Implement a more robust workaround for swiftlang/swift#70557.

Sources/Retry/Backoff/Algorithms/FullJitterExponentialBackoff.swift

@@ -79,22 +79,21 @@ where ClockType: Clock, RandomNumberGeneratorType: RandomNumberGenerator {
       let maxDelayInClockTicks = min(baseDelayInClockTicks * Double(1 << exponent),
                                      maxDelayInClockTicks)
 
-      let delayInClockTicks = Double.random(in: 0...maxDelayInClockTicks,
-                                            using: &randomNumberGenerator)
+      let delayInClockTicks = randomNumberGenerator.random(in: 0...maxDelayInClockTicks)
 
       // Unfortunately, `DurationProtocol` does not have a `Duration * Double` operator, so we need to cast to `Int`.
       // We make sure to cast to `Int` at the end rather than at the beginning so that the imprecision is bounded.
       return clockMinResolution * Int(clamping: UInt(delayInClockTicks.rounded()))
    }
 }
 
-extension FullJitterExponentialBackoff where RandomNumberGeneratorType == SystemRandomNumberGenerator {
+extension FullJitterExponentialBackoff where RandomNumberGeneratorType == StandardRandomNumberGenerator {
    init(clock: ClockType,
         baseDelay: ClockType.Duration,
         maxDelay: ClockType.Duration?) {
       self.init(clock: clock,
                 baseDelay: baseDelay,
                 maxDelay: maxDelay,
-                randomNumberGenerator: SystemRandomNumberGenerator())
+                randomNumberGenerator: StandardRandomNumberGenerator())
    }
 }

Sources/Retry/Backoff/Algorithms/Random Number Generator/RandomNumberGenerator.swift

@@ -0,0 +1,31 @@
+// MIT License
+//
+// Copyright © 2023 Darren Mo.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+/// A protocol that allows one to specify a different implementation from the standard one (e.g. for automated tests).
+///
+/// - Remark: Cannot use the Swift standard library’s `RandomNumberGenerator` protocol for this purpose as detailed here:
+///    https://github.com/apple/swift/issues/70557
+protocol RandomNumberGenerator {
+   func random<T>(
+      in range: ClosedRange<T>
+   ) -> T where T: BinaryFloatingPoint, T.RawSignificand: FixedWidthInteger
+}

Sources/Retry/Backoff/Algorithms/Random Number Generator/StandardRandomNumberGenerator.swift

@@ -0,0 +1,29 @@
+// MIT License
+//
+// Copyright © 2023 Darren Mo.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+struct StandardRandomNumberGenerator: RandomNumberGenerator {
+   func random<T>(
+      in range: ClosedRange<T>
+   ) -> T where T: BinaryFloatingPoint, T.RawSignificand: FixedWidthInteger {
+      return T.random(in: range)
+   }
+}

Tests/RetryTests/Fakes/RandomNumberGeneratorFake.swift

@@ -20,6 +20,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
+@testable import Retry
+
 class RandomNumberGeneratorFake: RandomNumberGenerator {
    enum Mode {
       case min
@@ -31,15 +33,15 @@ class RandomNumberGeneratorFake: RandomNumberGenerator {
       self.mode = mode
    }
 
-   func next() -> UInt64 {
+   func random<T>(
+      in range: ClosedRange<T>
+   ) -> T where T : BinaryFloatingPoint, T.RawSignificand : FixedWidthInteger {
       switch mode {
       case .min:
-         // Add `1` to work around the following issue with Swift’s random number generator implementation:
-         // https://github.com/apple/swift/issues/70557
-         return .min + 1
+         return range.lowerBound
 
       case .max:
-         return .max
+         return range.upperBound
       }
    }
 }