Proposal to generate UUIDs using RandomNumberGenerators

Proposals/NNNN-random-uuid.md

@@ -0,0 +1,57 @@
+# Generating UUIDs using RandomNumberGenerators
+
+* Proposal: [SF-NNNN](NNNN-random-uuid.md)
+* Authors: [FranzBusch](https://github.com/FranzBusch)
+* Review Manager: TBD
+* Status: **Awaiting review**
+* Implementation: [swiftlang/swift-foundation#1271](https://github.com/swiftlang/swift-foundation/pull/1271)
+* Review: ([pitch](https://forums.swift.org/...))
+
+## Introduction
+
+UUIDs (Universally Unique IDentifiers) are 128 bits long and is intended to
+guarantee uniqueness across space and time. This proposal adds APIs to generate
+UUIDs from Swift's random number generators.
+
+## Motivation
+
+UUIDs often need to be randomly generated. This is currently possible by calling
+the `UUID` initializer. However, this initializer doesn't allow providing a
+custom source from which the `UUID` is generated. Swift's standard library
+provides a common abstraction for random number generators through the
+`RandomNumberGenerator` protocol. Providing methods to generate `UUID`s using a
+`RandomNumberGenerator` allows developers to customize their source of randomness.
+
+An example where this is useful is where a system needs to generate UUIDs using a
+deterministically seeded random number generator.
+
+## Proposed solution
+
+This proposal adds a new static method to the `UUID` type to generate new random `UUIDs` using a `RandomNumberGenerator`.
+
+```swift
+/// Generates a new random UUID.
+///
+/// - Parameter generator: The random number generator to use when creating the new random value.
+/// - Returns: A random UUID.
+@available(FoundationPreview 6.2, *)
+public static func random(
+    using generator: inout some RandomNumberGenerator
+) -> UUID
+```
+
+## Source compatibility
+
+The new API is purely additive and ha no impact on the existing API.
+
+## Implications on adoption
+
+This feature can be freely adopted and un-adopted in source code with no deployment constraints and without affecting source compatibility.
+
+## Alternatives considered
+
+### Initializer based random UUID generation
+
+The existing `UUID.init()` is already generating new random `UUID`s and a new
+`UUID(using: &rng)` method would be a good alternative to the proposed static method.
+However, the static `random` method has precedence on various types such as [Int.random](https://developer.apple.com/documentation/swift/int/random(in:)-9mjpw).

Sources/FoundationEssentials/UUID.swift

@@ -76,6 +76,50 @@ public struct UUID : Hashable, Equatable, CustomStringConvertible, Sendable {
             hasher.combine(bytes: buffer)
         }
     }
+
+    /// Generates a new random UUID.
+    ///
+    /// - Parameter generator: The random number generator to use when creating the new random value.
+    /// - Returns: A random UUID.
+    @available(FoundationPreview 6.2, *)
+    public static func random(
+        using generator: inout some RandomNumberGenerator
+    ) -> UUID {
+        let first = UInt64.random(in: .min ... .max, using: &generator)
+        let second = UInt64.random(in: .min ... .max, using: &generator)
+
+        var firstBits = first
+        var secondBits = second
+
+        // Set the version to 4 (0100 in binary)
+        firstBits &= 0xFFFFFFFFFFFF0FFF // Clear bits 48 through 51
+        firstBits |= 0x0000000000004000 // Set the version bits to '0100' at the correct position
+
+        // Set the variant to '10' (RFC9562 variant)
+        secondBits &= 0x3FFFFFFFFFFFFFFF // Clear the 2 most significant bits
+        secondBits |= 0x8000000000000000 // Set the two MSB to '10'
+
+        let uuidBytes = (
+            UInt8(truncatingIfNeeded: firstBits >> 56),
+            UInt8(truncatingIfNeeded: firstBits >> 48),
+            UInt8(truncatingIfNeeded: firstBits >> 40),
+            UInt8(truncatingIfNeeded: firstBits >> 32),
+            UInt8(truncatingIfNeeded: firstBits >> 24),
+            UInt8(truncatingIfNeeded: firstBits >> 16),
+            UInt8(truncatingIfNeeded: firstBits >> 8),
+            UInt8(truncatingIfNeeded: firstBits),
+            UInt8(truncatingIfNeeded: secondBits >> 56),
+            UInt8(truncatingIfNeeded: secondBits >> 48),
+            UInt8(truncatingIfNeeded: secondBits >> 40),
+            UInt8(truncatingIfNeeded: secondBits >> 32),
+            UInt8(truncatingIfNeeded: secondBits >> 24),
+            UInt8(truncatingIfNeeded: secondBits >> 16),
+            UInt8(truncatingIfNeeded: secondBits >> 8),
+            UInt8(truncatingIfNeeded: secondBits)
+        )
+
+        return UUID(uuid: uuidBytes)
+    }
 
