Merge remote-tracking branch 'origin/develop' into claude/iterative-diff

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/features/arbitrary_types.md

@@ -79,7 +79,6 @@ Some important things:
 * When using `get<your_type>()`, `your_type` **MUST** be [DefaultConstructible](https://en.cppreference.com/w/cpp/named_req/DefaultConstructible). (There is a way to bypass this requirement described later.)
 * In function `from_json`, use function [`at()`](../api/basic_json/at.md) to access the object values rather than `operator[]`. In case a key does not exist, `at` throws an exception that you can handle, whereas `operator[]` exhibits undefined behavior.
 * You do not need to add serializers or deserializers for STL types like `std::vector`: the library already implements these.
-* If you control the type, consider defining `to_json`/`from_json` as `friend` functions inside the class ("hidden friends"). Argument-dependent lookup then only finds them for your type, which also avoids a [GCC < 11 compilation error](../home/faq.md#incomplete-detector-type-with-gcc-11).
 
 
 ## Simplify your life with macros

docs/mkdocs/docs/home/faq.md

@@ -307,51 +307,6 @@ APP_CPPFLAGS += -frtti -fexceptions
 The code compiles successfully with [Android NDK](https://developer.android.com/ndk/index.html?hl=ml), Revision 9 - 11 (and possibly later) and [CrystaX's Android NDK](https://www.crystax.net/en/android/ndk) version 10.
 
 
-### Incomplete `detector` type with GCC < 11
-
-!!! question
-
-    Why does GCC 10 or older fail with `invalid use of incomplete type 'struct nlohmann::detail::detector<..., to_json_function, ...>'` for a type that holds an `optional` member?
-
-This happens with GCC 10 and older in C++11/C++14 mode when all of these hold:
-
-- a class `Holder` has an `optional<Dummy>` member (e.g., `boost::optional`),
-- `Dummy` has a constructor taking a `json` value, and
-- `to_json` for `Holder` is a free function in the namespace of `Dummy`.
-
-```cpp
-class Dummy {
-  public:
-    explicit Dummy(const nlohmann::json& j);
-};
-
-class Holder {
-    boost::optional<Dummy> d;
-};
-
-void to_json(nlohmann::json& j, const Holder& h);  // triggers the error
-```
-
-To decide whether `Dummy` is copyable, the compiler checks whether a `Dummy` can be converted to `json`. That check
-looks up `to_json` via argument-dependent lookup, finds the unrelated `to_json` for `Holder`, and eventually asks again
-whether `Dummy` is copyable. GCC before version 11 turns this cycle into a hard error; GCC 11 and later, Clang, and
-C++17 mode compile the code. The same error shows up without this library whenever a constrained converting constructor
-is involved, so the library can't avoid it.
-
-To work around this, define `to_json` (and `from_json`) as a *hidden friend* inside the class. That way,
-argument-dependent lookup only finds it for `Holder`:
-
-```cpp
-class Holder {
-    boost::optional<Dummy> d;
-
-    friend void to_json(nlohmann::json& j, const Holder& h) { /* ... */ }
-};
-```
-
-The [`NLOHMANN_DEFINE_TYPE_INTRUSIVE`](../api/macros/nlohmann_define_type_intrusive.md) macros define hidden friends as
-well. See [#3669](https://github.com/nlohmann/json/issues/3669) for details.
-
 ### Missing STL function
 
 !!! question "Questions"

include/nlohmann/json.hpp

@@ -6061,21 +6061,240 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     {
         // the patch
         basic_json result(value_t::array);
+        diff_recursively(result, source, target, path, 0);
+        return result;
+    }
 
