test/Prototypes/Algorithms.swift.gyb

//===--- Algorithms.swift.gyb ---------------------------------*- 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
//
//===----------------------------------------------------------------------===//
// RUN: rm -rf %t && mkdir -p %t && %gyb -DWORD_BITS=%target-ptrsize %s -o %t/out.swift
// RUN: %line-directive %t/out.swift -- %target-build-swift -parse-stdlib %t/out.swift -o %t/a.out -Onone
// RUN: %line-directive %t/out.swift -- %target-run %t/a.out

import Swift
import StdlibUnittest

%{

from gyb_stdlib_support import (
    TRAVERSALS,
    collectionForTraversal,
    defaultIndicesForTraversal,
    sliceTypeName
)

}%

//===--- Rotate -----------------------------------------------------------===//
//===----------------------------------------------------------------------===//

// In the stdlib, this would simply be MutableCollection
public protocol MutableCollectionAlgorithms : MutableCollection {
  /// Rotates the elements of the collection so that the element
  /// at `middle` ends up first.
  ///
  /// - Returns: The new index of the element that was first
  ///   pre-rotation.
  /// - Complexity: O(*n*)
  @discardableResult
  mutating func rotate(shiftingToStart middle: Index) -> Index

  /// Rotates the elements in `bounds` so that the element
  /// at `middle` ends up first in `bounds`.
  ///
  /// - Returns: The new index of the element that was first
  ///   pre-rotation.
  /// - Complexity: O(*n*)
  @discardableResult
  mutating func rotateSubrange(
    _ bounds: Range<Index>, shiftingToStart middle: Index
  ) -> Index
}

// In the stdlib, this conformance wouldn't be needed
extension Array : MutableCollectionAlgorithms {  }

/// In the stdlib, this would simply be MutableCollection
extension MutableCollectionAlgorithms {
  @inline(__always)
  internal mutating func _swapNonemptySubrangePrefixes(
    _ lhs: Range<Index>, _ rhs: Range<Index>
  ) -> (Index, Index) {
    _sanityCheck(!lhs.isEmpty)
    _sanityCheck(!rhs.isEmpty)
    
    var p = lhs.lowerBound
    var q = rhs.lowerBound
    repeat {
      swap(&self[p], &self[q])
      formIndex(after: &p)
      formIndex(after: &q)
    }
    while p != lhs.upperBound && q != rhs.upperBound
    return (p, q)
  }
  
  @inline(__always)
  internal mutating func _swapSubrangePrefixes(
    _ lhs: Range<Index>, with rhs: Range<Index>
  ) -> (Index, Index) {
    return lhs.isEmpty || rhs.isEmpty
      ? (lhs.lowerBound, rhs.lowerBound)
      : _swapNonemptySubrangePrefixes(lhs, rhs)
  }
  
  /// Rotates the elements of the collection so that the element
  /// at `middle` ends up first.
  ///
  /// - Returns: The new index of the element that was first
  ///   pre-rotation.
  /// - Complexity: O(*n*)
  @discardableResult
  public mutating func rotate(shiftingToStart middle: Index) -> Index {
    return rotateSubrange(startIndex..<endIndex, shiftingToStart: middle)
  }

  /// Rotates the elements in `bounds` so that the element
  /// at `middle` ends up first.
  ///
  /// - Returns: The new index of the element that was first
  ///   pre-rotation.
  /// - Complexity: O(*n*)
  @discardableResult
  public mutating func rotateSubrange(
    _ bounds: Range<Index>, shiftingToStart middle: Index
  ) -> Index {
    return _rotateSubrangeForward(bounds, shiftingToStart: middle)
  }

  // Broken out of the method above for testability purposes
  @discardableResult
  internal mutating func _rotateSubrangeForward(
    _ bounds: Range<Index>, shiftingToStart middle: Index
  ) -> Index {
    var m = middle, s = bounds.lowerBound
    let e = bounds.upperBound

    // Handle the trivial cases
    if s == m { return e }
    if m == e { return s }

