Implement Summable, Invertible, Comparable for Duration

source/ceylon/time/Duration.ceylon

@@ -10,10 +10,20 @@
 import ceylon.time.base { ReadableDuration }
 
 "Duration specifies a discreet amount of milliseconds between two instances of time."
-shared serializable class Duration(milliseconds) satisfies ReadableDuration & Scalable<Integer, Duration> {
+shared serializable class Duration satisfies ReadableDuration
+        & Comparable<Duration>
+        & Summable<Duration>
+        & Scalable<Integer, Duration>
+        & Invertible<Duration> {
+
+    shared static Duration zero = Duration(0);
 
     "Number of milliseconds of this duration."
     shared actual Integer milliseconds;
+
+    shared new(Integer milliseconds) {
+        this.milliseconds = milliseconds;
+    }
 
     "Returns this duration as a period of milliseconds."
     shared Period period => Period { milliseconds = milliseconds; }; 
@@ -35,6 +45,14 @@ shared serializable class Duration(milliseconds) satisfies ReadableDuration & Sc
         value result = 3;
         return prime * result + milliseconds.hash;
     }
+
+    shared actual Comparison compare(Duration other) => milliseconds.compare(other.milliseconds);
+
+    shared actual Duration plus(Duration other) => Duration(milliseconds.plus(other.milliseconds));
+
+    shared actual Duration negated => Duration(milliseconds.negated);
+
+    shared actual Duration minus(Duration other) => Duration(milliseconds.minus(other.milliseconds));
 
     "Returns a new [[Duration]] with it´s milliseconds scaled."
     shared actual Duration scale(Integer scale) => Duration( scale * milliseconds );

test-source/test/ceylon/time/testDuration.ceylon

@@ -13,4 +13,25 @@ import ceylon.time {
 ];
 parameters (`value scalableDurationTests`)
 shared test void testScalableDuration(Duration expectedDuration, Integer scale)
-        => assertEquals { expected = expectedDuration; actual = scale ** Duration(1000); };
+        => assertEquals { actual = scale ** Duration(1000); expected = expectedDuration; };
+
+shared test void testPlus()
+        => assertEquals { actual = Duration(-4567) + Duration(4567); expected= Duration.zero; };
+
+shared test void testInv()
+        => assertEquals { actual = -Duration(99); expected= Duration(-99); };
+
+shared test void testInvZero()
+        => assertEquals { actual = -Duration.zero; expected= Duration.zero; };
+
+shared test void testMinus()
+        => assertEquals { actual = Duration(4567) - Duration(4567); expected= Duration.zero; };
+
+shared test void testCompare()
+        => assertEquals { actual = Duration(345) <=> Duration(4567); expected= Comparison.smaller; };
+
+shared test void testCompare()
+        => assertEquals { actual = Duration(3450) <=> Duration(456); expected= Comparison.greater; };
+
+shared test void testCompare()
+        => assertEquals { actual = Duration(345) <=> Duration(345); expected= Comparison.equal; };