-        // if the values are the same, return an empty patch
+  private:
+    /// @brief two arrays or two objects @ref diff_iteratively is diffing
+    struct diff_frame
+    {
+        diff_frame(const basic_json* source_, const basic_json* target_, const std::size_t path_length_) noexcept
+            : source(source_), target(target_), path_length(path_length_)
+        {}
+
+        // declared for GCC's -Weffc++, which asks for them in a class with
+        // pointer members and a non-trivial destructor; the exception
+        // specifications are left implicit, as GCC 4.8 rejects explicit ones
+        // that differ from them
+        diff_frame(const diff_frame&) = default;
+        diff_frame(diff_frame&&) = default;
+        diff_frame& operator=(const diff_frame&) = default;
+        diff_frame& operator=(diff_frame&&) = default;
+        ~diff_frame() = default;
+
+        /// the values being diffed, both arrays or both objects
+        const basic_json* source;
+        const basic_json* target;
+        /// the length of their path in `current_path`
+        std::size_t path_length;
+        /// arrays: the next index to diff
+        std::size_t index = 0;
+        /// objects: the next member of source to look at
+        const_iterator member{}; // NOLINT(readability-redundant-member-init)
+        /// objects: the keys common to both, in source's order
+        std::vector<typename object_t::key_type> common_keys{}; // NOLINT(readability-redundant-member-init)
+        /// objects: the next entry of common_keys
+        std::size_t next_common = 0;
+        /// objects: the "add" operations for keys only target has
+        basic_json added_ops{}; // NOLINT(readability-redundant-member-init)
+    };
+
+    // The operations of a diff are built by the functions below rather than
+    // where they are needed: building one takes several temporaries, and
+    // unoptimized builds give each temporary a stack slot of its own in the
+    // function it appears in. In diff_recursively, which is on the call stack
+    // once per nesting level, that made every level cost kilobytes of stack.
+
+    /// @brief append a "replace" operation for @a path with @a value to @a result
+    static void diff_replace(basic_json& result, const string_t& path, const basic_json& value)
+    {
+        result.push_back(
+        {
+            {"op", "replace"}, {"path", path}, {"value", value}
+        });
+    }
+
+    /// @brief append a "remove" operation for @a path to @a result
+    static void diff_remove(basic_json& result, const string_t& path)
+    {
+        result.push_back(object(
+        {
+            {"op", "remove"}, {"path", path}
+        }));
+    }
+
+    /// @brief append an "add" operation for @a path with @a value to @a result
+    static void diff_add(basic_json& result, const string_t& path, const basic_json& value)
+    {
+        result.push_back(
+        {
+            {"op", "add"}, {"path", path}, {"value", value}
+        });
+    }
+
+    /// @brief append the "remove" operations for the elements of array
+    ///        @a source from @a index on, and the "add" operations for the
+    ///        elements of array @a target from source's size on, to @a result
+    static void diff_array_tails(basic_json& result, const basic_json& source, const basic_json& target,
+                                 const string_t& path, const std::size_t index)
+    {
+        // remove my remaining elements, highest index first; appending
+        // in that order avoids the quadratic reinsertion done before
+        for (std::size_t j = source.size(); j > index; --j)
+        {
+            diff_remove(result, detail::concat<string_t>(path, '/', detail::to_string<string_t>(j - 1)));
+        }
+
+        // add other remaining elements
+        for (std::size_t i = source.size(); i < target.size(); ++i)
+        {
+            diff_add(result, detail::concat<string_t>(path, "/-"), target[i]);
+        }
+    }
+
+    /*!
+    @brief compare the keys of objects @a source and @a target
+
+    If the keys both objects have are in the same order in both, and the keys
+    only @a target has come after them, stores the keys common to both in
+    source's order in @a common_keys, stores the "add" operations for the keys
+    only @a target has in @a added_ops, and returns true: the caller then diffs
+    the objects member by member. Otherwise, appends operations that remove
+    every member of @a source and add every member of @a target to @a result,
+    and returns false.
+    */
+    static bool diff_object_keys(basic_json& result, const basic_json& source, const basic_json& target,
+                                 const string_t& path, std::vector<typename object_t::key_type>& common_keys,
+                                 basic_json& added_ops)
+    {
+        // first pass: record, for every source key, whether it is
+        // common to both objects (in source's iteration order) or
+        // was deleted (i.e., in source but not in target) -- this is
+        // a by-product of the target.find() call already needed to
+        // tell the two cases apart, so it adds no extra lookups. The
+        // "remove" ops themselves are emitted later, interleaved
+        // with the per-key diffs in the caller's fast path, to match
+        // source's original iteration order (as the original,
+        // pre-reordering-aware implementation did) instead of
+        // grouping all removes before all per-key diffs.
+        std::vector<typename object_t::key_type> common_keys_source_order;
+        for (auto it = source.cbegin(); it != source.cend(); ++it)
+        {
+            if (target.find(it.key()) != target.end())
+            {
+                common_keys_source_order.push_back(it.key());
+            }
+        }
+
+        // second pass: find keys that were added (i.e., in target but
+        // not in source), and record the keys common to both, in
+        // target's iteration order -- again a by-product of the
+        // source.find() call already needed to detect added keys. At
+        // the same time, determine whether every added key comes
+        // after every common key in target's order (a precondition
+        // for the fast path, which only ever appends new keys
+        // at the very end): for an object_t whose iteration order is
+        // a pure function of the key set (e.g. the default std::map,
+        // which always iterates in sorted key order), the order
+        // check further below is always true and this whole
+        // mechanism is effectively a no-op; it only matters for a
+        // reorderable object_t such as the one backing `ordered_json`.
+        // The patch ops for keys that were added (i.e., in target but not
+        // in source) are built here so the fast path can reuse
+        // them without a second source.find() per target key. Only
+        // used by the fast path -- the slow (reordering) path
+        // rebuilds "add" ops for every key itself.
+        std::vector<typename object_t::key_type> common_keys_target_order;
+        bool new_keys_form_suffix = true;
+        bool seen_new_key = false;
+        for (auto it = target.cbegin(); it != target.cend(); ++it)
+        {
+            if (source.find(it.key()) == source.end())
+            {
+                seen_new_key = true;
+                diff_add(added_ops, detail::concat<string_t>(path, '/', detail::escape(it.key())), it.value());
+            }
+            else
+            {
+                common_keys_target_order.push_back(it.key());
+                if (seen_new_key)
+                {
+                    new_keys_form_suffix = false;
+                }
+            }
+        }
+
+        if (common_keys_source_order == common_keys_target_order && new_keys_form_suffix)
+        {
+            // fast path: order of common keys already matches (or the
+            // object_t's iteration order does not depend on
+            // insertion history), so a plain per-key diff is correct
+            // and minimal, as before
+            common_keys = std::move(common_keys_source_order);
+            return true;
+        }
+
+        // slow path: the common keys are in a different relative
+        // order in source and target (only possible for a
+        // reorderable object_t like ordered_map). Building a
+        // minimal reordering patch is a nontrivial (LCS-like)
+        // problem; instead, remove every source key -- both
+        // deleted keys (which must be removed regardless) and
+        // common keys (removed so they can be re-added in
+        // target's order) -- and re-add every key that should
+        // remain, with its final target value, in target's
+        // order. basic_json::patch()'s "add" operation on an
+        // object uses operator[], which appends at the end for a
+        // vector-backed insertion-ordered map when the key does
+        // not already exist -- so removing a key and then adding
+        // it moves it to the end, fixing its position.
+        for (auto it = source.cbegin(); it != source.cend(); ++it)
+        {
+            diff_remove(result, detail::concat<string_t>(path, '/', detail::escape(it.key())));
+        }
+
+        // add every key that is either common (just removed
+        // above) or brand new, in target's iteration order, so
+        // that the final order after applying the patch matches
+        // target exactly
+        for (auto it = target.cbegin(); it != target.cend(); ++it)
+        {
+            diff_add(result, detail::concat<string_t>(path, '/', detail::escape(it.key())), it.value());
+        }
+        return false;
+    }
+
+    /*!
+    @brief @ref diff, for values at nesting level @a depth, appending the
+           operations to @a result
+
+    Diffing two arrays or objects calls this function again, once per nesting
+    level, so values nested deeply enough used to exhaust the call stack and
+    terminate the process. The descent is bounded here: once @ref
+    detail::recursion_depth_limit levels have been entered, @ref
+    diff_iteratively diffs what is left without the call stack.
+    */
+    static void diff_recursively(basic_json& result, const basic_json& source, const basic_json& target,
+                                 const string_t& path, const std::size_t depth)
+    {
+        // if the values are the same, there is nothing to do
         if (source == target)
         {
-            return result;
+            return;
+        }
+
+        if (JSON_HEDLEY_UNLIKELY(depth >= detail::recursion_depth_limit()))
+        {
+            diff_iteratively(result, source, target, path);
+            return;
         }
 
         if (source.type() != target.type())
         {
             // different types: replace value
-            result.push_back(
-            {
-                {"op", "replace"}, {"path", path}, {"value", target}
-            });
-            return result;
+            diff_replace(result, path, target);
+            return;
         }
 
         switch (source.type())
@@ -6087,185 +6306,50 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                 while (i < source.size() && i < target.size())
                 {
                     // recursive call to compare array values at index i
-                    auto temp_diff = diff(source[i], target[i], detail::concat<string_t>(path, '/', detail::to_string<string_t>(i)));
-                    result.insert(result.end(), temp_diff.begin(), temp_diff.end());
+                    diff_recursively(result, source[i], target[i], detail::concat<string_t>(path, '/', detail::to_string<string_t>(i)), depth + 1);
                     ++i;
                 }
 
