//===--- LazyCollection.swift ---------------------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2016 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See http://swift.org/LICENSE.txt for license information
// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//

/// A collection on which normally-eager operations such as `map` and
/// `filter` are implemented lazily.
///
/// Please see `LazySequenceProtocol` for background; `LazyCollectionProtocol`
/// is an analogous component, but for collections.
///
/// To add new lazy collection operations, extend this protocol with
/// methods that return lazy wrappers that are themselves
/// `LazyCollectionProtocol`s.
///
/// - See Also: `LazySequenceProtocol`, `LazyCollection`
public protocol LazyCollectionProtocol
  : Collection, LazySequenceProtocol {
  /// A `Collection` that can contain the same elements as this one,
  /// possibly with a simpler type.
  ///
  /// - See also: `elements`
  associatedtype Elements : Collection = Self
}

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

/// A collection containing the same elements as a `Base` collection,
/// but on which some operations such as `map` and `filter` are
/// implemented lazily.
///
/// - See also: `LazySequenceProtocol`, `LazyCollection`
public struct LazyCollection<Base : Collection>
  : LazyCollectionProtocol {

  /// The type of the underlying collection
  public typealias Elements = Base

  /// The underlying collection
  public var elements: Elements { return _base }

  /// A type that represents a valid position in the collection.
  ///
  /// Valid indices consist of the position of every element and a
  /// "past the end" position that's not valid for use as a subscript.
  public typealias Index = Base.Index
  
  /// Construct an instance with `base` as its underlying Collection
  /// instance.
  internal init(_base: Base) {
    self._base = _base
  }

  internal var _base: Base
}

/// Forward implementations to the base collection, to pick up any
/// optimizations it might implement.
extension LazyCollection : Sequence {
  
  /// Returns an iterator over the elements of this sequence.
  ///
  /// - Complexity: O(1).
  public func makeIterator() -> Base.Iterator {
    return _base.makeIterator()
  }
  
  /// Returns a value less than or equal to the number of elements in
  /// `self`, **nondestructively**.
  ///
  /// - Complexity: O(N).
  public var underestimatedCount: Int { return _base.underestimatedCount }

  public func _copyToNativeArrayBuffer() 
     -> _ContiguousArrayBuffer<Base.Iterator.Element> {
    return _base._copyToNativeArrayBuffer()
  }
  
  public func _copyContents(
    initializing ptr: UnsafeMutablePointer<Base.Iterator.Element>
  ) -> UnsafeMutablePointer<Base.Iterator.Element> {
    return _base._copyContents(initializing: ptr)
  }

  public func _customContainsEquatableElement(
    element: Base.Iterator.Element
  ) -> Bool? { 
    return _base._customContainsEquatableElement(element)
  }
}

extension LazyCollection : Collection {
  /// The position of the first element in a non-empty collection.
  ///
  /// In an empty collection, `startIndex == endIndex`.
  public var startIndex: Base.Index {
    return _base.startIndex
  }
  
  /// The collection's "past the end" position.
  ///
  /// `endIndex` is not a valid argument to `subscript`, and is always
  /// reachable from `startIndex` by zero or more applications of
  /// `successor()`.
  public var endIndex: Base.Index {
    return _base.endIndex
  }

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

  /// Returns a collection representing a contiguous sub-range of
  /// `self`'s elements.
  ///
  /// - Complexity: O(1)
  public subscript(bounds: Range<Index>) -> LazyCollection<Slice<Base>> {
    return Slice(_base: _base, bounds: bounds).lazy
  }
  
  /// Returns `true` iff `self` is empty.
  public var isEmpty: Bool {
    return _base.isEmpty
  }

  /// Returns the number of elements.
  ///
  /// - Complexity: O(1) if `Index` conforms to `RandomAccessIndex`;
  ///   O(N) otherwise.
  public var count: Index.Distance {
    return _base.count
  }
  
  // The following requirement enables dispatching for index(of:) when
  // the element type is Equatable.
  
  /// Returns `Optional(Optional(index))` if an element was found;
  /// `nil` otherwise.
  ///
  /// - Complexity: O(N).
  public func _customIndexOfEquatableElement(
    element: Base.Iterator.Element
  ) -> Index?? {
    return _base._customIndexOfEquatableElement(element)
  }

  /// Returns the first element of `self`, or `nil` if `self` is empty.
  public var first: Base.Iterator.Element? {
    return _base.first
  }
}

/// Augment `self` with lazy methods such as `map`, `filter`, etc.
extension Collection {
  /// A collection with contents identical to `self`, but on which
  /// normally-eager operations such as `map` and `filter` are
  /// implemented lazily.
  ///
  /// - See Also: `LazySequenceProtocol`, `LazyCollectionProtocol`.
  public var lazy: LazyCollection<Self> {
    return LazyCollection(_base: self)
  }
}

extension LazyCollectionProtocol {
  /// Identical to `self`.
  public var lazy: Self { // Don't re-wrap already-lazy collections
    return self
  }
}

@available(*, unavailable, renamed="LazyCollectionProtocol")
public typealias LazyCollectionType = LazyCollectionProtocol

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