Update and simplify GCD and associated tests.

Sources/IntegerUtilities/CMakeLists.txt

@@ -9,7 +9,7 @@ See https://swift.org/LICENSE.txt for license information
 
 add_library(IntegerUtilities
   DivideWithRounding.swift
-  GCD.swift
+  GreatestCommonDivisor.swift
   Rotate.swift
   RoundingRule.swift
   SaturatingArithmetic.swift

Sources/IntegerUtilities/GreatestCommonDivisor.swift

@@ -0,0 +1,35 @@
+//===--- GreatestCommonDivisor.swift --------------------------*- swift -*-===//
+//
+// This source file is part of the Swift Numerics open source project
+//
+// Copyright (c) 2021-2025 Apple Inc. and the Swift Numerics project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+/// The [greatest common divisor][gcd] of `a` and `b`.
+///
+/// If both inputs are zero, the result is zero. If one input is zero, the
+/// result is the absolute value of the other input.
+///
+/// [gcd]: https://en.wikipedia.org/wiki/Greatest_common_divisor
+@inlinable
+public func gcd<T: BinaryInteger>(_ a: T, _ b: T) -> T.Magnitude {
+    var x = a
+    var y = b
+
+    if x.magnitude < y.magnitude {
+        swap(&x, &y)
+    }
+
+    // Euclidean algorithm for GCD. It's worth using Lehmer instead for larger
+    // integer types, but for now this is good and dead-simple and faster than
+    // the other obvious choice, the binary algorithm.
+    while y != 0 {
+        (x, y) = (y, x % y)
+    }
+
+    return x.magnitude
+}

Tests/IntegerUtilitiesTests/CMakeLists.txt

@@ -10,7 +10,7 @@ See https://swift.org/LICENSE.txt for license information
 add_library(IntegerUtilitiesTests
   DivideTests.swift
   DoubleWidthTests.swift
-  GCDTests.swift
+  GreatestCommonDivisorTests.swift
   RotateTests.swift
   SaturatingArithmeticTests.swift
   ShiftTests.swift)

Tests/IntegerUtilitiesTests/GreatestCommonDivisorTests.swift

@@ -0,0 +1,42 @@
+//===--- GreatestCommonDivisorTests.swift ---------------------*- swift -*-===//
+//
+// This source file is part of the Swift Numerics open source project
+//
+// Copyright (c) 2021-2025 Apple Inc. and the Swift Numerics project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import IntegerUtilities
+import Testing
+
+struct `Greatest Common Divisor Tests` {
+    @Test func `gcd<BinaryInteger>`() async throws {
+        #expect(gcd(0, 0) == 0)
+        #expect(gcd(0, 1) == 1)
+        #expect(gcd(1, 0) == 1)
+        #expect(gcd(0, -1) == 1)
+        #expect(gcd(-1, 0) == 1)
+        #expect(gcd(1, 1) == 1)
+        #expect(gcd(1, 2) == 1)
+        #expect(gcd(2, 2) == 2)
+        #expect(gcd(4, 2) == 2)
+        #expect(gcd(6, 8) == 2)
+        #expect(gcd(77, 91) == 7)
+        #expect(gcd(24, -36) == 12)
+        #expect(gcd(-24, -36) == 12)
+        #expect(gcd(51, 34) == 17)
+        #expect(gcd(64, 96) == 32)
+        #expect(gcd(-64, 96) == 32)
+        #expect(gcd(4*7*19, 27*25) == 1)
+        #expect(gcd(16*315, 11*315) == 315)
+        #expect(gcd(97*67*53*27*8, 83*67*53*9*32) == 67*53*9*8)
+        #expect(gcd(Int.max, Int.max) == Int.max)
+        #expect(gcd(0, Int.min) == Int.min.magnitude)
+        #expect(gcd(Int.min, 0) == Int.min.magnitude)
+        #expect(gcd(Int.min, Int.min) == Int.min.magnitude)
+    }
+}