                 // We now reached the end of at least one array
                 // in a second pass, traverse the remaining elements
-
-                // remove my remaining elements, highest index first; appending
-                // in that order avoids the quadratic reinsertion done before
-                for (std::size_t j = source.size(); j > i; --j)
-                {
-                    result.push_back(object(
-                    {
-                        {"op", "remove"},
-                        {"path", detail::concat<string_t>(path, '/', detail::to_string<string_t>(j - 1))}
-                    }));
-                }
-                i = source.size();
-
-                // add other remaining elements
-                while (i < target.size())
-                {
-                    result.push_back(
-                    {
-                        {"op", "add"},
-                        {"path", detail::concat<string_t>(path, "/-")},
-                        {"value", target[i]}
-                    });
-                    ++i;
-                }
-
+                diff_array_tails(result, source, target, path, i);
                 break;
             }
 
             case value_t::object:
             {
-                // first pass: record, for every source key, whether it is
-                // common to both objects (in source's iteration order) or
-                // was deleted (i.e., in source but not in target) -- this is
-                // a by-product of the target.find() call already needed to
-                // tell the two cases apart, so it adds no extra lookups. The
-                // "remove" ops themselves are emitted later, interleaved
-                // with the recursive per-key diffs in the fast path below,
-                // to match source's original iteration order (as the
-                // original, pre-reordering-aware implementation did) instead
-                // of grouping all removes before all recursive diffs.
-                std::vector<typename object_t::key_type> common_keys_source_order;
-                for (auto it = source.cbegin(); it != source.cend(); ++it)
-                {
-                    if (target.find(it.key()) != target.end())
-                    {
-                        common_keys_source_order.push_back(it.key());
-                    }
-                }
-
-                // second pass: find keys that were added (i.e., in target but
-                // not in source), and record the keys common to both, in
-                // target's iteration order -- again a by-product of the
-                // source.find() call already needed to detect added keys. At
-                // the same time, determine whether every added key comes
-                // after every common key in target's order (a precondition
-                // for the fast path below, which only ever appends new keys
-                // at the very end): for an object_t whose iteration order is
-                // a pure function of the key set (e.g. the default std::map,
-                // which always iterates in sorted key order), the order
-                // check further below is always true and this whole
-                // mechanism is effectively a no-op; it only matters for a
-                // reorderable object_t such as the one backing `ordered_json`.
-                // patch ops for keys that were added (i.e., in target but not
-                // in source); built here so the fast path below can reuse
-                // them without a second source.find() per target key. Only
-                // used by the fast path -- the slow (reordering) path
-                // rebuilds "add" ops for every key itself.
-                std::vector<typename object_t::key_type> common_keys_target_order;
+                std::vector<typename object_t::key_type> common_keys;
                 basic_json added_ops(value_t::array);
-                bool new_keys_form_suffix = true;
-                bool seen_new_key = false;
-                for (auto it = target.cbegin(); it != target.cend(); ++it)
+                if (diff_object_keys(result, source, target, path, common_keys, added_ops))
                 {
-                    if (source.find(it.key()) == source.end())
-                    {
-                        seen_new_key = true;
-                        const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
-                        added_ops.push_back(
-                        {
-                            {"op", "add"}, {"path", path_key},
-                            {"value", it.value()}
-                        });
-                    }
-                    else
-                    {
-                        common_keys_target_order.push_back(it.key());
-                        if (seen_new_key)
-                        {
-                            new_keys_form_suffix = false;
-                        }
-                    }
-                }
-
-                if (common_keys_source_order == common_keys_target_order && new_keys_form_suffix)
-                {
-                    // fast path: order of common keys already matches (or the
-                    // object_t's iteration order does not depend on
-                    // insertion history), so a plain per-key recursive diff
-                    // is correct and minimal, as before. common_keys_source_order
-                    // is, by construction, the subsequence of source's keys
-                    // that are common to both objects, in source's iteration
-                    // order -- so it can be walked in lockstep with `source`
-                    // using a cheap key comparison instead of another lookup.
-                    // Deleted keys (those source keys not in common_keys_source_order)
-                    // are interleaved here too, in source's original order, to
-                    // match the historical (pre-reordering-aware) output order.
-                    auto common_it = common_keys_source_order.cbegin();
+                    // fast path: common_keys is, by construction, the
+                    // subsequence of source's keys that are common to both
+                    // objects, in source's iteration order -- so it can be
+                    // walked in lockstep with `source` using a cheap key
+                    // comparison instead of another lookup. Deleted keys
+                    // (those source keys not in common_keys) are interleaved
+                    // here too, in source's original order, to match the
+                    // historical (pre-reordering-aware) output order.
+                    auto common_it = common_keys.cbegin();
                     for (auto it = source.cbegin(); it != source.cend(); ++it)
                     {
-                        if (common_it != common_keys_source_order.cend() && it.key() == *common_it)
+                        if (common_it != common_keys.cend() && it.key() == *common_it)
                         {
-                            const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
-                            auto temp_diff = diff(it.value(), target[it.key()], path_key);
-                            result.insert(result.end(), temp_diff.begin(), temp_diff.end());
+                            diff_recursively(result, it.value(), target[it.key()], detail::concat<string_t>(path, '/', detail::escape(it.key())), depth + 1);
                             ++common_it;
                         }
                         else
                         {
                             // found a key that is not in target -> remove it
-                            const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
-                            result.push_back(object(
-                            {
-                                {"op", "remove"}, {"path", path_key}
-                            }));
+                            diff_remove(result, detail::concat<string_t>(path, '/', detail::escape(it.key())));
                         }
                     }
 
-                    // append the "add" ops for brand-new keys collected above
-                    // during the pass over target -- no second source.find()
-                    // per target key needed
+                    // append the "add" ops for brand-new keys collected by
+                    // diff_object_keys -- no second source.find() per target
+                    // key needed
                     result.insert(result.end(), added_ops.begin(), added_ops.end());
                 }
-                else
-                {
-                    // slow path: the common keys are in a different relative
-                    // order in source and target (only possible for a
-                    // reorderable object_t like ordered_map). Building a
-                    // minimal reordering patch is a nontrivial (LCS-like)
-                    // problem; instead, remove every source key -- both
-                    // deleted keys (which must be removed regardless) and
-                    // common keys (removed so they can be re-added in
-                    // target's order) -- and re-add every key that should
-                    // remain, with its final target value, in target's
-                    // order. basic_json::patch()'s "add" operation on an
-                    // object uses operator[], which appends at the end for a
-                    // vector-backed insertion-ordered map when the key does
-                    // not already exist -- so removing a key and then adding
-                    // it moves it to the end, fixing its position.
-                    for (auto it = source.cbegin(); it != source.cend(); ++it)
-                    {
-                        const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
-                        result.push_back(object(
-                        {
-                            {"op", "remove"}, {"path", path_key}
-                        }));
-                    }
-
-                    // add every key that is either common (just removed
-                    // above) or brand new, in target's iteration order, so
-                    // that the final order after applying the patch matches
-                    // target exactly
-                    for (auto it = target.cbegin(); it != target.cend(); ++it)
-                    {
-                        const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
-                        result.push_back(
-                        {
-                            {"op", "add"}, {"path", path_key},
-                            {"value", it.value()}
-                        });
-                    }
-                }
-
                 break;
             }
 
@@ -6280,16 +6364,170 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             default:
             {
                 // both primitive types: replace value
-                result.push_back(
-                {
-                    {"op", "replace"}, {"path", path}, {"value", target}
-                });
+                diff_replace(result, path, target);
                 break;
             }
         }
-
-        return result;
     }
+
+    /*!
+    @brief @ref diff without the call stack, appending the operations to
+           @a result
+
+    Produces the same operations as @ref diff_recursively. Only reached for
+    values nested more deeply than @ref detail::recursion_depth_limit.
+    */
+    static void diff_iteratively(basic_json& result, const basic_json& source, const basic_json& target,
+                                 const string_t& path)
+    {
+        // The arrays and objects being diffed are kept on an explicit stack,
+        // and every pair of elements is still diffed completely before the
+        // next one, so the operations come out in the same order as in
+        // diff_recursively. The path of the values being diffed is kept in
+        // one buffer that grows and shrinks with the stack, rather than in a
+        // new string per level.
+        std::vector<diff_frame> stack;
+        string_t current_path = path;
+
+        // diff `s` against `t`, whose path is current_path: primitives,
+        // values of different types, and objects whose members were reordered
+        // are handled right away; arrays and other objects get a frame
+        const auto enter = [&result, &stack, &current_path](const basic_json & s, const basic_json & t)
+        {
+            // if the values are the same, there is nothing to do. Arrays and
+            // objects are not compared up front: comparing them visits
+            // everything below them, so doing that at every level would take
+            // quadratic time in the nesting depth - equal ones yield no
+            // operations anyway.
+            if ((!s.is_structured() || !t.is_structured()) && s == t)
+            {
+                return;
+            }
+
+            if (s.type() != t.type())
+            {
+                // different types: replace value
+                diff_replace(result, current_path, t);
+                return;
+            }
+
+            switch (s.type())
+            {
+                case value_t::array:
+                {
+                    stack.emplace_back(&s, &t, current_path.size());
+                    return;
+                }
+
+                case value_t::object:
+                {
+                    std::vector<typename object_t::key_type> common_keys;
+                    basic_json added_ops(value_t::array);
+                    if (diff_object_keys(result, s, t, current_path, common_keys, added_ops))
+                    {
+                        // fast path: the frame walks source in lockstep with
+                        // common_keys, as diff_recursively does, and appends
+                        // added_ops once all members are done
+                        stack.emplace_back(&s, &t, current_path.size());
+                        stack.back().member = s.cbegin();
+                        stack.back().common_keys = std::move(common_keys);
+                        stack.back().added_ops = std::move(added_ops);
+                    }
+                    return;
+                }
+
+                case value_t::null:
+                case value_t::string:
+                case value_t::boolean:
+                case value_t::number_integer:
+                case value_t::number_unsigned:
+                case value_t::number_float:
+                case value_t::binary:
+                case value_t::discarded:
+                default:
+                {
+                    // both primitive types: replace value
+                    diff_replace(result, current_path, t);
+                    return;
+                }
+            }
+        };
+
+        enter(source, target);
+        while (!stack.empty())
+        {
+            // the frame is copied out member by member and changed through
+            // stack.back(): enter() may push a frame and the end of the loop
+            // pops it, either of which would invalidate a reference to it
+            const basic_json* const s = stack.back().source;
+            const basic_json* const t = stack.back().target;
+            const std::size_t path_length = stack.back().path_length;
+            const std::size_t depth = stack.size();
+
+            if (s->is_array())
+            {
+                const auto& source_array = *s->m_data.m_value.array;
+                const auto& target_array = *t->m_data.m_value.array;
+
+                // first pass: traverse common elements
+                const std::size_t i = stack.back().index;
+                if (i < source_array.size() && i < target_array.size())
+                {
+                    ++stack.back().index;
+                    detail::concat_into(current_path, '/', detail::to_string<string_t>(i));
+                    enter(source_array[i], target_array[i]);
+                    if (stack.size() == depth)
+                    {
+                        current_path.resize(path_length);
+                    }
+                    continue;
+                }
+
+                // We now reached the end of at least one array
+                // in a second pass, traverse the remaining elements
+                diff_array_tails(result, *s, *t, current_path, i);
+            }
+            else
+            {
+                const const_iterator it = stack.back().member;
+                if (it != s->cend())
+                {
+                    ++stack.back().member;
+                    const std::size_t next_common = stack.back().next_common;
+                    if (next_common < stack.back().common_keys.size() && it.key() == stack.back().common_keys[next_common])
+                    {
+                        ++stack.back().next_common;
+                        const basic_json& target_value = (*t)[it.key()];
+                        detail::concat_into(current_path, '/', detail::escape(it.key()));
+                        enter(it.value(), target_value);
+                        if (stack.size() == depth)
+                        {
+                            current_path.resize(path_length);
+                        }
+                    }
+                    else
+                    {
+                        // found a key that is not in target -> remove it
+                        diff_remove(result, detail::concat<string_t>(current_path, '/', detail::escape(it.key())));
+                    }
+                    continue;
+                }
+
+                // append the "add" ops for brand-new keys collected when the
+                // object was entered
+                result.insert(result.end(), stack.back().added_ops.begin(), stack.back().added_ops.end());
+            }
+
+            // this array or object is done: continue with the one it is in
+            stack.pop_back();
+            if (!stack.empty())
+            {
+                current_path.resize(stack.back().path_length);
+            }
+        }
+    }
+
+  public:
     /// @}
 
     ////////////////////////////////

single_include/nlohmann/json.hpp

@@ -31944,21 +31944,240 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     {
         // the patch
         basic_json result(value_t::array);
+        diff_recursively(result, source, target, path, 0);
+        return result;
+    }
 
