump(4) == "[\n"
              "    \"value\",\n"
              "    1,\n"
              "    {\n"
              "        \"bytes\": [1, 2, 3, 4],\n"
              "        \"subtype\": null\n"
              "    }\n"
              "]");
        CHECK(array_empty.dump(4) == "[\n"
              "    \"value\",\n"
              "    1,\n"
              "    {\n"
              "        \"bytes\": [],\n"
              "        \"subtype\": null\n"
              "    }\n"
              "]");
        CHECK(array_with_subtype.dump(4) == "[\n"
              "    \"value\",\n"
              "    1,\n"
              "    {\n"
              "        \"bytes\": [1, 2, 3, 4],\n"
              "        \"subtype\": 128\n"
              "    }\n"
              "]");
        CHECK(array_empty_with_subtype.dump(4) == "[\n"
              "    \"value\",\n"
              "    1,\n"
              "    {\n"
              "        \"bytes\": [],\n"
              "        \"subtype\": 128\n"
              "    }\n"
              "]");
    }
}

TEST_CASE("dump for basic_json with long double number_float_t")
{
    // Custom basic_json instantiation with long double as NumberFloatType.
    // On platforms where long double is wider than double (e.g. GCC/Clang on
    // Linux/macOS x86_64), dump() goes through the snprintf path in
    // serializer::dump_float(x, std::false_type). That branch must use the
    // "%.*Lg" format specifier; using "%.*g" with a long double argument is
    // undefined behavior and corrupts the output.
    using long_double_json = nlohmann::basic_json<std::map, std::vector, std::string,
          bool, std::int64_t, std::uint64_t, long double>;

    SECTION("round-trip dump/parse")
    {
        constexpr std::array<long double, 13> values =
        {
            {
                0.0L, -0.0L, 1.0L, -1.0L,
                0.5L, -0.5L, 1.5L, -2.25L,
                1.23e45L, 1.23e-45L,
                (std::numeric_limits<long double>::min)(),
                std::numeric_limits<long double>::lowest(),
                (std::numeric_limits<long double>::max)()
            }
        };

        for (long double v : values)
        {
            const long_double_json j = v;
            const auto s = j.dump();
            const auto j2 = long_double_json::parse(s);
            CHECK(j2.template get<long double>() == v);
        }
    }

    SECTION("exact dump string for simple values")
    {
        CHECK(long_double_json(0.5L).dump()   == "0.5");
        CHECK(long_double_json(-0.5L).dump()  == "-0.5");
        CHECK(long_double_json(1.5L).dump()   == "1.5");
        CHECK(long_double_json(-2.25L).dump() == "-2.25");
        CHECK(long_double_json(0.0L).dump()   == "0.0");
        CHECK(long_double_json(1.0L).dump()   == "1.0");
        CHECK(long_double_json(-1.0L).dump()  == "-1.0");
        CHECK(long_double_json(100.0L).dump() == "100.0");
    }

    SECTION("NaN and infinity dump as null")
    {
        CHECK(long_double_json(std::numeric_limits<long double>::quiet_NaN()).dump() == "null");

        // Probe the platform's runtime behavior — `volatile` forces a runtime
        // call rather than constexpr-folding to a known answer at compile time.
        // Skip the infinity assertions if std::isfinite() doesn't actually
        // recognize long double infinity on this platform (notably, Valgrind
        // 3.22's x87 80-bit emulation reports +/-inf as a large finite value).
        // TODO(rusloker): remove this guard once Valgrind's 80-bit long double
        // support ships (Valgrind bug https://bugs.kde.org/show_bug.cgi?id=197915,
        // ASSIGNED since 2009 — the Valgrind project tracks its bugs on
        // bugs.kde.org) and the minimum supported Valgrind version contains it.
        const volatile long double inf_probe = std::numeric_limits<long double>::infinity();
        if (!std::isfinite(inf_probe))
        {
            CHECK(long_double_json(std::numeric_limits<long double>::infinity()).dump() == "null");
            CHECK(long_double_json(-std::numeric_limits<long double>::infinity()).dump() == "null");
        }
    }

    SECTION("dump output matches double for exactly-representable values")
    {
        auto check_same = [](long double v_ld, double v_d)
        {
            const long_double_json j_ld = v_ld;
            const json j_d = v_d;
            CHECK(j_ld.dump() == j_d.dump());
        };

        check_same(0.0L,    0.0);
        check_same(0.5L,    0.5);
        check_same(-0.5L,  -0.5);
        check_same(1.5L,    1.5);
        check_same(-2.25L, -2.25);
        check_same(1.0L,    1.0);
        check_same(100.0L,  100.0);
    }
}

TEST_CASE("serialization of strings (bulk fast path)")
{
    // These cases exercise the SWAR bulk-copy fast path in dump_escaped and the
    // internal write buffer: long runs, escapes interrupting runs, 0x7F/DEL,
    // multibyte UTF-8 under both ensure_ascii settings, and payloads larger than
    // the write buffer.

