Reverting iter to previous position

[avekarpov]

In the case of using a one-pass version of the parser (using "find_fileld"), if the field is not found, then the object has to be reset, which is logical, but not for the case when we expect that there may not be a value with the passed keys. This results in performance degradation as we reprocess the keys. This can be avoided by adding the ability to remember the position of the iterator.

Result result;

auto error = object.find_field(key).get(result);

switch (error) {
    case simdjson::error_code::SUCCESS:
        break;

    case simdjson::error_code::NO_SUCH_FIELD:
        object.reset();
        break;

    default:
        ...
}

Make something like this

auto position = object.get_current_position();

Result result;

auto error = object.find_field(key).get(result);

switch (error) {
    case simdjson::error_code::SUCCESS:
        break;

    case simdjson::error_code::NO_SUCH_FIELD:
        object.revert_position(position);
        break;

    default:
        ...
}

[lemire]

With all issues having to do with performance, we really want to have measurements (benchmarks). This being said, what you describe is not too difficult to implement but we really would like to have to benchmarks supporting our beliefs especially if we make the API more complex.

[lemire]

Pull request invited.

[jkeiser]

It's been a minute, but I think it does actually return to the existing field position. To find out whether the field exists, it has to cycle through all the fields, including the initial ones.

[lemire]

@jkeiser The find_field assumes that you provide the keys in order, so if a key is missing, you end up at the end, and if you want to continue, you have to reset.

The default (operator[]) is find_field_unordered and it definitively remembers the location.

It is a matter of semantics at this point because the current behaviour is that `find_field("x")...find_field("y")...find_field("z")..." only succeeds if you have the fields x, y, and z in sequence.

If the field y is missing, then you have to reset the object.

You don't have to do that with the default (find_field_unordered).

(I think all of that was written and designed by @jkeiser, to be clear.)