stdlib/public/core/CollectionAlgorithms.swift - third_party/swift - Git at Google

 //===--- CollectionAlgorithms.swift.gyb -----------------------*- swift -*-===//
 //
 // This source file is part of the Swift.org open source project
 //
 // Copyright (c) 2014 - 2018 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 //
 //===----------------------------------------------------------------------===//

 //===----------------------------------------------------------------------===//
 // last
 //===----------------------------------------------------------------------===//

 extension BidirectionalCollection {
   /// The last element of the collection.
   ///
   /// If the collection is empty, the value of this property is `nil`.
   ///
   ///     let numbers = [10, 20, 30, 40, 50]
   ///     if let lastNumber = numbers.last {
   ///         print(lastNumber)
   ///     }
   ///     // Prints "50"
   ///
   /// - Complexity: O(1)
   @inlinable
   public var last: Element? {
     return isEmpty ? nil : self[index(before: endIndex)]
   }
 }

 //===----------------------------------------------------------------------===//
 // firstIndex(of:)/firstIndex(where:)
 //===----------------------------------------------------------------------===//

 extension Collection where Element : Equatable {
   /// Returns the first index where the specified value appears in the
   /// collection.
   ///
   /// After using `firstIndex(of:)` to find the position of a particular element
   /// in a collection, you can use it to access the element by subscripting.
   /// This example shows how you can modify one of the names in an array of
   /// students.
   ///
   ///     var students = ["Ben", "Ivy", "Jordell", "Maxime"]
   ///     if let i = students.firstIndex(of: "Maxime") {
   ///         students[i] = "Max"
   ///     }
   ///     print(students)
   ///     // Prints "["Ben", "Ivy", "Jordell", "Max"]"
   ///
   /// - Parameter element: An element to search for in the collection.
   /// - Returns: The first index where `element` is found. If `element` is not
   ///   found in the collection, returns `nil`.
   ///
   /// - Complexity: O(*n*), where *n* is the length of the collection.
   @inlinable
   public func firstIndex(of element: Element) -> Index? {
     if let result = _customIndexOfEquatableElement(element) {
       return result
     }

     var i = self.startIndex
     while i != self.endIndex {
       if self[i] == element {
         return i
       }
       self.formIndex(after: &i)
     }
     return nil
   }
 }

 extension Collection {
   /// Returns the first index in which an element of the collection satisfies
   /// the given predicate.
   ///
   /// You can use the predicate to find an element of a type that doesn't
   /// conform to the `Equatable` protocol or to find an element that matches
   /// particular criteria. Here's an example that finds a student name that
   /// begins with the letter "A":
   ///
   ///     let students = ["Kofi", "Abena", "Peter", "Kweku", "Akosua"]
   ///     if let i = students.firstIndex(where: { $0.hasPrefix("A") }) {
   ///         print("\(students[i]) starts with 'A'!")
   ///     }
   ///     // Prints "Abena starts with 'A'!"
   ///
   /// - Parameter predicate: A closure that takes an element as its argument
   ///   and returns a Boolean value that indicates whether the passed element
   ///   represents a match.
   /// - Returns: The index of the first element for which `predicate` returns
   ///   `true`. If no elements in the collection satisfy the given predicate,
   ///   returns `nil`.
   ///
   /// - Complexity: O(*n*), where *n* is the length of the collection.
   @inlinable
   public func firstIndex(
     where predicate: (Element) throws -> Bool
   ) rethrows -> Index? {
     var i = self.startIndex
     while i != self.endIndex {
       if try predicate(self[i]) {
         return i
       }
       self.formIndex(after: &i)
     }
     return nil
   }
 }

 //===----------------------------------------------------------------------===//
 // lastIndex(of:)/lastIndex(where:)
 //===----------------------------------------------------------------------===//

 extension BidirectionalCollection {
   /// Returns the last element of the sequence that satisfies the given
   /// predicate.
   ///
   /// This example uses the `last(where:)` method to find the last
   /// negative number in an array of integers:
   ///
   ///     let numbers = [3, 7, 4, -2, 9, -6, 10, 1]
   ///     if let lastNegative = numbers.last(where: { $0 < 0 }) {
   ///         print("The last negative number is \(lastNegative).")
   ///     }
   ///     // Prints "The last negative number is -6."
   ///
   /// - Parameter predicate: A closure that takes an element of the sequence as
   ///   its argument and returns a Boolean value indicating whether the
   ///   element is a match.
   /// - Returns: The last element of the sequence that satisfies `predicate`,
   ///   or `nil` if there is no element that satisfies `predicate`.
   ///
   /// - Complexity: O(*n*), where *n* is the length of the collection.
   @inlinable
   public func last(
     where predicate: (Element) throws -> Bool
   ) rethrows -> Element? {
     return try lastIndex(where: predicate).map { self[$0] }
   }

