//===--- Stride.swift.gyb - Components for stride(...) iteration ----------===//
//
// 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 https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//

// FIXME(ABI)#69 (Overload Resolution): Remove `_Strideable`.
// WORKAROUND rdar://25214598 - should be:
// protocol Strideable : Comparable {...}

% for Self in ['_Strideable', 'Strideable']:

%{
    Conformance = (
      'Equatable' if Self == '_Strideable' else '_Strideable, Comparable'
    )
}%
/// Conforming types are notionally continuous, one-dimensional
/// values that can be offset and measured.
public protocol ${Self} : ${Conformance} {
  /// A type that can represent the distance between two values of `Self`.
  associatedtype Stride : SignedNumber

  /// Returns a stride `x` such that `self.advanced(by: x)` approximates
  /// `other`.
  ///
  /// If `Stride` conforms to `Integer`, then `self.advanced(by: x) == other`.
  ///
  /// - Complexity: O(1).
  func distance(to other: Self) -> Stride

  /// Returns a `Self` `x` such that `self.distance(to: x)` approximates `n`.
  ///
  /// If `Stride` conforms to `Integer`, then `self.distance(to: x) == n`.
  ///
  /// - Complexity: O(1).
  func advanced(by n: Stride) -> Self

  /// `_step` is an implementation detail of Strideable; do not use it directly.
  static func _step(
    after current: (index: Int?, value: Self),
    from start: Self, by distance: Self.Stride
  ) -> (index: Int?, value: Self)

  associatedtype _DisabledRangeIndex = _DisabledRangeIndex_
}

% end

/// Compare two `Strideable`s.
public func < <T : Strideable>(x: T, y: T) -> Bool {
  return x.distance(to: y) > 0
}

public func == <T : Strideable>(x: T, y: T) -> Bool {
  return x.distance(to: y) == 0
}

public func + <T : Strideable>(lhs: T, rhs: T.Stride) -> T {
  return lhs.advanced(by: rhs)
}

public func + <T : Strideable>(lhs: T.Stride, rhs: T) -> T {
  return rhs.advanced(by: lhs)
}

public func - <T : Strideable>(lhs: T, rhs: T.Stride) -> T {
  return lhs.advanced(by: -rhs)
}

public func - <T : Strideable>(lhs: T, rhs: T) -> T.Stride {
  return rhs.distance(to: lhs)
}

public func += <T : Strideable>(lhs: inout T, rhs: T.Stride) {
  lhs = lhs.advanced(by: rhs)
}

public func -= <T : Strideable>(lhs: inout T, rhs: T.Stride) {
  lhs = lhs.advanced(by: -rhs)
}

//===--- Deliberately-ambiguous operators for UnsignedIntegerTypes --------===//
// The UnsignedIntegerTypes all have a signed Stride type.  Without these     //
// overloads, expressions such as UInt(2) + Int(3) would compile.             //
//===----------------------------------------------------------------------===//

public func + <T : UnsignedInteger>(
  lhs: T, rhs: T._DisallowMixedSignArithmetic
) -> T {
  _sanityCheckFailure("Should not be callable.")
}

public func + <T : UnsignedInteger>(
  lhs: T._DisallowMixedSignArithmetic, rhs: T
) -> T {
  _sanityCheckFailure("Should not be callable.")
}

public func - <T : _DisallowMixedSignArithmetic>(
  lhs: T, rhs: T._DisallowMixedSignArithmetic
) -> T {
  _sanityCheckFailure("Should not be callable.")
}

public func - <T : _DisallowMixedSignArithmetic>(
  lhs: T, rhs: T
) -> T._DisallowMixedSignArithmetic {
  _sanityCheckFailure("Should not be callable.")
}

public func += <T : UnsignedInteger>(
  lhs: inout T, rhs: T._DisallowMixedSignArithmetic
) {
  _sanityCheckFailure("Should not be callable.")
}

public func -= <T : UnsignedInteger>(
  lhs: inout T, rhs: T._DisallowMixedSignArithmetic
) {
  _sanityCheckFailure("Should not be callable.")
}

//===----------------------------------------------------------------------===//

extension Strideable {
  public static func _step(
    after current: (index: Int?, value: Self),
    from start: Self, by distance: Self.Stride
  ) -> (index: Int?, value: Self) {
    return (nil, current.value + distance)
  }
}

extension Strideable where Stride : FloatingPoint {
  public static func _step(
    after current: (index: Int?, value: Self),
    from start: Self, by distance: Self.Stride
  ) -> (index: Int?, value: Self) {
    if let i = current.index {
      return (i + 1, start + Stride(i + 1) * distance)
    }
    // If current.index == nil, either we're just starting out (in which case
    // the next index is 1), or we should proceed without an index just as
    // though this floating point specialization doesn't exist.
    return (current.value == start ? 1 : nil, current.value + distance)
  }
}

