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

 //===--- LazySequence.swift -----------------------------------------------===//
 //
 // This source file is part of the Swift.org open source project
 //
 // Copyright (c) 2014 - 2017 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
 //
 //===----------------------------------------------------------------------===//

 /// A sequence on which normally-eager sequence operations are implemented
 /// lazily.
 ///
 /// Lazy sequences can be used to avoid needless storage allocation
 /// and computation, because they use an underlying sequence for
 /// storage and compute their elements on demand. For example, `doubled` in
 /// this code sample is a sequence containing the values `2`, `4`, and `6`.
 ///
 ///     let doubled = [1, 2, 3].lazy.map { $0 * 2 }
 ///
 /// Each time an element of the lazy sequence `doubled` is accessed, the
 /// closure accesses and transforms an element of the underlying array.
 ///
 /// Sequence operations that take closure arguments, such as `map(_:)` and
 /// `filter(_:)`, are normally eager: They use the closure immediately and
 /// return a new array. When you use the `lazy` property, you give the standard
 /// library explicit permission to store the closure and the sequence
 /// in the result, and defer computation until it is needed.
 ///
 /// ## Adding New Lazy Operations
 ///
 /// To add a new lazy sequence operation, extend this protocol with
 /// a method that returns a lazy wrapper that itself conforms to
 /// `LazySequenceProtocol`.  For example, an eager `scan(_:_:)`
 /// method is defined as follows:
 ///
 ///     extension Sequence {
 ///         /// Returns an array containing the results of
 ///         ///
 ///         ///   p.reduce(initial, nextPartialResult)
 ///         ///
 ///         /// for each prefix `p` of `self`, in order from shortest to
 ///         /// longest. For example:
 ///         ///
 ///         ///     (1..<6).scan(0, +) // [0, 1, 3, 6, 10, 15]
 ///         ///
 ///         /// - Complexity: O(n)
 ///         func scan<Result>(
 ///             _ initial: Result,
 ///             _ nextPartialResult: (Result, Element) -> Result
 ///         ) -> [Result] {
 ///             var result = [initial]
 ///             for x in self {
 ///                 result.append(nextPartialResult(result.last!, x))
 ///             }
 ///             return result
 ///         }
 ///     }
 ///
 /// You can build a sequence type that lazily computes the elements in the
 /// result of a scan:
 ///
 ///     struct LazyScanSequence<Base: Sequence, Result>
 ///         : LazySequenceProtocol
 ///     {
 ///         let initial: Result
 ///         let base: Base
 ///         let nextPartialResult:
 ///             (Result, Base.Element) -> Result
 ///
 ///         struct Iterator: IteratorProtocol {
 ///             var base: Base.Iterator
 ///             var nextElement: Result?
 ///             let nextPartialResult:
 ///                 (Result, Base.Element) -> Result
 ///
 ///             mutating func next() -> Result? {
 ///                 return nextElement.map { result in
 ///                     nextElement = base.next().map {
 ///                         nextPartialResult(result, $0)
 ///                     }
 ///                     return result
 ///                 }
 ///             }
 ///         }
 ///
 ///         func makeIterator() -> Iterator {
 ///             return Iterator(
 ///                 base: base.makeIterator(),
 ///                 nextElement: initial as Result?,
 ///                 nextPartialResult: nextPartialResult)
 ///         }
 ///     }
 ///
 /// Finally, you can give all lazy sequences a lazy `scan(_:_:)` method:
 ///
 ///     extension LazySequenceProtocol {
 ///         func scan<Result>(
 ///             _ initial: Result,
 ///             _ nextPartialResult: @escaping (Result, Element) -> Result
 ///         ) -> LazyScanSequence<Self, Result> {
 ///             return LazyScanSequence(
 ///                 initial: initial, base: self, nextPartialResult: nextPartialResult)
 ///         }
 ///     }
 ///
 /// With this type and extension method, you can call `.lazy.scan(_:_:)` on any
 /// sequence to create a lazily computed scan. The resulting `LazyScanSequence`
 /// is itself lazy, too, so further sequence operations also defer computation.
 ///
 /// The explicit permission to implement operations lazily applies
 /// only in contexts where the sequence is statically known to conform to
 /// `LazySequenceProtocol`. In the following example, because the extension
 /// applies only to `Sequence`, side-effects such as the accumulation of
 /// `result` are never unexpectedly dropped or deferred:
 ///
 ///     extension Sequence where Element == Int {
 ///         func sum() -> Int {
 ///             var result = 0
 ///             _ = self.map { result += $0 }
 ///             return result
 ///         }
 ///     }
 ///
 /// Don't actually use `map` for this purpose, however, because it creates
 /// and discards the resulting array. Instead, use `reduce` for summing
 /// operations, or `forEach` or a `for`-`in` loop for operations with side
 /// effects.
 public protocol LazySequenceProtocol: Sequence {
   /// A `Sequence` that can contain the same elements as this one,
   /// possibly with a simpler type.
   ///
   /// - See also: `elements`
   associatedtype Elements: Sequence = Self where Elements.Element == Element

