Fix RandomSample crash when 0 was out from random.

Author: sidepelican

Sources/Algorithms/RandomSample.swift

@@ -87,14 +87,14 @@ extension Collection {
 internal func nextW<G: RandomNumberGenerator>(
   k: Int, using rng: inout G
 ) -> Double {
-  Double.root(.random(in: 0..<1, using: &rng), k)
+  Double.root(1 - .random(in: 0..<1, using: &rng), k)
 }
 
 @usableFromInline
 internal func nextOffset<G: RandomNumberGenerator>(
   w: Double, using rng: inout G
 ) -> Int {
-  Int(Double.log(.random(in: 0..<1, using: &rng)) / .log(1 - w))
+  Int(Double.log(1 - .random(in: 0..<1, using: &rng)) / .log(1 - w))
 }
 
 extension Collection {

Tests/SwiftAlgorithmsTests/RandomSampleTests.swift

@@ -95,4 +95,9 @@ final class RandomSampleTests: XCTestCase {
     let sample2c = c.randomStableSample(count: k, using: &generator)
     XCTAssertEqual(sample1c, sample2c)
   }
+
+  func testRandomSampleZeroRandom() {
+    var generator = SplitMix64(seed: 0x61c8864680b583eb) // this seed starts with 0
+    _ = c.randomSample(count: k, using: &generator) // must not crash
+  }
 }