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Implement iterator facade for external sort.
The `iterator_facade` class (in tiledb/common/iterator_facade.h) implements, well, an iterator facade. An iterator must provide a fairly rich set of functions to be fully conformant to the C++20 ranges iterator concepts (and to conform to the `LegacyIterator` named requirements). Providing that functionality involves an enormous amount of boilerplate and can have subtle issues (e.g., with `const`, with the required type aliases, etc). The `iterator_facade` class generates all of the boilerplate, based on just a handful of functions that the user has to provide. Use of the `iterator_facade` class is based on CRTP. A simple iterator only needs to provide three functions in the simplest case to be able to meet all of the requirements of an iterator. For example, we can define an iterator that wraps up a raw pointer and strides through its data by 10 elements at a time.
```
template <class Value>a
class pointer_wrapper
: public iterator_facade<pointer_wrapper<Value>> {
Value *current_ = nullptr;
public:
pointer_wrapper() = default;
explicit pointer_wrapper(Value* i)
: current_(i) {}
Value &dereference() const noexcept { return *current_; }
void advance(int off) noexcept { current_ += 10 * off; }
auto distance_to(pointer_wrapper o) const noexcept {
return o.current_ - current_;
}
};
```
To do the port, I removed all dependencies on other parts of the `neo-fun` library, primarily by substituting standard library functions, type traits, and concepts for those in the `neo-fun` library. I also added some functionality to support assignment to an iterator.
I also ported the unit testing from the `neo-fun` repository. I added a significant number of new tests, primarily to verify that an iterator based on `iterator_facade` would conform to the expected range and iterator concepts.
Note that there are a few tests that return slightly unexpected results (in that they differ from , e.g., what should be an equivalent iterator from `std::vector`). Those should have no impact on our immediate intended usage of `iterator_facade`, but at some point, we should address those issues
* The `iterator_category` of some const variants does not appear to be correct
* `operator[]` seems to return nonsense in one usage mode in unit_iterator_facade.cc
The implementation of `iterator_facade` is ported from the implementation at vector-of-bool/neo-fun@b6c38c8, which I cloned on Mar 21, 2024 to use as the beginning of the port. The downloaded code is licensed under the Boost Software License 1.0 (which essentially the same as the MIT license). This `iterator_facade` is a modernized approach to Boost.Iterator and is greatly simplified via the use of concepts and other modern C++ features (Boost.Iterator dates from the early 2000s). (One of the authors of Boost.Graph is Jeremy Siek, a former PhD student in my group at Indiana University.)
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TYPE: IMPROVEMENT
DESC: Implement iterator facade for external sort.- Loading branch information