   /// Returns the index of the last element in the collection that matches the
   /// given predicate.
   ///
   /// You can use the predicate to find an element of a type that doesn't
   /// conform to the `Equatable` protocol or to find an element that matches
   /// particular criteria. This example finds the index of the last name that
   /// begins with the letter *A:*
   ///
   ///     let students = ["Kofi", "Abena", "Peter", "Kweku", "Akosua"]
   ///     if let i = students.lastIndex(where: { $0.hasPrefix("A") }) {
   ///         print("\(students[i]) starts with 'A'!")
   ///     }
   ///     // Prints "Akosua starts with 'A'!"
   ///
   /// - Parameter predicate: A closure that takes an element as its argument
   ///   and returns a Boolean value that indicates whether the passed element
   ///   represents a match.
   /// - Returns: The index of the last element in the collection that matches
   ///   `predicate`, or `nil` if no elements match.
   ///
   /// - Complexity: O(*n*), where *n* is the length of the collection.
   @inlinable
   public func lastIndex(
     where predicate: (Element) throws -> Bool
   ) rethrows -> Index? {
     var i = endIndex
     while i != startIndex {
       formIndex(before: &i)
       if try predicate(self[i]) {
         return i
       }
     }
     return nil
   }
 }

 extension BidirectionalCollection where Element : Equatable {
   /// Returns the last index where the specified value appears in the
   /// collection.
   ///
   /// After using `lastIndex(of:)` to find the position of the last instance of
   /// a particular element in a collection, you can use it to access the
   /// element by subscripting. This example shows how you can modify one of
   /// the names in an array of students.
   ///
   ///     var students = ["Ben", "Ivy", "Jordell", "Ben", "Maxime"]
   ///     if let i = students.lastIndex(of: "Ben") {
   ///         students[i] = "Benjamin"
   ///     }
   ///     print(students)
   ///     // Prints "["Ben", "Ivy", "Jordell", "Benjamin", "Max"]"
   ///
   /// - Parameter element: An element to search for in the collection.
   /// - Returns: The last index where `element` is found. If `element` is not
   ///   found in the collection, this method returns `nil`.
   ///
   /// - Complexity: O(*n*), where *n* is the length of the collection.
   @inlinable
   public func lastIndex(of element: Element) -> Index? {
     if let result = _customLastIndexOfEquatableElement(element) {
       return result
     }
     return lastIndex(where: { $0 == element })
   }
 }

 //===----------------------------------------------------------------------===//
 // partition(by:)
 //===----------------------------------------------------------------------===//

 extension MutableCollection {
   /// Reorders the elements of the collection such that all the elements
   /// that match the given predicate are after all the elements that don't
   /// match.
   ///
   /// After partitioning a collection, there is a pivot index `p` where
   /// no element before `p` satisfies the `belongsInSecondPartition`
   /// predicate and every element at or after `p` satisfies
   /// `belongsInSecondPartition`.
   ///
   /// In the following example, an array of numbers is partitioned by a
   /// predicate that matches elements greater than 30.
   ///
   ///     var numbers = [30, 40, 20, 30, 30, 60, 10]
   ///     let p = numbers.partition(by: { $0 > 30 })
   ///     // p == 5
   ///     // numbers == [30, 10, 20, 30, 30, 60, 40]
   ///
   /// The `numbers` array is now arranged in two partitions. The first
   /// partition, `numbers[..<p]`, is made up of the elements that
   /// are not greater than 30. The second partition, `numbers[p...]`,
   /// is made up of the elements that *are* greater than 30.
   ///
   ///     let first = numbers[..<p]
   ///     // first == [30, 10, 20, 30, 30]
   ///     let second = numbers[p...]
   ///     // second == [60, 40]
   ///
   /// - Parameter belongsInSecondPartition: A predicate used to partition
   ///   the collection. All elements satisfying this predicate are ordered
   ///   after all elements not satisfying it.
   /// - Returns: The index of the first element in the reordered collection
   ///   that matches `belongsInSecondPartition`. If no elements in the
   ///   collection match `belongsInSecondPartition`, the returned index is
   ///   equal to the collection's `endIndex`.
   ///
   /// - Complexity: O(*n*), where *n* is the length of the collection.
   @inlinable
   public mutating func partition(
     by belongsInSecondPartition: (Element) throws -> Bool
   ) rethrows -> Index {
     return try _halfStablePartition(isSuffixElement: belongsInSecondPartition)
   }