/// An iterator for `StrideTo<Element>`.
public struct StrideToIterator<Element : Strideable> : IteratorProtocol {
  internal let _start: Element
  internal let _end: Element
  internal let _stride: Element.Stride
  internal var _current: (index: Int?, value: Element)

  internal init(_start: Element, end: Element, stride: Element.Stride) {
    self._start = _start
    _end = end
    _stride = stride
    _current = (nil, _start)
  }

  /// 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`.
  public mutating func next() -> Element? {
    let result = _current.value
    if _stride > 0 ? result >= _end : result <= _end {
      return nil
    }
    _current = Element._step(after: _current, from: _start, by: _stride)
    return result
  }
}

/// A `Sequence` of values formed by striding over a half-open interval.
public struct StrideTo<Element : Strideable> : Sequence, CustomReflectable {
  // FIXME: should really be a Collection, as it is multipass

  /// Returns an iterator over the elements of this sequence.
  ///
  /// - Complexity: O(1).
  public func makeIterator() -> StrideToIterator<Element> {
    return StrideToIterator(_start: _start, end: _end, stride: _stride)
  }

  internal init(_start: Element, end: Element, stride: Element.Stride) {
    _precondition(stride != 0, "stride size must not be zero")
    // At start, striding away from end is allowed; it just makes for an
    // already-empty Sequence.
    self._start = _start
    self._end = end
    self._stride = stride
  }

  internal let _start: Element
  internal let _end: Element
  internal let _stride: Element.Stride

  public var customMirror: Mirror {
    return Mirror(self, children: ["from": _start, "to": _end, "by": _stride])
  }
}

/// Returns the sequence of values (`self`, `self + stride`, `self +
/// 2 * stride`, ... *last*) where *last* is the last value in the
/// progression that is less than `end`.
public func stride<T : Strideable>(
  from start: T, to end: T, by stride: T.Stride
) -> StrideTo<T> {
  return StrideTo(_start: start, end: end, stride: stride)
}

/// An iterator for `StrideThrough<Element>`.
public struct StrideThroughIterator<Element : Strideable> : IteratorProtocol {
  internal let _start: Element
  internal let _end: Element
  internal let _stride: Element.Stride
  internal var _current: (index: Int?, value: Element)
  internal var _didReturnEnd: Bool = false

  internal init(_start: Element, end: Element, stride: Element.Stride) {
    self._start = _start
    _end = end
    _stride = stride
    _current = (nil, _start)
  }

  /// 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`.
  public mutating func next() -> Element? {
    let result = _current.value
    if _stride > 0 ? result >= _end : result <= _end {
      // This check is needed because if we just changed the above operators
      // to > and <, respectively, we might advance current past the end
      // and throw it out of bounds (e.g. above Int.max) unnecessarily.
      if result == _end && !_didReturnEnd {
        _didReturnEnd = true
        return result
      }
      return nil
    }
    _current = Element._step(after: _current, from: _start, by: _stride)
    return result
  }
}

/// A `Sequence` of values formed by striding over a closed interval.
public struct StrideThrough<
  Element : Strideable
> : Sequence, CustomReflectable {
  // FIXME: should really be a CollectionType, as it is multipass

  /// Returns an iterator over the elements of this sequence.
  ///
  /// - Complexity: O(1).
  public func makeIterator() -> StrideThroughIterator<Element> {
    return StrideThroughIterator(_start: _start, end: _end, stride: _stride)
  }

  internal init(_start: Element, end: Element, stride: Element.Stride) {
    _precondition(stride != 0, "stride size must not be zero")
    self._start = _start
    self._end = end
    self._stride = stride
  }

  internal let _start: Element
  internal let _end: Element
  internal let _stride: Element.Stride

  public var customMirror: Mirror {
    return Mirror(self,
      children: ["from": _start, "through": _end, "by": _stride])
  }
}

/// Returns the sequence of values (`self`, `self + stride`, `self +
/// 2 * stride`, ... *last*) where *last* is the last value in the
/// progression less than or equal to `end`.
///
/// - Note: There is no guarantee that `end` is an element of the sequence.
public func stride<T : Strideable>(
  from start: T, through end: T, by stride: T.Stride
) -> StrideThrough<T> {
  return StrideThrough(_start: start, end: end, stride: stride)
}

@available(*, unavailable, renamed: "StrideToIterator")
public struct StrideToGenerator<Element : Strideable> {}

@available(*, unavailable, renamed: "StrideThroughIterator")
public struct StrideThroughGenerator<Element : Strideable> {}

extension Strideable {
  @available(*, unavailable, message: "Use stride(from:to:by:) free function instead")
  public func stride(to end: Self, by stride: Stride) -> StrideTo<Self> {
    Builtin.unreachable()
  }

  @warn_unqualified_access
  @available(*, unavailable, message: "Use stride(from:through:by:) free function instead")
  public func stride(
    through end: Self, by stride: Stride
  ) -> StrideThrough<Self> {
    Builtin.unreachable()
  }
}