Removing duplicates

RemoveDuplicates.swift

// removes all but first occurrence from a sequence, returning an array.
// requires elements to be hashable, not just equatable, but the alternative
// of using contains is very inefficient
// alternatively, could require comparable, sort, and remove adjacent dupes
func uniq<S: SequenceType, E: Hashable where E==S.Generator.Element>(seq: S) -> [E] {
    var seen: [S.Generator.Element:Int] = [:]
    return filter(seq) { !seen.updateValue(1, forKey: $0).hasValue }
}

// TODO: a version that takes a custom comparator function, say for lexicographic deduping


// give String an initializer of a sequence of characters.
// (I'm not missing an existing one am I?)
extension String {
    init<S: SequenceType where S.Generator.Element == Character>(_ seq: S) {
        self.init()
        self.extend(seq)
    }
}

// removeDuplicates returns a version of the String with 
// all duplicates removed
extension String {
    func removeDuplicates() -> String {
        return String(uniq(self))
    }
}

let s = "hello, I must be going".removeDuplicates()
// => "helo, Imustbgin"

Cool!