    SECTION("long unescaped ASCII exceeds the write buffer")
    {
        const std::string big(3000, 'a');
        const json j = big;
        CHECK(j.dump() == '"' + big + '"');
        CHECK(j.dump(-1, ' ', true) == '"' + big + '"');
        // round-trips
        CHECK(json::parse(j.dump()) == j);
    }

    SECTION("runs interrupted by escapes")
    {
        const json j = std::string(500, 'x') + "\n\"\\" + std::string(500, 'y');
        const std::string out = j.dump();
        CHECK(out == '"' + std::string(500, 'x') + "\\n\\\"\\\\" + std::string(500, 'y') + '"');
        CHECK(json::parse(out) == j);
    }

    SECTION("DEL (0x7F) depends on ensure_ascii")
    {
        const json j = std::string("a\x7f" "b");
        CHECK(j.dump(-1, ' ', false) == "\"a\x7f" "b\"");   // copied verbatim
        CHECK(j.dump(-1, ' ', true) == "\"a\\u007fb\"");    // escaped
    }

    SECTION("multibyte UTF-8 under both ensure_ascii settings")
    {
        const json j = std::string("A\xc3\xa9\xe4\xbd\xa0\xf0\x9f\x98\x80Z"); // A é 你 😀 Z
        // not escaping non-ASCII: bytes are copied through the bulk validator
        CHECK(j.dump(-1, ' ', false) == "\"A\xc3\xa9\xe4\xbd\xa0\xf0\x9f\x98\x80Z\"");
        // ensure_ascii: escaped (with a surrogate pair for the emoji)
        CHECK(j.dump(-1, ' ', true) == "\"A\\u00e9\\u4f60\\ud83d\\ude00Z\"");
        CHECK(json::parse(j.dump(-1, ' ', true)) == j);
    }

    SECTION("many small structural writes exceed the write buffer")
    {
        json arr = json::array();
        for (int i = 0; i < 2000; ++i)
        {
            arr.push_back(i);
        }
        const std::string out = arr.dump();
        CHECK(out.front() == '[');
        CHECK(out.back() == ']');
        CHECK(json::parse(out) == arr);

        json obj = json::object();
        for (int i = 0; i < 500; ++i)
        {
            obj["key" + std::to_string(i)] = i;
        }
        CHECK(json::parse(obj.dump()) == obj);
        CHECK(json::parse(obj.dump(2)) == obj);

        // an array of many empty strings emits a long run of single-character
        // writes ('"', '"', ',') at shallow nesting depth, so the write buffer
        // fills and flushes mid-run without the deep recursion that would
        // overflow the stack on some debug builds
        json many_empty = json::array();
        for (int i = 0; i < 500; ++i)
        {
            many_empty.push_back("");
        }
        const std::string out2 = many_empty.dump();
        CHECK(out2.size() > 1024); // spans multiple write-buffer flushes
        CHECK(out2.front() == '[');
        CHECK(out2.back() == ']');
        CHECK(json::parse(out2) == many_empty);
    }

    SECTION("invalid UTF-8 handling is unaffected by the fast path")
    {
        const json j = std::string("valid\xff" "more");
        CHECK_THROWS_WITH_AS(j.dump(), "[json.exception.type_error.316] invalid UTF-8 byte at index 5: 0xFF", json::type_error&);
        CHECK(j.dump(-1, ' ', false, json::error_handler_t::replace) == "\"valid\xef\xbf\xbd" "more\"");
        CHECK(j.dump(-1, ' ', true, json::error_handler_t::replace) == "\"valid\\ufffdmore\"");
        CHECK(j.dump(-1, ' ', false, json::error_handler_t::ignore) == "\"validmore\"");
    }
}

TEST_CASE("indentation is written straight into the write buffer")
{
    // put_indent() memsets the indentation into the write buffer instead of
    // copying it out of a pre-grown indentation string. These cases cover an
    // indentation wider than the buffer, a non-space indentation character, and
    // nesting deep enough that the accumulated indentation spans several
    // buffer-fulls - the situations the old grow-a-string approach got wrong.

