swift-mirror/stdlib/public/core/CollectionDifference.swift

//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2015 - 2019 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
//
//===----------------------------------------------------------------------===//

/// A type that represents the difference between two ordered collection states.
@available(swift, introduced: 5.1)
public struct CollectionDifference<ChangeElement> {
  /// A type that represents a single change to a collection.
  ///
  /// The `offset` of each `insert` refers to the offset of its `element` in
  /// the final state after the difference is fully applied. The `offset` of
  /// each `remove` refers to the offset of its `element` in the original
  /// state. Non-`nil` values of `associatedWith` refer to the offset of the
  /// complementary change.
  public enum Change {
    case insert(offset: Int, element: ChangeElement, associatedWith: Int?)
    case remove(offset: Int, element: ChangeElement, associatedWith: Int?)

    // Internal common field accessors
    var offset: Int {
      get {
        switch self {
        case .insert(offset: let o, element: _, associatedWith: _):
          return o
        case .remove(offset: let o, element: _, associatedWith: _):
          return o
        }
      }
    }
    var element: ChangeElement {
      get {
        switch self {
        case .insert(offset: _, element: let e, associatedWith: _):
          return e
        case .remove(offset: _, element: let e, associatedWith: _):
          return e
        }
      }
    }
    var associatedOffset: Int? {
      get {
        switch self {
        case .insert(offset: _, element: _, associatedWith: let o):
          return o
        case .remove(offset: _, element: _, associatedWith: let o):
          return o
        }
      }
    }
  }

  // The public initializer calls this function to ensure that its parameter
  // meets the conditions set in its documentation.
  private static func validateChanges<C>(_ changes : C) -> Bool where C:Collection, C.Element == Change {
    if changes.count == 0 { return true }

    var insertAssocToOffset = Dictionary<Int,Int>()
    var removeOffsetToAssoc = Dictionary<Int,Int>()
    var insertOffset = Set<Int>()
    var removeOffset = Set<Int>()

    for c in changes {
      let offset = c.offset
      if offset < 0 { return false }

      switch c {
      case .remove(_, _, _):
        if removeOffset.contains(offset) { return false }
        removeOffset.insert(offset)
      case .insert(_, _, _):
        if insertOffset.contains(offset) { return false }
        insertOffset.insert(offset)
      }

      if let assoc = c.associatedOffset {
        if assoc < 0 { return false }
        switch c {
        case .remove(_, _, _):
          if removeOffsetToAssoc[offset] != nil { return false }
          removeOffsetToAssoc[offset] = assoc
        case .insert(_, _, _):
          if insertAssocToOffset[assoc] != nil { return false }
          insertAssocToOffset[assoc] = offset
        }
      }
    }

    return removeOffsetToAssoc == insertAssocToOffset
  }

  /// Creates an instance from a collection of changes.
  ///
  /// For clients interested in the difference between two collections, see
  /// `BidirectionalCollection.difference(from:)`.
  ///
  /// To guarantee that instances are unambiguous and safe for compatible base
  /// states, this initializer will fail unless its parameter meets to the
  /// following requirements:
  ///
  /// 1) All insertion offsets are unique
  /// 2) All removal offsets are unique
  /// 3) All offset associations between insertions and removals are symmetric
  ///
  /// - Parameter changes: A collection of changes that represent a transition
  ///   between two states.
  ///
  /// - Complexity: O(*n* * log(*n*)), where *n* is the length of the
  ///   parameter.
  public init?<C>(_ c: C) where C:Collection, C.Element == Change {
    if !CollectionDifference<ChangeElement>.validateChanges(c) {
      return nil
    }

    self.init(validatedChanges: c)
  }

  // Internal initializer for use by algorithms that cannot produce invalid
  // collections of changes. These include the Myers' diff algorithm and
  // the move inferencer.
  init<C>(validatedChanges c: C) where C:Collection, C.Element == Change {
    let changes = c.sorted { (a, b) -> Bool in
      switch (a, b) {
      case (.remove(_, _, _), .insert(_, _, _)):
        return true
      case (.insert(_, _, _), .remove(_, _, _)):
        return false
      default:
        return a.offset < b.offset
      }
    }

    // Find first insertion via binary search
    let firstInsertIndex: Int
    if changes.count == 0 {
      firstInsertIndex = 0
    } else {
      var range = 0...changes.count
      while range.lowerBound != range.upperBound {
        let i = (range.lowerBound + range.upperBound) / 2
        switch changes[i] {
        case .insert(_, _, _):
          range = range.lowerBound...i
        case .remove(_, _, _):
          range = (i + 1)...range.upperBound
        }
      }
      firstInsertIndex = range.lowerBound
    }

    removals = Array(changes[0..<firstInsertIndex])
    insertions = Array(changes[firstInsertIndex..<changes.count])
  }

  /// The `.insert` changes contained by this difference, from lowest offset to highest
  public let insertions: [Change]
  
  /// The `.remove` changes contained by this difference, from lowest offset to highest
  public let removals: [Change]
}

