By doing a fast check for CR-LF, we can speed up forwards and
backwards character iteration on ASCII strings by ~2-3x. This speedup
can be seen in the stringTests suite (previous PR).
It is done as a fast path inside of
_measureExtendedGraphemeClusterForward/Backward, which is never
inlined. Doing the fast path inline would yield even more dramatic
improvements, and might be an area for future exploration, but would
slightly pessimize non-ASCII strings due to code bloat. Additionally,
the current implementation has not been micro-optimized yet. It's
possible that more optimal code would be smaller and have less of an
impact on the general case.
Eliminate the vestigial type `LazyFilterIndex`, which was
necessary pre-Swift-3 to allow the index to move. Swift 3's indexing
model means that the movement of indices is on the collection itself,
so we no longer need `LazyFilterIndex`: instead, the `Index` type of
the lazy filtered collection is simply the `Index` type of the base
collection, which is a nice convenience: it means you can take indices
from a lazy wrapper around a given collection C and use them with the
collection C (and, with care, vice-versa) without jumping through
extra hoops.
Part of ABI FIXME #99, this gives us some nice consistency that
ensures that slicing a SubSequence gives us another SubSequence. There
are two source-compatibility implications to this change:
* Collections now need to satisfy this property, which could not be
expressed in Swift 3. There might be some Collections that don't
satisfy this property, and will break with the Swift 4 compiler
*even in Swift 3 compatibility mode*. Case in point...
* The Lazy collection types were formulated as a lazy collection of
the base slice (e.g., LazyCollection<ArraySlice<T>>) rather than as
a slice of the lazy collection (e.g.,
Slice<LazyCollection<Array<T>>). The former doesn't meet the new
requirements, so change to the latter.
There isn't any work related to SE-0142 associated type where clauses
for these particular ABI FIXMEs. Rather, we get all of the constraints
we need from Collection and the recursive constraint we cannot yet
express, all of which are covered by other ABI FIXMEs.
Address ABI FIXME #68 by using same-type constraints directly on an
associated type to describe the requirements on the Indices associated
type of the Collection protocol. ABI FIXMEs #89, #90, #91 are all in
StdlibUnittest, and provoke warnings once #68 is fixed, but it's nice
to clear them out.
Fixes SR-2121.
In Swift 3 shifts used to be defined on the concrete integer types, so
the right-hand-side value in the shift expression could define a type
for the result, as in `1 << i32` would have the type Int32. Swift 4
makes shift operators heterogeneous, so now `1 << i32` will result in an
Int, according to the type of the left-hand-side value, which gets a
default type for integer literals.
