Fix the Meson CMake target for includedir or datadir outside the prefix

nlohmann_jsonTargets.cmake always prepended the prefix computed from the
file's location to includedir. With an absolute includedir outside the
prefix (as Nix passes for packages with a separate dev output), this gave
"<prefix>//abs/include"; with an absolute datadir outside the prefix, the
number of ".." was derived from the absolute path and the prefix resolved
to "/". In both cases find_package(nlohmann_json) failed with "Imported
target includes non-existent path". Use the absolute include directory in
these cases, as CMake's install(EXPORT) does, and check it in CI.

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

.github/workflows/ubuntu.yml

@@ -31,6 +31,45 @@ jobs:
       - name: Build
         run: cmake --build build --target ci_test_gcc
 
+  ci_meson_install:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+      - name: Get latest CMake and ninja
+        uses: lukka/get-cmake@fffaaafeea488556c2c12dad60690008bc1caacb # v4.4.2
+      - name: Install Meson
+        run: pip install meson
+      - name: Install with Meson
+        run: |
+          meson setup build-meson --prefix=${{ github.workspace }}/install
+          meson install -C build-meson
+      - name: Use the installed package with find_package
+        run: |
+          cmake -S tests/cmake_import/project -B build-import -DCMAKE_PREFIX_PATH=${{ github.workspace }}/install
+          cmake --build build-import
+      - name: Install with Meson and non-default options
+        run: |
+          meson setup build-meson-options --prefix=${{ github.workspace }}/install-options -DMultipleHeaders=true -DDiagnostics=true -DGlobalUDLs=false
+          meson install -C build-meson-options
+      - name: Check that the options reach the installed files
+        run: |
+          test -d install-options/include/nlohmann/detail
+          cflags=$(PKG_CONFIG_PATH=${{ github.workspace }}/install-options/share/pkgconfig pkg-config --cflags nlohmann_json)
+          echo "$cflags"
+          echo "$cflags" | grep -q -- '-DJSON_DIAGNOSTICS=1'
+          echo "$cflags" | grep -q -- '-DJSON_USE_GLOBAL_UDLS=0'
+          grep -q 'JSON_USE_GLOBAL_UDLS=0;JSON_DIAGNOSTICS=1' install-options/share/cmake/nlohmann_json/nlohmann_jsonTargets.cmake
+          cmake -S tests/cmake_import/project -B build-import-options -DCMAKE_PREFIX_PATH=${{ github.workspace }}/install-options
+          cmake --build build-import-options
+      - name: Install with Meson and the include directory outside the prefix
+        run: |
+          meson setup build-meson-split --prefix=${{ github.workspace }}/install-split --includedir=${{ github.workspace }}/install-split-dev/include
+          meson install -C build-meson-split
+          cmake -S tests/cmake_import/project -B build-import-split -DCMAKE_PREFIX_PATH=${{ github.workspace }}/install-split
+          cmake --build build-import-split
+
   ci_infer:
     runs-on: ubuntu-latest
     steps:

CMakeLists.txt

@@ -168,7 +168,30 @@ if (MSVC)
     )
 endif()
 
-# Install a pkg-config file, so other tools can find this.
+# Install a pkg-config file, so other tools can find this. It carries the same
+# compile definitions as the target above.
+set(NLOHMANN_JSON_PKGCONFIG_CFLAGS "")
+if (NOT JSON_GlobalUDLs)
+    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_USE_GLOBAL_UDLS=0")
+endif()
+if (NOT JSON_ImplicitConversions)
+    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_USE_IMPLICIT_CONVERSIONS=0")
+endif()
+if (JSON_DisableEnumSerialization)
+    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_DISABLE_ENUM_SERIALIZATION=1")
+endif()
+if (JSON_Diagnostics)
+    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_DIAGNOSTICS=1")
+endif()
+if (JSON_Diagnostic_Positions)
+    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_DIAGNOSTIC_POSITIONS=1")
+endif()
+if (JSON_LegacyDiscardedValueComparison)
+    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON=1")
+endif()
+if (JSON_StrictNulHandling)
+    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_STRICT_NUL_HANDLING=1")
+endif()
 configure_file(
     "${CMAKE_CURRENT_SOURCE_DIR}/cmake/pkg-config.pc.in"
     "${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc"

FILES.md

@@ -260,9 +260,29 @@ make BUILD.bazel
 
 The "Check amalgamation" workflow fails if the file is out of date.
 