    // We have two regions of possibly-unequal length that need to be
    // exchanged.  The return value of this method is going to be the
    // position following that of the element that is currently last
    // (element j).
    //
    //   [a b c d e f g|h i j]   or   [a b c|d e f g h i j]
    //   ^             ^     ^        ^     ^             ^
    //   s             m     e        s     m             e
    //
    var ret = e // start with a known incorrect result.
    while true {
      // Exchange the leading elements of each region (up to the
      // length of the shorter region).
      //
      //   [a b c d e f g|h i j]   or   [a b c|d e f g h i j]
      //    ^^^^^         ^^^^^          ^^^^^ ^^^^^
      //   [h i j d e f g|a b c]   or   [d e f|a b c g h i j]
      //   ^     ^       ^     ^         ^    ^     ^       ^
      //   s    s1       m    m1/e       s   s1/m   m1      e
      //
      let (s1, m1) = _swapNonemptySubrangePrefixes(s..<m, m..<e)
      
      if m1 == e {
        // Left-hand case: we have moved element j into position.  if
        // we haven't already, we can capture the return value which
        // is in s1.
        //
        // Note: the STL breaks the loop into two just to avoid this
        // comparison once the return value is known.  I'm not sure
        // it's a worthwhile optimization, though.
        if ret == e { ret = s1 }
        
        // If both regions were the same size, we're done.
        if s1 == m { break }
      }
      
      // Now we have a smaller problem that is also a rotation, so we
      // can adjust our bounds and repeat.
      //
      //    h i j[d e f g|a b c]   or    d e f[a b c|g h i j]
      //         ^       ^     ^              ^     ^       ^
      //         s       m     e              s     m       e
      s = s1
      if s == m { m = m1 }
    }
    
    return ret
  }
}

extension MutableCollection where Self: BidirectionalCollection {

  // This could be internal, but until we have pinned accessors for
  // slices, every mutating algorithm needs a version that takes
  // indices in order to get performance.
  
  /// Reverses the elements in the given subrange in place.
  ///
  ///     var characters: [Character] = ["^", "C", "a", "f", "é", "$""]
  ///     let r = characters.index(after: characters.startIndex)
  ///             ..< characters.index(before: characters.endIndex)
  ///     characters.reverseSubrange(r)
  ///     print(cafe.characters)
  ///     // Prints "["^", "é", "f", "a", "C", "$"]
  ///
  /// - Complexity: O(*n*), where *n* is the number of elements in the
  ///   subrange.
  public mutating func reverseSubrange(_ bounds: Range<Index>) {
    if bounds.isEmpty { return }
    var f = bounds.lowerBound
    var l = index(before: bounds.upperBound)
    while f < l {
      swap(&self[f], &self[l])
      formIndex(after: &f)
      formIndex(before: &l)
    }
  }
  
  @inline(__always)
  @discardableResult
  internal mutating func _reverseUntil(_ limit: Index) -> (Index, Index) {
    var f = startIndex
    var l = endIndex
    while f != limit && l != limit {
      formIndex(before: &l)
      swap(&self[f], &self[l])
      formIndex(after: &f)
    }
    return (f, l)
  }
  
  /// Rotates the elements of the collection so that the element
  /// at `middle` ends up first.
  ///
  /// - Returns: The new index of the element that was first
  ///   pre-rotation.
  /// - Complexity: O(*n*)
  @discardableResult
  public mutating func rotate(shiftingToStart middle: Index) -> Index {
    // FIXME: this algorithm should be benchmarked on arrays against
    // the forward Collection algorithm above to prove that it's
    // actually faster.  The other one sometimes does more swaps, but
    // has better locality properties.  Similarly, we've omitted a
    // specialization of rotate for RandomAccessCollection that uses
    // cycles per section 11.4 in "From Mathematics to Generic
    // Programming" by A. Stepanov because it has *much* worse
    // locality properties than either of the other implementations.
    // Benchmarks should be performed for that algorithm too, just to
    // be sure.
    reverseSubrange(startIndex..<middle)
    reverseSubrange(middle..<endIndex)
    let (p, q) = _reverseUntil(middle)
    reverseSubrange(p..<q)
    return middle == p  ? q : p
  }
}

