[Issue] Collection.distance() works not very well in some situations
The question is that, whether the original Collection Protocol requires index
from lower to higher (startIndex < endIndex).
If the original Collection Protocol requires index from lower to higher, than
this infinity loop should not happen:
> class MyCollectionA : Collection {
>
> var startIndex: Int {
> return 1
> }
>
> var endIndex: Int {
> return 100
> }
>
> func index(after i: Int) -> Int {
> return i + 2
> }
>
> subscript(position: Int) -> Void {
> return ()
> }
> }
>
>
> let a = MyCollectionA()
> a.distance(from: 0, to: 100) // 50
> a.distance(from: 1, to: 100) // !! Infinity Loop
> a.count // !! Infinity Loop
if we assume the index is from lower to higher, the implementation of the
distance(from:to:) should use "<" with "while" to count distance
>
>
>
> var start = start
> var count: IndexDistance = 0
> while start < end { // instead of using !=
> count = count + 1
> formIndex(after: &start)
> }
> return count
> }
If the original Collection Protocol not requires index from to higher, than the
code below should works, but unfortunately, the code in Collection.distance
does not allow.
> class MyCollectionB : Collection {
>
> var startIndex: Int {
> return 100
> }
>
> var endIndex: Int {
> return 0
> }
>
> func index(after i: Int) -> Int {
> return i - 2
> }
>
> // start, end, index(after i: Int) -> indices: DefaultIndices
>
> subscript(position: Int) -> Void {
> return ()
> }
> }
>
> let b = MyCollectionB()
> b.count // !! Fetal Error: Only BidirectionalCollections can have end come
> before start
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