Algorithms.swift

//===--- Algorithms.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
//
//===----------------------------------------------------------------------===//
// RUN: %empty-directory(%t)
// RUN: %target-build-swift -g -Onone -DUSE_STDLIBUNITTEST %s -o %t/a.out
// RUN: %target-codesign %t/a.out
// RUN: %target-run %t/a.out
// REQUIRES: executable_test

#if USE_STDLIBUNITTEST
import Swift
import StdlibUnittest
#endif

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

/// Provides customization points for `MutableCollection` algorithms.
///
/// If incorporated into the standard library, these requirements would just be
/// part of `MutableCollection`.  In the meantime, you can declare conformance
/// of a collection to `MutableCollectionAlgorithms` to get these customization
/// points to be used from other algorithms defined on
/// `MutableCollectionAlgorithms`.
public protocol MutableCollectionAlgorithms : MutableCollection
  where SubSequence : MutableCollectionAlgorithms
{
  /// 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
}

// Conformances of common collection types to MutableCollectionAlgorithms.
// If rotate was a requirement of MutableCollection, these would not be needed.
extension Array : MutableCollectionAlgorithms {  }
extension ArraySlice : MutableCollectionAlgorithms {  }
extension Slice : MutableCollectionAlgorithms
  where Base: MutableCollection { }

extension MutableCollection {
  /// Swaps the elements of the two given subranges, up to the upper bound of
  /// the smaller subrange. The returned indices are the ends of the two ranges
  /// that were actually swapped.
  ///
  ///     Input:
  ///     [a b c d e f g h i j k l m n o p]
  ///      ^^^^^^^         ^^^^^^^^^^^^^
  ///      lhs             rhs
  ///
  ///     Output:
  ///     [i j k l e f g h a b c d m n o p]
  ///             ^               ^
  ///             p               q
  ///
  /// - Precondition: !lhs.isEmpty && !rhs.isEmpty
  /// - Postcondition: For returned indices `(p, q)`:
  ///   - distance(from: lhs.lowerBound, to: p) ==
  ///       distance(from: rhs.lowerBound, to: q)
  ///   - p == lhs.upperBound || q == rhs.upperBound
  @inline(__always)
  internal mutating func _swapNonemptySubrangePrefixes(
    _ lhs: Range<Index>, _ rhs: Range<Index>
  ) -> (Index, Index) {
    assert(!lhs.isEmpty)
    assert(!rhs.isEmpty)

    var p = lhs.lowerBound
    var q = rhs.lowerBound
    repeat {
      swapAt(p, q)
      formIndex(after: &p)
      formIndex(after: &q)
    }
    while p != lhs.upperBound && q != rhs.upperBound
    return (p, q)
  }

  /// 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 {
    var m = middle, s = startIndex
    let e = endIndex

    // 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 {

  /// Reverses the elements of the collection, moving from each end until
  /// `limit` is reached from either direction. The returned indices are the
  /// start and end of the range of unreversed elements.
  ///
  ///     Input:
  ///     [a b c d e f g h i j k l m n o p]
  ///             ^
  ///           limit
  ///     Output:
  ///     [p o n m e f g h i j k l d c b a]
  ///             ^               ^
  ///             f               l
  ///
  /// - Postcondition: For returned indices `(f, l)`:
  ///   `f == limit || l == limit`
  @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)
      swapAt(f, 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.
    self[..<middle].reverse()
    self[middle...].reverse()
    let (p, q) = _reverseUntil(middle)
    self[p..<q].reverse()
    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 {

  /// Rotates elements through a cycle, using `sourceForIndex` to generate
  /// the source index for each movement.
  @inline(__always)
  internal mutating func _rotateCycle(
    start: Index,
    sourceOffsetForIndex: (Index) -> Int
  ) {
    let tmp = self[start]
    var i = start
    var j = index(start, offsetBy: sourceOffsetForIndex(start))
    while j != start {
      self[i] = self[j]
      i = j
      j = index(j, offsetBy: sourceOffsetForIndex(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)),
        sourceOffsetForIndex: { $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)`.

/// A concatenation of two collections with the same element type.
public struct Concatenation<C1 : Collection, C2: Collection>: Collection
  where C1.Element == C2.Element
{
  let _base1: C1
  let _base2: C2

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

  /// A position in a `Concatenation`.
  public struct Index : Comparable {
    internal enum _Representation : Equatable {
      case first(C1.Index)
      case second(C2.Index)
    }

    /// 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: _Representation

    public static func < (lhs: Index, rhs: Index) -> 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 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
      ? Index(second: _base2.startIndex)
      : Index(first: _base1.startIndex)
  }

