Run diff on 2 sequences

SequenceDiff.swift

extension SequenceType where Self.Generator.Element : Equatable {
    
    typealias Diff = (added: [Self.Generator.Element], removed: [Self.Generator.Element])
    
    func diff(changed:[Self.Generator.Element]) -> Diff {
        let added:[Self.Generator.Element] = changed.reduce([]) {
            !self.contains($1) ? $0 + [$1] : $0
        }
        
        let removed:[Self.Generator.Element] = self.reduce([]) {
            !changed.contains($1) ? $0 + [$1] : $0
        }
        
        return (added: added, removed: removed)
    }
}

Basic Tests:

let arr1 = [3,4,5,6,7,8]

func assertDiff(diff: (added: [Int], removed:[Int]), expectedAdded: [Int], expectedRemoved: [Int]) {
    assert(diff.added == expectedAdded)
    assert(diff.removed == expectedRemoved)
}

assertDiff(arr1.diff([]), expectedAdded: [], expectedRemoved: [3,4,5,6,7,8])
assertDiff(arr1.diff([1]), expectedAdded: [1], expectedRemoved: [3,4,5,6,7,8])
assertDiff(arr1.diff([7,8,9,0]), expectedAdded: [9,0], expectedRemoved: [3,4,5,6])