   /// Moves all elements satisfying `isSuffixElement` into a suffix of the
   /// collection, returning the start position of the resulting suffix.
   ///
   /// - Complexity: O(*n*) where n is the length of the collection.
   @inlinable
   internal mutating func _halfStablePartition(
     isSuffixElement: (Element) throws -> Bool
   ) rethrows -> Index {
     guard var i = try firstIndex(where: isSuffixElement)
     else { return endIndex }

     var j = index(after: i)
     while j != endIndex {
       if try !isSuffixElement(self[j]) { swapAt(i, j); formIndex(after: &i) }
       formIndex(after: &j)
     }
     return i
   }
 }

 extension MutableCollection where Self : BidirectionalCollection {
   /// Reorders the elements of the collection such that all the elements
   /// that match the given predicate are after all the elements that don't
   /// match.
   ///
   /// After partitioning a collection, there is a pivot index `p` where
   /// no element before `p` satisfies the `belongsInSecondPartition`
   /// predicate and every element at or after `p` satisfies
   /// `belongsInSecondPartition`.
   ///
   /// In the following example, an array of numbers is partitioned by a
   /// predicate that matches elements greater than 30.
   ///
   ///     var numbers = [30, 40, 20, 30, 30, 60, 10]
   ///     let p = numbers.partition(by: { $0 > 30 })
   ///     // p == 5
   ///     // numbers == [30, 10, 20, 30, 30, 60, 40]
   ///
   /// The `numbers` array is now arranged in two partitions. The first
   /// partition, `numbers[..<p]`, is made up of the elements that
   /// are not greater than 30. The second partition, `numbers[p...]`,
   /// is made up of the elements that *are* greater than 30.
   ///
   ///     let first = numbers[..<p]
   ///     // first == [30, 10, 20, 30, 30]
   ///     let second = numbers[p...]
   ///     // second == [60, 40]
   ///
   /// - Parameter belongsInSecondPartition: A predicate used to partition
   ///   the collection. All elements satisfying this predicate are ordered
   ///   after all elements not satisfying it.
   /// - Returns: The index of the first element in the reordered collection
   ///   that matches `belongsInSecondPartition`. If no elements in the
   ///   collection match `belongsInSecondPartition`, the returned index is
   ///   equal to the collection's `endIndex`.
   ///
   /// - Complexity: O(*n*), where *n* is the length of the collection.
   @inlinable
   public mutating func partition(
     by belongsInSecondPartition: (Element) throws -> Bool
   ) rethrows -> Index {
     let maybeOffset = try _withUnsafeMutableBufferPointerIfSupported {
       (bufferPointer) -> Int in
       let unsafeBufferPivot = try bufferPointer._partitionImpl(
         by: belongsInSecondPartition)
       return unsafeBufferPivot - bufferPointer.startIndex
     }
     if let offset = maybeOffset {
       return index(startIndex, offsetBy: offset)
     } else {
       return try _partitionImpl(by: belongsInSecondPartition)
     }
   }

   @usableFromInline
   internal mutating func _partitionImpl(
     by belongsInSecondPartition: (Element) throws -> Bool
   ) rethrows -> Index {
     var lo = startIndex
     var hi = endIndex

     // 'Loop' invariants (at start of Loop, all are true):
     // * lo < hi
     // * predicate(self[i]) == false, for i in startIndex ..< lo
     // * predicate(self[i]) == true, for i in hi ..< endIndex

     Loop: while true {
       FindLo: repeat {
         while lo < hi {
           if try belongsInSecondPartition(self[lo]) { break FindLo }
           formIndex(after: &lo)
         }
         break Loop
       } while false

