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[SR-11447] Infinite loop in RandomNumberGenerator using drand48 #53848

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swift-ci opened this issue Sep 10, 2019 · 4 comments
Closed

[SR-11447] Infinite loop in RandomNumberGenerator using drand48 #53848

swift-ci opened this issue Sep 10, 2019 · 4 comments
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. standard library Area: Standard library umbrella

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@swift-ci
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Previous ID SR-11447
Radar None
Original Reporter dbplunkett (JIRA User)
Type Bug
Status Closed
Resolution Invalid
Environment

Xcode 10.3 (10G8)
Swift 5.0.1

Additional Detail from JIRA
Votes 0
Component/s Standard Library
Labels Bug
Assignee None
Priority Medium

md5: 71caa2ca8b9939510bf7e4ee0645d128

Issue Description:

A simple implementation of RandomNumberGenerator using drand48 results in infinite calls to next() when used to find a random 8- or 16-bit Int or UInt.

This issue does not occur with other simple implementations, e.g. using arc4random, nor if it's used to find a random 32- or 64-bit Int or UInt.

The range passed to the random method doesn't seem to have any impact.

Uncomment lines below to see an example.

struct RNG: RandomNumberGenerator {

    //  is infinitely called in the Int8, Int16, UInt8, and UInt16 examples
    func next() -> UInt64 {
        return .init(drand48() * .init(UInt64.max))
    }

    //  works with all examples
//    func next() -> UInt64 {
//        return .init(Double(arc4random()) / .init(UInt32.max) * .init(UInt64.max))
//    }
}

var rng = RNG()

//Int64.random(in: 1...10, using: &rng)
//Int32.random(in: 1...10, using: &rng)
//Int16.random(in: 1...10, using: &rng)
//Int8.random(in: 1...10, using: &rng)

//UInt64.random(in: 1...10, using: &rng)
//UInt32.random(in: 1...10, using: &rng)
//UInt16.random(in: 1...10, using: &rng)
//UInt8.random(in: 1...10, using: &rng)
@belkadan
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@stephentyrone, any ideas?

@stephentyrone
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.init(drand48() * .init(UInt64.max)) will always result in a UInt64 whose low-order 16 bits are all zero. The .random(in) method first creates a UInt(8|16) using init(truncatingIfNeeded), which always results in all-zeros. That implementation could be somewhat more sophisticated to avoid wasting calls to next(), but even so this is an inadequate base RNG, and some failure is expected to occur.

TL;DR: don't use drand48() like this. It does not satisfy the requirements of Swift's RandomNumberGenerator protocol.

@swift-ci
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Comment by David Plunkett (JIRA)

Ahh, I see. That implementation is a surprise - however, I suppose it's better that my bad randomness caused an obvious failure, rather than simply producing a working but subtly non-random instance of a RandomNumberGenerator.

Thanks.

@stephentyrone
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Some follow-on work:

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. standard library Area: Standard library umbrella
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@stephentyrone @belkadan @swift-ci