/// Returns the greatest common denominator for `m` and `n`.
internal func _gcd(_ m: Int, _ n: Int) -> Int {
  var (m, n) = (m, n)
  while n != 0 {
    let t = m % n
    m = n
    n = t
  }
  return m
}

extension MutableCollection where Self: RandomAccessCollection,
  SubSequence: MutableCollection, SubSequence: RandomAccessCollection {

  /// Rotates elements through a cycle, using `sourceForIndex` to generate
  /// the source index for each movement.
  @inline(__always)
  internal mutating func _rotateCycle(start: Index,
    transform sourceForIndex: (Index) -> Index)
  {
    let tmp = self[start]
    var (i, j) = (start, sourceForIndex(start))
    while j != start {
      self[i] = self[j]
      i = j
      j = sourceForIndex(j)
    }
    self[i] = tmp
  }
  
  /// Rotates the elements of the collection so that the element
  /// at `middle` ends up first.
  ///
  /// - Returns: The new index of the element that was first
  ///   pre-rotation.
  /// - Complexity: O(*n*)
  @discardableResult
  public mutating func rotateRandomAccess(
    shiftingToStart middle: Index) -> Index
  {
    if middle == startIndex { return endIndex }
    if middle == endIndex { return startIndex }
    
    // The distance to move an element that is moving ->
    let plus = distance(from: startIndex, to: middle)
    // The distance to move an element that is moving <-
    let minus = distance(from: endIndex, to: middle)
    // The new pivot point, aka the destination for the first element
    let pivot = index(startIndex, offsetBy: -minus)
    
    // If the difference moving forward and backward are relative primes,
    // the entire rotation will be completed in one cycle. Otherwise, repeat
    // cycle, moving the start point forward with each cycle.
    let cycles = _gcd(numericCast(plus), -numericCast(minus))
    
    for cycle in 1...cycles {
      _rotateCycle(start: index(startIndex, offsetBy: numericCast(cycle))) {
        index($0, offsetBy: $0 < pivot ? plus : minus)
      }
    }
    return pivot
  }
}

//===--- ConcatenatedCollection -------------------------------------------===//
//===----------------------------------------------------------------------===//

// ConcatenatedCollection improves on a flattened array or other collection by
// allowing random-access traversal if the underlying collections are
// random-access.
//
// Q: Add a ConcatenatedSequence for consistency? Would be nice to be able to
// call `let seqAB = concatenate(seqA, seqB)`.

/// Represents a position in either the first or second collection of a
/// `ConcatenatedCollection`.
internal enum _ConcatenatedCollectionIndexRepresentation<
  I1 : Comparable, I2 : Comparable
> {
  case first(I1)
  case second(I2)
}

/// A position in a `ConcatenatedCollection` collection.
public struct ConcatenatedCollectionIndex<
  C1 : Collection, C2 : Collection
> : Comparable {
  /// Creates a new index into the first underlying collection.
  internal init(first i: C1.Index) {
    _position = .first(i)
  }

  /// Creates a new index into the second underlying collection.
  internal init(second i: C2.Index) {
    _position = .second(i)
  }

  internal let _position:
    _ConcatenatedCollectionIndexRepresentation<C1.Index, C2.Index>
}

public func < <C1: Collection, C2: Collection>(
  lhs: ConcatenatedCollectionIndex<C1, C2>,
  rhs: ConcatenatedCollectionIndex<C1, C2>
) -> Bool {
  switch (lhs._position, rhs._position) {
  case (.first, .second):
    return true
  case (.second, .first):
    return false
  case let (.first(l), .first(r)):
    return l < r
  case let (.second(l), .second(r)):
    return l < r
  }
}

public func == <C1: Collection, C2: Collection>(
  lhs: ConcatenatedCollectionIndex<C1, C2>,
  rhs: ConcatenatedCollectionIndex<C1, C2>
) -> Bool {
  switch (lhs._position, rhs._position) {
  case let (.first(l), .first(r)):
    return l == r
  case let (.second(l), .second(r)):
    return l == r
  default:
    return false
  }
}