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

  public subscript(i: Index) -> C1.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):
      assert(i != _base1.endIndex)
      let next = _base1.index(after: i)
      return next == _base1.endIndex
        ? Index(second: _base2.startIndex)
        : Index(first: next)
    case let .second(i):
      return Index(second: _base2.index(after: i))
    }
  }
}

extension Concatenation : BidirectionalCollection
  where C1: BidirectionalCollection, C2: BidirectionalCollection
{
  public func index(before i: Index) -> Index {
    assert(i != startIndex, "Can't advance before startIndex")
    switch i._position {
    case let .first(i):
      return Index(first: _base1.index(before: i))
    case let .second(i):
      return i == _base2.startIndex
        ? Index(first: _base1.index(before: _base1.endIndex))
        : Index(second: _base2.index(before: i))
    }
  }
}

extension Concatenation : RandomAccessCollection
  where C1: RandomAccessCollection, C2: RandomAccessCollection
{
  public func index(_ i: Index, offsetBy n: Int) -> Index {
    if n == 0 { return i }
    return n > 0 ? _offsetForward(i, by: n) : _offsetBackward(i, by: -n)
  }

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

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

/// 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>(
  _ first: C1,
  _ second: C2)
  -> Concatenation<C1, C2> where C1.Element == C2.Element
{
  return Concatenation(_base1: first, base2: second)
}

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

/// A rotated view onto a collection.
public struct RotatedCollection<Base : Collection> : Collection {
  let _base: Base
  let _indices: Concatenation<Base.Indices, Base.Indices>

  init(_base: Base, shiftingToStart i: Base.Index) {
    self._base = _base
    self._indices = concatenate(_base.indices[i...], _base.indices[..<i])
  }

  /// A position in a rotated collection.
  public struct Index : Comparable {
    internal let _index:
      Concatenation<Base.Indices, Base.Indices>.Index

    public static func < (lhs: Index, rhs: Index) -> Bool {
      return lhs._index < rhs._index
    }
  }

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

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

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

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

  public func index(_ i: Index, offsetBy n: Int) -> Index {
    return Index(_index: _indices.index(i._index, offsetBy: n))
  }

  public func distance(from start: Index, to end: Index) -> Int {
    return _indices.distance(from: start._index, to: end._index)
  }

  /// The shifted position of the base collection's `startIndex`.
  public var shiftedStartIndex: Index {
    return Index(
      _index: Concatenation<Base.Indices, Base.Indices>.Index(
        second: _indices._base2.startIndex)
    )
  }

  public func rotated(shiftingToStart i: Index) -> RotatedCollection<Base> {
    return RotatedCollection(_base: _base, shiftingToStart: _indices[i._index])
  }
}

extension RotatedCollection : BidirectionalCollection
  where Base : BidirectionalCollection {
  public func index(before i: Index) -> Index {
    return Index(_index: _indices.index(before: i._index))
  }
}

extension RotatedCollection : RandomAccessCollection
  where Base : RandomAccessCollection {}

extension 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) -> RotatedCollection<Self> {
    return RotatedCollection(_base: self, shiftingToStart: i)
  }
}

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

extension MutableCollectionAlgorithms {
  /// Moves all elements satisfying `isSuffixElement` into a suffix of the
  /// collection, preserving their relative order, and returns the start of the
  /// resulting suffix.
  ///
  /// - Complexity: O(n) where n is the number of elements.
  @discardableResult
  mutating func stablePartition(
    isSuffixElement: (Element) throws -> Bool
  ) rethrows -> Index {
    return try stablePartition(count: count, isSuffixElement: isSuffixElement)
  }

  /// Moves all elements satisfying `isSuffixElement` into a suffix of the
  /// collection, preserving their relative order, and returns the start of the
  /// resulting suffix.
  ///
  /// - Complexity: O(n) where n is the number of elements.
  /// - Precondition: `n == self.count`
  fileprivate mutating func stablePartition(
    count n: Int, isSuffixElement: (Element) throws-> Bool
  ) rethrows -> Index {
    if n == 0 { return startIndex }
    if n == 1 {
      return try isSuffixElement(self[startIndex]) ? startIndex : endIndex
    }
    let h = n / 2, i = index(startIndex, offsetBy: h)
    let j = try self[..<i].stablePartition(
      count: h, isSuffixElement: isSuffixElement)
    let k = try self[i...].stablePartition(
      count: n - h, isSuffixElement: isSuffixElement)
    return self[j..<k].rotate(shiftingToStart: i)
  }
}

extension Collection {
  func stablyPartitioned(
    isSuffixElement p: (Element) -> Bool
  ) -> [Element] {
    var a = Array(self)
    a.stablePartition(isSuffixElement: p)
    return a
  }
}

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

extension Collection {
  /// Returns the index of the first element in the collection
  /// that matches the predicate.
  ///
  /// The collection must already be partitioned according to the
  /// predicate, as if `self.partition(by: predicate)` had already
  /// been called.
  func partitionPoint(
    where predicate: (Element) throws -> Bool
  ) rethrows -> Index {
    var n = distance(from: startIndex, to: endIndex)
    var l = startIndex

    while n > 0 {
      let half = n / 2
      let mid = index(l, offsetBy: half)
      if try predicate(self[mid]) {
        n = half
      } else {
        l = index(after: mid)
        n -= half + 1
      }
    }
    return l
  }
}