-        // if the values are the same, return an empty patch
+  private:
+    /// @brief two arrays or two objects @ref diff_iteratively is diffing
+    struct diff_frame
+    {
+        diff_frame(const basic_json* source_, const basic_json* target_, const std::size_t path_length_) noexcept
+            : source(source_), target(target_), path_length(path_length_)
+        {}
+
+        // declared for GCC's -Weffc++, which asks for them in a class with
+        // pointer members and a non-trivial destructor; the exception
+        // specifications are left implicit, as GCC 4.8 rejects explicit ones
+        // that differ from them
+        diff_frame(const diff_frame&) = default;
+        diff_frame(diff_frame&&) = default;
+        diff_frame& operator=(const diff_frame&) = default;
+        diff_frame& operator=(diff_frame&&) = default;
+        ~diff_frame() = default;
+
+        /// the values being diffed, both arrays or both objects
+        const basic_json* source;
+        const basic_json* target;
+        /// the length of their path in `current_path`
+        std::size_t path_length;
+        /// arrays: the next index to diff
+        std::size_t index = 0;
+        /// objects: the next member of source to look at
+        const_iterator member{}; // NOLINT(readability-redundant-member-init)
+        /// objects: the keys common to both, in source's order
+        std::vector<typename object_t::key_type> common_keys{}; // NOLINT(readability-redundant-member-init)
+        /// objects: the next entry of common_keys
+        std::size_t next_common = 0;
+        /// objects: the "add" operations for keys only target has
+        basic_json added_ops{}; // NOLINT(readability-redundant-member-init)
+    };
+
+    // The operations of a diff are built by the functions below rather than
+    // where they are needed: building one takes several temporaries, and
+    // unoptimized builds give each temporary a stack slot of its own in the
+    // function it appears in. In diff_recursively, which is on the call stack
+    // once per nesting level, that made every level cost kilobytes of stack.
+
+    /// @brief append a "replace" operation for @a path with @a value to @a result
+    static void diff_replace(basic_json& result, const string_t& path, const basic_json& value)
+    {
+        result.push_back(
+        {
+            {"op", "replace"}, {"path", path}, {"value", value}
+        });
+    }
+
+    /// @brief append a "remove" operation for @a path to @a result
+    static void diff_remove(basic_json& result, const string_t& path)
+    {
+        result.push_back(object(
+        {
+            {"op", "remove"}, {"path", path}
+        }));
+    }
+
+    /// @brief append an "add" operation for @a path with @a value to @a result
+    static void diff_add(basic_json& result, const string_t& path, const basic_json& value)
+    {
+        result.push_back(
+        {
+            {"op", "add"}, {"path", path}, {"value", value}
+        });
+    }
+
+    /// @brief append the "remove" operations for the elements of array
+    ///        @a source from @a index on, and the "add" operations for the
+    ///        elements of array @a target from source's size on, to @a result
+    static void diff_array_tails(basic_json& result, const basic_json& source, const basic_json& target,
+                                 const string_t& path, const std::size_t index)
+    {
+        // remove my remaining elements, highest index first; appending
+        // in that order avoids the quadratic reinsertion done before
+        for (std::size_t j = source.size(); j > index; --j)
+        {
+            diff_remove(result, detail::concat<string_t>(path, '/', detail::to_string<string_t>(j - 1)));
+        }
+
+        // add other remaining elements
+        for (std::size_t i = source.size(); i < target.size(); ++i)
+        {
+            diff_add(result, detail::concat<string_t>(path, "/-"), target[i]);
+        }
+    }
+
+    /*!
+    @brief compare the keys of objects @a source and @a target
+
+    If the keys both objects have are in the same order in both, and the keys
+    only @a target has come after them, stores the keys common to both in
+    source's order in @a common_keys, stores the "add" operations for the keys
+    only @a target has in @a added_ops, and returns true: the caller then diffs
+    the objects member by member. Otherwise, appends operations that remove
+    every member of @a source and add every member of @a target to @a result,
+    and returns false.
+    */
+    static bool diff_object_keys(basic_json& result, const basic_json& source, const basic_json& target,
+                                 const string_t& path, std::vector<typename object_t::key_type>& common_keys,
+                                 basic_json& added_ops)
+    {
+        // first pass: record, for every source key, whether it is
+        // common to both objects (in source's iteration order) or
+        // was deleted (i.e., in source but not in target) -- this is
+        // a by-product of the target.find() call already needed to
+        // tell the two cases apart, so it adds no extra lookups. The
+        // "remove" ops themselves are emitted later, interleaved
+        // with the per-key diffs in the caller's fast path, to match
+        // source's original iteration order (as the original,
+        // pre-reordering-aware implementation did) instead of
+        // grouping all removes before all per-key diffs.
+        std::vector<typename object_t::key_type> common_keys_source_order;
+        for (auto it = source.cbegin(); it != source.cend(); ++it)
+        {
+            if (target.find(it.key()) != target.end())
+            {
+                common_keys_source_order.push_back(it.key());
+            }
+        }
+
+        // second pass: find keys that were added (i.e., in target but
+        // not in source), and record the keys common to both, in
+        // target's iteration order -- again a by-product of the
+        // source.find() call already needed to detect added keys. At
+        // the same time, determine whether every added key comes
+        // after every common key in target's order (a precondition
+        // for the fast path, which only ever appends new keys
+        // at the very end): for an object_t whose iteration order is
+        // a pure function of the key set (e.g. the default std::map,
+        // which always iterates in sorted key order), the order
+        // check further below is always true and this whole
+        // mechanism is effectively a no-op; it only matters for a
+        // reorderable object_t such as the one backing `ordered_json`.
+        // The patch ops for keys that were added (i.e., in target but not
+        // in source) are built here so the fast path can reuse
+        // them without a second source.find() per target key. Only
+        // used by the fast path -- the slow (reordering) path
+        // rebuilds "add" ops for every key itself.
+        std::vector<typename object_t::key_type> common_keys_target_order;
+        bool new_keys_form_suffix = true;
+        bool seen_new_key = false;
+        for (auto it = target.cbegin(); it != target.cend(); ++it)
+        {
+            if (source.find(it.key()) == source.end())
+            {
+                seen_new_key = true;
+                diff_add(added_ops, detail::concat<string_t>(path, '/', detail::escape(it.key())), it.value());
+            }
+            else
+            {
+                common_keys_target_order.push_back(it.key());
+                if (seen_new_key)
+                {
+                    new_keys_form_suffix = false;
+                }
+            }
+        }
+
+        if (common_keys_source_order == common_keys_target_order && new_keys_form_suffix)
+        {
+            // fast path: order of common keys already matches (or the
+            // object_t's iteration order does not depend on
+            // insertion history), so a plain per-key diff is correct
+            // and minimal, as before
+            common_keys = std::move(common_keys_source_order);
+            return true;
+        }
+
+        // slow path: the common keys are in a different relative
+        // order in source and target (only possible for a
+        // reorderable object_t like ordered_map). Building a
+        // minimal reordering patch is a nontrivial (LCS-like)
+        // problem; instead, remove every source key -- both
+        // deleted keys (which must be removed regardless) and
+        // common keys (removed so they can be re-added in
+        // target's order) -- and re-add every key that should
+        // remain, with its final target value, in target's
+        // order. basic_json::patch()'s "add" operation on an
+        // object uses operator[], which appends at the end for a
+        // vector-backed insertion-ordered map when the key does
+        // not already exist -- so removing a key and then adding
+        // it moves it to the end, fixing its position.
+        for (auto it = source.cbegin(); it != source.cend(); ++it)
+        {
+            diff_remove(result, detail::concat<string_t>(path, '/', detail::escape(it.key())));
+        }
+
+        // add every key that is either common (just removed
+        // above) or brand new, in target's iteration order, so
+        // that the final order after applying the patch matches
+        // target exactly
+        for (auto it = target.cbegin(); it != target.cend(); ++it)
+        {
+            diff_add(result, detail::concat<string_t>(path, '/', detail::escape(it.key())), it.value());
+        }
+        return false;
+    }
+
+    /*!
+    @brief @ref diff, for values at nesting level @a depth, appending the
+           operations to @a result
+
+    Diffing two arrays or objects calls this function again, once per nesting
+    level, so values nested deeply enough used to exhaust the call stack and
+    terminate the process. The descent is bounded here: once @ref
+    detail::recursion_depth_limit levels have been entered, @ref
+    diff_iteratively diffs what is left without the call stack.
+    */
+    static void diff_recursively(basic_json& result, const basic_json& source, const basic_json& target,
+                                 const string_t& path, const std::size_t depth)
+    {
+        // if the values are the same, there is nothing to do
         if (source == target)
         {
-            return result;
+            return;
+        }
+
+        if (JSON_HEDLEY_UNLIKELY(depth >= detail::recursion_depth_limit()))
+        {
+            diff_iteratively(result, source, target, path);
+            return;
         }
 
         if (source.type() != target.type())
         {
             // different types: replace value
-            result.push_back(
-            {
-                {"op", "replace"}, {"path", path}, {"value", target}
-            });
-            return result;
+            diff_replace(result, path, target);
+            return;
         }
 
         switch (source.type())
@@ -31970,185 +32189,50 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                 while (i < source.size() && i < target.size())
                 {
                     // recursive call to compare array values at index i
-                    auto temp_diff = diff(source[i], target[i], detail::concat<string_t>(path, '/', detail::to_string<string_t>(i)));
-                    result.insert(result.end(), temp_diff.begin(), temp_diff.end());
+                    diff_recursively(result, source[i], target[i], detail::concat<string_t>(path, '/', detail::to_string<string_t>(i)), depth + 1);
                     ++i;
                 }
 