% for Traversal in TRAVERSALS:
%   Collection = collectionForTraversal(Traversal)
%   Self = "Concatenated" + Collection

/// A concatenation of two collections with the same element type.
public struct ${Self}<C1 : ${Collection}, C2: ${Collection}
  where C1.Iterator.Element == C2.Iterator.Element>: ${Collection} {

  init(_base1: C1, base2: C2) {
    self._base1 = _base1
    self._base2 = base2
  }

  public typealias Index = ConcatenatedCollectionIndex<C1, C2>

  public var startIndex: Index {
    // If `_base1` is empty, then `_base2.startIndex` is either a valid position
    // of an element or equal to `_base2.endIndex`.
    return _base1.isEmpty
      ? ConcatenatedCollectionIndex(second: _base2.startIndex)
      : ConcatenatedCollectionIndex(first: _base1.startIndex)
  }

  public var endIndex: Index {
    return ConcatenatedCollectionIndex(second: _base2.endIndex)
  }

  public subscript(i: Index) -> C1.Iterator.Element {
    switch i._position {
    case let .first(i):
      return _base1[i]
    case let .second(i):
      return _base2[i]
    }
  }

  public func index(after i: Index) -> Index {
    switch i._position {
    case let .first(i):
      _sanityCheck(i != _base1.endIndex)
      let next = _base1.index(after: i)
      return next == _base1.endIndex
        ? ConcatenatedCollectionIndex(second: _base2.startIndex)
        : ConcatenatedCollectionIndex(first: next)
    case let .second(i):
      return ConcatenatedCollectionIndex(second: _base2.index(after: i))
    }
  }

%   if Traversal in ['Bidirectional', 'RandomAccess']:
  public func index(before i: Index) -> Index {
    assert(i != startIndex, "Can't advance before startIndex")
    switch i._position {
    case let .first(i):
      return ConcatenatedCollectionIndex(first: _base1.index(before: i))
    case let .second(i):
      return i == _base2.startIndex
        ? ConcatenatedCollectionIndex(
            first: _base1.index(before: _base1.endIndex))
        : ConcatenatedCollectionIndex(second: _base2.index(before: i))
    }
  }
%   end

%   if Traversal is 'RandomAccess':
  public func index(_ i: Index, offsetBy n: ${Self}.IndexDistance) -> Index {
    if n == 0 { return i }
    return n > 0 ? _offsetForward(i, by: n) : _offsetBackward(i, by: -n)
  }

  internal func _offsetForward(
    _ i: Index, by n: ${Self}.IndexDistance
  ) -> Index {
    switch i._position {
    case let .first(i):
      let d: ${Self}.IndexDistance = numericCast(
        _base1.distance(from: i, to: _base1.endIndex))
      if n < d {
        return ConcatenatedCollectionIndex(
          first: _base1.index(i, offsetBy: numericCast(n)))
      } else {
        return ConcatenatedCollectionIndex(
          second: _base2.index(_base2.startIndex, offsetBy: numericCast(n - d)))
      }
    case let .second(i):
      return ConcatenatedCollectionIndex(
        second: _base2.index(i, offsetBy: numericCast(n)))
    }
  }

  internal func _offsetBackward(
    _ i: Index, by n: ${Self}.IndexDistance
  ) -> Index {
    switch i._position {
    case let .first(i):
      return ConcatenatedCollectionIndex(
        first: _base1.index(i, offsetBy: -numericCast(n)))
    case let .second(i):
      let d: ${Self}.IndexDistance = numericCast(
        _base2.distance(from: _base2.startIndex, to: i))
      if n <= d {
        return ConcatenatedCollectionIndex(
          second: _base2.index(i, offsetBy: -numericCast(n)))
      } else {
        return ConcatenatedCollectionIndex(
          first: _base1.index(_base1.endIndex, offsetBy: -numericCast(n - d)))
      }
    }
  }
%   end

