Many lazy collections don't offer O(1) performance for `startIndex`,
`first`, or any method that depends on `startIndex`.
`LazyFilterCollection` already had a note to this effect (which I
tweaked a bit), but `FlattenCollection` didn't.
Related to SR-425.
...replacing it with the new, after passing API review!
* The lazy free function has become a property.
* Before we could extend protocols, we lacked a means for value types to
share implementations, and each new lazy algorithm had to be added to
each of up to four types: LazySequence, LazyForwardCollection,
LazyBidirectionalCollection, and LazyRandomAccessCollection. These
generic adapters hid the usual algorithms by defining their own
versions that returned new lazy generic adapters. Now users can extend
just one of two protocols to do the same thing: LazySequenceType or
LazyCollectionType.
* To avoid making the code duplication worse than it already was, the
generic adapters mentioned above were used to add the lazy generic
algorithms around simpler adapters such as MapSequence that just
provided the basic requirements of SequenceType by applying a
transformation to some base sequence, resulting in deeply nested
generic types as shown here. Now, MapSequence is an instance of
LazySequenceType (and is renamed LazyMapSequence), and thus transmits
laziness to its algorithms automatically.
* Documentation comments have been rewritten.
* The .array property was retired
* various renamings
* A bunch of Gyb files were retired.
Swift SVN r30902
An index of a FlattenCollection is composed of two: an index to the
current chunk and an index within that chunk. So you take the
startIndex, and advance it to the second chunk; call the result idx1.
You take startIndex again, and advance it the same number of steps; call
the result idx2:
let idx1 = flatMapCollection.startIndex.advancedBy(10)
let idx2 = flatMapCollection.startIndex.advancedBy(10)
idx1 and idx2 have to compare equal according to collection and index
axioms. But this can't be cheaply implemented. Every index contains an
index into the inner collection. So indices into two collections with
the same elements have to compare equal even though the collections
don't have any common history, they were merely created using the same
sequence of steps.
What currently happens is that you get random results depending on the
underlying collection kind.
rdar://21989896
Swift SVN r30617
FlattenCollection used to evaluate the closure three times per element.
That's acceptable from the technical standpoint, given its lazy
semantics, but not acceptable in practice. This commit fixes it to only
evaluate it two times when it is converted into an array.
This commit also improves tests. The new checks used to fail before the
standard library was fixed.
But there are larger issues with FlattenCollection -- it does not model
CollectionType properly. See next commit.
Swift SVN r30616
We used to be only propagating laziness through reverse collections when
the base collection was a LazyCollection<T>, rather than all
LazyCollectionType's. Also updated the flatten tests to check for the
same issue.
Swift SVN r30367