-### `meson.build`
+### `meson.build` and `meson_options.txt`
 
-The build definition for the [Meson](https://mesonbuild.com) build system.
+Meson build definitions suitable for use as a subproject ("wrap" in Meson terminology).
+
+Projects wishing to use the wrap can execute:
+```sh
+meson wrap install nlohmann_json
+```
+
+Which allows Meson to build from source when a system provided dependency isn't available.
+
+To build directly:
+```sh
+meson setup builddir
+ninja -C builddir
+```
+
+`meson_options.txt` defines the options, which mirror the CMake options that change the library's target (for example,
+`-DDiagnostics=true`). Meson requires this file next to `meson.build`, so it is also part of `include.zip`.
+
+When installing, `meson.build` installs the headers, a pkg-config file, and the CMake package config files, so that
+`find_package(nlohmann_json)` works. As Meson cannot generate `nlohmann_jsonTargets.cmake` itself, it is created from
+the template `cmake/nlohmann_jsonTargets.cmake.in`, which is only used by Meson.
 
 ### `Package.swift`
 

Makefile

@@ -222,7 +222,7 @@ json.tar.xz:
 # We use `-X` to make the resulting ZIP file reproducible, see
 # <https://content.pivotal.io/blog/barriers-to-deterministic-reproducible-zip-files>.
 include.zip: BUILD.bazel
-	zip -9 --recurse-paths -X include.zip $(SRCS) $(AMALGAMATED_FILE) $(AMALGAMATED_FWD_FILE) BUILD.bazel MODULE.bazel meson.build LICENSE.MIT
+	zip -9 --recurse-paths -X include.zip $(SRCS) $(AMALGAMATED_FILE) $(AMALGAMATED_FWD_FILE) BUILD.bazel MODULE.bazel meson.build meson_options.txt LICENSE.MIT
 
 # Create the files for a release and add signatures and hashes.
 release: include.zip json.tar.xz

cmake/nlohmann_jsonTargets.cmake.in

@@ -0,0 +1,38 @@
+# Imported target for installations made with Meson (see meson.build).
+#
+# CMake installations generate this file with install(EXPORT ...). Meson cannot
+# do that, but as the library is header-only, the target only needs an include
+# directory, the C++ standard, and the compile definitions of the options that
+# differ from their defaults. Paths are computed relative to this file so that
+# the installation can be relocated (e.g., into a sysroot), unless includedir or
+# datadir is outside the prefix.
+
+if(TARGET @PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@)
+    return()
+endif()
+
+get_filename_component(_IMPORT_PREFIX "${CMAKE_CURRENT_LIST_DIR}/@NLOHMANN_JSON_CONFIG_TO_PREFIX@" ABSOLUTE)
+
+add_library(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ INTERFACE IMPORTED)
+set_target_properties(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ PROPERTIES
+    INTERFACE_INCLUDE_DIRECTORIES "@NLOHMANN_JSON_INCLUDE_DIR@"
+)
+if(CMAKE_VERSION VERSION_LESS 3.8)
+    set_target_properties(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ PROPERTIES
+        INTERFACE_COMPILE_FEATURES cxx_range_for
+    )
+else()
+    set_target_properties(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ PROPERTIES
+        INTERFACE_COMPILE_FEATURES cxx_std_11
+    )
+endif()
+
+set(_NLOHMANN_JSON_COMPILE_DEFINITIONS "@NLOHMANN_JSON_COMPILE_DEFINITIONS@")
+if(_NLOHMANN_JSON_COMPILE_DEFINITIONS)
+    set_target_properties(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ PROPERTIES
+        INTERFACE_COMPILE_DEFINITIONS "${_NLOHMANN_JSON_COMPILE_DEFINITIONS}"
+    )
+endif()
+
+unset(_NLOHMANN_JSON_COMPILE_DEFINITIONS)
+unset(_IMPORT_PREFIX)

cmake/pkg-config.pc.in

@@ -4,4 +4,4 @@ includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@
 Name: @PROJECT_NAME@
 Description: JSON for Modern C++
 Version: @PROJECT_VERSION@
-Cflags: -I${includedir}
+Cflags: -I${includedir}@NLOHMANN_JSON_PKGCONFIG_CFLAGS@

docs/mkdocs/docs/integration/package_managers.md

@@ -121,10 +121,23 @@ meson wrap install nlohmann_json
 Please see the Meson project for any issues regarding the packaging.
 