  let _base1: C1
  let _base2: C2
}

/// Returns a new collection that presents a view onto the elements of the
/// first collection and then the elements of the second collection.
func concatenate<
  C1 : ${Collection}, C2 : ${Collection}
  where C1.Iterator.Element == C2.Iterator.Element
>(_ first: C1, _ second: C2) -> ${Self}<C1, C2> {
  return ${Self}(_base1: first, base2: second)
}

% end

//===--- RotatedCollection ------------------------------------------------===//
//===----------------------------------------------------------------------===//

/// A position in a rotated collection.
public struct RotatedCollectionIndex<
  Base : Collection where Base.SubSequence: Collection
> : Comparable {
  internal let _index:
    ConcatenatedCollectionIndex<Base.SubSequence, Base.SubSequence>
}

public func < <Base: Collection where Base.SubSequence: Collection>(
  lhs: RotatedCollectionIndex<Base>, rhs: RotatedCollectionIndex<Base>
) -> Bool {
  return lhs._index < rhs._index
}

public func == <Base: Collection where Base.SubSequence: Collection>(
  lhs: RotatedCollectionIndex<Base>, rhs: RotatedCollectionIndex<Base>
) -> Bool {
  return lhs._index == rhs._index
}

% for Traversal in TRAVERSALS:
%   Collection = collectionForTraversal(Traversal)
%   Self = "Rotated" + Collection

/// A rotated view onto a `${Collection}`.
public struct ${Self}<
  Base : ${Collection} where Base.SubSequence: ${Collection}
> : ${Collection} {
  let _concatenation: Concatenated${Collection}<
    Base.SubSequence, Base.SubSequence>

  init(_base: Base, shiftingToStart i: Base.Index) {
    _concatenation = concatenate(_base.suffix(from: i), _base.prefix(upTo: i))
  }

  public typealias Index = RotatedCollectionIndex<Base>

  public var startIndex: Index {
    return RotatedCollectionIndex(_index: _concatenation.startIndex)
  }

  public var endIndex: Index {
    return RotatedCollectionIndex(_index: _concatenation.endIndex)
  }

  public subscript(i: Index) -> Base.SubSequence.Iterator.Element {
    return _concatenation[i._index]
  }

  public func index(after i: Index) -> Index {
    return RotatedCollectionIndex(_index: _concatenation.index(after: i._index))
  }

%   if Traversal in ['Bidirectional', 'RandomAccess']:
  public func index(before i: Index) -> Index {
    return RotatedCollectionIndex(
      _index: _concatenation.index(before: i._index))
  }
%   end

  public func index(_ i: Index, offsetBy n: ${Self}.IndexDistance) -> Index {
    return RotatedCollectionIndex(
      _index: _concatenation.index(i._index, offsetBy: n))
  }

  /// The shifted position of the base collection's `startIndex`.
  public var shiftedStartIndex: Index {
    return RotatedCollectionIndex(
      _index: ConcatenatedCollectionIndex(
        second: _concatenation._base2.startIndex)
    )
  }
}

extension ${Collection} where SubSequence: ${Collection} {
  /// Returns a view of this collection with the elements reordered such the
  /// element at the given position ends up first.
  ///
  /// The subsequence of the collection up to `i` is shifted to after the
  /// subsequence starting at `i`. The order of the elements within each
  /// partition is otherwise unchanged.
  ///
  ///     let a = [10, 20, 30, 40, 50, 60, 70]
  ///     let r = a.rotated(shiftingToStart: 3)
  ///     // r.elementsEqual([40, 50, 60, 70, 10, 20, 30])
  ///
  /// - Parameter i: The position in the collection that should be first in the
  ///   result. `i` must be a valid index of the collection.
  /// - Returns: A rotated view on the elements of this collection, such that
  ///   the element at `i` is first.
  func rotated(shiftingToStart i: Index) -> ${Self}<Self> {
    return ${Self}(_base: self, shiftingToStart: i)
  }
}

