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[stdlib] Add a comment about use of Lemire's nearly divisionless method #38263

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Jul 6, 2021
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[stdlib] Add a comment about use of Lemire's nearly divisionless method #38263

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4 changes: 4 additions & 0 deletions stdlib/public/core/Random.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -71,6 +71,7 @@ extension RandomNumberGenerator {
// unsigned integer will be used, recursing infinitely and probably blowing
// the stack.
@available(*, unavailable)
@_alwaysEmitIntoClient
public mutating func next() -> UInt64 { fatalError() }

/// Returns a value from a uniform, independent distribution of binary data.
Expand Down Expand Up @@ -103,6 +104,9 @@ extension RandomNumberGenerator {
upperBound: T
) -> T {
_precondition(upperBound != 0, "upperBound cannot be zero.")
// We use Lemire's "nearly divisionless" method for generating random
// integers in an interval. For a detailed explanation, see:
// https://arxiv.org/abs/1805.10941
var random: T = next()
var m = random.multipliedFullWidth(by: upperBound)
if m.low < upperBound {
Expand Down