Merge the array and object branches of the BON8 iterative writer

write_bon8_iterative and write_bon8_value_or_push handled arrays and
objects in separate branches that repeated the end-of-container check,
the 0xFE closer and the marker computation. Share those parts and branch
only where arrays and objects really differ. The output is unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Hz7VJi1FTKr6gpseLErbbS
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/output/binary_writer.hpp

@@ -878,9 +878,9 @@ class binary_writer
         }
     }
 
-    /// @brief a CBOR or MessagePack array or object whose elements
-    ///        @ref write_cbor_iterative or @ref write_msgpack_iterative is
-    ///        still writing
+    /// @brief a CBOR, MessagePack, or BON8 array or object whose elements
+    ///        @ref write_cbor_iterative, @ref write_msgpack_iterative, or
+    ///        @ref write_bon8_iterative is still writing
     struct binary_container_frame
     {
         explicit binary_container_frame(const BasicJsonType* value_) noexcept
@@ -2570,9 +2570,26 @@ class binary_writer
     @param[in] j                JSON value to serialize
     @param[in,out] string_open  whether the output ends with a non-empty
                                 string that has not been terminated with 0xFF
+    @param[in] depth            nesting level of @a j, counted from the
+                                top-level value passed to @ref basic_json::to_bon8
+    @throw type_error.316 if a string value or an object key is not valid
+           UTF-8
+    @throw out_of_range.407 if an unsigned integer does not fit int64
+
+    Values nested deeper than @ref recursion_depth_limit are written by
+    @ref write_bon8_iterative without the call stack.
+
+    @sa @ref write_cbor
+    @sa https://github.com/nlohmann/json/issues/5392
     */
-    void write_bon8_value(const BasicJsonType& j, bool& string_open)
+    void write_bon8_value(const BasicJsonType& j, bool& string_open, const std::size_t depth = 0)
     {
+        if (JSON_HEDLEY_UNLIKELY(depth >= recursion_depth_limit()) && (j.is_array() || j.is_object()))
+        {
+            write_bon8_iterative(j, string_open);
+            return;
+        }
+
         switch (j.type())
         {
             case value_t::null:
@@ -2626,7 +2643,7 @@ class binary_writer
 
                 for (const auto& el : *j.m_data.m_value.array)
                 {
-                    write_bon8_value(el, string_open);
+                    write_bon8_value(el, string_open, depth + 1);
                 }
 
                 if (N > 4)
@@ -2645,7 +2662,7 @@ class binary_writer
                 for (const auto& el : *j.m_data.m_value.object)
                 {
                     write_bon8_string(el.first, string_open, j);
-                    write_bon8_value(el.second, string_open);
+                    write_bon8_value(el.second, string_open, depth + 1);
                 }
 
                 if (N > 4)
@@ -2682,6 +2699,83 @@ class binary_writer
         }
     }
 
+    /*!
+    @brief write @a j with @ref write_bon8_value, or write its marker and push
+           a frame for @ref write_bon8_iterative to continue with its elements
+
+    A scalar, and an empty array or object, are written out in full and not
+    pushed.
+
+    @sa @ref write_cbor_value_or_push
+    */
+    void write_bon8_value_or_push(const BasicJsonType& j, bool& string_open, std::vector<binary_container_frame>& stack)
+    {
+        if (j.is_array() || j.is_object())
+        {
+            // arrays use the markers 0x80..0x85, objects 0x86..0x8B; the last
+            // one stands for more than four elements, closed later with 0xFE
+            const auto N = j.size();
+            const std::size_t base = j.is_array() ? 0x80 : 0x86;
+            write_bon8_marker(static_cast<std::uint8_t>(base + (N <= 4 ? N : 5)), string_open);
+            if (N != 0)
+            {
+                stack.emplace_back(&j);
+            }
+            return;
+        }
+
+        write_bon8_value(j, string_open);
+    }
+
+    /*!
+    @brief write out @a root and everything below it without the call stack
+
+    A container with more than four elements is closed with 0xFE once its
+    last element is written, as in @ref write_bon8_value.
+
+    @sa @ref write_cbor_iterative
+    */
+    void write_bon8_iterative(const BasicJsonType& root, bool& string_open)
+    {
+        // only a container with elements is ever pushed; see write_bon8_value_or_push
+        std::vector<binary_container_frame> stack;
+        write_bon8_value_or_push(root, string_open, stack);
+
+        while (!stack.empty())
+        {
+            const binary_container_frame current = stack.back();
+            const bool is_array = current.value->is_array();
+            const bool at_end = is_array
+                                ? current.array_it == current.value->m_data.m_value.array->cend()
+                                : current.object_it == current.value->m_data.m_value.object->cend();
+
+            if (at_end)
+            {
+                if (current.value->size() > 4)
+                {
+                    write_bon8_marker(0xFE, string_open);
+                }
+                stack.pop_back();
+                continue;
+            }
+
+            // read the child before pushing: entering it can move every frame
+            const BasicJsonType* child = nullptr;
+            if (is_array)
+            {
+                child = &(*current.array_it);
+                ++stack.back().array_it;
+            }
+            else
+            {
+                write_bon8_string(current.object_it->first, string_open, *current.value);
+                child = &(current.object_it->second);
+                ++stack.back().object_it;
+            }
+            write_bon8_value_or_push(*child, string_open, stack);
+        }
+    }
+
     /*!
     @brief write a single byte that is not part of a string
 