 The provided `meson.build` can also be used as an alternative to CMake for installing `nlohmann_json` system-wide in
-which case a pkg-config file is installed. To use it, have your build system require the `nlohmann_json`
-pkg-config dependency. In Meson, it is preferred to use the
-[`dependency()`](https://mesonbuild.com/Reference-manual.html#dependency) object with a subproject fallback, rather than
-using the subproject directly.
+which case a pkg-config file and the CMake package config files are installed. To use it, have your build system require
+the `nlohmann_json` pkg-config dependency, or use [`find_package(nlohmann_json)`](cmake.md#external) in CMake. In Meson,
+it is preferred to use the [`dependency()`](https://mesonbuild.com/Reference-manual.html#dependency) object with a
+subproject fallback, rather than using the subproject directly.
+
+The options that change the library's configuration are available in Meson as well, named like the
+[CMake options](cmake.md#cmake-options) without the `JSON_` prefix: `MultipleHeaders`, `GlobalUDLs`,
+`ImplicitConversions`, `DisableEnumSerialization`, `Diagnostics`, `Diagnostic_Positions`,
+`LegacyDiscardedValueComparison`, and `StrictNulHandling`. They have the same defaults as in CMake, except that
+`MultipleHeaders` is `false`. Set them with `-D` when setting up the build, or with the subproject name as prefix when
+the library is used as a subproject:
+
+```shell
+meson setup build -Dnlohmann_json:Diagnostics=true
+```
+
+The resulting compile definitions are part of the Meson dependency, the pkg-config file, and the CMake target.
 
 ??? example "Example: Wrap"
 

include/nlohmann/json.hpp

@@ -6061,240 +6061,21 @@ 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;
-    }
 
-  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 the values are the same, return an empty patch
         if (source == target)
         {
-            return;
-        }
-
-        if (JSON_HEDLEY_UNLIKELY(depth >= detail::recursion_depth_limit()))
-        {
-            diff_iteratively(result, source, target, path);
-            return;
+            return result;
         }
 
         if (source.type() != target.type())
         {
             // different types: replace value
-            diff_replace(result, path, target);
-            return;
+            result.push_back(
+            {
+                {"op", "replace"}, {"path", path}, {"value", target}
+            });
+            return result;
         }
 
         switch (source.type())
@@ -6306,50 +6087,185 @@ 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
-                    diff_recursively(result, source[i], target[i], detail::concat<string_t>(path, '/', detail::to_string<string_t>(i)), depth + 1);
+                    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());
                     ++i;
                 }
 
                 // We now reached the end of at least one array
                 // in a second pass, traverse the remaining elements
-                diff_array_tails(result, source, target, path, i);
+
+                // 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;
+                }
+
                 break;
             }
 
             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, source, target, path, common_keys, 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 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)
                 {
-                    // 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();
+                    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;
+                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 (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();
                     for (auto it = source.cbegin(); it != source.cend(); ++it)
                     {
-                        if (common_it != common_keys.cend() && it.key() == *common_it)
+                        if (common_it != common_keys_source_order.cend() && it.key() == *common_it)
                         {
-                            diff_recursively(result, it.value(), target[it.key()], detail::concat<string_t>(path, '/', detail::escape(it.key())), depth + 1);
+                            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());
                             ++common_it;
                         }
                         else
                         {
                             // found a key that is not in target -> remove it
-                            diff_remove(result, detail::concat<string_t>(path, '/', detail::escape(it.key())));
+                            const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
+                            result.push_back(object(
+                            {
+                                {"op", "remove"}, {"path", path_key}
+                            }));
                         }
                     }
 
