Index and Iterate over CollectionType in swift - swift

Question

I have code which is basically like this:
func arrayHalvesEqual(data:[UInt8]) -> Bool {
let midPoint = data.count / 2
for i in 0..<midPoint {
let b = data[i]
let b2 = data[i + midPoint]
if b != b2 {
return false
}
}
return true
}
This works fine, but sometimes I want to pass in Arrays, and other times ArraySlice. I thought I'd change it to use generics and the CollectionType protocol, which converts as follows:
func arrayHalvesEqual<ByteArray : CollectionType where ByteArray.Generator.Element == UInt8>(data:ByteArray) -> Bool {
let midPoint = data.count / 2
for i in 0..<midPoint {
let b = data[i]
let b2 = data[i + midPoint]
if b != b2 {
return false
}
}
return true
}
However, I get the following compiler error:
error: binary operator '..<' cannot be applied to operands of type 'Int' and 'ByteArray.Index.Distance'
for i in 0..<midPoint {
I can switch the for loop to for i in data.indices which makes that compile, but then I can no longer divide it by 2 to get the midPoint, as data.indices returns the abstract CollectionType.Index whereas / 2 is an Int.
Is it possible to do something like this in Swift? Can I bridge between an abstract protocol Index type and some real type I can do maths on?
P.S: I've seen and found other examples for iterating over the whole collection by using indices and enumerate, but I explicitly only want to iterate over half the collection which requires some sort of division by 2
Thanks

Answer 1

You can restrict the method to collections which are indexed
by Int:
func arrayHalvesEqual<ByteArray : CollectionType where ByteArray.Index == Int, ByteArray.Generator.Element == UInt8>
(data:ByteArray) -> Bool { ... }
This covers both Array and ArraySlice.
And if you use indices.startIndex instead of 0 as initial index
then it suffices to restrict the index type to IntegerType.
Also the data type UInt8 can be replaced by a generic Equatable,
and the entire method shortened to
func arrayHalvesEqual<ByteArray : CollectionType where ByteArray.Index : IntegerType, ByteArray.SubSequence.Generator.Element : Equatable>
(data:ByteArray) -> Bool {
let midPoint = (data.indices.endIndex - data.indices.startIndex)/2
let firstHalf = data[data.indices.startIndex ..< midPoint]
let secondHalf = data[midPoint ..< data.indices.endIndex]
return !zip(firstHalf, secondHalf).contains { $0 != $1 }
}