Pre-Parse strftime string

[Kixiron]

A way to pre-parse a format/parse string would be really nice, I call DateTime::parse_from_str in a hot loop and I'd like not to re-parse the format string in every iteration

[pitdicker]

It is possible, but not well-documented yet. With StrftimeItems you can parse the format string, and then use it with parse() and Parsed::to_datetime. I don't think the API is great though 😄.

Just happened to figure this out for myself yesterday, with an example in this comment #1049 (comment).

[Kixiron]

I guess StrftimeItems lazily parses the string and then allows you to later reuse it? I don't see any sort of .reset() method on it, how do you restore it to a state that allows you to call parse() with it again?

[pitdicker]

Ai, it doesn't even allow reuse?
Is it for you an option to manually create an array with formatting items (again, pretty much undocumented)? https://docs.rs/chrono/latest/chrono/format/enum.Item.html

In Numeric and Literal you can find the items that would make up the fields of the format string. parse can accept an iterator over such an array.

[pitdicker]

Collecting the elements of the StrftimeItems iterator should also get you that array.

[jtmoon79]

@Kixiron if you figure it out, would you please repost the reduced recipe here?

[Kixiron]

You just do what pitdicker said, StrftimeItems::new("%Y-%m-%d").collect::<Vec<_>>()

[djc]

@Kixiron thanks for following up!