   /// A sequence containing the same elements as this one, possibly with
   /// a simpler type.
   ///
   /// When implementing lazy operations, wrapping `elements` instead
   /// of `self` can prevent result types from growing an extra
   /// `LazySequence` layer.  For example,
   ///
   /// _prext_ example needed
   ///
   /// Note: this property need not be implemented by conforming types,
   /// it has a default implementation in a protocol extension that
   /// just returns `self`.
   var elements: Elements { get }
 }

 /// When there's no special associated `Elements` type, the `elements`
 /// property is provided.
 extension LazySequenceProtocol where Elements == Self {
   /// Identical to `self`.
   @inlinable // protocol-only
   public var elements: Self { return self }
 }

 extension LazySequenceProtocol {
   @inlinable // protocol-only
   public var lazy: LazySequence<Elements> {
     return elements.lazy
   }
 }

 extension LazySequenceProtocol where Elements: LazySequenceProtocol {
   @inlinable // protocol-only
   public var lazy: Elements {
     return elements
   }
 }

 /// A sequence containing the same elements as a `Base` sequence, but
 /// on which some operations such as `map` and `filter` are
 /// implemented lazily.
 ///
 /// - See also: `LazySequenceProtocol`
 @frozen // lazy-performance
 public struct LazySequence<Base: Sequence> {
   @usableFromInline
   internal var _base: Base

   /// Creates a sequence that has the same elements as `base`, but on
   /// which some operations such as `map` and `filter` are implemented
   /// lazily.
   @inlinable // lazy-performance
   internal init(_base: Base) {
     self._base = _base
   }
 }

 extension LazySequence: Sequence {
   public typealias Element = Base.Element
   public typealias Iterator = Base.Iterator

   @inlinable
   public __consuming func makeIterator() -> Iterator {
     return _base.makeIterator()
   }

   @inlinable // lazy-performance
   public var underestimatedCount: Int {
     return _base.underestimatedCount
   }

   @inlinable // lazy-performance
   @discardableResult
   public __consuming func _copyContents(
     initializing buf: UnsafeMutableBufferPointer<Element>
   ) -> (Iterator, UnsafeMutableBufferPointer<Element>.Index) {
     return _base._copyContents(initializing: buf)
   }

   @inlinable // lazy-performance
   public func _customContainsEquatableElement(_ element: Element) -> Bool? {
     return _base._customContainsEquatableElement(element)
   }

   @inlinable // generic-performance
   public __consuming func _copyToContiguousArray() -> ContiguousArray<Element> {
     return _base._copyToContiguousArray()
   }
 }

 extension LazySequence: LazySequenceProtocol {
   public typealias Elements = Base

