swift-mirror/stdlib/core/Arrays.swift.gyb

%# -*- mode: swift -*-
//===--- Arrays.swift.gyb - ContiguousArray, Array, and Slice -------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2015 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
//
//===----------------------------------------------------------------------===//
//
//  Three generic, mutable array-like types with value semantics.
//
//  - ContiguousArray<T> is a fast, contiguous array of T with a known
//    backing store.
//
//  - Slice<T> presents an arbitrary subsequence of some contiguous sequence
//    of Ts.
//
//  - Array<T> is like ContiguousArray<T> when T is not an ObjC type.
//    Otherwise, it may use an NSArray bridged from Cocoa for storage
//
//===----------------------------------------------------------------------===//

% arrayTypes = ['ContiguousArray', 'Slice', 'Array']
% for Self in arrayTypes:
struct ${Self}<T> : MutableCollection, Sliceable {
  typealias Element = T
  var startIndex: Int {
    return 0
  }

  var endIndex: Int {
    return _buffer.count
  }

  subscript(index: Int) -> Element {
    get {
      _precondition(index < count, "${Self} index out of range")
      _precondition(index >= 0, "Negative ${Self} index is out of range")
      return _buffer[index]
    }
    set {
      _precondition(index < count, "${Self} index out of range")
      _precondition(index >= 0, "Negative array index is out of range")
      if _buffer.isMutableAndUniquelyReferenced() {
        _buffer[index] = newValue
      }
      else {
        // Slow path is written in a closure to put it out of line and prevent
        // performance inliner from inlining it back.
        // FIXME: extract into a '@noretun' function.
        {
          (inout buffer: _Buffer, e: Element)->() in
          var newBuffer = ContiguousArrayBuffer<T>(
            count: buffer.count, minimumCapacity: buffer.count)
          let target = buffer._uninitializedCopy(
            0..<index, target: newBuffer.elementStorage)
          target.initialize(e)
          buffer._uninitializedCopy(
            (index + 1)..<buffer.count, target: target + 1)
          buffer = _Buffer(newBuffer)
        }(&_buffer, newValue)
      }
    }
  }

  func generate() -> IndexingGenerator<${Self}> {
    return IndexingGenerator(self)
  }
  
  typealias SliceType = Slice<T>
  subscript(subRange: Range<Int>) -> SliceType {
    get {
      return Slice(_buffer[subRange])
    }
    set(rhs) {
      self.replaceRange(subRange, with: rhs)
    }
  }

  //===--- private --------------------------------------------------------===//
  typealias _Buffer = ${'Array' if Self.startswith('New') else Self}Buffer<T>

  init(_ buffer: _Buffer) {
    self._buffer = buffer
  }

  var _buffer: _Buffer
}

extension ${Self} : ArrayLiteralConvertible {
  static func convertFromArrayLiteral(elements: Element...) -> ${Self} {
    return ${Self}(_extractOrCopyToNativeArrayBuffer(elements._buffer))
  }
}

extension ${Self} {
  func _asCocoaArray() -> _CocoaArray {
    return _buffer._asCocoaArray()
  }
}

extension ${Self} : ArrayType {
  /// Construct an empty ${Self}
  init() {
    _buffer = _Buffer()
  }

  init<S: Sequence where S.GeneratorType.Element == _Buffer.Element>(_ s: S) {
    self = ${Self}(_Buffer(s~>_copyToNativeArrayBuffer()))
  }

  /// Construct an array of count elements, each initialized to value
  init(count: Int, repeatedValue: T) {
    _precondition(count >= 0, "Can't construct ${Self} with count < 0")
    _buffer = _Buffer()
    reserveCapacity(count)
    var p = _buffer.elementStorage
    for _ in 0..<count {
      p++.initialize(repeatedValue)
    }
    _buffer.count = count
  }
  
  /// How many elements the ${Self} stores
  var count: Int {
    return _buffer.count
  }

  /// How many elements the ${Self} can store without reallocation
  var capacity: Int {
    return _buffer.capacity
  }

  /// true if and only if the ${Self} is empty
  var isEmpty: Bool {
    return count == 0
  }
  
  /// An object that guarantees the lifetime of this array's elements
  var _owner: AnyObject? {
    return _buffer.owner
  }
  