       FindHi: repeat {
         formIndex(before: &hi)
         while lo < hi {
           if try !belongsInSecondPartition(self[hi]) { break FindHi }
           formIndex(before: &hi)
         }
         break Loop
       } while false

       swapAt(lo, hi)
       formIndex(after: &lo)
     }

     return lo
   }
 }

 //===----------------------------------------------------------------------===//
 // shuffled()/shuffle()
 //===----------------------------------------------------------------------===//

 extension Sequence {
   /// Returns the elements of the sequence, shuffled using the given generator
   /// as a source for randomness.
   ///
   /// You use this method to randomize the elements of a sequence when you are
   /// using a custom random number generator. For example, you can shuffle the
   /// numbers between `0` and `9` by calling the `shuffled(using:)` method on
   /// that range:
   ///
   ///     let numbers = 0...9
   ///     let shuffledNumbers = numbers.shuffled(using: &myGenerator)
   ///     // shuffledNumbers == [8, 9, 4, 3, 2, 6, 7, 0, 5, 1]
   ///
   /// - Parameter generator: The random number generator to use when shuffling
   ///   the sequence.
   /// - Returns: An array of this sequence's elements in a shuffled order.
   ///
   /// - Complexity: O(*n*), where *n* is the length of the sequence.
   /// - Note: The algorithm used to shuffle a sequence may change in a future
   ///   version of Swift. If you're passing a generator that results in the
   ///   same shuffled order each time you run your program, that sequence may
   ///   change when your program is compiled using a different version of
   ///   Swift.
   @inlinable
   public func shuffled<T: RandomNumberGenerator>(
     using generator: inout T
   ) -> [Element] {
     var result = ContiguousArray(self)
     result.shuffle(using: &generator)
     return Array(result)
   }

   /// Returns the elements of the sequence, shuffled.
   ///
   /// For example, you can shuffle the numbers between `0` and `9` by calling
   /// the `shuffled()` method on that range:
   ///
   ///     let numbers = 0...9
   ///     let shuffledNumbers = numbers.shuffled()
   ///     // shuffledNumbers == [1, 7, 6, 2, 8, 9, 4, 3, 5, 0]
   ///
   /// This method is equivalent to calling `shuffled(using:)`, passing in the
   /// system's default random generator.
   ///
   /// - Returns: A shuffled array of this sequence's elements.
   ///
   /// - Complexity: O(*n*), where *n* is the length of the sequence.
   @inlinable
   public func shuffled() -> [Element] {
     var g = SystemRandomNumberGenerator()
     return shuffled(using: &g)
   }
 }

 extension MutableCollection where Self : RandomAccessCollection {
   /// Shuffles the collection in place, using the given generator as a source
   /// for randomness.
   ///
   /// You use this method to randomize the elements of a collection when you
   /// are using a custom random number generator. For example, you can use the
   /// `shuffle(using:)` method to randomly reorder the elements of an array.
   ///
   ///     var names = ["Alejandro", "Camila", "Diego", "Luciana", "Luis", "Sofía"]
   ///     names.shuffle(using: &myGenerator)
   ///     // names == ["Sofía", "Alejandro", "Camila", "Luis", "Diego", "Luciana"]
   ///
   /// - Parameter generator: The random number generator to use when shuffling
   ///   the collection.
   ///
   /// - Complexity: O(*n*), where *n* is the length of the collection.
   /// - Note: The algorithm used to shuffle a collection may change in a future
   ///   version of Swift. If you're passing a generator that results in the
   ///   same shuffled order each time you run your program, that sequence may
   ///   change when your program is compiled using a different version of
   ///   Swift.
   @inlinable
   public mutating func shuffle<T: RandomNumberGenerator>(
     using generator: inout T
   ) {
     guard count > 1 else { return }
     var amount = count
     var currentIndex = startIndex
     while amount > 1 {
       let random = Int.random(in: 0 ..< amount, using: &generator)
       amount -= 1
       swapAt(
         currentIndex,
         index(currentIndex, offsetBy: random)
       )
       formIndex(after: &currentIndex)
     }
   }