% end

//===--- Stable Partition -------------------------------------------------===//
//===----------------------------------------------------------------------===//

extension BidirectionalCollection
  where Self : MutableCollectionAlgorithms, 
  SubSequence : BidirectionalCollection, 
  SubSequence.Index == Self.Index {

  @discardableResult
  mutating func stablePartition(
    choosingStartGroupBy p: (Iterator.Element) -> Bool
  ) -> Index {
    return stablyPartitionSubrange(
      startIndex..<endIndex,
      choosingStartGroupBy: p
    )
  }
  
  mutating func stablyPartitionSubrange(
    _ bounds: Range<Index>,
    choosingStartGroupBy p: (Iterator.Element) -> Bool
  ) -> Index {
    return _stablyPartitionSubrange(
      bounds,
      distance: distance(from: bounds.lowerBound, to: bounds.upperBound),
      choosingStartGroupBy: p
    )
  }
  
  mutating func _stablyPartitionSubrange(
    _ bounds: Range<Index>,
    distance n: IndexDistance,
    choosingStartGroupBy p: (Iterator.Element) -> Bool
  ) -> Index {
    assert(n >= 0)
    let (start, end) = (bounds.lowerBound, bounds.upperBound)
    assert(n == distance(from: start, to: end))
    if n == 0 { return start }
    if n == 1 {
      return p(self[start]) ? index(after: start) : start
    }


    // divide and conquer.
    let d = n / numericCast(2)
    let m = index(start, offsetBy: d)
    
    // TTTTTTTTT s FFFFFFF m ?????????????
    let s = _stablyPartitionSubrange(
      start..<m, distance: d, choosingStartGroupBy: p)
    
    // TTTTTTTTT s FFFFFFF m TTTTTTT e FFFFFFFF
	  let e = _stablyPartitionSubrange(
      m..<end, distance: n - d, choosingStartGroupBy: p)
    
    // TTTTTTTTT s TTTTTTT m  FFFFFFF e FFFFFFFF
    return self.rotateSubrange(s..<e, shiftingToStart: m)
  }
}
  
extension Collection {
  func stablyPartitioned(
    choosingStartGroupBy p: (Iterator.Element) -> Bool
  ) -> [Iterator.Element] {
    var a = Array(self)
    a.stablePartition(choosingStartGroupBy: p)
    return a
  }
}

extension LazyCollectionProtocol 
  where Iterator.Element == Elements.Iterator.Element {
  func stablyPartitioned(
    choosingStartGroupBy p: (Iterator.Element) -> Bool
  ) -> LazyCollection<[Iterator.Element]> {
    return elements.stablyPartitioned(choosingStartGroupBy: p).lazy
  }
}

//===--- Tests ------------------------------------------------------------===//
//===----------------------------------------------------------------------===//

var suite = TestSuite("Algorithms")

suite.test("reverseSubrange") {
  for l in 0..<10 {
    let a = Array(0..<l)
    
    for p in a.startIndex...a.endIndex {
      let prefix = a.prefix(upTo: p)
      for q in p...l {
        let suffix = a.suffix(from: q)
        var b = a
        b.reverseSubrange(p..<q)
        expectEqual(
          b,
          Array([prefix, ArraySlice(a[p..<q].reversed()), suffix].joined()))
        }
    }
  }
}

suite.test("rotate") {
  for l in 0..<11 {
    let a = Array(0..<l)
    
    for p in a.startIndex...a.endIndex {
      let prefix = a.prefix(upTo: p)
      for q in p...l {
        let suffix = a.suffix(from: q)

        for m in p...q {
          var b = a
          let r0 = b._rotateSubrangeForward(p..<q, shiftingToStart: m)
          let rotated = Array([prefix, a[m..<q], a[p..<m], suffix].joined())
          expectEqual(b, rotated)
          expectEqual(r0, a.index(p, offsetBy: a[m..<q].count))

