support JSON Merge Patch (RFC 7396) diff creation (#4965)

* support JSON Merge Patch (RFC 7396) diff creation

The JSON merge patch document format describes the set of modifications
to a resource's content, that more closely mimics the syntax of the
resource being modified.

However, and in contrast to JSON Patch (RFC 6902), a JSON Merge Patch
cannot express certain modifications, e.g., changing an array element
at a specific index, or setting a specific object value to null.
The null value in a JSON Merge Patch is used to remove the key from the
object.

The diff algorithm is not part of the RFC 7396, but it was tested
against all examples provided, plus additional cases on how null values
are handled.

Signed-off-by: Luís Murta <luis@murta.dev>

* improve algorithm efficiency

Signed-off-by: Luís Murta <luis@murta.dev>

* fix inconsistent nulls and empty arrays

Also add CHECKs for the patch, documentation and run `make amalgamate`.

Signed-off-by: Luís Murta <luis@murta.dev>

* documentation cleanups

Signed-off-by: Luís Murta <luis@murta.dev>

* add an iterative fallback to merge_diff

Bound the recursion depth like diff() and merge_patch(), and test values
nested past the bound.

Signed-off-by: Luís Murta <luis@murta.dev>

* address the remaining review comments

Signed-off-by: Luís Murta <luis@murta.dev>

* move multi-line raw strings out of CHECK macros

GCC 4.8/5, ICPC and NVHPC mis-tokenize multi-line raw string literals
inside macro arguments, breaking the build or the parsed JSON.

Signed-off-by: Luís Murta <luis@murta.dev>

---------

Signed-off-by: Luís Murta <luis@murta.dev>

tests/src/unit-diagnostics.cpp

@@ -567,9 +567,9 @@ TEST_CASE("Regression tests for extended diagnostics")
     }
 }
 
-TEST_CASE("Better diagnostics past the descent bound of update() and merge_patch()")
+TEST_CASE("Better diagnostics past the descent bound of update(), merge_patch() and merge_diff()")
 {
-    // Both merge objects nested more than detail::recursion_depth_limit()
+    // All three handle objects nested more than detail::recursion_depth_limit()
     // (128) levels deep without recursing; the values they add or replace
     // there must still know their parents.
     // The values are built rather than parsed, so that the expected messages
@@ -615,4 +615,19 @@ TEST_CASE("Better diagnostics past the descent bound of update() and merge_patch
         CHECK_THROWS_WITH_AS(p->at("x").at(0), expected_x.c_str(), json::type_error);
         CHECK_THROWS_WITH_AS(p->at("y").at(0), expected_y.c_str(), json::type_error);
     }
+
+    SECTION("merge_diff()")
+    {
+        // target and patch differ only in their innermost members, so the
+        // diff of the two is nested as deeply: {"a": ... {"x": null, "y": 2}}
+        const json diff = json::merge_diff(target, patch); // NOLINT(readability-suspicious-call-argument)
+
+        const json* p = &diff;
+        for (std::size_t i = 0; i < depth; ++i)
+        {
+            p = &p->at("a");
+        }
+        CHECK(p->at("x").is_null());
+        CHECK_THROWS_WITH_AS(p->at("y").at(0), expected_y.c_str(), json::type_error);
+    }
 }