                 // We now reached the end of at least one array
                 // in a second pass, traverse the remaining elements
-
-                // remove my remaining elements, highest index first; appending
-                // in that order avoids the quadratic reinsertion done before
-                for (std::size_t j = source.size(); j > i; --j)
-                {
-                    result.push_back(object(
-                    {
-                        {"op", "remove"},
-                        {"path", detail::concat<string_t>(path, '/', detail::to_string<string_t>(j - 1))}
-                    }));
-                }
-                i = source.size();
-
-                // add other remaining elements
-                while (i < target.size())
-                {
-                    result.push_back(
-                    {
-                        {"op", "add"},
-                        {"path", detail::concat<string_t>(path, "/-")},
-                        {"value", target[i]}
-                    });
-                    ++i;
-                }
-
+                diff_array_tails(result, source, target, path, i);
                 break;
             }
 
             case value_t::object:
             {
-                // first pass: record, for every source key, whether it is
-                // common to both objects (in source's iteration order) or
-                // was deleted (i.e., in source but not in target) -- this is
-                // a by-product of the target.find() call already needed to
-                // tell the two cases apart, so it adds no extra lookups. The
-                // "remove" ops themselves are emitted later, interleaved
-                // with the recursive per-key diffs in the fast path below,
-                // to match source's original iteration order (as the
-                // original, pre-reordering-aware implementation did) instead
-                // of grouping all removes before all recursive diffs.
-                std::vector<typename object_t::key_type> common_keys_source_order;
-                for (auto it = source.cbegin(); it != source.cend(); ++it)
-                {
-                    if (target.find(it.key()) != target.end())
-                    {
-                        common_keys_source_order.push_back(it.key());
-                    }
-                }
-
-                // second pass: find keys that were added (i.e., in target but
-                // not in source), and record the keys common to both, in
-                // target's iteration order -- again a by-product of the
-                // source.find() call already needed to detect added keys. At
-                // the same time, determine whether every added key comes
-                // after every common key in target's order (a precondition
-                // for the fast path below, which only ever appends new keys
-                // at the very end): for an object_t whose iteration order is
-                // a pure function of the key set (e.g. the default std::map,
-                // which always iterates in sorted key order), the order
-                // check further below is always true and this whole
-                // mechanism is effectively a no-op; it only matters for a
-                // reorderable object_t such as the one backing `ordered_json`.
-                // patch ops for keys that were added (i.e., in target but not
-                // in source); built here so the fast path below can reuse
-                // them without a second source.find() per target key. Only
-                // used by the fast path -- the slow (reordering) path
-                // rebuilds "add" ops for every key itself.
-                std::vector<typename object_t::key_type> common_keys_target_order;
+                std::vector<typename object_t::key_type> common_keys;
                 basic_json added_ops(value_t::array);
-                bool new_keys_form_suffix = true;
-                bool seen_new_key = false;
-                for (auto it = target.cbegin(); it != target.cend(); ++it)
+                if (diff_object_keys(result, source, target, path, common_keys, added_ops))
                 {
-                    if (source.find(it.key()) == source.end())
-                    {
-                        seen_new_key = true;
-                        const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
-                        added_ops.push_back(
-                        {
-                            {"op", "add"}, {"path", path_key},
-                            {"value", it.value()}
-                        });
-                    }
-                    else
-                    {
-                        common_keys_target_order.push_back(it.key());
-                        if (seen_new_key)
-                        {
-                            new_keys_form_suffix = false;
-                        }
-                    }
-                }
-
-                if (common_keys_source_order == common_keys_target_order && new_keys_form_suffix)
-                {
-                    // fast path: order of common keys already matches (or the
-                    // object_t's iteration order does not depend on
-                    // insertion history), so a plain per-key recursive diff
-                    // is correct and minimal, as before. common_keys_source_order
-                    // is, by construction, the subsequence of source's keys
-                    // that are common to both objects, in source's iteration
-                    // order -- so it can be walked in lockstep with `source`
-                    // using a cheap key comparison instead of another lookup.
-                    // Deleted keys (those source keys not in common_keys_source_order)
-                    // are interleaved here too, in source's original order, to
-                    // match the historical (pre-reordering-aware) output order.
-                    auto common_it = common_keys_source_order.cbegin();
+                    // fast path: common_keys is, by construction, the
+                    // subsequence of source's keys that are common to both
+                    // objects, in source's iteration order -- so it can be
+                    // walked in lockstep with `source` using a cheap key
+                    // comparison instead of another lookup. Deleted keys
+                    // (those source keys not in common_keys) are interleaved
+                    // here too, in source's original order, to match the
+                    // historical (pre-reordering-aware) output order.
+                    auto common_it = common_keys.cbegin();
                     for (auto it = source.cbegin(); it != source.cend(); ++it)
                     {
-                        if (common_it != common_keys_source_order.cend() && it.key() == *common_it)
+                        if (common_it != common_keys.cend() && it.key() == *common_it)
                         {
-                            const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
-                            auto temp_diff = diff(it.value(), target[it.key()], path_key);
-                            result.insert(result.end(), temp_diff.begin(), temp_diff.end());
+                            diff_recursively(result, it.value(), target[it.key()], detail::concat<string_t>(path, '/', detail::escape(it.key())), depth + 1);
                             ++common_it;
                         }
                         else
                         {
                             // found a key that is not in target -> remove it
-                            const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
-                            result.push_back(object(
-                            {
-                                {"op", "remove"}, {"path", path_key}
-                            }));
+                            diff_remove(result, detail::concat<string_t>(path, '/', detail::escape(it.key())));
                         }
                     }
 
-                    // append the "add" ops for brand-new keys collected above
-                    // during the pass over target -- no second source.find()
-                    // per target key needed
+                    // append the "add" ops for brand-new keys collected by
+                    // diff_object_keys -- no second source.find() per target
+                    // key needed
                     result.insert(result.end(), added_ops.begin(), added_ops.end());
                 }
-                else
-                {
-                    // slow path: the common keys are in a different relative
-                    // order in source and target (only possible for a
-                    // reorderable object_t like ordered_map). Building a
-                    // minimal reordering patch is a nontrivial (LCS-like)
-                    // problem; instead, remove every source key -- both
-                    // deleted keys (which must be removed regardless) and
-                    // common keys (removed so they can be re-added in
-                    // target's order) -- and re-add every key that should
-                    // remain, with its final target value, in target's
-                    // order. basic_json::patch()'s "add" operation on an
-                    // object uses operator[], which appends at the end for a
-                    // vector-backed insertion-ordered map when the key does
-                    // not already exist -- so removing a key and then adding
-                    // it moves it to the end, fixing its position.
-                    for (auto it = source.cbegin(); it != source.cend(); ++it)
-                    {
-                        const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
-                        result.push_back(object(
-                        {
-                            {"op", "remove"}, {"path", path_key}
-                        }));
-                    }
-
-                    // add every key that is either common (just removed
-                    // above) or brand new, in target's iteration order, so
-                    // that the final order after applying the patch matches
-                    // target exactly
-                    for (auto it = target.cbegin(); it != target.cend(); ++it)
-                    {
-                        const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
-                        result.push_back(
-                        {
-                            {"op", "add"}, {"path", path_key},
-                            {"value", it.value()}
-                        });
-                    }
-                }
-
                 break;
             }
 
