Add base() method for sequence iterators

[void2012]

Some standard library ranges provide the base() iterator move/factory conversion for their iterators which returns the iterator for previous base range, which is VERY useful for views like transform and filter. Sadly, flux has this feature only for some sequences and not for sequence iterators and cursors.

See

https://en.cppreference.com/w/cpp/ranges/transform_view/iterator/base.html

https://en.cppreference.com/w/cpp/ranges/filter_view/iterator.html#base

https://en.cppreference.com/w/cpp/ranges/enumerate_view/iterator/base.html

and so on.

Thanks!

[tcbrindle]

Yes, we should definitely be more consistent about this.

Currently, some sequence adaptors provide a base() member, but there are others which don't have this even though they potentially could.

For sequence adaptors which additionally adapt cursors, we're pretty consistent about using a public data member named base_cur, so you usually can access the base cursor today. But it would be better to provide a base() member on such cursors (or in future, positions) for consistency.

[void2012]

Well, sadly not all cursor_types provide it:

No member named 'base_cur' in 'flux::detail::range_sequence<std::ranges::ref_view<const std::string>, true>::flux_sequence_traits::cursor_type' clang(no_member)

And also my request applies rather to sequence iterators, like flux::detail::sequence_iterator
So I could do the following:

const auto removeEntryExtension = [](const auto& str) {return removeExtension(str);};
			auto entryNames = projection_to<cistring_view>(
				std::as_const(fileEntries), 
				&SoundHeaderStruct::filename)
			.map(removeEntryExtension);
			const auto entry = std::ranges::find(entryNames, removeExtension(entryName));

On std::ranges version of this code, I can do
entry.base().base().base() to get the 3rd underlying iterator. And access the element if the element was found. Thanks to std::ranges::transform_view::iterator::base
https://en.cppreference.com/w/cpp/ranges/transform_view/iterator/base.html

On flux I can't to that currently.

[tcbrindle]

In the case of range_sequence, there is no underlying cursor; it directly wraps an iterator. I guess that range_sequence::cursor_type could provide an iter() method which returns of copy of the iterator, though.

For sequence_iterator, I'm afraid I don't really understand what you're asking for. A sequence_iterator<Seq> looks like

template <sequence Seq>
struct sequence_iterator {
    Seq* seq_ptr;
    cursor_t<Seq> cur;
};

along with a bunch of member functions defining the iterator API. That is, sequence_iterator just contains a sequence pointer and a cursor: there is no base iterator for it to return.