fix: single-element .dt.time() and .dt.date() should always preserve sortedness

[MarcoGorelli]

closes #13806

[MarcoGorelli]

this shortens the code - but also ends up hitting

polars/crates/polars-core/src/chunked_array/ops/chunkops.rs

Lines 102 to 104 in 2b43fc1

	if self.length <= 1 {
	self.set_sorted_flag(IsSorted::Ascending)
	}

[orlp]

Honestly? I'd say add an if statement to the set_sorted_flag function itself.

pub fn set_sorted_flag(&mut self, sorted: IsSorted) {
    if self.len() <= 1 {
        self.bit_settings.insert(Settings::SORTED_ASC);
        self.bit_settings.insert(Settings::SORTED_DSC);
        return;
    }
    
    // ...
}

Then we can benefit from this in many more places.

[MarcoGorelli]

thanks - even simpler, could be to just do the early return in is_sorted_flag?

[MarcoGorelli]

a single element would now always be sorted, so I'm adding an extra one to try to preserve the spirit of the test

[stinodego]

Technically it is sorted both Ascending and Descending at the same time 🤔 it would be some refactoring to get that done though...

[orlp]

@stinodego Yes I've brought up this limitation of the current sorted flag to Ritchie before. You can also run into this for larger arrays, [0, 0, 0, 0, 0] is sorted both ascending and descending.

[ritchie46]

Why do we replace this from compute_len to here?

[MarcoGorelli]

Expressions don't necessarily go down that path, e.g. the original implementation of casting Datetime to Time didn't (see #13812), so this just seemed simpler