   /// The `Base` (presumably non-lazy) sequence from which `self` was created.
   @inlinable // lazy-performance
   public var elements: Elements { return _base }
 }

 extension Sequence {
   /// A sequence containing the same elements as this sequence,
   /// but on which some operations, such as `map` and `filter`, are
   /// implemented lazily.
   @inlinable // protocol-only
   public var lazy: LazySequence<Self> {
     return LazySequence(_base: self)
   }
 }
	//===--- LazySequence.swift -----------------------------------------------===//
	//
	// This source file is part of the Swift.org open source project
	//
	// Copyright (c) 2014 - 2017 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
	//
	//===----------------------------------------------------------------------===//

	/// A sequence on which normally-eager sequence operations are implemented
	/// lazily.
	///
	/// Lazy sequences can be used to avoid needless storage allocation
	/// and computation, because they use an underlying sequence for
	/// storage and compute their elements on demand. For example, `doubled` in
	/// this code sample is a sequence containing the values `2`, `4`, and `6`.
	///
	/// let doubled = [1, 2, 3].lazy.map { $0 * 2 }
	///
	/// Each time an element of the lazy sequence `doubled` is accessed, the
	/// closure accesses and transforms an element of the underlying array.
	///
	/// Sequence operations that take closure arguments, such as `map(_:)` and
	/// `filter(_:)`, are normally eager: They use the closure immediately and
	/// return a new array. When you use the `lazy` property, you give the standard
	/// library explicit permission to store the closure and the sequence
	/// in the result, and defer computation until it is needed.
	///
	/// ## Adding New Lazy Operations
	///
	/// To add a new lazy sequence operation, extend this protocol with
	/// a method that returns a lazy wrapper that itself conforms to
	/// `LazySequenceProtocol`. For example, an eager `scan(_:_:)`
	/// method is defined as follows:
	///
	/// extension Sequence {
	/// /// Returns an array containing the results of
	/// ///
	/// /// p.reduce(initial, nextPartialResult)
	/// ///
	/// /// for each prefix `p` of `self`, in order from shortest to
	/// /// longest. For example:
	/// ///
	/// /// (1..<6).scan(0, +) // [0, 1, 3, 6, 10, 15]
	/// ///
	/// /// - Complexity: O(n)
	/// func scan<Result>(
	/// _ initial: Result,
	/// _ nextPartialResult: (Result, Element) -> Result
	/// ) -> [Result] {
	/// var result = [initial]
	/// for x in self {
	/// result.append(nextPartialResult(result.last!, x))
	/// }
	/// return result
	/// }
	/// }
	///
	/// You can build a sequence type that lazily computes the elements in the
	/// result of a scan:
	///
	/// struct LazyScanSequence<Base: Sequence, Result>
	/// : LazySequenceProtocol
	/// {
	/// let initial: Result
	/// let base: Base
	/// let nextPartialResult:
	/// (Result, Base.Element) -> Result
	///
	/// struct Iterator: IteratorProtocol {
	/// var base: Base.Iterator
	/// var nextElement: Result?
	/// let nextPartialResult:
	/// (Result, Base.Element) -> Result
	///
	/// mutating func next() -> Result? {
	/// return nextElement.map { result in
	/// nextElement = base.next().map {
	/// nextPartialResult(result, $0)
	/// }
	/// return result
	/// }
	/// }
	/// }
	///
	/// func makeIterator() -> Iterator {
	/// return Iterator(
	/// base: base.makeIterator(),
	/// nextElement: initial as Result?,
	/// nextPartialResult: nextPartialResult)
	/// }
	/// }
	///
	/// Finally, you can give all lazy sequences a lazy `scan(_:_:)` method:
	///
	/// extension LazySequenceProtocol {
	/// func scan<Result>(
	/// _ initial: Result,
	/// _ nextPartialResult: @escaping (Result, Element) -> Result
	/// ) -> LazyScanSequence<Self, Result> {
	/// return LazyScanSequence(
	/// initial: initial, base: self, nextPartialResult: nextPartialResult)
	/// }
	/// }
	///
	/// With this type and extension method, you can call `.lazy.scan(_:_:)` on any
	/// sequence to create a lazily computed scan. The resulting `LazyScanSequence`
	/// is itself lazy, too, so further sequence operations also defer computation.
	///
	/// The explicit permission to implement operations lazily applies
	/// only in contexts where the sequence is statically known to conform to
	/// `LazySequenceProtocol`. In the following example, because the extension
	/// applies only to `Sequence`, side-effects such as the accumulation of
	/// `result` are never unexpectedly dropped or deferred:
	///
	/// extension Sequence where Element == Int {
	/// func sum() -> Int {
	/// var result = 0
	/// _ = self.map { result += $0 }
	/// return result
	/// }
	/// }
	///
	/// Don't actually use `map` for this purpose, however, because it creates
	/// and discards the resulting array. Instead, use `reduce` for summing
	/// operations, or `forEach` or a `for`-`in` loop for operations with side
	/// effects.
	public protocol LazySequenceProtocol: Sequence {
	/// A `Sequence` that can contain the same elements as this one,
	/// possibly with a simpler type.
	///
	/// - See also: `elements`
	associatedtype Elements: Sequence = Self where Elements.Element == Element