@@ -32163,16 +32247,170 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             default:
             {
                 // both primitive types: replace value
-                result.push_back(
-                {
-                    {"op", "replace"}, {"path", path}, {"value", target}
-                });
+                diff_replace(result, path, target);
                 break;
             }
         }
-
-        return result;
     }
+
+    /*!
+    @brief @ref diff without the call stack, appending the operations to
+           @a result
+
+    Produces the same operations as @ref diff_recursively. Only reached for
+    values nested more deeply than @ref detail::recursion_depth_limit.
+    */
+    static void diff_iteratively(basic_json& result, const basic_json& source, const basic_json& target,
+                                 const string_t& path)
+    {
+        // The arrays and objects being diffed are kept on an explicit stack,
+        // and every pair of elements is still diffed completely before the
+        // next one, so the operations come out in the same order as in
+        // diff_recursively. The path of the values being diffed is kept in
+        // one buffer that grows and shrinks with the stack, rather than in a
+        // new string per level.
+        std::vector<diff_frame> stack;
+        string_t current_path = path;
+
+        // diff `s` against `t`, whose path is current_path: primitives,
+        // values of different types, and objects whose members were reordered
+        // are handled right away; arrays and other objects get a frame
+        const auto enter = [&result, &stack, &current_path](const basic_json & s, const basic_json & t)
+        {
+            // if the values are the same, there is nothing to do. Arrays and
+            // objects are not compared up front: comparing them visits
+            // everything below them, so doing that at every level would take
+            // quadratic time in the nesting depth - equal ones yield no
+            // operations anyway.
+            if ((!s.is_structured() || !t.is_structured()) && s == t)
+            {
+                return;
+            }
+
+            if (s.type() != t.type())
+            {
+                // different types: replace value
+                diff_replace(result, current_path, t);
+                return;
+            }
+
+            switch (s.type())
+            {
+                case value_t::array:
+                {
+                    stack.emplace_back(&s, &t, current_path.size());
+                    return;
+                }
+
+                case value_t::object:
+                {
+                    std::vector<typename object_t::key_type> common_keys;
+                    basic_json added_ops(value_t::array);
+                    if (diff_object_keys(result, s, t, current_path, common_keys, added_ops))
+                    {
+                        // fast path: the frame walks source in lockstep with
+                        // common_keys, as diff_recursively does, and appends
+                        // added_ops once all members are done
+                        stack.emplace_back(&s, &t, current_path.size());
+                        stack.back().member = s.cbegin();
+                        stack.back().common_keys = std::move(common_keys);
+                        stack.back().added_ops = std::move(added_ops);
+                    }
+                    return;
+                }
+
+                case value_t::null:
+                case value_t::string:
+                case value_t::boolean:
+                case value_t::number_integer:
+                case value_t::number_unsigned:
+                case value_t::number_float:
+                case value_t::binary:
+                case value_t::discarded:
+                default:
+                {
+                    // both primitive types: replace value
+                    diff_replace(result, current_path, t);
+                    return;
+                }
+            }
+        };
+
+        enter(source, target);
+        while (!stack.empty())
+        {
+            // the frame is copied out member by member and changed through
+            // stack.back(): enter() may push a frame and the end of the loop
+            // pops it, either of which would invalidate a reference to it
+            const basic_json* const s = stack.back().source;
+            const basic_json* const t = stack.back().target;
+            const std::size_t path_length = stack.back().path_length;
+            const std::size_t depth = stack.size();
+
+            if (s->is_array())
+            {
+                const auto& source_array = *s->m_data.m_value.array;
+                const auto& target_array = *t->m_data.m_value.array;
+
+                // first pass: traverse common elements
+                const std::size_t i = stack.back().index;
+                if (i < source_array.size() && i < target_array.size())
+                {
+                    ++stack.back().index;
+                    detail::concat_into(current_path, '/', detail::to_string<string_t>(i));
+                    enter(source_array[i], target_array[i]);
+                    if (stack.size() == depth)
+                    {
+                        current_path.resize(path_length);
+                    }
+                    continue;
+                }
+
+                // We now reached the end of at least one array
+                // in a second pass, traverse the remaining elements
+                diff_array_tails(result, *s, *t, current_path, i);
+            }
+            else
+            {
+                const const_iterator it = stack.back().member;
+                if (it != s->cend())
+                {
+                    ++stack.back().member;
+                    const std::size_t next_common = stack.back().next_common;
+                    if (next_common < stack.back().common_keys.size() && it.key() == stack.back().common_keys[next_common])
+                    {
+                        ++stack.back().next_common;
+                        const basic_json& target_value = (*t)[it.key()];
+                        detail::concat_into(current_path, '/', detail::escape(it.key()));
+                        enter(it.value(), target_value);
+                        if (stack.size() == depth)
+                        {
+                            current_path.resize(path_length);
+                        }
+                    }
+                    else
+                    {
+                        // found a key that is not in target -> remove it
+                        diff_remove(result, detail::concat<string_t>(current_path, '/', detail::escape(it.key())));
+                    }
+                    continue;
+                }
+
+                // append the "add" ops for brand-new keys collected when the
+                // object was entered
+                result.insert(result.end(), stack.back().added_ops.begin(), stack.back().added_ops.end());
+            }
+
+            // this array or object is done: continue with the one it is in
+            stack.pop_back();
+            if (!stack.empty())
+            {
+                current_path.resize(stack.back().path_length);
+            }
+        }
+    }
+
+  public:
     /// @}
 
     ////////////////////////////////

tests/src/unit-json_patch.cpp

@@ -15,8 +15,65 @@ using nlohmann::json;
 #endif
 
 #include <fstream>
+#include <string>
+#include <vector>
 #include "make_test_data_available.hpp"
 