-                    // append the "add" ops for brand-new keys collected by
-                    // diff_object_keys -- no second source.find() per target
-                    // key needed
+                    // append the "add" ops for brand-new keys collected above
+                    // during the pass over target -- 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;
             }
 
@@ -6364,170 +6280,16 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             default:
             {
                 // both primitive types: replace value
-                diff_replace(result, path, target);
+                result.push_back(
+                {
+                    {"op", "replace"}, {"path", path}, {"value", 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:
     /// @}
 
     ////////////////////////////////

meson.build

@@ -2,23 +2,111 @@ project('nlohmann_json',
     'cpp',
     version : '3.12.0',
     license : 'MIT',
+    meson_version : '>= 0.64',
+    default_options: ['cpp_std=c++11'],
 )
 
+if get_option('MultipleHeaders')
+    incdir = 'include'
+else
+    incdir = 'single_include'
+endif
+
+# The same compile definitions as the CMake target (see target_compile_definitions
+# in CMakeLists.txt): only an option that differs from its default adds one.
+json_defines = []
+if not get_option('GlobalUDLs')
+    json_defines += 'JSON_USE_GLOBAL_UDLS=0'
+endif
+if not get_option('ImplicitConversions')
+    json_defines += 'JSON_USE_IMPLICIT_CONVERSIONS=0'
+endif
+if get_option('DisableEnumSerialization')
+    json_defines += 'JSON_DISABLE_ENUM_SERIALIZATION=1'
+endif
+if get_option('Diagnostics')
+    json_defines += 'JSON_DIAGNOSTICS=1'
+endif
+if get_option('Diagnostic_Positions')
+    json_defines += 'JSON_DIAGNOSTIC_POSITIONS=1'
+endif
+if get_option('LegacyDiscardedValueComparison')
+    json_defines += 'JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON=1'
+endif
+if get_option('StrictNulHandling')
+    json_defines += 'JSON_STRICT_NUL_HANDLING=1'
+endif
+
+cpp_args = []
+foreach define : json_defines
+    cpp_args += '-D' + define
+endforeach
+
 nlohmann_json_dep = declare_dependency(
-    include_directories: include_directories('single_include')
+    compile_args: cpp_args,
+    include_directories: include_directories(incdir)
 )
+meson.override_dependency('nlohmann_json', nlohmann_json_dep)
 
+# The multi-header version under the name earlier versions of this file used
 nlohmann_json_multiple_headers = declare_dependency(
+    compile_args: cpp_args,
     include_directories: include_directories('include')
 )
 
 if not meson.is_subproject()
-install_headers('single_include/nlohmann/json.hpp', subdir: 'nlohmann')
-install_headers('single_include/nlohmann/json_fwd.hpp', subdir: 'nlohmann')
+    install_subdir(
+        incdir / 'nlohmann',
+        install_dir: get_option('includedir'),
+        install_tag: 'devel',
+    )
 