single_include/nlohmann/json.hpp

@@ -22495,9 +22495,9 @@ class binary_writer
         }
     }
 
-    /// @brief a CBOR or MessagePack array or object whose elements
-    ///        @ref write_cbor_iterative or @ref write_msgpack_iterative is
-    ///        still writing
+    /// @brief a CBOR, MessagePack, or BON8 array or object whose elements
+    ///        @ref write_cbor_iterative, @ref write_msgpack_iterative, or
+    ///        @ref write_bon8_iterative is still writing
     struct binary_container_frame
     {
         explicit binary_container_frame(const BasicJsonType* value_) noexcept
@@ -24187,9 +24187,26 @@ class binary_writer
     @param[in] j                JSON value to serialize
     @param[in,out] string_open  whether the output ends with a non-empty
                                 string that has not been terminated with 0xFF
+    @param[in] depth            nesting level of @a j, counted from the
+                                top-level value passed to @ref basic_json::to_bon8
+    @throw type_error.316 if a string value or an object key is not valid
+           UTF-8
+    @throw out_of_range.407 if an unsigned integer does not fit int64
+
+    Values nested deeper than @ref recursion_depth_limit are written by
+    @ref write_bon8_iterative without the call stack.
+
+    @sa @ref write_cbor
+    @sa https://github.com/nlohmann/json/issues/5392
     */
-    void write_bon8_value(const BasicJsonType& j, bool& string_open)
+    void write_bon8_value(const BasicJsonType& j, bool& string_open, const std::size_t depth = 0)
     {
+        if (JSON_HEDLEY_UNLIKELY(depth >= recursion_depth_limit()) && (j.is_array() || j.is_object()))
+        {
+            write_bon8_iterative(j, string_open);
+            return;
+        }
+
         switch (j.type())
         {
             case value_t::null:
@@ -24243,7 +24260,7 @@ class binary_writer
 
                 for (const auto& el : *j.m_data.m_value.array)
                 {
-                    write_bon8_value(el, string_open);
+                    write_bon8_value(el, string_open, depth + 1);
                 }
 
                 if (N > 4)
@@ -24262,7 +24279,7 @@ class binary_writer
                 for (const auto& el : *j.m_data.m_value.object)
                 {
                     write_bon8_string(el.first, string_open, j);
-                    write_bon8_value(el.second, string_open);
+                    write_bon8_value(el.second, string_open, depth + 1);
                 }
 
                 if (N > 4)
@@ -24299,6 +24316,83 @@ class binary_writer
         }
     }
 
+    /*!
+    @brief write @a j with @ref write_bon8_value, or write its marker and push
+           a frame for @ref write_bon8_iterative to continue with its elements
+
+    A scalar, and an empty array or object, are written out in full and not
+    pushed.
+
+    @sa @ref write_cbor_value_or_push
+    */
+    void write_bon8_value_or_push(const BasicJsonType& j, bool& string_open, std::vector<binary_container_frame>& stack)
+    {
+        if (j.is_array() || j.is_object())
+        {
+            // arrays use the markers 0x80..0x85, objects 0x86..0x8B; the last
+            // one stands for more than four elements, closed later with 0xFE
+            const auto N = j.size();
+            const std::size_t base = j.is_array() ? 0x80 : 0x86;
+            write_bon8_marker(static_cast<std::uint8_t>(base + (N <= 4 ? N : 5)), string_open);
+            if (N != 0)
+            {
+                stack.emplace_back(&j);
+            }
+            return;
+        }
+
+        write_bon8_value(j, string_open);
+    }
+
+    /*!
+    @brief write out @a root and everything below it without the call stack
+
+    A container with more than four elements is closed with 0xFE once its
+    last element is written, as in @ref write_bon8_value.
+
+    @sa @ref write_cbor_iterative
+    */
+    void write_bon8_iterative(const BasicJsonType& root, bool& string_open)
+    {
+        // only a container with elements is ever pushed; see write_bon8_value_or_push
+        std::vector<binary_container_frame> stack;
+        write_bon8_value_or_push(root, string_open, stack);
+
+        while (!stack.empty())
+        {
+            const binary_container_frame current = stack.back();
+            const bool is_array = current.value->is_array();
+            const bool at_end = is_array
+                                ? current.array_it == current.value->m_data.m_value.array->cend()
+                                : current.object_it == current.value->m_data.m_value.object->cend();
+
+            if (at_end)
+            {
+                if (current.value->size() > 4)
+                {
+                    write_bon8_marker(0xFE, string_open);
+                }
+                stack.pop_back();
+                continue;
+            }
+
+            // read the child before pushing: entering it can move every frame
+            const BasicJsonType* child = nullptr;
+            if (is_array)
+            {
+                child = &(*current.array_it);
+                ++stack.back().array_it;
+            }
+            else
+            {
+                write_bon8_string(current.object_it->first, string_open, *current.value);
+                child = &(current.object_it->second);
+                ++stack.back().object_it;
+            }
+            write_bon8_value_or_push(*child, string_open, stack);
+        }
+    }
+
     /*!
     @brief write a single byte that is not part of a string
 

