Ensure the last element of reduction in the throttle is emitted and use appropriate delay (apple#292)

Author: phausler

Sources/AsyncAlgorithms/AsyncThrottleSequence.swift

@@ -81,7 +81,16 @@ extension AsyncThrottleSequence: AsyncSequence {
       let start = last ?? clock.now
       repeat {
         guard let element = try await base.next() else {
-          return nil
+          if reduced != nil, let last {
+            // ensure the rate of elements never exceeds the given interval
+            let amount = interval - last.duration(to: clock.now)
+            if amount > .zero {
+              try? await clock.sleep(for: amount)
+            }
+          }
+          // the last value is unable to have any subsequent
+          // values so always return the last reduction
+          return reduced
         }
         let reduction = await reducing(reduced, element)
         let now = clock.now

Tests/AsyncAlgorithmsTests/TestThrottle.swift

@@ -72,7 +72,7 @@ final class TestThrottle: XCTestCase {
     validate {
       "abcdefghijk|"
       $0.inputs[0].throttle(for: .steps(3), clock: $0.clock)
-      "a--d--g--j-|"
+      "a--d--g--j--[k|]"
     }
   }
 
@@ -81,7 +81,7 @@ final class TestThrottle: XCTestCase {
     validate {
       "abcdefghijk|"
       $0.inputs[0].throttle(for: .steps(3), clock: $0.clock, latest: false)
-      "a--b--e--h-|"
+      "a--b--e--h--[k|]"
     }
   }
 
@@ -138,4 +138,22 @@ final class TestThrottle: XCTestCase {
       "-a---c---e---g---i---k-|"
     }
   }
+  
+  func test_trailing_delay_without_latest() throws {
+    guard #available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *) else { throw XCTSkip("Skipped due to Clock/Instant/Duration availability") }
+    validate {
+       "abcdefghijkl|"
+       $0.inputs[0].throttle(for: .steps(3), clock: $0.clock, latest: false)
+       "a--b--e--h--[k|]"
+     }
+  }
+  
+  func test_trailing_delay_with_latest() throws {
+    guard #available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *) else { throw XCTSkip("Skipped due to Clock/Instant/Duration availability") }
+    validate {
+       "abcdefghijkl|"
+       $0.inputs[0].throttle(for: .steps(3), clock: $0.clock, latest: true)
+       "a--d--g--j--[l|]"
+     }
+  }
 }