Conform TimeAmount to AdditiveArithmetic (#1691)

Conforms TimeAmount to AdditiveArithmetic.

Motivation:
TimeAmount does not support -=, +=. Sometimes it is useful to manipulate time amounts when building up a delay and if that iteration fails, we would want to delay += .milliseconds(5) to add 5 milliseconds to our delay and try again.

Modifications:
Conformed TimeAmount to AdditiveArithmetic: added a static zero property and required operators.

Result:
TimeAmount conforms to AdditiveArithmetic.

Co-authored-by: Josh <jrtkski@icloud.com>
Co-authored-by: Cory Benfield <lukasa@apple.com>

Sources/NIO/EventLoop.swift

@@ -349,14 +349,27 @@ extension TimeAmount: Comparable {
     }
 }
 
-extension TimeAmount {
+extension TimeAmount: AdditiveArithmetic {
+    /// The zero value for `TimeAmount`.
+    public static var zero: TimeAmount {
+        return TimeAmount.nanoseconds(0)
+    }
+
     public static func + (lhs: TimeAmount, rhs: TimeAmount) -> TimeAmount {
         return TimeAmount(lhs.nanoseconds + rhs.nanoseconds)
     }
+    
+    public static func +=(lhs: inout TimeAmount, rhs: TimeAmount) {
+        lhs = lhs + rhs
+    }
 
     public static func - (lhs: TimeAmount, rhs: TimeAmount) -> TimeAmount {
         return TimeAmount(lhs.nanoseconds - rhs.nanoseconds)
     }
+    
+    public static func -=(lhs: inout TimeAmount, rhs: TimeAmount) {
+        lhs = lhs - rhs
+    }
 
     public static func * <T: BinaryInteger>(lhs: T, rhs: TimeAmount) -> TimeAmount {
         return TimeAmount(Int64(lhs) * rhs.nanoseconds)

Tests/NIOTests/TimeAmountTests+XCTest.swift

@@ -29,6 +29,8 @@ extension TimeAmountTests {
       return [
                 ("testTimeAmountConversion", testTimeAmountConversion),
                 ("testTimeAmountIsHashable", testTimeAmountIsHashable),
+                ("testTimeAmountDoesAddTime", testTimeAmountDoesAddTime),
+                ("testTimeAmountDoesSubtractTime", testTimeAmountDoesSubtractTime),
            ]
    }
 }

Tests/NIOTests/TimeAmountTests.swift

@@ -22,10 +22,25 @@ class TimeAmountTests: XCTestCase {
         XCTAssertEqual(TimeAmount.seconds(9), .nanoseconds(9_000_000_000))
         XCTAssertEqual(TimeAmount.minutes(2), .nanoseconds(120_000_000_000))
         XCTAssertEqual(TimeAmount.hours(6), .nanoseconds(21_600_000_000_000))
+        XCTAssertEqual(TimeAmount.zero, .nanoseconds(0))
     }
 
     func testTimeAmountIsHashable() {
         let amounts: Set<TimeAmount> = [.seconds(1), .milliseconds(4), .seconds(1)]
         XCTAssertEqual(amounts, [.seconds(1), .milliseconds(4)])
     }
+    
+    func testTimeAmountDoesAddTime() {
+        var lhs = TimeAmount.nanoseconds(0)
+        let rhs = TimeAmount.nanoseconds(5)
+        lhs += rhs
+        XCTAssertEqual(lhs, .nanoseconds(5))
+    }
+
+    func testTimeAmountDoesSubtractTime() {
+        var lhs = TimeAmount.nanoseconds(5)
+        let rhs = TimeAmount.nanoseconds(5)
+        lhs -= rhs
+        XCTAssertEqual(lhs, .nanoseconds(0))
+    }
 }