   /// Shuffles the collection in place.
   ///
   /// Use the `shuffle()` method to randomly reorder the elements of an array.
   ///
   ///     var names = ["Alejandro", "Camila", "Diego", "Luciana", "Luis", "Sofía"]
   ///     names.shuffle(using: myGenerator)
   ///     // names == ["Luis", "Camila", "Luciana", "Sofía", "Alejandro", "Diego"]
   ///
   /// This method is equivalent to calling `shuffle(using:)`, passing in the
   /// system's default random generator.
   ///
   /// - Complexity: O(*n*), where *n* is the length of the collection.
   @inlinable
   public mutating func shuffle() {
     var g = SystemRandomNumberGenerator()
     shuffle(using: &g)
   }
 }
	//===--- CollectionAlgorithms.swift.gyb ------------------------ swift --===//
	//
	// This source file is part of the Swift.org open source project
	//
	// Copyright (c) 2014 - 2018 Apple Inc. and the Swift project authors
	// Licensed under Apache License v2.0 with Runtime Library Exception
	//
	// See https://swift.org/LICENSE.txt for license information
	// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
	//
	//===----------------------------------------------------------------------===//

	//===----------------------------------------------------------------------===//
	// last
	//===----------------------------------------------------------------------===//

	extension BidirectionalCollection {
	/// The last element of the collection.
	///
	/// If the collection is empty, the value of this property is `nil`.
	///
	/// let numbers = [10, 20, 30, 40, 50]
	/// if let lastNumber = numbers.last {
	/// print(lastNumber)
	/// }
	/// // Prints "50"
	///
	/// - Complexity: O(1)
	@inlinable
	public var last: Element? {
	return isEmpty ? nil : self[index(before: endIndex)]
	}
	}

	//===----------------------------------------------------------------------===//
	// firstIndex(of:)/firstIndex(where:)
	//===----------------------------------------------------------------------===//

	extension Collection where Element : Equatable {
	/// Returns the first index where the specified value appears in the
	/// collection.
	///
	/// After using `firstIndex(of:)` to find the position of a particular element
	/// in a collection, you can use it to access the element by subscripting.
	/// This example shows how you can modify one of the names in an array of
	/// students.
	///
	/// var students = ["Ben", "Ivy", "Jordell", "Maxime"]
	/// if let i = students.firstIndex(of: "Maxime") {
	/// students[i] = "Max"
	/// }
	/// print(students)
	/// // Prints "["Ben", "Ivy", "Jordell", "Max"]"
	///
	/// - Parameter element: An element to search for in the collection.
	/// - Returns: The first index where `element` is found. If `element` is not
	/// found in the collection, returns `nil`.
	///
	/// - Complexity: O(n), where n is the length of the collection.
	@inlinable
	public func firstIndex(of element: Element) -> Index? {
	if let result = _customIndexOfEquatableElement(element) {
	return result
	}

	var i = self.startIndex
	while i != self.endIndex {
	if self[i] == element {
	return i
	}
	self.formIndex(after: &i)
	}
	return nil
	}
	}

	extension Collection {
	/// Returns the first index in which an element of the collection satisfies
	/// the given predicate.
	///
	/// You can use the predicate to find an element of a type that doesn't
	/// conform to the `Equatable` protocol or to find an element that matches
	/// particular criteria. Here's an example that finds a student name that
	/// begins with the letter "A":
	///
	/// let students = ["Kofi", "Abena", "Peter", "Kweku", "Akosua"]
	/// if let i = students.firstIndex(where: { $0.hasPrefix("A") }) {
	/// print("\(students[i]) starts with 'A'!")
	/// }
	/// // Prints "Abena starts with 'A'!"
	///
	/// - Parameter predicate: A closure that takes an element as its argument
	/// and returns a Boolean value that indicates whether the passed element
	/// represents a match.
	/// - Returns: The index of the first element for which `predicate` returns
	/// `true`. If no elements in the collection satisfy the given predicate,
	/// returns `nil`.
	///
	/// - Complexity: O(n), where n is the length of the collection.
	@inlinable
	public func firstIndex(
	where predicate: (Element) throws -> Bool
	) rethrows -> Index? {
	var i = self.startIndex
	while i != self.endIndex {
	if try predicate(self[i]) {
	return i
	}
	self.formIndex(after: &i)
	}
	return nil
	}
	}