  /// If the elements are stored contiguously, a pointer to the first
  /// element. Otherwise, nil.
  var _elementStorageIfContiguous: UnsafePointer<Element> {
    return _buffer.elementStorage
  }

%if Self != 'Array': # // Array does not necessarily have contiguous storage
  var _elementStorage: UnsafePointer<Element> {
    return _buffer.elementStorage
  }
%end
  //===--- basic mutations ------------------------------------------------===//


  /// Ensure the array has enough mutable contiguous storage to store
  /// minimumCapacity elements in.  Note: does not affect count.
  /// Complexity: O(N)
  mutating func reserveCapacity(minimumCapacity: Int) {
    if !_buffer.requestUniqueMutableBuffer(minimumCapacity) {
      
      var newBuffer = ContiguousArrayBuffer<T>(
        count: count, minimumCapacity: minimumCapacity)
      
      _buffer._uninitializedCopy(0..<count, target: newBuffer.elementStorage)
      _buffer = _Buffer(newBuffer)
    }
    _sanityCheck(capacity >= minimumCapacity)
  }
  
  /// Append newElement to the ${Self} in O(1) (amortized)
  mutating func append(newElement: T) {
    _arrayAppend(&_buffer, newElement)
  }

  /// Append elements from `sequence` to the Array
  mutating func extend<
      S : Sequence
      where S.GeneratorType.Element == T
  >(sequence: S) {
    // Calling a helper free function instead of writing the code inline
    // because of:
    //
    // <rdar://problem/16954386> Type checker assertion: Unable to solve for
    // call to witness?

    _${Self}Extend(&self, sequence)
  }

  /// Remove an element from the end of the ${Self} in O(1).
  /// Requires: count > 0
  mutating func removeLast() -> T {
    _precondition(count > 0, "can't removeLast from an empty ${Self}")
    let c = count
    let result = self[c - 1]
    self.replaceRange((c - 1)..<c, with: EmptyCollection())
    return result
  }
  
  /// Insert an element at the given index in O(N).  Requires: atIndex
  /// <= count
  mutating func insert(newElement: T, atIndex: Int) {
    self.replaceRange(atIndex..<atIndex, with: CollectionOfOne(newElement))
  }

  /// Remove the element at the given index.  Worst case complexity:
  /// O(N).  Requires: index < count
  mutating func removeAtIndex(index: Int) -> T {
    let result = self[index]
    self.replaceRange(index..<(index + 1), with: EmptyCollection())
    return result
  }

  /// Erase all the elements.  If `keepCapacity` is `true`, `capacity`
  /// will not change
  mutating func removeAll(keepCapacity: Bool = false) {
    if !keepCapacity {
      _buffer = _Buffer()
    }
    else {
      self.replaceRange(indices(self), with: EmptyCollection())
    }
  }
  
  //===--- algorithms -----------------------------------------------------===//

  func join<
      S : Sequence where S.GeneratorType.Element == ${Self}<T>
  >(elements: S) -> ${Self}<T> {
    return Swift.join(self, elements)
  }

  func reduce<U>(initial: U, combine: (U, T)->U) -> U {
    return Swift.reduce(self, initial, combine)
  }

  mutating func sort(isOrderedBefore: (T, T)->Bool) {
    Swift.sort(&self, isOrderedBefore)
  }

  func sorted(isOrderedBefore: (T, T)->Bool) -> ${Self} {
    var result = self
    result.sort(isOrderedBefore)
    return result
  }

  /// Return a ${Self} containing the results of calling
  /// `transform(x)` on each element `x` of `self`
  func map<U>(transform: (T)->U) -> ${Self}<U> {
    return ${Self}<U>(Swift.map(self, transform))
  }

  /// A ${Self} containing the elements of `self` in reverse order
  func reverse() -> ${Self} {
    return ${Self}(Swift.reverse(self))
  }

  /// Return a ${Self} containing the elements `x` of `self` for which
  /// `includeElement(x)` is `true`
  func filter(includeElement: (T)->Bool) -> ${Self} {
    return ${Self}(Swift.filter(self, includeElement))
  }
}

func _${Self}Extend<
    T, S : Sequence
    where S.GeneratorType.Element == T
>(inout a: ${Self}<T>, sequence: S) {
  a += sequence
}

extension ${Self} : Reflectable {
  func getMirror() -> Mirror {
    return _ArrayTypeMirror(self)
  }
}

extension ${Self} : Printable, DebugPrintable {
  func _makeDescription(#isDebug: Bool) -> String {
    var result = "["
    var first = true
    for item in self {
      if first {
        first = false
      } else {
        result += ", "
      }
      if isDebug {
        debugPrint(item, &result)
      } else {
        print(item, &result)
      }
    }
    result += "]"
    return result
  }

  var description: String {
    return _makeDescription(isDebug: false)
  }

  var debugDescription: String {
    return _makeDescription(isDebug: true)
  }
}

extension ${Self} {
  @transparent
  func _cPointerArgs() -> (AnyObject?, Builtin.RawPointer) {
    let p = _elementStorageIfContiguous
    if _fastPath(p != nil || count == 0) {
      return (_owner, p.value)
    }
    let n = _extractOrCopyToNativeArrayBuffer(self._buffer)
    return (n.owner, n.elementStorage.value)
  }
  
  // Conversion to C pointer arguments
  @transparent @conversion
  func __conversion() -> CConstPointer<T> {
    return CConstPointer(_cPointerArgs())
  }
  
  @transparent @conversion
  func __conversion() -> CConstVoidPointer {
    return CConstVoidPointer(_cPointerArgs())
  }
}

extension ${Self} {
  /// Call body(p), where p is a pointer to the ${Self}'s contiguous storage
%if Self != 'Array':
  /// Requires: the Array's storage is not provided by an opaque NSArray
%end
  func withUnsafePointerToElements<R>(body: (UnsafePointer<T>)->R) -> R {
    return _buffer.withUnsafePointerToElements(body)
  }
}

extension ${Self} {
  mutating func withMutableStorage<R>(
    body: (inout UnsafeMutableArray<T>)->R
  ) -> R {
    _arrayReserve(&_buffer, 0)
    var a = UnsafeMutableArray(start: _buffer.elementStorage, length: count)
    let ret = body(&a)
    _fixLifetime(_buffer)
    return ret
  }
}
%end

extension Array {
  /// This function "seeds" the ArrayLiteralConvertible protocol
  static func convertFromHeapArray(
    base: Builtin.RawPointer,
    owner: Builtin.NativeObject,
    count: Builtin.Word
  ) -> Array {
    let elements = UnsafeArray(
      start: reinterpretCast(base) as UnsafePointer<T>,
      length: reinterpretCast(count) as Int
    )
    let r = Array(elements)
    _fixLifetime(owner)
    return r
  }
}

struct _InitializeMemoryFromCollection<C: Collection> : _PointerFunction {
  func call(rawMemory: UnsafePointer<C.GeneratorType.Element>) {
    var p = rawMemory
    for x in newValues {
      p++.initialize(x)
    }    
  }

  init(_ newValues: C) {
    self.newValues = newValues
  }

  var newValues: C
}

func _arrayReplace2<
  A: ArrayType, C: Collection
  where C.GeneratorType.Element == A._Buffer.Element, A.IndexType == Int
  >(
  inout target: A, subRange: Range<Int>, newValues: C
) {
  _precondition(
    subRange.startIndex >= 0,
    "${Self} replace: subRange start is negative")
  
  _precondition(
    subRange.endIndex >= subRange.startIndex,
    "${Self} replace: subRange is inside-out")
  
  _precondition(
    subRange.endIndex <= target.endIndex,
    "${Self} replace: subRange is inside-out")
  
  let oldCount = target.count
  let eraseCount = countElements(subRange)
  let insertCount = numericCast(countElements(newValues)) as Int
  var newBuffer = _demandUniqueMutableBuffer(
    &target._buffer, oldCount + insertCount - eraseCount)

  if _fastPath(!newBuffer) {
    let growth = insertCount - eraseCount
    
    let elements = target._buffer.elementStorage
    _sanityCheck(elements != nil)
    
    let oldTailIndex = subRange.endIndex
    let oldTailStart = elements + oldTailIndex
    let newTailIndex = oldTailIndex + growth
    let newTailStart = oldTailStart + growth
    let tailCount = oldCount - subRange.endIndex

    if growth > 0 {
      // Slide the tail part of the buffer forwards, in reverse order
      // so as not to self-clobber.
      newTailStart.moveInitializeBackwardFrom(oldTailStart, count: tailCount)

      // Assign over the original subRange
      var i = newValues.startIndex
      for j in subRange {
        elements[j] = newValues[i++]
      }
      // Initialize the hole left by sliding the tail forward
      for j in oldTailIndex..<newTailIndex {
        (elements + j).initialize(newValues[i++])
      }
    }
    else { // We're not growing the buffer
      // Assign all the new elements into the start of the subRange
      var i = subRange.startIndex
      for j in indices(newValues) {
        elements[i++] = newValues[j]
      }

      // If the size didn't change, we're done.
      if growth == 0 {
        return
      }

      // Move the tail backward to cover the shrinkage.  
      let shrinkage = -growth
      if tailCount > shrinkage {   // If the tail length exceeds the shrinkage

        // Assign over the rest of the replaced range with the first
        // part of the tail.
        newTailStart.moveAssignFrom(oldTailStart, count: shrinkage)
        
        //  slide the rest of the tail back
        oldTailStart.moveInitializeFrom(
          oldTailStart + shrinkage, count: tailCount - shrinkage)
      }
      else {                      // tail fits within erased elements
        // Assign over the start of the replaced range with the tail
        newTailStart.moveAssignFrom(oldTailStart, count: tailCount)

        // destroy elements remaining after the tail in subRange
        (newTailStart + tailCount).destroy(shrinkage - tailCount)
      }
    }
  }
  else {
    _arrayOutOfPlaceUpdate(
      &target._buffer, &newBuffer,
      subRange.startIndex, insertCount,
      _InitializeMemoryFromCollection(newValues)
    )
  }
}

% for Self in arrayTypes:
func _arrayReplace1<
  T, C: Collection where C.GeneratorType.Element == T
  >(
  inout target: ${Self}<T>, subRange: Range<Int>, newValues: C
) {
  _arrayReplace2(&target, subRange, newValues)
}

extension ${Self} {
  mutating func replaceRange<
    C: Collection where C.GeneratorType.Element == _Buffer.Element
  >(
    subRange: Range<Int>, with newValues: C
  ) {
    _arrayReplace1(&self, subRange, newValues) 
  }
}
% end

@assignment
func += <
  A: ArrayType, S: Sequence
  where S.GeneratorType.Element == A._Buffer.Element
>(inout lhs: A, rhs: S) {
  _arrayAppendSequence(&lhs._buffer, rhs)
}

func _growArrayCapacity(capacity: Int) -> Int {
  return capacity * 2
}

@assignment
func += <
  A: ArrayType, C: Collection
where C.GeneratorType.Element == A._Buffer.Element
>(inout lhs: A, rhs: C) {
  let rhsCount = numericCast(countElements(rhs)) as Int
  
  let oldCount = lhs.count
  let capacity = lhs.capacity
  let newCount = oldCount + rhsCount
  
  // Ensure uniqueness, mutability, and sufficient storage.  Note that
  // for consistency, we need unique lhs even if rhs is empty.
  lhs.reserveCapacity(
    newCount > capacity ? max(newCount, _growArrayCapacity(capacity)) : newCount)
  
  var p = lhs._buffer.elementStorage + oldCount
  for x in rhs {
    (p++).initialize(x)
  }
  lhs._buffer.count = newCount
}

@assignment
func += <
  A: ArrayType
>(inout lhs: A, rhs: A._Buffer.Element) {
  lhs += CollectionOfOne(rhs)
}

//===--- generic helpers --------------------------------------------------===//
/// Ensure there's a ContiguousArrayBuffer capable of storing
/// max(newCount, minimumCapacity) elements, with count set to
/// newCount.
///
/// If source has sufficient capacity, returns nil.  Otherwise,
/// returns a new buffer.
///
/// NOTE: does not initialize or destroy any elements.  In general,
/// the buffer that satisfies the capacity request now has a count
/// that does not match its number of initialized elements, and that
/// needs to be corrected before the buffer can go back into circulation.
func _demandUniqueMutableBuffer<_Buffer: ArrayBufferType>(
  inout source: _Buffer, newCount: Int, minimumCapacity: Int = 0)
-> ContiguousArrayBuffer<_Buffer.Element>? {

  _sanityCheck(newCount >= 0)
  
  let requiredCapacity = max(newCount, minimumCapacity)
  
  if let b = source.requestUniqueMutableBuffer(requiredCapacity) {
    source.count = newCount
    return nil
  }

  let minimumCapacity = max(
    requiredCapacity,
    newCount > source.capacity
      ? _growArrayCapacity(source.capacity) : source.capacity)

  return ContiguousArrayBuffer(
    count: newCount, minimumCapacity: minimumCapacity)
}

protocol _PointerFunction {
  typealias Element
  func call(UnsafePointer<Element>)
}

/// initialize the elements of dest by copying the first headCount
/// items from source, calling initializeNewElements on the next
/// uninitialized element, and finally by copying the last N items
/// from source into the N remaining uninitialized elements of dest.
///
/// As an optimization, may move elements out of source rather than
/// copying when it isUniquelyReferenced.
func _arrayOutOfPlaceUpdate<
  _Buffer: ArrayBufferType, Initializer: _PointerFunction
    where Initializer.Element == _Buffer.Element
>(
  inout source: _Buffer, 
  inout dest: ContiguousArrayBuffer<_Buffer.Element>?,
  headCount: Int, // Count of initial source elements to copy/move
  newCount: Int,  // Count of new elements to insert
  initializeNewElements: Initializer
) {
  _sanityCheck(headCount >= 0)
  _sanityCheck(newCount >= 0)
  
  // Count of trailing source elements to copy/move
  let tailCount = dest!.count - headCount - newCount
  _sanityCheck(headCount + tailCount <= source.count)
  
  let sourceCount = source.count
  let oldCount = sourceCount - headCount - tailCount
  let destStart = dest!.elementStorage
  let newStart = destStart + headCount
  let newEnd = newStart + newCount
  
  // Check to see if we have storage we can move from
  if let backing = source.requestUniqueMutableBuffer(sourceCount) {
    let sourceStart = source.elementStorage
    let oldStart = sourceStart + headCount
    
    // Destroy any items that may be lurking in a SliceBuffer before
    // its real first element
    let backingStart = backing.elementStorage
    let sourceOffset = sourceStart - backingStart
    backingStart.destroy(sourceOffset)

    // Move the head items
    destStart.moveInitializeFrom(sourceStart, count: headCount)

    // Destroy unused source items
    oldStart.destroy(oldCount)

    initializeNewElements.call(newStart)

    // Move the tail items
    newEnd.moveInitializeFrom(oldStart + oldCount, count: tailCount)
    
    // Destroy any items that may be lurking in a SliceBuffer after
    // its real last element
    let backingEnd = backingStart + backing.count
    let sourceEnd = sourceStart + sourceCount
    sourceEnd.destroy(sourceEnd - backingEnd)
    backing.count = 0
  }
  else {
    let newStart = source._uninitializedCopy(0..<headCount, target: destStart)
    initializeNewElements.call(newStart)
    source._uninitializedCopy(headCount + oldCount..<sourceCount, 
                              target: newEnd)
  }
  source = _Buffer(dest!)
}

struct _InitializePointer<T> : _PointerFunction {
  func call(rawMemory: UnsafePointer<T>) {
    // FIXME: maybe we should move here instead of copying?
    rawMemory.initialize(newValue)
  }

  @transparent
  init(_ newValue: T) {
    self.newValue = newValue
  }

  var newValue: T
}

func _arrayAppend<_Buffer: ArrayBufferType>(
  inout buffer: _Buffer, newValue: _Buffer.Element
) {
  let oldCount = buffer.count
  var newBuffer = _demandUniqueMutableBuffer(&buffer, oldCount + 1)
  if _fastPath(!newBuffer) {
    (buffer.elementStorage + oldCount).initialize(newValue)
  }
  else {
    _arrayOutOfPlaceUpdate(
      &buffer, &newBuffer, oldCount, 1, _InitializePointer(newValue))
  }
}

struct _IgnorePointer<T> : _PointerFunction {
  func call(_:UnsafePointer<T>) {}
}

func _arrayReserve<_Buffer: ArrayBufferType>(
  inout buffer: _Buffer, minimumCapacity: Int
) {
  let oldCount = buffer.count
  var newBuffer = _demandUniqueMutableBuffer(
    &buffer, oldCount, minimumCapacity: minimumCapacity)
  if _slowPath(newBuffer) {
    _arrayOutOfPlaceUpdate(&buffer, &newBuffer, oldCount, 0, _IgnorePointer())
  }
}

func _extractOrCopyToNativeArrayBuffer<
  _Buffer: ArrayBufferType
    where _Buffer.GeneratorType.Element == _Buffer.Element
>(source: _Buffer) 
  -> ContiguousArrayBuffer<_Buffer.Element> 
{
  if let n = source.requestNativeBuffer() {
    return n
  }
  return _copyCollectionToNativeArrayBuffer(source)
}

/// Append items from newItems to buffer
func _arrayAppendSequence<
  _Buffer: ArrayBufferType,
  S: Sequence where S.GeneratorType.Element == _Buffer.Element
>(
  inout buffer: _Buffer, newItems: S
) {
  var stream = newItems.generate()
  var nextItem = stream.next()

  if !nextItem {
    return
  }

  // This will force uniqueness
  _arrayAppend(&buffer, nextItem!)
  var count = buffer.count
  nextItem = stream.next()
  while nextItem {
    let capacity = buffer.capacity
    let base = buffer.elementStorage
    
    while nextItem && count < capacity {
      (base + count++).initialize(nextItem!)
      nextItem = stream.next()
    }
    buffer.count = count
    if nextItem {
      _arrayReserve(&buffer, _growArrayCapacity(capacity))
    }
  }
}

% for Self in arrayTypes:
// NOTE: The '==' and '!=' below only handles array types
// that are the same, e.g. Array<Int> and Array<Int>, not
// Slice<Int> and Array<Int>.

/// Returns true if these arrays contain the same elements.
func ==<T: Equatable>(lhs: ${Self}<T>, rhs: ${Self}<T>) -> Bool {
  let lhsCount = lhs.count
  if lhsCount != rhs.count {
    return false
  }
  
  // Test referential equality.
  if lhsCount == 0 || lhs._buffer.identity == rhs._buffer.identity {
    return true
  }
  
  var streamLHS = lhs.generate()
  var streamRHS = rhs.generate()
  
  var nextLHS = streamLHS.next()
  while nextLHS {
    let nextRHS = streamRHS.next()
    if nextLHS != nextRHS {
      return false
    }
    nextLHS = streamLHS.next()
  }

  return true
}

/// Returns true if the arrays do not contain the same elements.
func !=<T: Equatable>(lhs: ${Self}<T>, rhs: ${Self}<T>) -> Bool {
  return !(lhs == rhs)
}
%end

/// Returns an Array<Base> containing the same elements as a in
/// O(1). Requires: Base is a base class or base @objc protocol (such
/// as AnyObject) of Derived.
func _arrayUpCast<Derived, Base>(a: Array<Derived>) -> Array<Base> {
  return Array(a._buffer.castToBufferOf(Base.self))
}

/// Implements the semantics of `x as Derived[]` where `x` has type
/// `Base[]` and `Derived` is a verbatim-bridged trivial subtype of
/// `Base`.
func _arrayDownCast<Base, Derived>(a: Array<Base>) -> Derived[] {
  _sanityCheck(isBridgedVerbatimToObjectiveC(Base.self))
  _sanityCheck(isBridgedVerbatimToObjectiveC(Derived.self))

  let native = a._buffer.requestNativeBuffer()
    
  // Fast path: a native buffer that already stores elements of the
  // Derived type.
  if _fastPath(native) {
    if _fastPath(native!.storesOnlyElementsOfType(Derived.self)) {
      return Array(a._buffer.castToBufferOf(Derived.self))
    }
  }

  // FIXME: Make these checks deferred.
  let result: Derived[]? = _arrayDownCastConditional(a)
  _precondition(result, "array cannot be downcast to array of derived")
  return result!
}

/// Implements the semantics of `x as? Derived[]` where `x` has type
/// `Base[]` and `Derived` is a verbatim-bridged trivial subtype of
/// `Base`.
///
/// Returns an Array<Derived> containing the same elements as a in
/// O(1) iff a's buffer elements are dynamically known to have
/// type Derived or a type derived from Derived.
func _arrayDownCastConditional<Base, Derived>(a: Array<Base>) -> Derived[]? {
  _sanityCheck(isBridgedVerbatimToObjectiveC(Base.self))
  _sanityCheck(isBridgedVerbatimToObjectiveC(Derived.self))
  
  if _fastPath(!a.isEmpty) {
    let native = a._buffer.requestNativeBuffer()
    
    if _fastPath(native) {
      if native!.storesOnlyElementsOfType(Derived.self) {
        return Array(a._buffer.castToBufferOf(Derived.self))
      }
      return nil
    }
    
    // slow path: we store an NSArray
    
    // We can skip the check if Derived happens to be AnyObject
    if !(AnyObject.self is Derived.Type) {
      for element in a {
        // FIXME: reinterpretCast works around <rdar://problem/16953026>
        if !(reinterpretCast(element) as AnyObject is Derived) {
          return nil
        }
      }
    }
    return Array(a._buffer.castToBufferOf(Derived.self))
  }
  return []
}

/// Convert a to its corresponding bridged array type.
/// Precondition: T is bridged non-verbatim to objective C
/// O(N), because each element must be bridged separately.
func _arrayBridgeToObjectiveC<BridgesToDerived, Base>(
  source: Array<BridgesToDerived>
) -> Array<Base> {
  _sanityCheck(isBridgedVerbatimToObjectiveC(Base.self))
  _sanityCheck(!isBridgedVerbatimToObjectiveC(BridgesToDerived.self))
  var buf = ContiguousArrayBuffer<Base>(count: source.count, minimumCapacity: 0)
  var p = buf._unsafeElementStorage
  for value in source {
    let bridged: AnyObject? = bridgeToObjectiveC(value)
    _precondition(bridged, "array element cannot be bridged to Objective-C")
    p++.initialize(reinterpretCast(bridged!))
  }
  return Array(ArrayBuffer(buf))
}

/// Try to convert the source array of objects to an array of values
/// produced by bridging the objects from Objective-C to \c
/// BridgesToDerived.
///
/// Precondition: Base is a class type.
/// Precondition: BridgesToDerived is bridged non-verbatim to Objective-C.
/// O(n), because each element must be bridged separately.
func _arrayBridgeFromObjectiveC<Base, BridgesToDerived>(source: Array<Base>)
       -> Array<BridgesToDerived> {
  let result: Array<BridgesToDerived>?
    = _arrayBridgeFromObjectiveCConditional(source);
  _precondition(result, "array cannot be bridged from Objective-C")
  return result!
}

/// Try to convert the source array of objects to an array of values
/// produced by bridging the objects from Objective-C to \c
/// BridgesToDerived.
///
/// Precondition: Base is a class type.
/// Precondition: BridgesToDerived is bridged non-verbatim to Objective-C.
/// O(n), because each element must be bridged separately.
func _arrayBridgeFromObjectiveCConditional<Base, BridgesToDerived>(
       source: Array<Base>
     ) -> Array<BridgesToDerived>? {
  _sanityCheck(isBridgedVerbatimToObjectiveC(Base.self))
  _sanityCheck(!isBridgedVerbatimToObjectiveC(BridgesToDerived.self))
  var buf = ContiguousArrayBuffer<BridgesToDerived>(count: source.count, 
                                                    minimumCapacity: 0)
  var p = buf._unsafeElementStorage
  
ElementwiseBridging:
  do {
    for object: Base in source {
      let value = Swift.bridgeFromObjectiveC(reinterpretCast(object), 
                                             BridgesToDerived.self)
      if _slowPath(!value) {
        break ElementwiseBridging
      }
      p++.initialize(value!)
    }
    return Array(ArrayBuffer(buf))
  }
  while false
  
  // Don't destroy anything we never created.
  buf.count = p - buf._unsafeElementStorage
  
  // Report failure
  return nil
}

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