tests/src/unit-large_json.cpp

@@ -400,26 +400,31 @@ TEST_CASE("issue #5392 - binary writers on deeply nested values")
         CHECK(json::from_ubjson(json::to_ubjson(deep_array, true, false)) == deep_array);
         CHECK(json::from_ubjson(json::to_ubjson(deep_array, true, true)) == deep_array);
         CHECK(json::from_bjdata(json::to_bjdata(deep_array)) == deep_array);
+        CHECK(json::from_bon8(json::to_bon8(deep_array)) == deep_array);
 
         CHECK(json::from_cbor(json::to_cbor(deep_object)) == deep_object);
         CHECK(json::from_msgpack(json::to_msgpack(deep_object)) == deep_object);
         CHECK(json::from_ubjson(json::to_ubjson(deep_object)) == deep_object);
         CHECK(json::from_ubjson(json::to_ubjson(deep_object, true, true)) == deep_object);
         CHECK(json::from_bjdata(json::to_bjdata(deep_object)) == deep_object);
+        CHECK(json::from_bon8(json::to_bon8(deep_object)) == deep_object);
 
         CHECK(json::from_cbor(json::to_cbor(empty_array)) == empty_array);
         CHECK(json::from_msgpack(json::to_msgpack(empty_array)) == empty_array);
         CHECK(json::from_ubjson(json::to_ubjson(empty_array)) == empty_array);
         CHECK(json::from_ubjson(json::to_ubjson(empty_array, true, true)) == empty_array);
+        CHECK(json::from_bon8(json::to_bon8(empty_array)) == empty_array);
 
         CHECK(json::from_cbor(json::to_cbor(empty_object)) == empty_object);
         CHECK(json::from_msgpack(json::to_msgpack(empty_object)) == empty_object);
         CHECK(json::from_ubjson(json::to_ubjson(empty_object)) == empty_object);
+        CHECK(json::from_bon8(json::to_bon8(empty_object)) == empty_object);
 
         CHECK(json::from_cbor(json::to_cbor(mixed)) == mixed);
         CHECK(json::from_msgpack(json::to_msgpack(mixed)) == mixed);
         CHECK(json::from_ubjson(json::to_ubjson(mixed)) == mixed);
         CHECK(json::from_bjdata(json::to_bjdata(mixed)) == mixed);
+        CHECK(json::from_bon8(json::to_bon8(mixed)) == mixed);
     }
 
     SECTION("the two ways of writing a value meet at the bound")
@@ -432,11 +437,13 @@ TEST_CASE("issue #5392 - binary writers on deeply nested values")
             CHECK(json::from_cbor(json::to_cbor(array)) == array);
             CHECK(json::from_msgpack(json::to_msgpack(array)) == array);
             CHECK(json::from_ubjson(json::to_ubjson(array, true, true)) == array);
+            CHECK(json::from_bon8(json::to_bon8(array)) == array);
 
             const json object = nested_object(depth, json(7));
             CHECK(json::from_cbor(json::to_cbor(object)) == object);
             CHECK(json::from_msgpack(json::to_msgpack(object)) == object);
             CHECK(json::from_bjdata(json::to_bjdata(object)) == object);
+            CHECK(json::from_bon8(json::to_bon8(object)) == object);
         }
     }
 
