Hashable.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
//
//===----------------------------------------------------------------------===//

/// A type that can be hashed into a `Hasher` to produce an integer hash value.
///
/// You can use any type that conforms to the `Hashable` protocol in a set or as
/// a dictionary key. Many types in the standard library conform to `Hashable`:
/// Strings, integers, floating-point and Boolean values, and even sets are
/// hashable by default. Some other types, such as optionals, arrays and ranges
/// automatically become hashable when their type arguments implement the same.
///
/// Your own custom types can be hashable as well. When you define an
/// enumeration without associated values, it gains `Hashable` conformance
/// automatically, and you can add `Hashable` conformance to your other custom
/// types by implementing the `hash(into:)` method. For structs whose stored
/// properties are all `Hashable`, and for enum types that have all-`Hashable`
/// associated values, the compiler is able to provide an implementation of
/// `hash(into:)` automatically.
///
/// Hashing a value means feeding its essential components into a hash function,
/// represented by the `Hasher` type. Essential components are those that
/// contribute to the type's implementation of `Equatable`. Two instances that
/// are equal must feed the same values to `Hasher` in `hash(into:)`, in the
/// same order.
///
/// Conforming to the Hashable Protocol
/// ===================================
///
/// To use your own custom type in a set or as the key type of a dictionary,
/// add `Hashable` conformance to your type. The `Hashable` protocol inherits
/// from the `Equatable` protocol, so you must also satisfy that protocol's
/// requirements.
///
/// The compiler automatically synthesizes your custom type's `Hashable` and
/// requirements when you declare `Hashable` conformance in the type's original
/// declaration and your type meets these criteria:
///
/// - For a `struct`, all its stored properties must conform to `Hashable`.
/// - For an `enum`, all its associated values must conform to `Hashable`. (An
///   `enum` without associated values has `Hashable` conformance even without
///   the declaration.)
///
/// To customize your type's `Hashable` conformance, to adopt `Hashable` in a
/// type that doesn't meet the criteria listed above, or to extend an existing
/// type to conform to `Hashable`, implement the `hash(into:)` method in your
/// custom type.
///
/// In your `hash(into:)` implementation, call `combine(_:)` on the provided
/// `Hasher` instance with the essential components of your type. To ensure
/// that your type meets the semantic requirements of the `Hashable` and
/// `Equatable` protocols, it's a good idea to also customize your type's
/// `Equatable` conformance to match.
///
/// As an example, consider a `GridPoint` type that describes a location in a
/// grid of buttons. Here's the initial declaration of the `GridPoint` type:
///
///     /// A point in an x-y coordinate system.
///     struct GridPoint {
///         var x: Int
///         var y: Int
///     }
///
/// You'd like to create a set of the grid points where a user has already
/// tapped. Because the `GridPoint` type is not hashable yet, it can't be used
/// in a set. To add `Hashable` conformance, provide an `==` operator function
/// and implement the `hash(into:)` method.
///
///     extension GridPoint: Hashable {
///         static func == (lhs: GridPoint, rhs: GridPoint) -> Bool {
///             return lhs.x == rhs.x && lhs.y == rhs.y
///         }
///
///         func hash(into hasher: inout Hasher) {
///             hasher.combine(x)
///             hasher.combine(y)
///         }
///     }
///
/// The `hash(into:)` method in this example feeds the grid point's `x` and `y`
/// properties into the provided hasher. These properties are the same ones
/// used to test for equality in the `==` operator function.
///
/// Now that `GridPoint` conforms to the `Hashable` protocol, you can create a
/// set of previously tapped grid points.
///
///     var tappedPoints: Set = [GridPoint(x: 2, y: 3), GridPoint(x: 4, y: 1)]
///     let nextTap = GridPoint(x: 0, y: 1)
///     if tappedPoints.contains(nextTap) {
///         print("Already tapped at (\(nextTap.x), \(nextTap.y)).")
///     } else {
///         tappedPoints.insert(nextTap)
///         print("New tap detected at (\(nextTap.x), \(nextTap.y)).")
///     }
///     // Prints "New tap detected at (0, 1).")
public protocol Hashable: Equatable {
  /// The hash value.
  ///
  /// Hash values are not guaranteed to be equal across different executions of
  /// your program. Do not save hash values to use during a future execution.
  ///
  /// - Important: `hashValue` is deprecated as a `Hashable` requirement. To
  ///   conform to `Hashable`, implement the `hash(into:)` requirement instead.
  ///   The compiler provides an implementation for `hashValue` for you.
  var hashValue: Int { get }

  /// Hashes the essential components of this value by feeding them into the
  /// given hasher.
  ///
  /// Implement this method to conform to the `Hashable` protocol. The
  /// components used for hashing must be the same as the components compared
  /// in your type's `==` operator implementation. Call `hasher.combine(_:)`
  /// with each of these components.
  ///
  /// - Important: In your implementation of `hash(into:)`,
  ///   don't call `finalize()` on the `hasher` instance provided,
  ///   or replace it with a different instance.
  ///   Doing so may become a compile-time error in the future.
  ///
  /// - Parameter hasher: The hasher to use when combining the components
  ///   of this instance.
  func hash(into hasher: inout Hasher)

  // Raw top-level hashing interface. Some standard library types (mostly
  // primitives) specialize this to eliminate small resiliency overheads. (This
  // only matters for tiny keys.)
  func _rawHashValue(seed: Int) -> Int
}

extension Hashable {
  @inlinable
  @inline(__always)
  public func _rawHashValue(seed: Int) -> Int {
    var hasher = Hasher(_seed: seed)
    hasher.combine(self)
    return hasher._finalize()
  }
}

// Called by synthesized `hashValue` implementations.
@inlinable
@inline(__always)
public func _hashValue<H: Hashable>(for value: H) -> Int {
  return value._rawHashValue(seed: 0)
}

// Called by the SwiftValue implementation.
@_silgen_name("_swift_stdlib_Hashable_isEqual_indirect")
internal func Hashable_isEqual_indirect<T: Hashable>(
  _ lhs: UnsafePointer<T>,
  _ rhs: UnsafePointer<T>
) -> Bool {
  return lhs.pointee == rhs.pointee
}

// Called by the SwiftValue implementation.
@_silgen_name("_swift_stdlib_Hashable_hashValue_indirect")
internal func Hashable_hashValue_indirect<T: Hashable>(
  _ value: UnsafePointer<T>
) -> Int {
  return value.pointee.hashValue
}