-pkgc = import('pkgconfig')
-pkgc.generate(name: 'nlohmann_json',
-    version: meson.project_version(),
-    description: 'JSON for Modern C++'
-)
+    pkgc = import('pkgconfig')
+    pkgc.generate(name: 'nlohmann_json',
+        version: meson.project_version(),
+        description: 'JSON for Modern C++',
+        extra_cflags: cpp_args,
+        install_dir: get_option('datadir') / 'pkgconfig',
+    )
+
+    # CMake package config files, so that find_package(nlohmann_json) works. The
+    # include directory is given relative to the config files, so that the
+    # installation can be relocated. This is not possible if includedir or datadir
+    # is an absolute path outside the prefix (e.g., with the separate outputs of
+    # Nix); then the absolute include directory is used, as CMake does.
+    fs = import('fs')
+    cmake_install_dir = get_option('datadir') / 'cmake' / meson.project_name()
+    cmake_to_prefix = []
+    if fs.is_absolute(get_option('includedir')) or fs.is_absolute(cmake_install_dir)
+        cmake_include_dir = (get_option('prefix') / get_option('includedir')).replace('\\', '/')
+    else
+        foreach component : cmake_install_dir.split('/')
+            cmake_to_prefix += '..'
+        endforeach
+        cmake_include_dir = '${_IMPORT_PREFIX}/' + get_option('includedir')
+    endif
+
+    cmake_conf = configuration_data()
+    cmake_conf.set('PROJECT_NAME', meson.project_name())
+    cmake_conf.set('PROJECT_VERSION', meson.project_version())
+    cmake_conf.set('PROJECT_VERSION_MAJOR', meson.project_version().split('.')[0])
+    cmake_conf.set('NLOHMANN_JSON_TARGET_NAME', meson.project_name())
+    cmake_conf.set('NLOHMANN_JSON_TARGETS_EXPORT_NAME', meson.project_name() + 'Targets')
+    cmake_conf.set('NLOHMANN_JSON_INCLUDE_DIR', cmake_include_dir)
+    cmake_conf.set('NLOHMANN_JSON_CONFIG_TO_PREFIX', '/'.join(cmake_to_prefix))
+    cmake_conf.set('NLOHMANN_JSON_COMPILE_DEFINITIONS', ';'.join(json_defines))
+
+    foreach cmake_file : [
+        ['cmake/config.cmake.in', 'nlohmann_jsonConfig.cmake'],
+        ['cmake/nlohmann_jsonConfigVersion.cmake.in', 'nlohmann_jsonConfigVersion.cmake'],
+        ['cmake/nlohmann_jsonTargets.cmake.in', 'nlohmann_jsonTargets.cmake'],
+    ]
+        configure_file(
+            input: cmake_file[0],
+            output: cmake_file[1],
+            configuration: cmake_conf,
+            format: 'cmake@',
+            install_dir: cmake_install_dir,
+        )
+    endforeach
 endif

meson_options.txt

@@ -0,0 +1,48 @@
+option(
+    'MultipleHeaders',
+    type: 'boolean',
+    value: false,
+    description: 'Use non-amalgamated version of the library',
+)
+option(
+    'GlobalUDLs',
+    type: 'boolean',
+    value: true,
+    description: 'Place user-defined string literals in the global namespace',
+)
+option(
+    'ImplicitConversions',
+    type: 'boolean',
+    value: true,
+    description: 'Enable implicit conversions',
+)
+option(
+    'DisableEnumSerialization',
+    type: 'boolean',
+    value: false,
+    description: 'Disable default integer enum serialization',
+)
+option(
+    'Diagnostics',
+    type: 'boolean',
+    value: false,
+    description: 'Use extended diagnostic messages',
+)
+option(
+    'Diagnostic_Positions',
+    type: 'boolean',
+    value: false,
+    description: 'Enable diagnostic positions',
+)
+option(
+    'LegacyDiscardedValueComparison',
+    type: 'boolean',
+    value: false,
+    description: 'Enable legacy discarded value comparison',
+)
+option(
+    'StrictNulHandling',
+    type: 'boolean',
+    value: false,
+    description: 'Enable strict NUL-byte handling',
+)

single_include/nlohmann/json.hpp

@@ -31944,240 +31944,21 @@ 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;
-    }
 
-  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 the values are the same, return an empty patch
         if (source == target)
         {
-            return;
-        }
-
-        if (JSON_HEDLEY_UNLIKELY(depth >= detail::recursion_depth_limit()))
-        {
-            diff_iteratively(result, source, target, path);
-            return;
+            return result;
         }
 
         if (source.type() != target.type())
         {
             // different types: replace value
-            diff_replace(result, path, target);
-            return;
+            result.push_back(
+            {
+                {"op", "replace"}, {"path", path}, {"value", target}
+            });
+            return result;
         }
 
         switch (source.type())
@@ -32189,50 +31970,185 @@ 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
-                    diff_recursively(result, source[i], target[i], detail::concat<string_t>(path, '/', detail::to_string<string_t>(i)), depth + 1);
+                    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());
                     ++i;
                 }
 
                 // We now reached the end of at least one array
                 // in a second pass, traverse the remaining elements
-                diff_array_tails(result, source, target, path, i);
+
+                // 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;
+                }
+
                 break;
             }
 
