Hashable in Swift

Presentation about Swift Hasable protocol

Transcript

1. hashable ! Lukasz Pikor @pikor
2. What is Hashable
3. conforming to hashable
4. conforming to hashable 1. implement one property var hashValue: Int
5. conforming to hashable Simple?
6. conforming to hashable Simple? Yes
7. conforming to hashable Playground time!
8. Things to remember if a == b then a.hashValue == b.hashValue But reverse is not true!
9. Things to remember A = false // hash value = 0 b = true // hash value = 1
10. Things to remember There is no guarantee that a hash value will be the same for different executions of your code - don’t save iT
11. Things to remember But for given app runtime, hash for given object should be always the same hash(obj1) // 111 hash(obj1) // 111 hash(obj1) // 111
12. FUN part Hash collisions: HASH(obj1) / 111 HASH(obj2) /111
13. FUN part A ^ A = 0 A ^ B = B ^ A
14. conforming to hashable Simple? No
15. conforming to hashable Simple? No performance it’s very important to avoid collisions Cryptographic hash function Locality-sensitive hashing Avalanche effect uniformity Caveats with floating-points values
16. conforming to hashable Simple? No var hashValue: Int { return x.hashValue ^ y.hashValue } let (width, height) = (500, 500) let total = width * height var hashes = Set<Int>() for x in 0..<width { for y in 0..<height { hashes.insert(GridPoint(x: x, y: y).hashValue) } } print("\(hashes.count) unique hashes out of a total of \(total).”) // 1024 unique hashes out of a total of 1_000_000 99.9% of values triggers a hash collision.
17. Bonus: DDOS (link)
18. Better Way No need to reinvent the weel. use sourcery! var hashValue: Int { return combineHashes([firstName.hashValue,lastName.hashValue]) }
19. Better Way No need to reinvent the weel. Or Any other already tested method. extension String { var djb2hash: Int { let unicodeScalars = self.unicodeScalars.map { $0.value } return unicodeScalars.reduce(5381) { ($0 << 5) &+ $0 &+ Int($1) } } }
20. Even Better Way (not yet available)
21. Even Better Way HashVisitable
22. Even Better Way protocol Hasher { mutating func finish() -> Int mutating func write(bytes: UnsafeRawBufferPointer) } protocol HashVisitable: Equatable { func hash<H: Hasher>(_ hasher: inout H) }
23. New code extension GridPoint: HashVisitable { func hash<H: Hasher>(_ hasher: inout H) { self.x.hash(&hasher) self.y.hash(&hasher) } }
24. Backwards compatibility extension Hashable: HashVisitable { func hash<H: Hasher>(_ hasher: inout H) { self.hashValue.hash(&hasher) } }
25. 1. use your loaf 2. Advanced swift 3. Further reading: Harsh Visitors + Swift proposal
26. thank you!