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, 14 insertions, 0 deletions

diff --git a/redfish-core/include/utils/json_utils.hpp b/redfish-core/include/utils/json_utils.hpp
index 6e18157fc0..642ca802eb 100644
--- a/redfish-core/include/utils/json_utils.hpp
+++ b/redfish-core/include/utils/json_utils.hpp
@@ -16,6 +16,7 @@
 #pragma once
 
 #include "error_messages.hpp"
+#include "http_connection.hpp"
 #include "http_request.hpp"
 #include "http_response.hpp"
 #include "human_sort.hpp"
@@ -673,5 +674,18 @@ inline void sortJsonArrayByOData(nlohmann::json::array_t& array)
     std::sort(array.begin(), array.end(), ODataObjectLess());
 }
 
+// Returns the estimated size of the JSON value
+// The implementation walks through every key and every value, accumulates the
+//  total size of keys and values.
+// Ideally, we should use a custom allocator that nlohmann JSON supports.
+
+// Assumption made:
+//  1. number: 8 characters
+//  2. boolean: 5 characters (False)
+//  3. string: len(str) + 2 characters (quote)
+//  4. bytes: len(bytes) characters
+//  5. null: 4 characters (null)
+uint64_t getEstimatedJsonSize(const nlohmann::json& root);
+
 } // namespace json_util
 } // namespace redfish