Write the path of an array element in flatten() with a single resize

Replacing the path of the previous child by "/<index>" took a resize, a
push_back and an append. For array elements the path is now resized once
to its final length and the slash and the digits are written into it.
Object keys keep the three calls: a version that also wrote them directly
made flatten() of twitter 4 % slower in cycles at equal instructions.

flatten() (instructions on macOS, Linux Ir GCC 13 / Clang 18): canada 172.1M ->
164.7M, 123.8M -> 120.6M / 121.2M -> 118.9M; citm_catalog 40.4M -> 39.5M, 30.1M
-> 29.6M / 30.0M -> 29.6M; twitter unchanged.

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

single_include/nlohmann/json.hpp

@@ -23867,8 +23867,9 @@ class json_pointer
             // frame, which would invalidate a reference to it
             const BasicJsonType* const container = stack.back().container;
 
-            // drop the path of the previous child
-            path.resize(stack.back().path_length);
+            // the path of the previous child is replaced by the path of the
+            // next one; for an array element in one resize
+            const std::size_t path_length = stack.back().path_length;
 
             if (container->is_array())
             {
@@ -23882,8 +23883,20 @@ class json_pointer
 
                 // iterate array and use index as a reference string
                 ++stack.back().index;
-                path.push_back('/');
-                append_index(path, i);
+                std::array<char, 24> digits{};
+                std::size_t pos = digits.size();
+                std::size_t rest = i;
+                do
+                {
+                    digits[--pos] = static_cast<char>('0' + rest % 10);
+                    rest /= 10;
+                }
+                while (rest != 0);
+                const std::size_t count = digits.size() - pos;
+                path.resize(path_length + 1 + count);
+                char* const out = &path[path_length];
+                out[0] = '/';
+                std::memcpy(out + 1, digits.data() + pos, count);
                 enter(array[i]);
             }
             else
@@ -23897,6 +23910,7 @@ class json_pointer
 
                 // iterate object and use keys as reference string
                 ++stack.back().member;
+                path.resize(path_length);
                 path.push_back('/');
                 detail::escape_into(path, it->first);
                 enter(it->second);