             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, source, target, path, common_keys, 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 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)
                 {
-                    // 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();
+                    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;
+                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 (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();
                     for (auto it = source.cbegin(); it != source.cend(); ++it)
                     {
-                        if (common_it != common_keys.cend() && it.key() == *common_it)
+                        if (common_it != common_keys_source_order.cend() && it.key() == *common_it)
                         {
-                            diff_recursively(result, it.value(), target[it.key()], detail::concat<string_t>(path, '/', detail::escape(it.key())), depth + 1);
+                            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());
                             ++common_it;
                         }
                         else
                         {
                             // found a key that is not in target -> remove it
-                            diff_remove(result, detail::concat<string_t>(path, '/', detail::escape(it.key())));
+                            const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
+                            result.push_back(object(
+                            {
+                                {"op", "remove"}, {"path", path_key}
+                            }));
                         }
                     }
 
-                    // append the "add" ops for brand-new keys collected by
-                    // diff_object_keys -- no second source.find() per target
-                    // key needed
+                    // append the "add" ops for brand-new keys collected above
+                    // during the pass over target -- 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;
             }
 
@@ -32247,170 +32163,16 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             default:
             {
                 // both primitive types: replace value
-                diff_replace(result, path, target);
+                result.push_back(
+                {
+                    {"op", "replace"}, {"path", path}, {"value", 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

@@ -15,65 +15,8 @@ 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")
@@ -1809,102 +1752,6 @@ 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-msgpack.cpp

@@ -2165,6 +2165,13 @@ TEST_CASE("MessagePack with std::byte")
 #endif
 
 // the fake sizes below do not fit into a 32-bit std::size_t
+// with clang and libstdc++ 10, the std::filesystem::path conversion that
+// C++17 builds consider for every string type is ambiguous for a class
+// derived from std::string, so the string case is not tested there
+#if !(defined(__clang__) && defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE < 11)
+    #define JSON_TEST_BEYOND_UINT32_STRING 1
+#endif
+
 #if SIZE_MAX > UINT32_MAX
 template<typename T, typename A = std::allocator<T>>
 struct huge_array : std::vector<T, A>
@@ -2262,11 +2269,12 @@ TEST_CASE("MessagePack Size above uint32 for object")
     object.fake_size = false;
 }
 
+#ifdef JSON_TEST_BEYOND_UINT32_STRING
 struct huge_string : std::string
 {
     using std::string::string;
 
-    std::size_t size() const noexcept
+    std::size_t size() const noexcept // NOLINT(readability-convert-member-functions-to-static)
     {
         return static_cast<std::size_t>(UINT32_MAX) + 1ULL;
     }
@@ -2287,20 +2295,20 @@ using huge_string_json = nlohmann::basic_json <
 
 TEST_CASE("MessagePack Size above uint32 for string")
 {
-
-    huge_string_json j = "hello";
+    const huge_string_json j = "hello";
 
     CHECK_THROWS_WITH_AS(
         huge_string_json::to_msgpack(j),
         "[json.exception.out_of_range.412] MessagePack length 4294967296 exceeds maximum of 4294967295",
         json::out_of_range&);
 }
+#endif
 
 struct huge_binary : std::vector<std::uint8_t>
 {
     using std::vector<std::uint8_t>::vector;
 
-    std::size_t size() const noexcept
+    std::size_t size() const noexcept // NOLINT(readability-convert-member-functions-to-static)
     {
         return static_cast<std::size_t>(UINT32_MAX) + 1ULL;
     }
@@ -2355,13 +2363,6 @@ class beyond_uint32_binary_t : public std::vector<std::uint8_t>
     }
 };
 
-// with clang and libstdc++ 10, the std::filesystem::path conversion that
-// C++17 builds consider for every string type is ambiguous for a class
-// derived from std::string, so the string case is not tested there
-#if !(defined(__clang__) && defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE < 11)
-    #define JSON_TEST_BEYOND_UINT32_STRING 1
-#endif
-
 #ifdef JSON_TEST_BEYOND_UINT32_STRING
 class beyond_uint32_string_t : public std::string
 {