Stridable Protocol - swift

Question

I am trying to convert the following Swift 2.3 code:
//Example usage:
//(0 ..< 778).binarySearch { $0 < 145 } // 145
extension CollectionType where Index: RandomAccessIndexType {
/// Finds such index N that predicate is true for all elements up to
/// but not including the index N, and is false for all elements
/// starting with index N.
/// Behavior is undefined if there is no such N.
func binarySearch(predicate: Generator.Element -> Bool)
-> Index {
var low = startIndex
var high = endIndex
while low != high {
let mid = low.advancedBy(low.distanceTo(high) / 2)
if predicate(self[mid]) {
low = mid.advancedBy(1)
} else {
high = mid
}
}
return low
}
}
into Swift 3 as follows:
//Example usage:
//(0 ..< 778).binarySearch { $0 < 145 } // 145
extension Collection where Index: Strideable {
/// Finds such index N that predicate is true for all elements up to
/// but not including the index N, and is false for all elements
/// starting with index N.
/// Behavior is undefined if there is no such N.
func binarySearch(predicate: (Generator.Element) -> Bool)
-> Index {
var low = startIndex
var high = endIndex
while low != high {
let mid = low.advanced(by: low.distance(to: high) / 2)
if predicate(self[mid]) {
low = mid.advanced(to: 1)
} else {
high = mid
}
}
return low
}
}
The error
Binary operator '/' cannot be applied to operands of type 'self.Index.Stride' and 'Int'
is thrown at let mid = low.advanced(by: low.distance(to: high) / 2)
Any help on how to fix it?

Answer 1

In Swift 3, "Collections move their index", compare
A New Model for Collections and Indices on Swift evolution. In particular, you don't call advancedBy() on an index,
but an index() method on the collection to advance indices.
So your method would be implemented in Swift 3 as
extension RandomAccessCollection {
func binarySearch(predicate: (Iterator.Element) -> Bool) -> Index {
var low = startIndex
var high = endIndex
while low != high {
let mid = index(low, offsetBy: distance(from: low, to: high)/2)
if predicate(self[mid]) {
low = index(after: mid)
} else {
high = mid
}
}
return low
}
}
The same method would also compile (and work) as an extension of the
more general Collection type, but – as Vadim Yelagin correctly remarked,
is very inefficient if the index/offset calculations cannot be done in
constant time.