Map.swift

//===--- Map.swift - Lazily map over a Sequence ---------------*- 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` whose elements consist of those in a `Base`
/// `Sequence` passed through a transform function returning `Element`.
/// These elements are computed lazily, each time they're read, by
/// calling the transform function on a base element.
@frozen
public struct LazyMapSequence<Base : Sequence, Element> {

  public typealias Elements = LazyMapSequence

  @usableFromInline
  internal var _base: Base
  @usableFromInline
  internal let _transform: (Base.Element) -> Element

  /// Creates an instance with elements `transform(x)` for each element
  /// `x` of base.
  @inlinable
  internal init(_base: Base, transform: @escaping (Base.Element) -> Element) {
    self._base = _base
    self._transform = transform
  }
}

extension LazyMapSequence {
  @frozen
  public struct Iterator {
    @usableFromInline
    internal var _base: Base.Iterator
    @usableFromInline
    internal let _transform: (Base.Element) -> Element

    @inlinable
    public var base: Base.Iterator { return _base }

    @inlinable
    internal init(
      _base: Base.Iterator, 
      _transform: @escaping (Base.Element) -> Element
    ) {
      self._base = _base
      self._transform = _transform
    }
  }
}

extension LazyMapSequence.Iterator: IteratorProtocol, Sequence {
  /// Advances to the next element and returns it, or `nil` if no next element
  /// exists.
  ///
  /// Once `nil` has been returned, all subsequent calls return `nil`.
  ///
  /// - Precondition: `next()` has not been applied to a copy of `self`
  ///   since the copy was made.
  @inlinable
  public mutating func next() -> Element? {
    return _base.next().map(_transform)
  }
}

extension LazyMapSequence: LazySequenceProtocol {
  /// Returns an iterator over the elements of this sequence.
  ///
  /// - Complexity: O(1).
  @inlinable
  public __consuming func makeIterator() -> Iterator {
    return Iterator(_base: _base.makeIterator(), _transform: _transform)
  }

  /// A value less than or equal to the number of elements in the sequence,
  /// calculated nondestructively.
  ///
  /// The default implementation returns 0. If you provide your own
  /// implementation, make sure to compute the value nondestructively.
  ///
  /// - Complexity: O(1), except if the sequence also conforms to `Collection`.
  ///   In this case, see the documentation of `Collection.underestimatedCount`.
  @inlinable
  public var underestimatedCount: Int {
    return _base.underestimatedCount
  }
}

/// A `Collection` whose elements consist of those in a `Base`
/// `Collection` passed through a transform function returning `Element`.
/// These elements are computed lazily, each time they're read, by
/// calling the transform function on a base element.
public typealias LazyMapCollection<T: Collection,U> = LazyMapSequence<T,U>

extension LazyMapCollection: Collection {
  public typealias Index = Base.Index
  public typealias Indices = Base.Indices
  public typealias SubSequence = LazyMapCollection<Base.SubSequence, Element>

  @inlinable
  public var startIndex: Base.Index { return _base.startIndex }
  @inlinable
  public var endIndex: Base.Index { return _base.endIndex }

  @inlinable
  public func index(after i: Index) -> Index { return _base.index(after: i) }
  @inlinable
  public func formIndex(after i: inout Index) { _base.formIndex(after: &i) }

  /// Accesses the element at `position`.
  ///
  /// - Precondition: `position` is a valid position in `self` and
  ///   `position != endIndex`.
  @inlinable
  public subscript(position: Base.Index) -> Element {
    return _transform(_base[position])
  }

  @inlinable
  public subscript(bounds: Range<Base.Index>) -> SubSequence {
    return SubSequence(_base: _base[bounds], transform: _transform)
  }

  @inlinable
  public var indices: Indices {
    return _base.indices
  }

  /// A Boolean value indicating whether the collection is empty.
  @inlinable
  public var isEmpty: Bool { return _base.isEmpty }

  /// The number of elements in the collection.
  ///
  /// To check whether the collection is empty, use its `isEmpty` property
  /// instead of comparing `count` to zero. Unless the collection guarantees
  /// random-access performance, calculating `count` can be an O(*n*)
  /// operation.
  ///
  /// - Complexity: O(1) if `Index` conforms to `RandomAccessIndex`; O(*n*)
  ///   otherwise.
  @inlinable
  public var count: Int {
    return _base.count
  }

  @inlinable
  public func index(_ i: Index, offsetBy n: Int) -> Index {
    return _base.index(i, offsetBy: n)
  }

  @inlinable
  public func index(
    _ i: Index, offsetBy n: Int, limitedBy limit: Index
  ) -> Index? {
    return _base.index(i, offsetBy: n, limitedBy: limit)
  }

  @inlinable
  public func distance(from start: Index, to end: Index) -> Int {
    return _base.distance(from: start, to: end)
  }
}

extension LazyMapCollection : BidirectionalCollection
  where Base : BidirectionalCollection {

  /// A value less than or equal to the number of elements in the collection.
  ///
  /// - Complexity: O(1) if the collection conforms to
  ///   `RandomAccessCollection`; otherwise, O(*n*), where *n* is the length
  ///   of the collection.
  @inlinable
  public func index(before i: Index) -> Index { return _base.index(before: i) }

  @inlinable
  public func formIndex(before i: inout Index) {
    _base.formIndex(before: &i)
  }
}

extension LazyMapCollection: LazyCollectionProtocol { }

extension LazyMapCollection : RandomAccessCollection
  where Base : RandomAccessCollection { }

//===--- Support for s.lazy -----------------------------------------------===//

extension LazySequenceProtocol {
  /// Returns a `LazyMapSequence` over this `Sequence`.  The elements of
  /// the result are computed lazily, each time they are read, by
  /// calling `transform` function on a base element.
  @inlinable
  public func map<U>(
    _ transform: @escaping (Element) -> U
  ) -> LazyMapSequence<Elements, U> {
    return LazyMapSequence(_base: elements, transform: transform)
  }
}

extension LazyMapSequence {
  @inlinable
  @available(swift, introduced: 5)
  public func map<ElementOfResult>(
    _ transform: @escaping (Element) -> ElementOfResult
  ) -> LazyMapSequence<Base, ElementOfResult> {
    return LazyMapSequence<Base, ElementOfResult>(
      _base: _base,
      transform: { transform(self._transform($0)) })
  }
}

extension LazyMapCollection {
  @inlinable
  @available(swift, introduced: 5)
  public func map<ElementOfResult>(
    _ transform: @escaping (Element) -> ElementOfResult
  ) -> LazyMapCollection<Base, ElementOfResult> {
    return LazyMapCollection<Base, ElementOfResult>(
      _base: _base,
      transform: {transform(self._transform($0))})
  }
}