	/// A sequence containing the same elements as this one, possibly with
	/// a simpler type.
	///
	/// When implementing lazy operations, wrapping `elements` instead
	/// of `self` can prevent result types from growing an extra
	/// `LazySequence` layer. For example,
	///
	/// _prext_ example needed
	///
	/// Note: this property need not be implemented by conforming types,
	/// it has a default implementation in a protocol extension that
	/// just returns `self`.
	var elements: Elements { get }
	}

	/// When there's no special associated `Elements` type, the `elements`
	/// property is provided.
	extension LazySequenceProtocol where Elements == Self {
	/// Identical to `self`.
	@inlinable // protocol-only
	public var elements: Self { return self }
	}

	extension LazySequenceProtocol {
	@inlinable // protocol-only
	public var lazy: LazySequence<Elements> {
	return elements.lazy
	}
	}

	extension LazySequenceProtocol where Elements: LazySequenceProtocol {
	@inlinable // protocol-only
	public var lazy: Elements {
	return elements
	}
	}

	/// A sequence containing the same elements as a `Base` sequence, but
	/// on which some operations such as `map` and `filter` are
	/// implemented lazily.
	///
	/// - See also: `LazySequenceProtocol`
	@frozen // lazy-performance
	public struct LazySequence<Base: Sequence> {
	@usableFromInline
	internal var _base: Base

	/// Creates a sequence that has the same elements as `base`, but on
	/// which some operations such as `map` and `filter` are implemented
	/// lazily.
	@inlinable // lazy-performance
	internal init(_base: Base) {
	self._base = _base
	}
	}

	extension LazySequence: Sequence {
	public typealias Element = Base.Element
	public typealias Iterator = Base.Iterator

	@inlinable
	public __consuming func makeIterator() -> Iterator {
	return _base.makeIterator()
	}

	@inlinable // lazy-performance
	public var underestimatedCount: Int {
	return _base.underestimatedCount
	}

	@inlinable // lazy-performance
	@discardableResult
	public __consuming func _copyContents(
	initializing buf: UnsafeMutableBufferPointer<Element>
	) -> (Iterator, UnsafeMutableBufferPointer<Element>.Index) {
	return _base._copyContents(initializing: buf)
	}

	@inlinable // lazy-performance
	public func _customContainsEquatableElement(_ element: Element) -> Bool? {
	return _base._customContainsEquatableElement(element)
	}

	@inlinable // generic-performance
	public __consuming func _copyToContiguousArray() -> ContiguousArray<Element> {
	return _base._copyToContiguousArray()
	}
	}

	extension LazySequence: LazySequenceProtocol {
	public typealias Elements = Base

	/// The `Base` (presumably non-lazy) sequence from which `self` was created.
	@inlinable // lazy-performance
	public var elements: Elements { return _base }
	}

	extension Sequence {
	/// A sequence containing the same elements as this sequence,
	/// but on which some operations, such as `map` and `filter`, are
	/// implemented lazily.
	@inlinable // protocol-only
	public var lazy: LazySequence<Self> {
	return LazySequence(_base: self)
	}
	}