Unsafe Swift

Case study of using unsafe to implement hashable type. Presented at try! Swift 2017 Tokyo, Japan.

Transcript

1. THE SAFETY OF UNSAFE SWIFT @RAYFIX ⚡ try! Swift Japan 2017 1
2. UB UNDEFINED BEHAVIOR 2
3. UNDEFINED SCHEDULE = 3
4. SWIFT SAFETY 4
5. WORKING WITH C PERFORMANCE LOW LEVEL 5
6. SWIFT POINTERS UnsafeMutableRawBufferPointer<Pointee> Mutable Raw Buffer <Pointee> 6
7. 7
8. O(1) CONSTANT TIME LOOKUP DICTIONARIES AND SETS8
9. O(N) LINEAR TIME LOOKUP BAD HASH O(1) 9
10. struct Angle: Hashable { var radians: Double … var hashValue: Int { return radians.hashValue } } ANGLE 10
11. struct Point: Hashable { var x, y: Double var hashValue: Int { return x.hashValue ^ y.hashValue } } ^ COMPOSITION 11
12. struct Point: Hashable { var x, y: Double var hashValue: Int { return "\(x),\(y)".hashValue } } FAKE IT Heap Allocations are Expensive! 12
13. protocol HashAlgorithm { init() // 1 mutating func consume(bytes:) // 2 var finalValue: Int // 3 } ROBUST COMPOSITION13
14. struct FVN1AHash: HashAlgorithm { private var hash: UInt64 = 0xcbf29ce484222325 private let prime: UInt64 = 0x100000001b3 mutating func consume<S: Sequence>(bytes: S) where S.Iterator.Element == UInt8 { for byte in bytes { hash = (hash ^ UInt64(byte)) &* prime } } var finalValue: Int { return Int(truncatingBitPattern: hash) } } HASH ALGO AUTHORS14
15. var hashValue: Int { var hash = FVN1AHash() hash.consume(x) hash.consume(y) return hash.finalValue } SAFE EASY CLIENT CODE15
16. UNSAFE CODE SAFELY HIDDEN AWAY extension HashAlgorithm { mutating func consume<I: Integer>(_ value: I) { var temp = value withUnsafeBytes(of: &temp) { rawBufferPointer in consume(bytes: rawBufferPointer) } } } 16
17. Safe, Swifty API for Users Safe Customization Points for Library Developers Well Tested Unsafe Code UNSAFE CODE SAFELY HIDDEN AWAY 17
18. 18