import Foundation

/// A type that emits notifications to observers when underlying data changes.
///
/// Conforming to this protocol signals to other APIs that the value type supports observation.
/// However, applying the ``ObservableState`` protocol by itself to a type doesn’t add observation
/// functionality to the type. Instead, always use the ``ObservableState()`` macro when adding
/// observation support to a type.
#if !os(visionOS)
  public protocol ObservableState: Perceptible {
    var _$id: ObservableStateID { get }
    mutating func _$willModify()
  }
#else
  public protocol ObservableState: Observable {
    var _$id: ObservableStateID { get }
    mutating func _$willModify()
  }
#endif

/// A unique identifier for a observed value.
public struct ObservableStateID: Equatable, Hashable, Sendable {
  @usableFromInline
  var location: UUID {
    get { self.storage.id.location }
    set {
      if !isKnownUniquelyReferenced(&self.storage) {
        self.storage = Storage(id: self.storage.id)
      }
      self.storage.id.location = newValue
    }
  }

  private var storage: Storage

  private init(storage: Storage) {
    self.storage = storage
  }

  public init() {
    self.init(storage: Storage(id: .location(UUID())))
  }

  public static func == (lhs: Self, rhs: Self) -> Bool {
    lhs.storage === rhs.storage || lhs.storage.id == rhs.storage.id
  }

  public func hash(into hasher: inout Hasher) {
    hasher.combine(self.storage.id)
  }

  @inlinable
  public static func _$id<T>(for value: T) -> Self {
    (value as? any ObservableState)?._$id ?? Self()
  }

  @inlinable
  public static func _$id(for value: some ObservableState) -> Self {
    value._$id
  }

  public func _$tag(_ tag: Int) -> Self {
    Self(storage: Storage(id: .tag(tag, self.storage.id)))
  }

  @inlinable
  public mutating func _$willModify() {
    self.location = UUID()
  }

  private final class Storage: @unchecked Sendable {
    fileprivate var id: ID

    init(id: ID = .location(UUID())) {
      self.id = id
    }

    enum ID: Equatable, Hashable, Sendable {
      case location(UUID)
      indirect case tag(Int, ID)

      var location: UUID {
        get {
          switch self {
          case let .location(location):
            return location
          case let .tag(_, id):
            return id.location
          }
        }
        set {
          switch self {
          case .location:
            self = .location(newValue)
          case .tag(let tag, var id):
            id.location = newValue
            self = .tag(tag, id)
          }
        }
      }
    }
  }
}

@inlinable
public func _$isIdentityEqual<T: ObservableState>(
  _ lhs: T, _ rhs: T
) -> Bool {
  lhs._$id == rhs._$id
}

@inlinable
public func _$isIdentityEqual<ID: Hashable, T: ObservableState>(
  _ lhs: IdentifiedArray<ID, T>,
  _ rhs: IdentifiedArray<ID, T>
) -> Bool {
  areOrderedSetsDuplicates(lhs.ids, rhs.ids)
}

@inlinable
public func _$isIdentityEqual<T: ObservableState>(
  _ lhs: PresentationState<T>,
  _ rhs: PresentationState<T>
) -> Bool {
  lhs.wrappedValue?._$id == rhs.wrappedValue?._$id
}

@inlinable
public func _$isIdentityEqual<T: ObservableState>(
  _ lhs: StackState<T>,
  _ rhs: StackState<T>
) -> Bool {
  areOrderedSetsDuplicates(lhs.ids, rhs.ids)
}

@inlinable
public func _$isIdentityEqual<C: Collection>(
  _ lhs: C,
  _ rhs: C
) -> Bool
where C.Element: ObservableState {
  lhs.count == rhs.count && zip(lhs, rhs).allSatisfy { $0._$id == $1._$id }
}

// NB: This is a fast path so that String is not checked as a collection.
@inlinable
public func _$isIdentityEqual(_ lhs: String, _ rhs: String) -> Bool {
  false
}

@inlinable
public func _$isIdentityEqual<T>(_ lhs: T, _ rhs: T) -> Bool {
  guard !_isPOD(T.self) else { return false }

  func openCollection<C: Collection>(_ lhs: C, _ rhs: Any) -> Bool {
    guard C.Element.self is any ObservableState.Type else {
      return false
    }

    func openIdentifiable<Element: Identifiable>(_: Element.Type) -> Bool? {
      guard
        let lhs = lhs as? IdentifiedArrayOf<Element>,
        let rhs = rhs as? IdentifiedArrayOf<Element>
      else {
        return nil
      }
      return areOrderedSetsDuplicates(lhs.ids, rhs.ids)
    }

    if let identifiable = C.Element.self as? any Identifiable.Type,
      let result = openIdentifiable(identifiable)
    {
      return result
    } else if let rhs = rhs as? C {
      return lhs.count == rhs.count && zip(lhs, rhs).allSatisfy(_$isIdentityEqual)
    } else {
      return false
    }
  }

  if let lhs = lhs as? any ObservableState, let rhs = rhs as? any ObservableState {
    return lhs._$id == rhs._$id
  } else if let lhs = lhs as? any Collection {
    return openCollection(lhs, rhs)
  } else {
    return false
  }
}

@inlinable
public func _$willModify<T>(_: inout T) {}
@inlinable
public func _$willModify<T: ObservableState>(_ value: inout T) {
  value._$willModify()
}