json utils: add getEstimatedJsonSize

Add a utility function which estimates the size of the JSON tree.

It is used in the children change to limit the reponse size of expand
query.

Tested:
1. unit test passed;
2. tested on hardware, the following are real sizes and estimation
```
Real payload size, Estimation, query
15.69 KB,  10.21 KB, redfish/v1?$expand=.($levels=1)
95.76 KB,  62.11 KB, redfish/v1?$expand=.($levels=2)
117.14 KB, 72.71 KB, redfish/v1?$expand=.($levels=3)
127.65 KB, 77.64 KB, redfish/v1?$expand=.($levels=4)
```

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: Iae26d6732a6ec63ecc59eacf657b4bf33c07c046

1 files changed, 49 insertions, 0 deletions

diff --git a/redfish-core/src/utils/json_utils.cpp b/redfish-core/src/utils/json_utils.cpp
index 5daf95b058..5e44199d44 100644
--- a/redfish-core/src/utils/json_utils.cpp
+++ b/redfish-core/src/utils/json_utils.cpp
@@ -48,5 +48,54 @@ bool processJsonFromRequest(crow::Response& res, const crow::Request& req,
     return true;
 }
 
+uint64_t getEstimatedJsonSize(const nlohmann::json& root)
+{
+    if (root.is_null())
+    {
+        return 4;
+    }
+    if (root.is_number())
+    {
+        return 8;
+    }
+    if (root.is_boolean())
+    {
+        return 5;
+    }
+    if (root.is_string())
+    {
+        constexpr uint64_t quotesSize = 2;
+        return root.get<std::string>().size() + quotesSize;
+    }
+    if (root.is_binary())
+    {
+        return root.get_binary().size();
+    }
+    const nlohmann::json::array_t* arr =
+        root.get_ptr<const nlohmann::json::array_t*>();
+    if (arr != nullptr)
+    {
+        uint64_t sum = 0;
+        for (const auto& element : *arr)
+        {
+            sum += getEstimatedJsonSize(element);
+        }
+        return sum;
+    }
+    const nlohmann::json::object_t* object =
+        root.get_ptr<const nlohmann::json::object_t*>();
+    if (object != nullptr)
+    {
+        uint64_t sum = 0;
+        for (const auto& [k, v] : root.items())
+        {
+            constexpr uint64_t colonQuoteSpaceSize = 4;
+            sum += k.size() + getEstimatedJsonSize(v) + colonQuoteSpaceSize;
+        }
+        return sum;
+    }
+    return 0;
+}
+
 } // namespace json_util
 } // namespace redfish