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[4.2] Make upperBound != 0 a precondition for RNG.next(upperBound:) (#17627) #17631

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Merged
merged 1 commit into from
Jul 2, 2018
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[4.2] Make upperBound != 0 a precondition for RNG.next(upperBound:) (#17627) #17631

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3 changes: 2 additions & 1 deletion stdlib/public/core/Random.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -89,13 +89,14 @@ extension RandomNumberGenerator {
/// Returns a random value that is less than the given upper bound.
///
/// - Parameter upperBound: The upper bound for the randomly generated value.
/// Must be non-zero.
/// - Returns: A random value of `T` in the range `0..<upperBound`. Every
/// value in the range `0..<upperBound` is equally likely to be returned.
@inlinable
public mutating func next<T: FixedWidthInteger & UnsignedInteger>(
upperBound: T
) -> T {
guard upperBound != 0 else { return 0 }
_precondition(upperBound != 0, "upperBound cannot be zero.")
let tmp = (T.max % upperBound) + 1
let range = tmp == upperBound ? 0 : tmp
var random: T = 0
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