/// A CollectionDifference is itself a Collection.
///
/// The enumeration order of `Change` elements is:
///
/// 1. `.remove`s, from highest `offset` to lowest
/// 2. `.insert`s, from lowest `offset` to highest
///
/// This guarantees that applicators on compatible base states are safe when
/// written in the form:
///
/// ```
/// for c in diff {
///   switch c {
///   case .remove(offset: let o, element: _, associatedWith: _):
///     arr.remove(at: o)
///   case .insert(offset: let o, element: let e, associatedWith: _):
///     arr.insert(e, at: o)
///   }
/// }
/// ```
extension CollectionDifference : Collection {
  public typealias Element = CollectionDifference<ChangeElement>.Change

  // Opaque index type is isomorphic to Int
  public struct Index: Comparable, Hashable {
    public static func < (lhs: CollectionDifference<ChangeElement>.Index, rhs: CollectionDifference<ChangeElement>.Index) -> Bool {
      return lhs.i < rhs.i
    }
    
    let i: Int
    init(_ index: Int) {
      i = index
    }
  }

  public var startIndex: CollectionDifference<ChangeElement>.Index {
    return Index(0)
  }
  
  public var endIndex: CollectionDifference<ChangeElement>.Index {
    return Index(removals.count + insertions.count)
  }
  
  public func index(after index: CollectionDifference<ChangeElement>.Index) -> CollectionDifference<ChangeElement>.Index {
    return Index(index.i + 1)
  }
  
  public subscript(position: CollectionDifference<ChangeElement>.Index) -> Element {
    return position.i < removals.count ? removals[removals.count - (position.i + 1)] : insertions[position.i - removals.count]
  }
  
  public func index(before index: CollectionDifference<ChangeElement>.Index) -> CollectionDifference<ChangeElement>.Index {
    return Index(index.i - 1)
  }
  
  public func formIndex(_ index: inout CollectionDifference<ChangeElement>.Index, offsetBy distance: Int) {
    index = Index(index.i + distance)
  }
  
  public func distance(from start: CollectionDifference<ChangeElement>.Index, to end: CollectionDifference<ChangeElement>.Index) -> Int {
    return end.i - start.i
  }
}

extension CollectionDifference.Change: Equatable where ChangeElement: Equatable {}

extension CollectionDifference: Equatable where ChangeElement: Equatable {}

extension CollectionDifference.Change: Hashable where ChangeElement: Hashable {}

extension CollectionDifference: Hashable where ChangeElement: Hashable {

  /// Infers which `ChangeElement`s have been both inserted and removed only
  /// once and returns a new difference with those associations.
  ///
  /// - Returns: an instance with all possible moves inferred.
  ///
  /// - Complexity: O(*n*) where *n* is `self.count`
  public func inferringMoves() -> CollectionDifference<ChangeElement> {
    let removeDict: [ChangeElement:Int?] = {
      var res = [ChangeElement:Int?](minimumCapacity: Swift.min(removals.count, insertions.count))
      for r in removals {
        let element = r.element
        if res[element] != .none {
          res[element] = .some(.none)
        } else {
          res[element] = .some(r.offset)
        }
      }
      return res.filter { (_, v) -> Bool in v != .none }
    }()
    
    let insertDict: [ChangeElement:Int?] = {
      var res = [ChangeElement:Int?](minimumCapacity: Swift.min(removals.count, insertions.count))
      for i in insertions {
        let element = i.element
        if res[element] != .none {
          res[element] = .some(.none)
        } else {
          res[element] = .some(i.offset)
        }
      }
      return res.filter { (_, v) -> Bool in v != .none }
    }()

    return CollectionDifference.init(validatedChanges:map({ (c: CollectionDifference<ChangeElement>.Change) -> CollectionDifference<ChangeElement>.Change in
      switch c {
      case .remove(offset: let o, element: let e, associatedWith: _):
        if removeDict[e] == nil {
          return c
        }
        if let assoc = insertDict[e] {
          return .remove(offset: o, element: e, associatedWith: assoc)
        }
      case .insert(offset: let o, element: let e, associatedWith: _):
        if insertDict[e] == nil {
          return c
        }
        if let assoc = removeDict[e] {
          return .insert(offset: o, element: e, associatedWith: assoc)
        }
      }
      return c
    }))
  }
}

extension CollectionDifference.Change: Codable where ChangeElement: Codable {
  private enum CodingKeys: String, CodingKey {
    case offset
    case element
    case associatedOffset
    case isRemove
  }

  public init(from decoder: Decoder) throws {
    let values = try decoder.container(keyedBy: CodingKeys.self)
    let offset = try values.decode(Int.self, forKey: .offset)
    let element = try values.decode(ChangeElement.self, forKey: .element)
    let associatedOffset = try values.decode(Int?.self, forKey: .associatedOffset)
    let isRemove = try values.decode(Bool.self, forKey: .isRemove)
    if isRemove {
      self = .remove(offset: offset, element: element, associatedWith: associatedOffset)
    } else {
      self = .insert(offset: offset, element: element, associatedWith: associatedOffset)
    }
  }
  
  public func encode(to encoder: Encoder) throws {
    var container = encoder.container(keyedBy: CodingKeys.self)
    switch self {
    case .remove(_, _, _):
      try container.encode(true, forKey: .isRemove)
    case .insert(_, _, _):
      try container.encode(false, forKey: .isRemove)
    }
    
    try container.encode(offset, forKey: .offset)
    try container.encode(element, forKey: .element)
    try container.encode(associatedOffset, forKey: .associatedOffset)
  }
}

extension CollectionDifference: Codable where ChangeElement: Codable {}