Unique Extension Method For Swift SequenceTypes

Unique.swift

/*
The reason this does not return a value is that SequenceType
is very general. It has no methods for modifying the sequence,
only traversing it.

Instead, we pass our "unique" method a closure that tells it
how to add a value to our result. The result is not visible 
at all to this method. It is captured by our closure.

The reason we don't pass the result as a parameter is because it
has to be declare with var anyway, otherwise it is immutable. I.e.,
something like this won't work:

unique(inputSequence, [Int]()) { $0.append($1) }

The compiler will complain, trust me. (At least as of this writing.)

Lastly, this assumes your module name is Prosumma. Replace
this with your module name, otherwise it won't compile.
*/

public func unique<H: Hashable, S: SequenceType where S.Generator.Element == H>(sequence: S, append: (H) -> Void) -> Void {
    var hashMap = [H: Bool]()
    for elem in sequence {
        let key = hashMap[elem]
        if key == nil {
            hashMap[elem] = true
            append(elem)
        }
    }
}

extension Array {
    public func unique<H: Hashable>() -> [H] {
        var result = [H]()
        Prosumma.unique(map({ $0 as H })) { result.append($0) }
        return result
    }
}