          b = a
          let r1 = b.rotateSubrange(p..<q, shiftingToStart: m)
          expectEqual(b, rotated)
          expectEqual(r1, r0)
        }
      }
      var b = a
      b.rotate(shiftingToStart: p)
      expectEqual(b, Array(a.rotated(shiftingToStart: p)))
    }
  }
}

suite.test("rotateRandomAccess") {
  for l in 0..<11 {
    let a = Array(0..<l)
    
    for p in a.startIndex...a.endIndex {
      let prefix = a.prefix(upTo: p)
      for q in p...l {
        let suffix = a.suffix(from: q)

        for m in p...q {
          var b = a
          let r0 = b[p..<q].rotateRandomAccess(shiftingToStart: m)
          let rotated = Array([prefix, a[m..<q], a[p..<m], suffix].joined())
          expectEqual(b, rotated)
          expectEqual(r0, a.index(p, offsetBy: a[m..<q].count))

          b = a
          let r1 = b[p..<q].rotateRandomAccess(shiftingToStart: m)
          expectEqual(b, rotated)
          expectEqual(r1, r0)
        }
      }
      var b = a
      b.rotateRandomAccess(shiftingToStart: p)
      expectEqual(b, Array(a.rotated(shiftingToStart: p)))
    }
  }
}

suite.test("concatenate") {
  for x in 0...6 {
    for y in 0...x {
      let r1 = 0..<y
      let r2 = y..<x
      expectEqual(Array(0..<x), Array(concatenate(r1, r2)))
    }
  }

  let c1 = concatenate([1, 2, 3, 4, 5], 6...10)
  let c2 = concatenate(1...5, [6, 7, 8, 9, 10])
  expectEqual(Array(1...10), Array(c1))
  expectEqual(Array(1...10), Array(c2))

  let h = "Hello, "
  let w = "world!"
  let hw = concatenate(h.characters, w.characters)
  expectEqual("Hello, world!", String(hw))
}

suite.test("stablePartition") {
  for l in 0..<13 {
    let a = Array(0..<l)
    
    for p in a.startIndex...a.endIndex {
      let prefix = a.prefix(upTo: p)
      for q in p...l {
        let suffix = a.suffix(from: q)

        let subrange = a[p..<q]
        
        for modulus in 1...5 {
          let f = { $0 % modulus == 0 }
          let notf = { !f($0) }
          
          var b = a
          var r = b.stablyPartitionSubrange(p..<q, choosingStartGroupBy: f)
          expectEqual(b.prefix(upTo:p), prefix)
          expectEqual(b.suffix(from:q), suffix)
          expectEqual(b[p..<r], ArraySlice(subrange.filter(f)))
          expectEqual(b[r..<q], ArraySlice(subrange.filter(notf)))

          b = a
          r = b.stablyPartitionSubrange(p..<q, choosingStartGroupBy: notf)
          expectEqual(b.prefix(upTo:p), prefix)
          expectEqual(b.suffix(from:q), suffix)
          expectEqual(b[p..<r], ArraySlice(subrange.filter(notf)))
          expectEqual(b[r..<q], ArraySlice(subrange.filter(f)))
        }
      }
      
      for modulus in 1...5 {
        let f = { $0 % modulus == 0 }
        let notf = { !f($0) }
        var b = a
        var r = b.stablePartition(choosingStartGroupBy: f)
        expectEqual(b.prefix(upTo: r), ArraySlice(a.filter(f)))
        expectEqual(b.suffix(from: r), ArraySlice(a.filter(notf)))

        b = a
        r = b.stablePartition(choosingStartGroupBy: notf)
        expectEqual(b.prefix(upTo: r), ArraySlice(a.filter(notf)))
        expectEqual(b.suffix(from: r), ArraySlice(a.filter(f)))
      }
    }
  }
}

runAllTests()