[SR-8178] Fix BinaryFloatingPoint.random(in:) open range returning upperBound (#17794)

Author: Azoy

stdlib/public/core/FloatingPoint.swift.gyb

@@ -2461,7 +2461,13 @@ where Self.RawSignificand : FixedWidthInteger {
 %     end
     }
     let unitRandom = Self.init(rand) * Self.ulpOfOne / 2
-    return delta * unitRandom + range.lowerBound
+    let randFloat = delta * unitRandom + range.lowerBound
+%   if 'Closed' not in Range:
+    if randFloat == range.upperBound {
+      return Self.random(in: range, using: &generator)
+    }
+%   end
+    return randFloat
   }
 
   /// Returns a random value within the specified range.

validation-test/stdlib/Random.swift

@@ -43,8 +43,7 @@ RandomTests.test("basic random numbers") {
 
 // Random integers in ranges
 
-func integerRangeTest<T: FixedWidthInteger>(_ type: T.Type) 
-  where T.Stride: SignedInteger, T.Magnitude: UnsignedInteger {
+func integerRangeTest<T: FixedWidthInteger>(_ type: T.Type) {
 
   func testRange(_ range: Range<T>, iterations: Int = 1_000) {
     var integerSet: Set<T> = []
@@ -104,10 +103,8 @@ RandomTests.test("random integers in ranges") {
 // Random floating points in ranges
 
 func floatingPointRangeTest<T: BinaryFloatingPoint>(_ type: T.Type) 
-  where T.RawSignificand: FixedWidthInteger,
-        T.RawSignificand.Stride: SignedInteger & FixedWidthInteger,
-        T.RawSignificand.Magnitude: UnsignedInteger {
-          
+  where T.RawSignificand: FixedWidthInteger {
+  
   let testRange = 0 ..< 1_000
 
   // open range
@@ -230,6 +227,56 @@ RandomTests.test("different random number generators") {
   expectEqual(shufflePasses[0], shufflePasses[1])
 }
 
+// Random floating points with max values (SR-8178)
+
+var lcrng = LCRNG(seed: 1234567890)
+
+public struct RotatingRNG: RandomNumberGenerator {
+  public let rotation: [() -> UInt64] = [
+    { return .min },
+    { return .max },
+    { return lcrng.next() }
+  ]
+  public var rotationIndex = 0
+  
+  public mutating func next() -> UInt64 {
+    if rotationIndex == rotation.count {
+      rotationIndex = 0
+    }
+    
+    defer {
+      rotationIndex += 1
+    }
+    
+    return rotation[rotationIndex]()
+  }
+}
+
+func floatingPointRangeMaxTest<T: BinaryFloatingPoint>(_ type: T.Type)
+  where T.RawSignificand: FixedWidthInteger {
+  
+  let testRange = 0 ..< 1_000
+  
+  var rng = RotatingRNG()
+  let ranges: [Range<T>] = [0 ..< 1, 1 ..< 2, 10 ..< 11, 0 ..< 10]
+  for range in ranges {
+    for _ in testRange {
+      let random = T.random(in: range, using: &rng)
+      expectTrue(range.contains(random))
+      expectTrue(range.lowerBound <= random)
+      expectTrue(random < range.upperBound)
+    }
+  }
+}
+
+RandomTests.test("random floating point range maxes") {
+  floatingPointRangeMaxTest(Float.self)
+  floatingPointRangeMaxTest(Double.self)
+#if !os(Windows) && (arch(i386) || arch(x86_64))
+  floatingPointRangeMaxTest(Float80.self)
+#endif
+}
+
 // Uniform Distribution
 
 func chi2Test(_ samples: [Double]) -> Bool {
@@ -248,7 +295,7 @@ func chi2Test(_ samples: [Double]) -> Bool {
 
   if chi2 > cvHigh {
     return false
-  }else {
+  } else {
     return true
   }
 }