JSON patch diff for op=add formation is not as per standard (RFC 6902)

[harishmurkal]

Hi,

When result_json = diff(source, target) is called with

source = { "foo": [ "bar", "baz" ] }
dest = {"foo": [ "bar", "baz", "baa" ]}

NOTE: actual function is

static basic_json diff(const basic_json& source, const basic_json& target,
                       const std::string& path = "")

Obtained JSON is
result_json =
[
{ "op": "add", "path": "/foo/1", "value": "qux" }
]

This is wrong as per RFC 6902 (refer section 4.1)
https://tools.ietf.org/html/rfc6902#page-4

Expected is
result_json =
[
{ "op": "add", "path": "/foo/-", "value": "qux" }
]

Where is the problem here?

Or is it we need to interchange source and target to generate diff?
Waiting for your response.

Thanks and Regards,
Harish M

[dota17]

Hi @harishmurkal IMO, there is no problem here.
if

source = { "foo": [ "bar", "baz" ] }
dest = {"foo": [ "bar", "baz", "baa" ]}

the new element baa in the array is at the end of the array, so these two json patches below have the same effect on the source according to RFC6902:
1.

[
{ "op": "add", "path": "/foo/-", "value": "baa" }
]

[
{ "op": "add", "path": "/foo/2", "value": "baa" }
]

[harishmurkal]

Hi @dota17,

Thanks for responding. Unfortunately, the receiver of the PATCH request is throwing error saying that the patch is trying to access element that is out-of-range. In the example given, source "foo" has two elements (index 0 and 1). The PATCH informs to add an element at the index 2, technically, it's out-of-range.

I've pasted some snippet from the standards, RFC 6902 (which indicates that there is an issue):

An element to add to an existing array - whereupon the supplied
value is added to the array at the indicated location. Any
elements at or above the specified index are shifted one position
to the right. The specified index MUST NOT be greater than the
number of elements in the array. If the "-" character is used to
index the end of the array (see [RFC6901]), this has the effect of
appending the value to the array.

Thanks and Regards,
Harish M

[dota17]

Actually, the number of elements in that array is 2, so the index 2 is not greater than it, this is consistent with what is stated in the RFC 6902. And I test some examples online, the result is no problem.

[nlohmann]

Thanks for reporting. I'll check.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[nlohmann]

I added a fix, see https://github.com/nlohmann/json/tree/feature/issue1983.

Is this OK for you @harishmurkal @dota17?

[dota17]

I added a fix, see https://github.com/nlohmann/json/tree/feature/issue1983.

Is this OK for you @harishmurkal @dota17?

Yes, I think it fixes the problem.