OrderedSet.swift

struct OrderedSet<Element: Comparable & Hashable> {
    private let elements: [Element]
    
    init<S: Sequence>(_ elements: S) where S.Element == Element {
        let set = Set<Element>(elements)
        let array = Array(set)
        self.elements = array.sorted()
    }
}

extension OrderedSet: BidirectionalCollection {
    func index(before i: Int) -> Int {
        return elements.index(before: i)
    }
}

extension OrderedSet: RandomAccessCollection {
    public typealias Indices = DefaultRandomAccessIndices<OrderedSet<Element>>
    
    public var startIndex: Int {
        return elements.startIndex
    }
    
    public var endIndex: Int {
        return elements.endIndex
    }
    
    public subscript(index: Int) -> Element {
        return elements[index]
    }
    
    public func index(after i: Int) -> Int {
        return elements.index(after: i)
    }
}

func ==<S1: Sequence, S2: Sequence>(lhs: S1, rhs: S2) -> Bool
    where S1.Element == S2.Element, S1.Element: Equatable {
    for (x, y) in zip(lhs, rhs) where x != y {
        return false
    }
    
    return true
}

assert([1, 2, 3, 4, 5] == OrderedSet([5, 4, 3, 2, 1, 5, 4, 3, 2, 1, 5, 4, 3, 2, 1]))