Open Radar

 
Summary:
Implementing a generic algorithm over random-access collections is made substantially harder by a RandomAccessIndex's lack of reference to its start index.

I can't add indices. I can add a distance to an index by means of advance(), but to add two indices I'd need to convert one of them into a distance. I can get the distance *between* two indices with distance(), but I can't get the distance between an index and 0. This further means I can't multiply indices, because there's no base case.

For example, consider an OffsetView of a Collection (see Notes). I can pass the offset in generically as a T.IndexType.DistanceType, however:

- I can't operate on it generically so as to be able to provide a default offset of, say, half the collection's length (because I can't get the collection's length generically)
- I can't compute an index algebraically—or maybe I can. *Apparently* adding two RandomAccessIndex.DistanceTypes is well-defined (insofar as n + m satisfies the typechecker), but there's no way to find that in the headers currently: DistanceType is referred to but never declared, RandomAccessIndex has no public members, etc. So even if I could get the collection's length generically, as it stands it's unclear whether or not I could do division over it.

I suspect some of this is actually possible and just not documented, apparently as with the addition of DistanceType, but the lack of reference to the surrounding start/end indices hints at real trouble.

Finally, I think RandomAccessIndex, or at least its DistanceType, ought to be convertible to Int. countElements(...) returns DistanceType; DistanceType isn’t arbitrarily convertible to Int (as far as I can tell). This means that I can't make an array whose length is computed from an arbitrary RandomAccessIndex'd Collection's count and which is initialized with 0:

    Int[](count: 2 * state.max + 1, repeatedValue: 0)

(state.max is a DistanceType computed from countElements.)


Steps to Reproduce:
1. Try to do arithmetic with RandomAccessIndex & its DistanceType

Expected Results:
Success

Actual Results:
Failure :(

Regression:
N/A

Notes:
Here's the OffsetView:

struct OffsetView<T : Collection where T.IndexType : RandomAccessIndex> : Collection {
	typealias GeneratorType = IndexingGenerator<OffsetView<T>>
	
	var _collection: T
	var _offset: T.IndexType.DistanceType
	
	init(collection: T, offset: T.IndexType.DistanceType) {
		_collection = collection
		_offset = offset
	}
	
	func generate() -> IndexingGenerator<OffsetView<T>> {
		return IndexingGenerator(self)
	}
	
	var startIndex: T.IndexType {
		return _collection.startIndex
	}
	var endIndex: T.IndexType {
		return _collection.endIndex
	}
	
	subscript (i: T.IndexType) -> T.GeneratorType.Element {
		return _collection[_offset + i] // requires an overload of + calling advance(i, _offset)
	}
}