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protocol CircularEnum: CaseIterable where Self.AllCases.Element: Equatable {
    var currentIndex: Self.AllCases.Index { get }
    func next() -> Self.AllCases.Element
    func previous() -> Self.AllCases.Element
}

extension CircularEnum {
    var currentIndex: Self.AllCases.Index {
        return Self.allCases.firstIndex(of: self)!
    }

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enum NoteLetter: CaseIterable {
  case c
  case d
  case e
  case f
  case g
  case a
  case b
}

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struct EnumDictionary<Key: Hashable & CaseIterable, Value> {
    private let dictionary: [Key: Value]
    
    init(_ dictionary: [Key: Value]) {
        for key in Key.allCases {
            assert(dictionary[key] != nil, "All enum cases must be assigned a value.")
        }
        
        self.dictionary = dictionary
    }

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struct Scale {
    let notes = ["C", "D", "E", "F", "G", "A", "B"]
}

struct ScaleIterator: IteratorProtocol {
    private let scale: Scale
    private var index = 0
    
    init(_ scale: Scale) {
        self.scale = scale

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extension UIView {
    func reimplementedHitTest(_ point: CGPoint, with event: UIEvent?) -> UIView? {
        guard isUserInteractionEnabled && !isHidden && alpha >= 0.01 else {
            return nil
        }
        
        if point(inside: point, with: event) {
            for subview in subviews {
                let convertedPoint = subview.convert(point, from: self)
                if let view = subview.reimplementedHitTest(convertedPoint, with: event) {

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func knapSack(capacity: Int, items: [(weight: Int, value: Int)]) -> Int {
    var grid = Array(repeatElement(Array(repeatElement(0, count: capacity)), count: items.count))
    
    for row in 0 ..< grid.count {
        for column in 0 ..< capacity {
            let item = items[row]
            let maxWeight = column + 1
            let previousMax = row >= 1 ? grid[row - 1][column] : 0
            
            if item.weight <= maxWeight {

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func reverseKGroup(_ head: ListNode?, _ k: Int) -> ListNode? {
    var count = 0
    var current = head
    var previous: ListNode?
    var newHead: ListNode?

    while current != nil {
        count += 1
        current = current?.next
    }

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public class ListNode: ExpressibleByArrayLiteral {
    public var value: Int
    public var next: ListNode?
    public init(_ value: Int, next: ListNode? = nil) {
        self.value = value
        self.next = next
    }
    
    public required init(arrayLiteral elements: Int...) {
        self.value = elements[0]

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// Returns 30 or 31 (except for February)

NSCalendar.currentCalendar().rangeOfUnit(.Day, inUnit: .Month, forDate: NSDate()).length

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func ordinal(forNumber: Int) -> String {
        let tens = (number / 10) % 10
        let ones = number % 10
        
        let suffix: String
        
        switch (tens, ones) {
        case (1, _):
            suffix = "th"
        case (_, 1):