@@ -481,6 +488,10 @@ TEST_CASE("issue #5392 - binary writers on deeply nested values")
             expected_ubjson.append(depth, ']');
             const auto packed_ubjson = json::to_ubjson(array);
             CHECK(std::string(packed_ubjson.begin(), packed_ubjson.end()) == expected_ubjson);
+
+            std::vector<std::uint8_t> expected_bon8(depth, 0x81);
+            expected_bon8.push_back(0x90);
+            CHECK(json::to_bon8(array) == expected_bon8);
         }
     }
 
@@ -493,6 +504,7 @@ TEST_CASE("issue #5392 - binary writers on deeply nested values")
         CHECK(json::from_msgpack(json::to_msgpack(object)) == object);
         CHECK(json::from_ubjson(json::to_ubjson(object, true, true)) == object);
         CHECK(json::from_bjdata(json::to_bjdata(object)) == object);
+        CHECK(json::from_bon8(json::to_bon8(object)) == object);
 
         const json ndarray = json({{"_ArrayType_", "uint8"}, {"_ArraySize_", {2, 3}}, {"_ArrayData_", {1, 2, 3, 4, 5, 6}}});
         const json deep_ndarray = nested_array(depth, ndarray);
@@ -523,6 +535,27 @@ TEST_CASE("issue #5392 - binary writers on deeply nested values")
         CHECK_THROWS_WITH_AS(json::to_bjdata(deep_discarded), (prefix + "cannot serialize discarded value to BJData").c_str(), json::type_error);
     }
 
+    SECTION("BON8 past the recursion bound: discarded values and errors")
+    {
+        const std::size_t depth = nlohmann::detail::recursion_depth_limit() + 50;
+
+        // BON8 writes nothing for a discarded value, deep or not
+        const json deep_discarded = nested_array(depth, json(json::value_t::discarded));
+        CHECK(json::to_bon8(deep_discarded) == std::vector<std::uint8_t>(depth, 0x81));
+
+        // errors are thrown from below the bound as from above it
+        const json deep_invalid_string = nested_array(depth, json(std::string("\x80")));
+        CHECK_THROWS_AS(json::to_bon8(deep_invalid_string), json::type_error);
+
+        json deep_invalid_key = json::object();
+        deep_invalid_key[std::string("\x80")] = 1;
+        deep_invalid_key = nested_object(depth, deep_invalid_key);
+        CHECK_THROWS_AS(json::to_bon8(deep_invalid_key), json::type_error);
+
+        const json deep_too_large = nested_array(depth, json(9223372036854775808u));
+        CHECK_THROWS_AS(json::to_bon8(deep_too_large), json::out_of_range);
+    }
+
     SECTION("does not overflow the C++ stack")
     {
         const std::size_t depth = 100000;
@@ -543,6 +576,27 @@ TEST_CASE("issue #5392 - binary writers on deeply nested values")
 
         CHECK_NOTHROW(packed = json::to_bjdata(j));
         CHECK(json::from_bjdata(packed) == j);
+
+        CHECK_NOTHROW(packed = json::to_bon8(j));
+        CHECK(json::from_bon8(packed) == j);
+    }
+
+    SECTION("BON8 containers with more than four elements past the recursion bound")
+    {
+        // such containers are closed with 0xFE, which the iterative writer
+        // must emit when it leaves them; strings next to each other must
+        // still be separated with 0xFF
+        const std::size_t depth = nlohmann::detail::recursion_depth_limit() + 50;
+        json wide_array = json::array({1, "a", "b", json::array(), 5});
+        json wide_object = json({{"k1", "v"}, {"k2", 2}, {"k3", ""}, {"k4", json::object()}, {"k5", nullptr}});
+        for (std::size_t i = 0; i < depth; ++i)
+        {
+            wide_array = json::array({"s", wide_array, "t", wide_object, true});
+            wide_object = json({{"a", wide_object}, {"b", "x"}, {"c", "y"}, {"d", wide_array.size()}, {"e", "z"}});
+        }
+
+        CHECK(json::from_bon8(json::to_bon8(wide_array)) == wide_array);
+        CHECK(json::from_bon8(json::to_bon8(wide_object)) == wide_object);
     }
 
     SECTION("regression test for https://issues.oss-fuzz.com/issues/566583014")
@@ -561,5 +615,14 @@ TEST_CASE("issue #5392 - binary writers on deeply nested values")
         const json j_msgpack = json::from_msgpack(v_msgpack);
         CHECK(json::to_msgpack(j_msgpack) == v_msgpack);
     }
+
+    SECTION("regression test for https://issues.oss-fuzz.com/issues/572238015")
+    {
+        // the BON8 analogue: 200000 nested one-element arrays down to null
+        std::vector<std::uint8_t> v(200000, 0x81);
+        v.push_back(0xFA);
+        const json j = json::from_bon8(v);
+        CHECK(json::to_bon8(j) == v);
+    }
 }