//===--- LazyCollection.swift.gyb -----------------------------*- 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
//
//===----------------------------------------------------------------------===//

% from gyb_stdlib_support import TRAVERSALS, collectionForTraversal

/// 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.
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`.
  @_inlineable // FIXME(sil-serialize-all)
  public var elements: Self { return self }
}

% for Traversal in TRAVERSALS:
%   TraversalCollection = collectionForTraversal(Traversal)
%   Self = 'Lazy' + TraversalCollection
%   Slice = TraversalCollection.replace('Collection', 'Slice')

/// 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`
@_fixed_layout
public struct ${Self}<Base : ${TraversalCollection}> : LazyCollectionProtocol {

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

  /// The underlying collection.
  @_inlineable
  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

  /// Creates an instance with `base` as its underlying Collection
  /// instance.
  @_inlineable
  @_versioned
  internal init(_base: Base) {
    self._base = _base
  }

  @_versioned
  internal var _base: Base
}

/// Forward implementations to the base collection, to pick up any
/// optimizations it might implement.
extension ${Self} : Sequence {
  
  public typealias Iterator = Base.Iterator

  /// Returns an iterator over the elements of this sequence.
  ///
  /// - Complexity: O(1).
  @_inlineable
  public func makeIterator() -> Iterator {
    return _base.makeIterator()
  }

  /// Returns a value less than or equal to the number of elements in
  /// `self`, **nondestructively**.
  ///
  /// - Complexity: O(*n*)
  @_inlineable
  public var underestimatedCount: Int { return _base.underestimatedCount }

  @_inlineable
  public func _copyToContiguousArray()
     -> ContiguousArray<Base.Iterator.Element> {
    return _base._copyToContiguousArray()
  }

  @_inlineable
  public func _copyContents(
    initializing buf: UnsafeMutableBufferPointer<Iterator.Element>
  ) -> (Iterator,UnsafeMutableBufferPointer<Iterator.Element>.Index) {
    return _base._copyContents(initializing: buf)
  }

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

extension ${Self} : ${TraversalCollection} {
  /// The position of the first element in a non-empty collection.
  ///
  /// In an empty collection, `startIndex == endIndex`.
  @_inlineable
  public var startIndex: Base.Index {
    return _base.startIndex
  }

  /// The collection's "past the end" position---that is, the position one
  /// greater than the last valid subscript argument.
  ///
  /// `endIndex` is always reachable from `startIndex` by zero or more
  /// applications of `index(after:)`.
  @_inlineable
  public var endIndex: Base.Index {
    return _base.endIndex
  }

  @_inlineable
  public var indices: Base.Indices {
    return _base.indices
  }

  // TODO: swift-3-indexing-model - add docs
  @_inlineable
  public func index(after i: Base.Index) -> Base.Index {
    return _base.index(after: i)
  }

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

  /// Returns a collection representing a contiguous sub-range of
  /// `self`'s elements.
  ///
  /// - Complexity: O(1)
  @_inlineable
  public subscript(bounds: Range<Index>) -> Slice<${Self}<Base>> {
    return Slice(base: self, bounds: bounds)
  }

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

  /// Returns the number of elements.
  ///
  /// To check whether a 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 `Self` conforms to `RandomAccessCollection`;
  ///   O(*n*) otherwise.
  @_inlineable
  public var count: Base.IndexDistance {
    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*)
  @_inlineable
  public func _customIndexOfEquatableElement(
    _ element: Base.Iterator.Element
  ) -> Index?? {
    return _base._customIndexOfEquatableElement(element)
  }

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

  // TODO: swift-3-indexing-model - add docs
  @_inlineable
  public func index(_ i: Index, offsetBy n: Base.IndexDistance) -> Index {
    return _base.index(i, offsetBy: n)
  }

  // TODO: swift-3-indexing-model - add docs
  @_inlineable
  public func index(
    _ i: Index, offsetBy n: Base.IndexDistance, limitedBy limit: Index
  ) -> Index? {
    return _base.index(i, offsetBy: n, limitedBy: limit)
  }

  // TODO: swift-3-indexing-model - add docs
  @_inlineable
  public func distance(from start: Index, to end: Index) -> Base.IndexDistance {
    return _base.distance(from:start, to: end)
  }

%   if Traversal != 'Forward':

  @_inlineable
  public func index(before i: Base.Index) -> Base.Index {
    return _base.index(before: i)
  }

  @_inlineable
  public var last: Base.Element? {
    return _base.last
  }
%   end
}

/// Augment `self` with lazy methods such as `map`, `filter`, etc.
extension ${TraversalCollection} {
  /// A view onto this collection that provides lazy implementations of
  /// normally eager operations, such as `map` and `filter`.
  ///
  /// Use the `lazy` property when chaining operations to prevent
  /// intermediate operations from allocating storage, or when you only
  /// need a part of the final collection to avoid unnecessary computation.
  @_inlineable
  public var lazy: ${Self}<Self> {
    return ${Self}(_base: self)
  }
}

// Without this specific overload the non-re-wrapping extension on
// LazyCollectionProtocol (below) is not selected for some reason.
extension ${TraversalCollection} where Self : LazyCollectionProtocol {
  /// Identical to `self`.
  @_inlineable
  public var lazy: Self { // Don't re-wrap already-lazy collections
    return self
  }
}

% end

extension Slice: LazySequenceProtocol where Base: LazySequenceProtocol { }
extension Slice: LazyCollectionProtocol where Base: LazyCollectionProtocol { }

// ${'Local Variables'}:
// eval: (read-only-mode 1)
// End: