//===--- UnsafeBufferPointer.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
//
//===----------------------------------------------------------------------===//

/// An iterator for the elements in the buffer referenced by an
/// `UnsafeBufferPointer` or `UnsafeMutableBufferPointer` instance.
@_fixed_layout
public struct UnsafeBufferPointerIterator<Element>
  : 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`.
  @_inlineable
  public mutating func next() -> Element? {
    if _position == _end { return nil }

    let result = _position!.pointee
    _position! += 1
    return result
  }

  @_versioned
  internal var _position, _end: UnsafePointer<Element>?

  @_inlineable
  public init(_position: UnsafePointer<Element>?, _end: UnsafePointer<Element>?) {
      self._position = _position
      self._end = _end
  }
}

% for mutable in (True, False):
%  Self = 'UnsafeMutableBufferPointer' if mutable else 'UnsafeBufferPointer'
%  Mutable = 'Mutable' if mutable else ''
/// A non-owning collection interface to a buffer of ${Mutable.lower()}
/// elements stored contiguously in memory.
///
/// You can use an `${Self}` instance in low level operations to eliminate
/// uniqueness checks and, in release mode, bounds checks. Bounds checks are
/// always performed in debug mode.
///
/// ${Self} Semantics
/// =================
///
/// An `${Self}` instance is a view into memory and does not own the memory
/// that it references. Copying a value of type `${Self}` does not copy the
/// instances stored in the underlying memory. However, initializing another
/// collection with an `${Self}` instance copies the instances out of the
/// referenced memory and into the new collection.
@_fixed_layout
public struct Unsafe${Mutable}BufferPointer<Element>
  : _${Mutable}Indexable, ${Mutable}Collection, RandomAccessCollection {
  // FIXME: rdar://18157434 - until this is fixed, this has to be fixed layout
  // to avoid a hang in Foundation, which has the following setup:
  // struct A { struct B { let x: UnsafeMutableBufferPointer<...> } let b: B }

  public typealias Index = Int
  public typealias IndexDistance = Int
  public typealias Iterator = UnsafeBufferPointerIterator<Element>

  /// The index of the first element in a nonempty buffer.
  ///
  /// The `startIndex` property of an `Unsafe${Mutable}BufferPointer` instance
  /// is always zero.
  @_inlineable
  public var startIndex: Int {
    return 0
  }

  /// The "past the end" position---that is, the position one greater than the
  /// last valid subscript argument.
  ///
  /// The `endIndex` property of an `Unsafe${Mutable}BufferPointer` instance is
  /// always identical to `count`.
  @_inlineable
  public var endIndex: Int {
    return count
  }

  @_inlineable
  public func index(after i: Int) -> Int {
    // NOTE: this is a manual specialization of index movement for a Strideable
    // index that is required for UnsafeBufferPointer performance. The
    // optimizer is not capable of creating partial specializations yet.
    // NOTE: Range checks are not performed here, because it is done later by
    // the subscript function.
    return i + 1
  }

  @_inlineable
  public func formIndex(after i: inout Int) {
    // NOTE: this is a manual specialization of index movement for a Strideable
    // index that is required for UnsafeBufferPointer performance. The
    // optimizer is not capable of creating partial specializations yet.
    // NOTE: Range checks are not performed here, because it is done later by
    // the subscript function.
    i += 1
  }

  @_inlineable
  public func index(before i: Int) -> Int {
    // NOTE: this is a manual specialization of index movement for a Strideable
    // index that is required for UnsafeBufferPointer performance. The
    // optimizer is not capable of creating partial specializations yet.
    // NOTE: Range checks are not performed here, because it is done later by
    // the subscript function.
    return i - 1
  }

  @_inlineable
  public func formIndex(before i: inout Int) {
    // NOTE: this is a manual specialization of index movement for a Strideable
    // index that is required for UnsafeBufferPointer performance. The
    // optimizer is not capable of creating partial specializations yet.
    // NOTE: Range checks are not performed here, because it is done later by
    // the subscript function.
    i -= 1
  }

  @_inlineable
  public func index(_ i: Int, offsetBy n: Int) -> Int {
    // NOTE: this is a manual specialization of index movement for a Strideable
    // index that is required for UnsafeBufferPointer performance. The
    // optimizer is not capable of creating partial specializations yet.
    // NOTE: Range checks are not performed here, because it is done later by
    // the subscript function.
    return i + n
  }

  @_inlineable
  public func index(
    _ i: Int, offsetBy n: Int, limitedBy limit: Int
  ) -> Int? {
    // NOTE: this is a manual specialization of index movement for a Strideable
    // index that is required for UnsafeBufferPointer performance. The
    // optimizer is not capable of creating partial specializations yet.
    // NOTE: Range checks are not performed here, because it is done later by
    // the subscript function.
    let l = limit - i
    if n > 0 ? l >= 0 && l < n : l <= 0 && n < l {
      return nil
    }
    return i + n
  }

  @_inlineable
  public func distance(from start: Int, to end: Int) -> Int {
    // NOTE: this is a manual specialization of index movement for a Strideable
    // index that is required for UnsafeBufferPointer performance. The
    // optimizer is not capable of creating partial specializations yet.
    // NOTE: Range checks are not performed here, because it is done later by
    // the subscript function.
    return end - start
  }

  @_inlineable
  public func _failEarlyRangeCheck(_ index: Int, bounds: Range<Int>) {
    // NOTE: This method is a no-op for performance reasons.
  }

  @_inlineable
  public func _failEarlyRangeCheck(_ range: Range<Int>, bounds: Range<Int>) {
    // NOTE: This method is a no-op for performance reasons.
  }

  public typealias Indices = CountableRange<Int>

  @_inlineable
  public var indices: Indices {
    return startIndex..<endIndex
  }

  /// Initializes the memory at `destination.baseAddress` with elements of `self`,
  /// stopping when either `self` or `destination` is exhausted.
  ///
  /// - Returns: an iterator over any remaining elements of `self` and the
  ///   number of elements initialized.
  @_inlineable // FIXME(sil-serialize-all)
  public func _copyContents(
    initializing destination: UnsafeMutableBufferPointer<Element>
  ) -> (Iterator, UnsafeMutableBufferPointer<Element>.Index) {
    guard !isEmpty && !destination.isEmpty else { return (makeIterator(), 0) }
    let s = self.baseAddress._unsafelyUnwrappedUnchecked
    let d = destination.baseAddress._unsafelyUnwrappedUnchecked
    let n = Swift.min(destination.count, self.count)
    d.initialize(from: s, count: n)
    return (Iterator(_position: s + n, _end: _end), n)
  }

  /// Accesses the element at the specified position.
  ///
%if Mutable:
  /// The following example uses the buffer pointer's subscript to access and
  /// modifying the elements of a mutable buffer pointing to the contiguous
  /// contents of an array:
  ///
  ///     var numbers = [1, 2, 3, 4, 5]
  ///     numbers.withUnsafeMutableBufferPointer { buffer in
  ///         for i in stride(from: buffer.startIndex, to: buffer.endIndex - 1, by: 2) {
  ///             let x = buffer[i]
  ///             buffer[i + 1] = buffer[i]
  ///             buffer[i] = x
  ///         }
  ///     }
  ///     print(numbers)
  ///     // Prints "[2, 1, 4, 3, 5]"
%else:
  /// The following example uses the buffer pointer's subscript to access every
  /// other element of the buffer:
  ///
  ///     let numbers = [1, 2, 3, 4, 5]
  ///     let sum = numbers.withUnsafeBufferPointer { buffer -> Int in
  ///         var result = 0
  ///         for i in stride(from: buffer.startIndex, to: buffer.endIndex, by: 2) {
  ///             result += buffer[i]
  ///         }
  ///         return result
  ///     }
  ///     // 'sum' == 9
%end
  ///
  /// - Note: Bounds checks for `i` are performed only in debug mode.
  ///
  /// - Parameter i: The position of the element to access. `i` must be in the
  ///   range `0..<count`.
  @_inlineable
  public subscript(i: Int) -> Element {
    get {
      _debugPrecondition(i >= 0)
      _debugPrecondition(i < endIndex)
      return _position![i]
    }
%if Mutable:
    nonmutating set {
      _debugPrecondition(i >= 0)
      _debugPrecondition(i < endIndex)
      _position![i] = newValue
    }
%end
  }

  @_inlineable
  public subscript(bounds: Range<Int>)
    -> ${Mutable}RandomAccessSlice<Unsafe${Mutable}BufferPointer<Element>>
  {
    get {
      _debugPrecondition(bounds.lowerBound >= startIndex)
      _debugPrecondition(bounds.upperBound <= endIndex)
      return ${Mutable}RandomAccessSlice(
        base: self, bounds: bounds)
    }
%  if Mutable:
    set {
      _debugPrecondition(bounds.lowerBound >= startIndex)
      _debugPrecondition(bounds.upperBound <= endIndex)
      // FIXME: swift-3-indexing-model: tests.
      _writeBackMutableSlice(&self, bounds: bounds, slice: newValue)
    }
%  end
  }

  /// Creates a new buffer pointer over the specified number of contiguous
  /// instances beginning at the given pointer.
  ///
  /// - Parameters:
  ///   - start: A pointer to the start of the buffer, or `nil`. If `start` is
  ///     `nil`, `count` must be zero. However, `count` may be zero even for a
  ///     non-`nil` `start`. The pointer passed as `start` must be aligned to
  ///     `MemoryLayout<Element>.alignment`.
  ///   - count: The number of instances in the buffer. `count` must not be
  ///     negative.
  @_inlineable
  public init(start: Unsafe${Mutable}Pointer<Element>?, count: Int) {
    _precondition(
      count >= 0, "Unsafe${Mutable}BufferPointer with negative count")
    _precondition(
      count == 0 || start != nil,
      "Unsafe${Mutable}BufferPointer has a nil start and nonzero count")
    _position = start
    _end = start.map { $0 + count }
  }

  /// Creates a buffer pointer starting at `start` and extending up to the last
  /// addressable pointer to `Element` in the memory space
  @_inlineable
  public init(_unboundedStartingAt start: Unsafe${Mutable}Pointer<Element>) {
    _position = start
    _end = type(of: start)._max
  }

  @_inlineable
  public init(_empty: ()) {
    _position = Unsafe${Mutable}Pointer._max
    _end = _position
  }
  
%  if not Mutable:
  /// Creates a buffer over the same memory as the given buffer slice.
  ///
  /// The new buffer represents the same region of memory as `slice`, but is
  /// indexed starting at zero instead of sharing indices with the original
  /// buffer. For example:
  ///
  ///     let buffer = returnsABuffer()
  ///     let n = 5
  ///     let slice = buffer[n...]
  ///     let rebased = UnsafeBufferPointer(rebasing: slice)
  ///
  /// After rebasing `slice` as the `rebased` buffer, the following are true:
  ///
  /// - `rebased.startIndex == 0`
  /// - `rebased[0] == slice[n]`
  /// - `rebased[0] == buffer[n]`
  /// - `rebased.count == slice.count`
  ///
  /// - Parameter slice: The buffer slice to rebase.
  @_inlineable
  public init(rebasing slice: RandomAccessSlice<UnsafeBufferPointer<Element>>) {
    self.init(start: slice.base.baseAddress! + slice.startIndex,
      count: slice.count)
  }
%  end # !mutable

  /// Creates a buffer over the same memory as the given buffer slice.
  ///
  /// The new buffer represents the same region of memory as `slice`, but is
  /// indexed starting at zero instead of sharing indices with the original
  /// buffer. For example:
  ///
  ///     let buffer = returnsABuffer()
  ///     let n = 5
  ///     let slice = buffer[n...]
  ///     let rebased = Unsafe${Mutable}BufferPointer(rebasing: slice)
  ///
  /// After rebasing `slice` as the `rebased` buffer, the following are true:
  ///
  /// - `rebased.startIndex == 0`
  /// - `rebased[0] == slice[n]`
  /// - `rebased[0] == buffer[n]`
  /// - `rebased.count == slice.count`
  ///
  /// - Parameter slice: The buffer slice to rebase.
  @_inlineable
  public init(
    rebasing slice:
    MutableRandomAccessSlice<UnsafeMutableBufferPointer<Element>>
  ) {
    self.init(start: slice.base.baseAddress! + slice.startIndex,
      count: slice.count)
  }

  /// Returns an iterator over the elements of this buffer.
  ///
  /// - Returns: An iterator over the elements of this buffer.
  @_inlineable
  public func makeIterator() -> UnsafeBufferPointerIterator<Element> {
    return UnsafeBufferPointerIterator(_position: _position, _end: _end)
  }

  /// A pointer to the first element of the buffer.
  ///
  /// If the `baseAddress` of this buffer is `nil`, the count is zero. However,
  /// a buffer can have a `count` of zero even with a non-`nil` base address.
  @_inlineable
  public var baseAddress: Unsafe${Mutable}Pointer<Element>? {
    return _position
  }

  /// The number of elements in the buffer.
  ///
  /// If the `baseAddress` of this buffer is `nil`, the count is zero. However,
  /// a buffer can have a `count` of zero even with a non-`nil` base address.
  @_inlineable
  public var count: Int {
    switch(_position, _end) {
    case (.some(let pos), .some(let end)):
      return (end - pos)
    case _:
      return 0
    }
  }

  @_versioned
  let _position, _end: Unsafe${Mutable}Pointer<Element>?
}

extension Unsafe${Mutable}BufferPointer : CustomDebugStringConvertible {
  /// A textual representation of the buffer, suitable for debugging.
  @_inlineable // FIXME(sil-serialize-all)
  public var debugDescription: String {
    return "Unsafe${Mutable}BufferPointer"
      + "(start: \(_position.map(String.init(describing:)) ?? "nil"), count: \(count))"
  }
}
%end


extension UnsafeMutableBufferPointer {
  /// Initializes the buffer's memory with the given elements.
  ///
  /// When calling the `initialize(from:)` method on a buffer `b`, the memory
  /// referenced by `b` must be uninitialized or the `Element` type must be a
  /// trivial type. After the call, the memory referenced by this buffer up
  /// to, but not including, the returned index is initialized. The buffer
  /// must contain sufficient memory to accommodate
  /// `source.underestimatedCount`.
  ///
  /// The returned index is the position of the element in the buffer one past
  /// the last element written. If `source` contains no elements, the returned
  /// index is equal to the buffer's `startIndex`. If `source` contains an
  /// equal or greater number of elements than the buffer can hold, the
  /// returned index is equal to the buffer's `endIndex`.
  ///
  /// - Parameter source: A sequence of elements with which to initializer the
  ///   buffer.
  /// - Returns: An iterator to any elements of `source` that didn't fit in the
  ///   buffer, and an index to the point in the buffer one past the last
  ///   element written.
  @_inlineable
  public func initialize<S: Sequence>(from source: S) -> (S.Iterator, Index)
    where S.Element == Element {
    return source._copyContents(initializing: self)
  }
}

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