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//===--- 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)
}
}
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