	//===----------------------------------------------------------------------===//
	// lastIndex(of:)/lastIndex(where:)
	//===----------------------------------------------------------------------===//

	extension BidirectionalCollection {
	/// Returns the last element of the sequence that satisfies the given
	/// predicate.
	///
	/// This example uses the `last(where:)` method to find the last
	/// negative number in an array of integers:
	///
	/// let numbers = [3, 7, 4, -2, 9, -6, 10, 1]
	/// if let lastNegative = numbers.last(where: { $0 < 0 }) {
	/// print("The last negative number is \(lastNegative).")
	/// }
	/// // Prints "The last negative number is -6."
	///
	/// - Parameter predicate: A closure that takes an element of the sequence as
	/// its argument and returns a Boolean value indicating whether the
	/// element is a match.
	/// - Returns: The last element of the sequence that satisfies `predicate`,
	/// or `nil` if there is no element that satisfies `predicate`.
	///
	/// - Complexity: O(n), where n is the length of the collection.
	@inlinable
	public func last(
	where predicate: (Element) throws -> Bool
	) rethrows -> Element? {
	return try lastIndex(where: predicate).map { self[$0] }
	}

	/// Returns the index of the last element in the collection that matches the
	/// given predicate.
	///
	/// You can use the predicate to find an element of a type that doesn't
	/// conform to the `Equatable` protocol or to find an element that matches
	/// particular criteria. This example finds the index of the last name that
	/// begins with the letter A:
	///
	/// let students = ["Kofi", "Abena", "Peter", "Kweku", "Akosua"]
	/// if let i = students.lastIndex(where: { $0.hasPrefix("A") }) {
	/// print("\(students[i]) starts with 'A'!")
	/// }
	/// // Prints "Akosua starts with 'A'!"
	///
	/// - Parameter predicate: A closure that takes an element as its argument
	/// and returns a Boolean value that indicates whether the passed element
	/// represents a match.
	/// - Returns: The index of the last element in the collection that matches
	/// `predicate`, or `nil` if no elements match.
	///
	/// - Complexity: O(n), where n is the length of the collection.
	@inlinable
	public func lastIndex(
	where predicate: (Element) throws -> Bool
	) rethrows -> Index? {
	var i = endIndex
	while i != startIndex {
	formIndex(before: &i)
	if try predicate(self[i]) {
	return i
	}
	}
	return nil
	}
	}

	extension BidirectionalCollection where Element : Equatable {
	/// Returns the last index where the specified value appears in the
	/// collection.
	///
	/// After using `lastIndex(of:)` to find the position of the last instance of
	/// a particular element in a collection, you can use it to access the
	/// element by subscripting. This example shows how you can modify one of
	/// the names in an array of students.
	///
	/// var students = ["Ben", "Ivy", "Jordell", "Ben", "Maxime"]
	/// if let i = students.lastIndex(of: "Ben") {
	/// students[i] = "Benjamin"
	/// }
	/// print(students)
	/// // Prints "["Ben", "Ivy", "Jordell", "Benjamin", "Max"]"
	///
	/// - Parameter element: An element to search for in the collection.
	/// - Returns: The last index where `element` is found. If `element` is not
	/// found in the collection, this method returns `nil`.
	///
	/// - Complexity: O(n), where n is the length of the collection.
	@inlinable
	public func lastIndex(of element: Element) -> Index? {
	if let result = _customLastIndexOfEquatableElement(element) {
	return result
	}
	return lastIndex(where: { $0 == element })
	}
	}

	//===----------------------------------------------------------------------===//
	// partition(by:)
	//===----------------------------------------------------------------------===//

	extension MutableCollection {
	/// Reorders the elements of the collection such that all the elements
	/// that match the given predicate are after all the elements that don't
	/// match.
	///
	/// After partitioning a collection, there is a pivot index `p` where
	/// no element before `p` satisfies the `belongsInSecondPartition`
	/// predicate and every element at or after `p` satisfies
	/// `belongsInSecondPartition`.
	///
	/// In the following example, an array of numbers is partitioned by a
	/// predicate that matches elements greater than 30.
	///
	/// var numbers = [30, 40, 20, 30, 30, 60, 10]
	/// let p = numbers.partition(by: { $0 > 30 })
	/// // p == 5
	/// // numbers == [30, 10, 20, 30, 30, 60, 40]
	///
	/// The `numbers` array is now arranged in two partitions. The first
	/// partition, `numbers[..<p]`, is made up of the elements that
	/// are not greater than 30. The second partition, `numbers[p...]`,
	/// is made up of the elements that are greater than 30.
	///
	/// let first = numbers[..<p]
	/// // first == [30, 10, 20, 30, 30]
	/// let second = numbers[p...]
	/// // second == [60, 40]
	///
	/// - Parameter belongsInSecondPartition: A predicate used to partition
	/// the collection. All elements satisfying this predicate are ordered
	/// after all elements not satisfying it.
	/// - Returns: The index of the first element in the reordered collection
	/// that matches `belongsInSecondPartition`. If no elements in the
	/// collection match `belongsInSecondPartition`, the returned index is
	/// equal to the collection's `endIndex`.
	///
	/// - Complexity: O(n), where n is the length of the collection.
	@inlinable
	public mutating func partition(
	by belongsInSecondPartition: (Element) throws -> Bool
	) rethrows -> Index {
	return try _halfStablePartition(isSuffixElement: belongsInSecondPartition)
	}

