feat: Implement DispatchTime using Swift Concurrency.

Author: scottmarchant

Sources/DispatchAsync/DispatchTime.swift

@@ -0,0 +1,70 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift 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
+//
+//===----------------------------------------------------------------------===//
+
+@available(macOS 13, *)
+public typealias DispatchTime = ContinuousClock.Instant
+
+/// The very first time someone tries to reference a `uptimeNanoseconds` or a similar
+/// function that references a beginning point, this variable will be initialized as a beginning
+/// reference point. This guarantees that all calls to `uptimeNanoseconds` or similar
+/// will be 0 or greater.
+///
+/// By design, it is not possible to related `ContinuousClock.Instant` to
+/// `ProcessInfo.processInfo.systemUptime`, and even if one devised such
+/// a mechanism, it would open the door for fingerprinting. It's best to let the concept
+/// of uptime be relative to previous uptime calls.
+@available(macOS 13, *)
+private let uptimeBeginning: DispatchTime = DispatchTime.now()
+
+@available(macOS 13, *)
+extension DispatchTime {
+    public static func now() -> DispatchTime {
+        now
+    }
+
+    public var uptimeNanoseconds: UInt64 {
+        let beginning = uptimeBeginning
+        let rightNow = DispatchTime.now()
+        let uptimeDuration: Int64 = beginning.duration(to: rightNow).nanosecondsClamped
+        guard uptimeDuration >= 0 else {
+            assertionFailure("It shouldn't be possible to get a negative duration since uptimeBeginning.")
+            return 0
+        }
+        return UInt64(uptimeDuration)
+    }
+}
+
+// NOTE: The following was copied from swift-nio/Source/NIOCore/TimeAmount+Duration on June 27, 2025
+// It was copied rather than brought via dependencies to avoid introducing
+// a dependency on swift-nio for such a small piece of code.
+//
+// This library will need to have no depedendencies to be able to be integrated into GCD.
+@available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+extension Swift.Duration {
+    /// The duration represented as nanoseconds, clamped to maximum expressible value.
+    fileprivate var nanosecondsClamped: Int64 {
+        let components = self.components
+
+        let secondsComponentNanos = components.seconds.multipliedReportingOverflow(by: 1_000_000_000)
+        let attosCompononentNanos = components.attoseconds / 1_000_000_000
+        let combinedNanos = secondsComponentNanos.partialValue.addingReportingOverflow(attosCompononentNanos)
+
+        guard
+            !secondsComponentNanos.overflow,
+            !combinedNanos.overflow
+        else {
+            return .max
+        }
+
+        return combinedNanos.partialValue
+    }
+}