//===--- Minimal subset of StdlibUnittest for standalone testing ----------===//
//===----------------------------------------------------------------------===//
#if !USE_STDLIBUNITTEST
class TestSuite {
  let name: String
  var tests: [(name: String, body: ()->())] = []
  static var all: [TestSuite] = []
  init(_ name: String) {
    self.name = name
    TestSuite.all.append(self)
  }

  func test(_ name: String, body: @escaping ()->()) {
    tests.append((name, body))
  }
}

func runAllTests() {
  for s in TestSuite.all {
    for (testName, f) in s.tests {
      print("\(s.name)/\(testName)...")
      f()
      print("done.")
    }
  }
}

func expectEqual<T : Equatable>(
  _ expected: T, _ x: T, file: StaticString = #file, line: UInt = #line
) {
  precondition(
    x == expected, "Expected \(x) == \(expected)", file: file, line: line)
}

func expectGE<T: Comparable>(
  _ a: T, _ b: T, _ message: @autoclosure ()->String = "",
  file: StaticString = #file, line: UInt = #line
) {
  precondition(a >= b, message(), file: file, line: line)
}

func expectLE<T: Comparable>(
  _ a: T, _ b: T, _ message: @autoclosure ()->String = "",
  file: StaticString = #file, line: UInt = #line
) {
  precondition(a <= b, message(), file: file, line: line)
}
#endif


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


func address<T>(_ p: UnsafePointer<T>) -> UInt { return UInt(bitPattern: p )}

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[..<p]
      for q in p...l {
        let suffix = a[q...]

        var b = a
        b.reserveCapacity(b.count)  // guarantee unique storage
        let id = address(b)

        b[p..<q].reverse()
        expectEqual(
          b,
          Array([prefix, ArraySlice(a[p..<q].reversed()), suffix].joined()))
        expectEqual(address(b), id)
      }
    }
  }
}

suite.test("rotate") {
  for l in 0..<11 {
    let a = Array(0..<l)

    for p in a.startIndex...a.endIndex {
      let prefix = a[..<p]
      for q in p...l {
        let suffix = a[q...]

        for m in p...q {
          var b = a
          b.reserveCapacity(b.count)  // guarantee unique storage
          let id = address(b)

          let r = b[p..<q].rotate(shiftingToStart: m)
          let rotated = Array([prefix, a[m..<q], a[p..<m], suffix].joined())
          expectEqual(b, rotated)
          expectEqual(r, a.index(p, offsetBy: a[m..<q].count))
          expectEqual(address(b), id)
        }
      }
      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[..<p]
      for q in p...l {
        let suffix = a[q...]

        for m in p...q {
          var b = a
          b.reserveCapacity(b.count)  // guarantee unique storage
          let id = address(b)

          let r = b[p..<q].rotateRandomAccess(shiftingToStart: m)
          let rotated = Array([prefix, a[m..<q], a[p..<m], suffix].joined())
          expectEqual(b, rotated)
          expectEqual(r, a.index(p, offsetBy: a[m..<q].count))
          expectEqual(address(b), id)
        }
      }
      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, w)
  expectEqual("Hello, world!", String(hw))
}

suite.test("stablePartition") {
  // FIXME: add test for stability
  for l in 0..<13 {
    let a = Array(0..<l)

    for p in a.startIndex...a.endIndex {
      let prefix = a[..<p]
      for q in p...l {
        let suffix = a[q...]

        let subrange = a[p..<q]

        for modulus in 1...5 {
          let f = { $0 % modulus != 0 }
          let notf = { !f($0) }

          var b = a
          b.reserveCapacity(b.count)  // guarantee unique storage
          let id = address(b)

          var r = b[p..<q].stablePartition(isSuffixElement: f)
          expectEqual(b[..<p], prefix)
          expectEqual(b.suffix(from:q), suffix)
          expectEqual(b[p..<r], ArraySlice(subrange.filter(notf)))
          expectEqual(b[r..<q], ArraySlice(subrange.filter(f)))
          expectEqual(address(b), id)

          b = a
          r = b[p..<q].stablePartition(isSuffixElement: notf)
          expectEqual(b[..<p], prefix)
          expectEqual(b.suffix(from:q), suffix)
          expectEqual(b[p..<r], ArraySlice(subrange.filter(f)))
          expectEqual(b[r..<q], ArraySlice(subrange.filter(notf)))
        }
      }

      for modulus in 1...5 {
        let f = { $0 % modulus != 0 }
        let notf = { !f($0) }
        var b = a
        var r = b.stablePartition(isSuffixElement: f)
        expectEqual(b[..<r], ArraySlice(a.filter(notf)))
        expectEqual(b[r...], ArraySlice(a.filter(f)))

        b = a
        r = b.stablePartition(isSuffixElement: notf)
        expectEqual(b[..<r], ArraySlice(a.filter(f)))
        expectEqual(b[r...], ArraySlice(a.filter(notf)))
      }
    }
  }
}

suite.test("partitionPoint") {
  for i in 0..<7 {
    for j in i..<11 {
      for k in i...j {
        let p = (i..<j).partitionPoint { $0 >= k }
        expectGE(p, i, "\(p) >= \(i)")
        expectLE(p, j, "\(p) <= \(j)")
        expectEqual(p, k)
      }
    }
  }
}

runAllTests()