	/// Moves all elements satisfying `isSuffixElement` into a suffix of the
	/// collection, returning the start position of the resulting suffix.
	///
	/// - Complexity: O(n) where n is the length of the collection.
	@inlinable
	internal mutating func _halfStablePartition(
	isSuffixElement: (Element) throws -> Bool
	) rethrows -> Index {
	guard var i = try firstIndex(where: isSuffixElement)
	else { return endIndex }

	var j = index(after: i)
	while j != endIndex {
	if try !isSuffixElement(self[j]) { swapAt(i, j); formIndex(after: &i) }
	formIndex(after: &j)
	}
	return i
	}
	}

	extension MutableCollection where Self : BidirectionalCollection {
	/// Reorders the elements of the collection such that all the elements
	/// that match the given predicate are after all the elements that don't
	/// match.
	///
	/// After partitioning a collection, there is a pivot index `p` where
	/// no element before `p` satisfies the `belongsInSecondPartition`
	/// predicate and every element at or after `p` satisfies
	/// `belongsInSecondPartition`.
	///
	/// In the following example, an array of numbers is partitioned by a
	/// predicate that matches elements greater than 30.
	///
	/// var numbers = [30, 40, 20, 30, 30, 60, 10]
	/// let p = numbers.partition(by: { $0 > 30 })
	/// // p == 5
	/// // numbers == [30, 10, 20, 30, 30, 60, 40]
	///
	/// The `numbers` array is now arranged in two partitions. The first
	/// partition, `numbers[..<p]`, is made up of the elements that
	/// are not greater than 30. The second partition, `numbers[p...]`,
	/// is made up of the elements that are greater than 30.
	///
	/// let first = numbers[..<p]
	/// // first == [30, 10, 20, 30, 30]
	/// let second = numbers[p...]
	/// // second == [60, 40]
	///
	/// - Parameter belongsInSecondPartition: A predicate used to partition
	/// the collection. All elements satisfying this predicate are ordered
	/// after all elements not satisfying it.
	/// - Returns: The index of the first element in the reordered collection
	/// that matches `belongsInSecondPartition`. If no elements in the
	/// collection match `belongsInSecondPartition`, the returned index is
	/// equal to the collection's `endIndex`.
	///
	/// - Complexity: O(n), where n is the length of the collection.
	@inlinable
	public mutating func partition(
	by belongsInSecondPartition: (Element) throws -> Bool
	) rethrows -> Index {
	let maybeOffset = try _withUnsafeMutableBufferPointerIfSupported {
	(bufferPointer) -> Int in
	let unsafeBufferPivot = try bufferPointer._partitionImpl(
	by: belongsInSecondPartition)
	return unsafeBufferPivot - bufferPointer.startIndex
	}
	if let offset = maybeOffset {
	return index(startIndex, offsetBy: offset)
	} else {
	return try _partitionImpl(by: belongsInSecondPartition)
	}
	}

	@usableFromInline
	internal mutating func _partitionImpl(
	by belongsInSecondPartition: (Element) throws -> Bool
	) rethrows -> Index {
	var lo = startIndex
	var hi = endIndex

	// 'Loop' invariants (at start of Loop, all are true):
	// * lo < hi
	// * predicate(self[i]) == false, for i in startIndex ..< lo
	// * predicate(self[i]) == true, for i in hi ..< endIndex

	Loop: while true {
	FindLo: repeat {
	while lo < hi {
	if try belongsInSecondPartition(self[lo]) { break FindLo }
	formIndex(after: &lo)
	}
	break Loop
	} while false