+namespace
+{
+// alternating objects and arrays nested `depth` levels deep, with members that
+// depend on `variant` at some levels, so diffing two variants yields
+// operations on many levels: replacing the innermost value, adding, removing,
+// and (for ordered_json) reordering members, and changing array lengths
+template<typename BasicJsonType>
+BasicJsonType nested(const std::size_t depth, const int variant)
+{
+    BasicJsonType value = variant;
+    for (std::size_t i = 0; i < depth; ++i)
+    {
+        if (i % 2 == 0)
+        {
+            BasicJsonType object = BasicJsonType::object();
+            if ((i + static_cast<std::size_t>(variant)) % 7 == 0)
+            {
+                object["x"] = i;
+            }
+            if (variant == 2 && i % 11 == 0)
+            {
+                object["z"] = "z";
+            }
+            object["a"] = std::move(value);
+            if (variant == 1 && i % 5 == 0)
+            {
+                object["y"] = 1;
+            }
+            value = std::move(object);
+        }
+        else
+        {
+            BasicJsonType array = BasicJsonType::array({std::move(value)});
+            if ((i + static_cast<std::size_t>(variant)) % 3 == 0)
+            {
+                array.push_back(i);
+            }
+            value = std::move(array);
+        }
+    }
+    return value;
+}
+
+// a path of `depth` reference tokens, as nested() nests its values
+std::string nested_path(const std::size_t depth)
+{
+    std::string path;
+    for (std::size_t i = depth; i > 0; --i)
+    {
+        path += (i - 1) % 2 == 0 ? "/a" : "/0";
+    }
+    return path;
+}
+} // namespace
+
 TEST_CASE("JSON patch")
 {
     SECTION("examples from RFC 6902")
@@ -1752,6 +1809,102 @@ TEST_CASE("JSON patch - diff emits array removals in descending index order")
     }
 }
 
+TEST_CASE("JSON patch: diff of deeply nested values")
+{
+    SECTION("the diff reproduces the target at every depth")
+    {
+        // depths on either side of the nesting depth up to which diff()
+        // recurses (detail::recursion_depth_limit(), 128); not every depth up
+        // to 300, as the test would then time out under Valgrind
+        std::vector<std::size_t> depths;
+        for (std::size_t depth = 0; depth <= 16; ++depth)
+        {
+            depths.push_back(depth);
+        }
+        for (std::size_t depth = 120; depth <= 136; ++depth)
+        {
+            depths.push_back(depth);
+        }
+        depths.push_back(300);
+
+        for (const auto depth : depths)
+        {
+            CAPTURE(depth);
+            for (int from = 0; from < 3; ++from)
+            {
+                for (int to = 0; to < 3; ++to)
+                {
+                    CAPTURE(from);
+                    CAPTURE(to);
+                    const auto source = nested<json>(depth, from);
+                    const auto target = nested<json>(depth, to);
+                    const auto patch = json::diff(source, target);
+                    CHECK(source.patch(patch) == target);
+                    CHECK(patch.empty() == (from == to));
+
+                    const auto ordered_source = nested<nlohmann::ordered_json>(depth, from);
+                    const auto ordered_target = nested<nlohmann::ordered_json>(depth, to);
+                    CHECK(ordered_source.patch(nlohmann::ordered_json::diff(ordered_source, ordered_target)) == ordered_target);
+                }
+            }
+        }
+    }
+
+    SECTION("a difference only in the innermost value is one replace operation")
+    {
+        for (std::size_t depth = 0; depth <= 300; ++depth)
+        {
+            CAPTURE(depth);
+            json source = 1;
+            json target = 2;
+            for (std::size_t i = 0; i < depth; ++i)
+            {
+                source = i % 2 == 0 ? json::object({{"a", std::move(source)}}) : json::array({std::move(source)});
+                target = i % 2 == 0 ? json::object({{"a", std::move(target)}}) : json::array({std::move(target)});
+            }
+            CHECK(json::diff(source, target, "/root") == json::array({{{"op", "replace"}, {"path", "/root" + nested_path(depth)}, {"value", 2}}}));
+        }
+    }
+
+    SECTION("values nested too deeply for the call stack (#5393)")
+    {
+        // diff() used to recurse once per nesting level, and compared the
+        // values with operator== on every level. The values are only
+        // parsed and diffed, never copied or compared, since those recurse
+        // too.
+        const std::size_t depth = 100000;
+        for (const bool objects :
+                {
+                    false, true
+                })
+        {
+            CAPTURE(objects);
+            std::string source_text;
+            std::string target_text;
+            std::string equal_text;
+            std::string path;
+            for (std::size_t i = 0; i < depth; ++i)
+            {
+                source_text += objects ? "{\"a\":" : "[";
+                path += objects ? "/a" : "/0";
+            }
+            target_text = source_text + "2";
+            equal_text = source_text + "1";
+            source_text += "1";
+            const std::string closing(depth, objects ? '}' : ']');
+            const auto source = json::parse(source_text + closing);
+
+            const auto patch = json::diff(source, json::parse(target_text + closing));
+            REQUIRE(patch.size() == 1);
+            CHECK(patch[0]["op"] == "replace");
+            CHECK(patch[0]["path"] == path);
+            CHECK(patch[0]["value"] == 2);
+
+            CHECK(json::diff(source, json::parse(equal_text + closing)).empty());
+        }
+    }
+}
+
 TEST_CASE("JSON patch - every operation on ordered_json")
 {
     using nlohmann::ordered_json;

tests/src/unit-regression2.cpp

@@ -241,52 +241,6 @@ class my_allocator : public std::allocator<T>
     };
 };
 
-/////////////////////////////////////////////////////////////////////
-// for #3669
-/////////////////////////////////////////////////////////////////////
-
-// mimics boost::optional's converting constructor, whose SFINAE check asks
-// whether T is constructible from const U&
-template<class T, class Arg>
-struct issue3669_is_constructible
-{
-    template<class T2, class A2, class = decltype(T2(std::declval<A2>()))>
-    static char test(int);
-    template<class, class>
-    static long test(...);
-    static constexpr bool value = sizeof(test<T, Arg>(0)) == 1;
-};
-
-template<class T>
-class issue3669_optional
-{
-  public:
-    issue3669_optional() = default;
-    template<class U>
-    issue3669_optional(const issue3669_optional<U>& /*unused*/, // NOLINT(google-explicit-constructor,hicpp-explicit-conversions)
-                       typename std::enable_if<issue3669_is_constructible<T, const U&>::value, bool>::type /*unused*/ = true) {}
-};
-
-class Issue3669Dummy
-{
-  public:
-    explicit Issue3669Dummy(const json& /*unused*/) {}
-};
-
-class Issue3669Holder
-{
-    issue3669_optional<Issue3669Dummy> d{};
-
-    // GCC < 11 (C++11/14) rejects a free to_json(json&, const Issue3669Holder&)
-    // here, because ADL for Issue3669Dummy finds it and closes an instantiation
-    // cycle; a hidden friend is only visible to ADL for Issue3669Holder
-    friend void to_json(json& j, const Issue3669Holder& h)
-    {
-        static_cast<void>(h.d); // silence -Wunused-private-field
-        j = "holder";
-    }
-};
-
 TEST_CASE("regression tests 2")
 {
     SECTION("issue #1001 - Fix memory leak during parser callback")
@@ -812,14 +766,6 @@ TEST_CASE("regression tests 2")
         CHECK(j == k);
     }
 
-    SECTION("issue #3669 - invalid use of incomplete type with optional member and to_json")
-    {
-        const Issue3669Holder h{};
-        const Issue3669Holder h2(h); // NOLINT(performance-unnecessary-copy-initialization)
-        const json j = h2;
-        CHECK(j == "holder");
-    }
-
 }
 
 TEST_CASE("regression test - parser callback must not lose a duplicate key's prior value")