     public var description: String {
         return uuidString

Tests/FoundationEssentialsTests/UUIDTests.swift

@@ -115,4 +115,64 @@ final class UUIDTests : XCTestCase {
         XCTAssertFalse(uuid2 > uuid1)
         XCTAssertTrue(uuid2 == uuid1)
     }
+
+    func testRandomVersionAndVariant() {
+        var generator = SystemRandomNumberGenerator()
+        for _ in 0..<10000 {
+            let uuid = UUID.random(using: &generator)
+            XCTAssertEqual(uuid.versionNumber, 0b0100)
+            XCTAssertEqual(uuid.varint, 0b10)
+        }
+    }
+
+    func testDeterministicRandomGeneration() {
+        var generator = PCGRandomNumberGenerator(seed: 123456789)
+
+        let firstUUID = UUID.random(using: &generator)
+        XCTAssertEqual(firstUUID, UUID(uuidString: "9492BAC4-F353-49E7-ACBB-A40941CA65DE"))
+
+        let secondUUID = UUID.random(using: &generator)
+        XCTAssertEqual(secondUUID, UUID(uuidString: "392C44E5-EB3E-4455-85A7-AF9556722B9A"))
+
+        let thirdUUID = UUID.random(using: &generator)
+        XCTAssertEqual(thirdUUID, UUID(uuidString: "9ABFCCE9-AA85-485C-9CBF-C62F0C8D1D1A"))
+
+        let fourthUUID = UUID.random(using: &generator)
+        XCTAssertEqual(fourthUUID, UUID(uuidString: "2B29542E-F719-4D58-87B9-C6291ADD4541"))
+    }
 }
+
+extension UUID {
+    fileprivate var versionNumber: Int {
+        Int(self.uuid.6 >> 4)
+    }
+
+    fileprivate var varint: Int {
+        Int(self.uuid.8 >> 6 & 0b11)
+    }
+}
+
+fileprivate struct PCGRandomNumberGenerator: RandomNumberGenerator {
+    private static let multiplier: UInt128 = 47_026_247_687_942_121_848_144_207_491_837_523_525
+    private static let increment: UInt128 = 117_397_592_171_526_113_268_558_934_119_004_209_487
+
+    private var state: UInt128
+
+    fileprivate init(seed: UInt64) {
+        self.state = UInt128(seed)
+    }
+
+    fileprivate mutating func next() -> UInt64 {
+        self.state = self.state &* Self.multiplier &+ Self.increment
+
+        return rotr64(
+            value: UInt64(truncatingIfNeeded: self.state &>> 64) ^ UInt64(truncatingIfNeeded: self.state),
+            rotation: UInt64(truncatingIfNeeded: self.state &>> 122)
+        )
+    }
+
+    private func rotr64(value: UInt64, rotation: UInt64) -> UInt64 {
+        (value &>> rotation) | value &<< ((~rotation &+ 1) & 63)
+    }
+}
+