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, 54 insertions, 0 deletions

diff --git a/test/redfish-core/include/utils/json_utils_test.cpp b/test/redfish-core/include/utils/json_utils_test.cpp
index 3fca4e72b1..5485bba91e 100644
--- a/test/redfish-core/include/utils/json_utils_test.cpp
+++ b/test/redfish-core/include/utils/json_utils_test.cpp
@@ -406,6 +406,60 @@ TEST(SortJsonArrayByKey, SortedByStringValueOnSuccessArrayIsSorted)
                             R"({"@odata.id" : "/redfish/v1/20"})"_json,
                             R"({"@odata.id" : "/redfish/v1/100"})"_json));
 }
+TEST(GetEstimatedJsonSize, NumberIs8Bytpes)
+{
+    EXPECT_EQ(getEstimatedJsonSize(nlohmann::json(123)), 8);
+    EXPECT_EQ(getEstimatedJsonSize(nlohmann::json(77777777777)), 8);
+    EXPECT_EQ(getEstimatedJsonSize(nlohmann::json(3.14)), 8);
+}
+
+TEST(GetEstimatedJsonSize, BooleanIs5Byte)
+{
+    EXPECT_EQ(getEstimatedJsonSize(nlohmann::json(true)), 5);
+    EXPECT_EQ(getEstimatedJsonSize(nlohmann::json(false)), 5);
+}
+
+TEST(GetEstimatedJsonSize, NullIs4Byte)
+{
+    EXPECT_EQ(getEstimatedJsonSize(nlohmann::json()), 4);
+}
+
+TEST(GetEstimatedJsonSize, StringAndBytesReturnsLengthAndQuote)
+{
+    EXPECT_EQ(getEstimatedJsonSize(nlohmann::json("1234")), 6);
+    EXPECT_EQ(getEstimatedJsonSize(nlohmann::json::binary({1, 2, 3, 4})), 4);
+}
+
+TEST(GetEstimatedJsonSize, ArrayReturnsSum)
+{
+    nlohmann::json arr = {1, 3.14, "123", nlohmann::json::binary({1, 2, 3, 4})};
+    EXPECT_EQ(getEstimatedJsonSize(arr), 8 + 8 + 5 + 4);
+}
+
+TEST(GetEstimatedJsonSize, ObjectsReturnsSumWithKeyAndValue)
+{
+    nlohmann::json obj = R"(
+{
+  "key0": 123,
+  "key1": "123",
+  "key2": [1, 2, 3],
+  "key3": {"key4": "123"}
+}
+)"_json;
+
+    uint64_t expected = 0;
+    // 5 keys of length 4
+    expected += uint64_t(5) * 4;
+    // 5 colons, 5 quote pairs, and 5 spaces for 5 keys
+    expected += uint64_t(5) * (1 + 2 + 1);
+    // 2 string values of length 3
+    expected += uint64_t(2) * (3 + 2);
+    // 1 number value
+    expected += 8;
+    // 1 array value of 3 numbers
+    expected += uint64_t(3) * 8;
+    EXPECT_EQ(getEstimatedJsonSize(obj), expected);
+}
 
 } // namespace
 } // namespace redfish::json_util