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let xs = [0,1,2,3,4,5]

let double: (Int) -> Int = { $0 * 2 }

let isEven: (Int) -> Bool = { $0 % 2 == 0 }

// Goal: [0,2,4,6,4,10]. Double all but the last even element.
extension Collection {
    func mapExceptLast(matching predicate: (Element) -> Bool, transform: (Element) -> Element) -> [Element] {
        var result: [Element] = []

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/// Returns a binary predicate using the given key path to create an ascending order
/// for elements of type `Root`.
func ascending<Root, Value: Comparable>(_ path: KeyPath<Root, Value>) -> (Root, Root) -> Bool {
  return { $0[keyPath: path] < $1[keyPath: path] }
}

/// Returns a binary predicate using the given key path to create a descending order
/// for elements of type `Root`.
func descending<Root, Value: Comparable>(_ path: KeyPath<Root, Value>) -> (Root, Root) -> Bool {
  return { $0[keyPath: path] > $1[keyPath: path] }

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extension Sequence {
  func interspersed(with element: Element) -> [Element] {
    var result: [Element] = []
    var iterator = makeIterator()
    if let first = iterator.next() {
      result.append(first)
    }
    while let elementFromSelf = iterator.next() {
      result.append(element)
      result.append(elementFromSelf)

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extension BidirectionalCollection {
  func drop(first: Int, last: Int) -> SubSequence {
    return self.dropFirst(first).dropLast(last)
  }
}

let numbers = 0..<10
numbers.drop(first: 1, last: 1)  // 1..<9
numbers.drop(first: 2, last: 4)  // 2..<6

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extension Collection {
  /// Chooses at random the given number of elements, using reservoir sampling
  /// aka Algorithm R.
  ///
  /// - Complexity: O(*n*) where *n* is the collection's count.
  func choose<T>(upTo max: Int, using generator: T) -> [Element]
    where T: RandomNumberGenerator
  {
    var i = index(startIndex, offsetBy: max, limitedBy: endIndex) ?? endIndex
    var result = self[..<i].shuffled(using: generator)

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/// This is a useful index that can store a comparable element or the end of
/// a collection. Similar to https://github.com/apple/swift/pull/15193
enum IndexWithEnd<T : Comparable> : Comparable {
  case element(T)
  case end
  
  static func < (lhs: IndexWithEnd, rhs: IndexWithEnd) -> Bool {
    switch (lhs, rhs) {
    case (.element(let l), .element(let r)):
      return l < r

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extension Sequence {
  func last(where predicate: (Element) throws -> Bool) rethrows -> Element? {
    var result: Element? = nil
    for el in self {
      if try predicate(el) {
        result = el
      }
    }
    return result
  }

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// Speculation in regards to https://forums.swift.org/t/shorthand-for-offsetting-startindex-and-endindex/9397/133

/// A pair of offsets that can be used to slice a collection.
///
/// In order to have `RangeExpression` conformance, this would need
/// a phantom `Bound` type, which would unnecessarily restrict its
/// usage. In addition, the `RangeExpression.contains(_:)` method
/// couldn't be implemented without access to a collection with the
/// actual indices to resolve.
struct Offsets {

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public struct PeekingIterator<T : IteratorProtocol> : IteratorProtocol {
  fileprivate var _iterator: T
  public fileprivate(set) var buffered: T.Element?
  
  public init(_ iterator: T) {
    self._iterator = iterator
    self.buffered = _iterator.next()
  }
  
  public init<U: Sequence>(_ sequence: U) where U.Iterator == T {

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/// Performs an exponential search to find the number of steps from `start` to `end`
/// without using division.
func count<T: Numeric & Comparable>(from start: T, to end: T, by step: T) -> Int {
  var count = 1
  var current = start
  var stepMultipliers = [1]
  while current < end {
    count += stepMultipliers.last!
    current += step * T(exactly: stepMultipliers.last!)!
    stepMultipliers.append(stepMultipliers.last! * 2)