	FindHi: repeat {
	formIndex(before: &hi)
	while lo < hi {
	if try !belongsInSecondPartition(self[hi]) { break FindHi }
	formIndex(before: &hi)
	}
	break Loop
	} while false

	swapAt(lo, hi)
	formIndex(after: &lo)
	}

	return lo
	}
	}

	//===----------------------------------------------------------------------===//
	// shuffled()/shuffle()
	//===----------------------------------------------------------------------===//

	extension Sequence {
	/// Returns the elements of the sequence, shuffled using the given generator
	/// as a source for randomness.
	///
	/// You use this method to randomize the elements of a sequence when you are
	/// using a custom random number generator. For example, you can shuffle the
	/// numbers between `0` and `9` by calling the `shuffled(using:)` method on
	/// that range:
	///
	/// let numbers = 0...9
	/// let shuffledNumbers = numbers.shuffled(using: &myGenerator)
	/// // shuffledNumbers == [8, 9, 4, 3, 2, 6, 7, 0, 5, 1]
	///
	/// - Parameter generator: The random number generator to use when shuffling
	/// the sequence.
	/// - Returns: An array of this sequence's elements in a shuffled order.
	///
	/// - Complexity: O(n), where n is the length of the sequence.
	/// - Note: The algorithm used to shuffle a sequence may change in a future
	/// version of Swift. If you're passing a generator that results in the
	/// same shuffled order each time you run your program, that sequence may
	/// change when your program is compiled using a different version of
	/// Swift.
	@inlinable
	public func shuffled<T: RandomNumberGenerator>(
	using generator: inout T
	) -> [Element] {
	var result = ContiguousArray(self)
	result.shuffle(using: &generator)
	return Array(result)
	}

	/// Returns the elements of the sequence, shuffled.
	///
	/// For example, you can shuffle the numbers between `0` and `9` by calling
	/// the `shuffled()` method on that range:
	///
	/// let numbers = 0...9
	/// let shuffledNumbers = numbers.shuffled()
	/// // shuffledNumbers == [1, 7, 6, 2, 8, 9, 4, 3, 5, 0]
	///
	/// This method is equivalent to calling `shuffled(using:)`, passing in the
	/// system's default random generator.
	///
	/// - Returns: A shuffled array of this sequence's elements.
	///
	/// - Complexity: O(n), where n is the length of the sequence.
	@inlinable
	public func shuffled() -> [Element] {
	var g = SystemRandomNumberGenerator()
	return shuffled(using: &g)
	}
	}

	extension MutableCollection where Self : RandomAccessCollection {
	/// Shuffles the collection in place, using the given generator as a source
	/// for randomness.
	///
	/// You use this method to randomize the elements of a collection when you
	/// are using a custom random number generator. For example, you can use the
	/// `shuffle(using:)` method to randomly reorder the elements of an array.
	///
	/// var names = ["Alejandro", "Camila", "Diego", "Luciana", "Luis", "Sofía"]
	/// names.shuffle(using: &myGenerator)
	/// // names == ["Sofía", "Alejandro", "Camila", "Luis", "Diego", "Luciana"]
	///
	/// - Parameter generator: The random number generator to use when shuffling
	/// the collection.
	///
	/// - Complexity: O(n), where n is the length of the collection.
	/// - Note: The algorithm used to shuffle a collection may change in a future
	/// version of Swift. If you're passing a generator that results in the
	/// same shuffled order each time you run your program, that sequence may
	/// change when your program is compiled using a different version of
	/// Swift.
	@inlinable
	public mutating func shuffle<T: RandomNumberGenerator>(
	using generator: inout T
	) {
	guard count > 1 else { return }
	var amount = count
	var currentIndex = startIndex
	while amount > 1 {
	let random = Int.random(in: 0 ..< amount, using: &generator)
	amount -= 1
	swapAt(
	currentIndex,
	index(currentIndex, offsetBy: random)
	)
	formIndex(after: &currentIndex)
	}
	}

	/// Shuffles the collection in place.
	///
	/// Use the `shuffle()` method to randomly reorder the elements of an array.
	///
	/// var names = ["Alejandro", "Camila", "Diego", "Luciana", "Luis", "Sofía"]
	/// names.shuffle(using: myGenerator)
	/// // names == ["Luis", "Camila", "Luciana", "Sofía", "Alejandro", "Diego"]
	///
	/// This method is equivalent to calling `shuffle(using:)`, passing in the
	/// system's default random generator.
	///
	/// - Complexity: O(n), where n is the length of the collection.
	@inlinable
	public mutating func shuffle() {
	var g = SystemRandomNumberGenerator()
	shuffle(using: &g)
	}
	}