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Adds a ResetMonoid which handles situations requiring resets #133

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41 algebird-core/src/main/scala/com/twitter/algebird/ResetAlgebra.scala
@@ -0,0 +1,41 @@
+/*
+Copyright 2012 Twitter, Inc.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+package com.twitter.algebird
+
+/** Used to represent cases where we need to periodically reset
+ * a + b = a + b
+ * |a + b = |(a + b)
+ * a + |b = |b
+ * |a + |b = |b
+ */
+sealed trait ResetState[+A] { def get: A }
+case class SetValue[+A](get: A) extends ResetState[A]
+case class ResetValue[+A](get: A) extends ResetState[A]
+
+object ResetState {
+ implicit def monoid[A:Monoid]: Monoid[ResetState[A]] = new ResetStateMonoid[A]
+}
+
+class ResetStateMonoid[A](implicit monoid: Monoid[A]) extends Monoid[ResetState[A]] {
+ def zero = SetValue(monoid.zero)
+ def plus(left: ResetState[A], right: ResetState[A]) = {
+ (left, right) match {
+ case (SetValue(l), SetValue(r)) => SetValue(monoid.plus(l, r))
+ case (ResetValue(l), SetValue(r)) => ResetValue(monoid.plus(l, r))
+ case (_, ResetValue(r)) => right
+ }
+ }
+}
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50 algebird-test/src/test/scala/com/twitter/algebird/ResetTest.scala
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+/*
+Copyright 2012 Twitter, Inc.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package com.twitter.algebird
+
+import org.scalacheck.Arbitrary
+import org.scalacheck.Arbitrary.arbitrary
+import org.scalacheck.Prop.forAll
+import org.scalacheck.Properties
+import org.scalacheck.Gen.choose
+
+object ResetTest extends Properties("ResetAlgebra") {
+ import BaseProperties._
+
+ implicit def resetArb[T:Arbitrary]: Arbitrary[ResetState[T]] = Arbitrary {
+ Arbitrary.arbitrary[T].map { t =>
+ if (scala.math.random < 0.1) {
+ ResetValue(t)
+ }
+ else {
+ SetValue(t)
+ }
+ }
+ }
+
+ property("ResetState[Int] forms a Monoid") = monoidLaws[ResetState[Int]]
+ property("ResetState[String] forms a Monoid") = monoidLaws[ResetState[String]]
+ property("ResetState[Int] works as expected") = forAll { (a: ResetState[Int], b: ResetState[Int], c: ResetState[Int]) =>
+ val result = Monoid.plus(Monoid.plus(a,b), c)
+ ((a,b,c) match {
+ case (SetValue(x), SetValue(y), SetValue(z)) => SetValue(x+y+z)
+ case (ResetValue(x), SetValue(y), SetValue(z)) => ResetValue(x+y+z)
+ case (_, ResetValue(y), SetValue(z)) => ResetValue(y+z)
+ case (_, _, ResetValue(z)) => ResetValue(z)
+ }) == result
+ }
+}
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