    SECTION("indent_step wider than the write buffer")
    {
        const json j = {{"a", 1}};
        // 2000 > the 1024-byte write buffer, and > the 512 the indentation
        // string used to start at
        CHECK(j.dump(2000) == "{\n" + std::string(2000, ' ') + "\"a\": 1\n}");
        // several whole buffer-fulls, so the buffer is refilled once and then
        // flushed repeatedly
        CHECK(j.dump(5000) == "{\n" + std::string(5000, ' ') + "\"a\": 1\n}");
        CHECK(j.dump(5000, '\t') == "{\n" + std::string(5000, '\t') + "\"a\": 1\n}");
        // an exact multiple of the buffer size
        CHECK(j.dump(4096) == "{\n" + std::string(4096, ' ') + "\"a\": 1\n}");
    }

    SECTION("a non-space indentation character is used throughout")
    {
        const json j = {{"a", 1}};
        // 600 is past the point where the indentation used to be grown, which
        // is where a hard-coded space would have shown up
        CHECK(j.dump(600, '\t') == "{\n" + std::string(600, '\t') + "\"a\": 1\n}");
        CHECK(j.dump(3, '.') == "{\n...\"a\": 1\n}");
    }

    SECTION("accumulated indentation spans several buffer-fulls")
    {
        // five levels deep at 400 per level: the innermost value is indented by
        // 2000 characters, reached in steps that each straddle the buffer end
        json j = json::array({1});
        for (int i = 0; i < 4; ++i)
        {
            j = json::array({j});
        }

        const std::string out = j.dump(400);
        CHECK(out.find(std::string("\n") + std::string(2000, ' ') + "1\n") != std::string::npos);
        CHECK(json::parse(out) == j);
    }

    SECTION("indentation is unchanged for ordinary widths")
    {
        const json j = {{"a", {1, 2}}, {"b", nullptr}};
        CHECK(j.dump(2) == "{\n  \"a\": [\n    1,\n    2\n  ],\n  \"b\": null\n}");
        CHECK(j.dump(0) == "{\n\"a\": [\n1,\n2\n],\n\"b\": null\n}");
    }
}

TEST_CASE("serialization of deeply nested values")
{
    // dump() descends into a bounded number of levels and writes out whatever
    // is nested deeper than that without the call stack; see
    // https://github.com/nlohmann/json/issues/5387

    SECTION("nested deeper than the call stack could follow")
    {
        // parsing is iterative, so building these costs little
        const std::size_t depth = 100000;

        const std::string array_text = std::string(depth, '[') + '0' + std::string(depth, ']');
        CHECK(json::parse(array_text).dump() == array_text);

        std::string object_text;
        object_text.reserve((6 * depth) + 1);
        for (std::size_t i = 0; i < depth; ++i)
        {
            object_text += "{\"a\":";
        }
        object_text += '1';
        object_text.append(depth, '}');
        CHECK(json::parse(object_text).dump() == object_text);
    }

    SECTION("depths around the bound of the descent")
    {
        // Cover every depth around the bound, so that the two ways of writing a
        // value are known to meet cleanly - wherever the bound is set.
        for (std::size_t d = 1; d <= 300; ++d)
        {
            CAPTURE(d);

            const std::string array_text = std::string(d, '[') + '7' + std::string(d, ']');
            CHECK(json::parse(array_text).dump() == array_text);

            std::string object_text;
            for (std::size_t i = 0; i < d; ++i)
            {
                object_text += "{\"k\":";
            }
            object_text += '7';
            object_text.append(d, '}');
            CHECK(json::parse(object_text).dump() == object_text);
        }
    }

    SECTION("pretty-printing across the bound")
    {
        for (std::size_t d = 120; d <= 140; ++d)
        {
            CAPTURE(d);

            const json j = json::parse(std::string(d, '[') + '7' + std::string(d, ']'));

            std::string expected;
            for (std::size_t i = 0; i < d; ++i)
            {
                expected += std::string(2 * i, ' ') + "[\n";
            }
            expected += std::string(2 * d, ' ') + '7';
            for (std::size_t i = d; i > 0; --i)
            {
                expected += '\n' + std::string(2 * (i - 1), ' ') + ']';
            }

            CHECK(j.dump(2) == expected);
        }
    }

    SECTION("an empty container below the bound")
    {
        // an empty container is written out in full and never descended into,
        // so it must not gain a newline when it is reached iteratively
        for (std::size_t d = 125; d <= 135; ++d)
        {
            CAPTURE(d);

            const std::string compact = std::string(d, '[') + "[]" + std::string(d, ']');
            CHECK(json::parse(compact).dump() == compact);

            const std::string with_object = std::string(d, '[') + "{}" + std::string(d, ']');
            CHECK(json::parse(with_object).dump() == with_object);
        }
    }
}