Swift 4.2+ seeding a random number generator

Question

I\'m trying to generate seeded random numbers with Swift 4.2+, with the Int.random() function, however there is no given implementation that allows for the random n

Answer 1

I ended up using srand48() and drand48() to generate a pseudo-random number with a seed for a specific test.

class SeededRandomNumberGenerator : RandomNumberGenerator {

    let range: ClosedRange = Double(UInt64.min) ... Double(UInt64.max)

    init(seed: Int) {
        // srand48() — Pseudo-random number initializer
        srand48(seed)
    }

    func next() -> UInt64 {
        // drand48() — Pseudo-random number generator
        return UInt64(range.lowerBound + (range.upperBound - range.lowerBound) * drand48())
    }
    
}

So, in production the implementation uses the SystemRandomNumberGenerator but in the test suite it uses the SeededRandomNumberGenerator.

Example:

let messageFixtures: [Any] = [
    "a string",
    ["some", ["values": 456]],
]

var seededRandomNumberGenerator = SeededRandomNumberGenerator(seed: 13)

func randomMessageData() -> Any {
    return messageFixtures.randomElement(using: &seededRandomNumberGenerator)!
}

// Always return the same element in the same order
randomMessageData() //"a string"
randomMessageData() //["some", ["values": 456]]
randomMessageData() //["some", ["values": 456]]
randomMessageData() //["some", ["values": 456]]
randomMessageData() //"a string"