but do you happen whether it is possible to automatically re-write a subset of JSON Path queries into JSON Pointers queries?

Basically JSON Pointer provides forward queries...

Given

{ "c" :{ "foo": { "a": [ 10, 20, 30 ] }}, "d": { "foo2": { "a": [ 10, 20, 30 ] }} , "e": 120 }

You have the following JSON Pointer queries...

"/c/foo/a/1" is 2
"/d/foo2/a/2" is 30
"/e" is 120

The equivalent in JSON Path might be... (up to potential semantics differences)

$c.foo.a[1]
$d.foo2.a[2]
$e

JSON Pointer is a well-established standard.

See https://www.rfc-editor.org/rfc/rfc6901

I should stress that JSON Pointer queries are very much still used in production and the standard is very much alive.

We also support an extension whereas you can apply a JSON Pointer from the current node, as in...

auto cars_json = R"( [
        { "make": "Toyota", "model": "Camry",  "year": 2018, "tire_pressure": [ 40.1, 39.9, 37.7, 40.4 ] },
        { "make": "Kia",    "model": "Soul",   "year": 2012, "tire_pressure": [ 30.1, 31.0, 28.6, 28.7 ] },
        { "make": "Toyota", "model": "Tercel", "year": 1999, "tire_pressure": [ 29.8, 30.0, 30.2, 30.5 ] }
        ] )"_padded;

        ondemand::parser parser;
        ondemand::document cars = parser.iterate(cars_json);
        std::vector<double> measured;
        for (auto car_element : cars) {
            double x = (double) car_element.at_pointer("/tire_pressure/1");
            measured.push_back(x);
        }

       //  measured.push_back == {39.9, 31, 30};

I'm curious what are the challenges

We support JSON Pointer highly efficiently. There is no head memory allocation and no need for additional dependencies.

As far as I can tell, JSON Path implementations are currently not guaranteed to be efficient.

The current state-of-the-art with respect to attempting to implement JSON Path efficiently is JSONSki but they provide only a partial implementation... It has no support for descendant selectors, and their wildcard selector implements only a part of the JSONPath specification, stepping into every entry of an array, but not into every field of an object.

Jiang, L., & Zhao, Z. (2022, February). JSONSki: streaming semi-structured data with bit-parallel fast-forwarding. In Proceedings of the 27th ACM International Conference on Architectural Support for Programming Languages and Operating Systems (pp. 200-211).

The type of JSON Path queries that would be challenging to implement efficiently are queries such as$.*[?@.price < 10]..a[?search(@.b, | {"b": "j"}].

It is doable if you have enough engineering effort, and I am not closing this issue. In fact, I am marking it as 'help needed' and 'good first issue'. A couple of talented engineers could implement JSON Path on top of, say, the On Demand API. But it would take more than a few days. I would be interested in working on this, and I might still work on this, but it is not trivial.

Add full support for JSONPath #2070

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