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[minor]: Binomial guarantees #15

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Merged
tayloraswift merged 1 commit into from
Jun 17, 2026
Merged

[minor]: Binomial guarantees #15

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9 changes: 8 additions & 1 deletion Sources/Random/Binomial.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -17,9 +17,16 @@ import RealModule
}
}
extension Binomial {
@inlinable public static subscript(n: Int64, p: Double) -> Self { .init(n: n, p: p) }
@inlinable public static subscript(n: Int64, p: Double) -> Self {
guard p.isFinite else {
fatalError("Binomial.subscript cannot be called with `p` = \(p)")
}
return .init(n: n, p: p)
}
Comment on lines +20 to +25

@gemini-code-assist gemini-code-assist Bot Jun 17, 2026

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high

If n is negative, calling pdf(_:) will attempt to construct an invalid range 0 ... self.n, which triggers a runtime fatal error in Swift (Can't form Range with upperBound < lowerBound). Guarding that n >= 0 in the subscript prevents this crash and ensures the distribution is always initialized in a valid state.

Suggested change
@inlinable public static subscript(n: Int64, p: Double) -> Self {
guard p.isFinite else {
fatalError("Binomial.subscript cannot be called with `p` = \(p)")
}
return .init(n: n, p: p)
}
@inlinable public static subscript(n: Int64, p: Double) -> Self {
guard n >= 0 else {
fatalError("Binomial.subscript cannot be called with negative `n` = \(n)")
}
guard p.isFinite else {
fatalError("Binomial.subscript cannot be called with `p` = \(p)")
}
return .init(n: n, p: p)
}

}
extension Binomial {
/// Sample the binomial distribution. As long as ``n`` is non-negative, the result is
/// guaranteed to lie in the range `0 ... n`.
@inlinable public func sample(using generator: inout some RandomNumberGenerator) -> Int64 {
if self.p <= 0 { return 0 }
if self.p >= 1 { return self.n }
Expand Down
20 changes: 20 additions & 0 deletions Sources/RandomTests/Distributions/BinomialGuarantees.swift
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Original file line number Diff line number Diff line change
@@ -0,0 +1,20 @@
import Random
import Testing

@Suite struct BinomialGuarantees {
private var random: PseudoRandom

init() {
self.random = .init(seed: 10)
}

@Test mutating func OutputRange() {
let n: Int64 = Int64.max - 1
let distribution = Binomial[n, 1.nextDown]
for _: Int in 0 ..< 1000 {
let result: Int64 = distribution.sample(using: &self.random.generator)
#expect(result >= 0)
#expect(result <= n)
}
}
Comment on lines +11 to +19

@gemini-code-assist gemini-code-assist Bot Jun 17, 2026

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medium

To thoroughly test the binomial guarantees and boundary conditions, consider adding test cases for edge cases such as n = 0, p = 0, and p = 1.

    @Test mutating func OutputRange() {
        let n: Int64 = Int64.max - 1
        let distribution = Binomial[n, 1.nextDown]
        for _: Int in 0 ..< 1000 {
            let result: Int64 = distribution.sample(using: &self.random.generator)
            #expect(result >= 0)
            #expect(result <= n)
        }

        // Test boundary edge cases
        #expect(Binomial[0, 0.5].sample(using: &self.random.generator) == 0)
        #expect(Binomial[10, 0.0].sample(using: &self.random.generator) == 0)
        #expect(Binomial[10, 1.0].sample(using: &self.random.generator) == 10)
    }

}