Added functionality to find opposite reviews between two users.

Recommender.Tests/Common/FindOppositeReviewsTests.cs

@@ -0,0 +1,47 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using NUnit.Framework;
+using Recommender.Common;
+using Recommender.Data;
+
+namespace Recommender.Tests.Common
+{
+    [TestFixture]
+    class FindOppositeReviewsTests
+    {
+
+        [Test]
+        public void TwoDistinctReviewersShouldHaveOneOppositeReview()
+        {
+            var r1 = ReviewerBuilder.BuildOneReviewMax();
+            var r2 = ReviewerBuilder.BuildAReviewerThatReviewedSomethingUnique();
+            Assert.AreEqual(1, new FindOppositeReviews(r1.Reviews, r2.Reviews).Calculate().Count);
+        }
+
+        [Test]
+        public void ShouldReturnSecondUsersUniqueReview()
+        {
+            var r1 = ReviewerBuilder.BuildOneReviewMax();
+            var r2 = ReviewerBuilder.BuildAReviewerThatReviewedSomethingUnique();
+            Assert.AreEqual("Chess", new FindOppositeReviews(r1.Reviews, r2.Reviews).Calculate().First());
+        }
+
+        [Test]
+        public void ReviewersThatHaveTheExactSameTasteShouldHaveNoOppositeItems()
+        {
+            var r1 = ReviewerBuilder.BuildReviewer1();
+            var r2 = ReviewerBuilder.BuildReviewer1();
+            Assert.AreEqual(0, new FindOppositeReviews(r1.Reviews, r2.Reviews).Calculate().Count);
+        }
+
+        [Test]
+        public void ReviewsWithAScoreOfZeroShouldBeIgnored()
+        {
+            var r1 = ReviewerBuilder.BuildAReviewerThatReviewedSomethingUnique();
+            var invalidReviewer = ReviewerBuilder.BuildReviewerWithInvalidReviews();
+            Assert.AreEqual(0, new FindOppositeReviews(r1.Reviews, invalidReviewer.Reviews).Calculate().Count);
+        }
+    }
+}

Recommender.Tests/Common/FindSimilarReviewsTests.cs

@@ -35,16 +35,16 @@ public void ReviewersThatHaveTheSameTasteShouldHaveAllSimilarItems()
         public void ReviewsWithAScoreOfZeroShouldBeIgnored()
         {
             var r1 = ReviewerBuilder.BuildReviewer1();
-            var empty = ReviewerBuilder.BuildReviewerWithInvalidReviews();
-            Assert.AreEqual(0, new FindSimilarReviews(r1.Reviews, empty.Reviews).Calculate().Count);
+            var invalidReviewer = ReviewerBuilder.BuildReviewerWithInvalidReviews();
+            Assert.AreEqual(0, new FindSimilarReviews(r1.Reviews, invalidReviewer.Reviews).Calculate().Count);
         }
 
         [Test]
         public void ReviewsWithAScoreOfZeroShouldBeIgnoredSwitchUsers()
         {
-            var r1 = ReviewerBuilder.BuildReviewer1();
-            var empty = ReviewerBuilder.BuildReviewerWithInvalidReviews();
-            Assert.AreEqual(0, new FindSimilarReviews(empty.Reviews, r1.Reviews).Calculate().Count);
+            var r1 = ReviewerBuilder.BuildAReviewerThatReviewedSomethingUnique();
+            var invalidReviewer = ReviewerBuilder.BuildReviewerWithInvalidReviews();
+            Assert.AreEqual(0, new FindSimilarReviews(invalidReviewer.Reviews, r1.Reviews).Calculate().Count);
         }
     }
 }

Recommender/Common/FindOppositeReviews.cs

@@ -0,0 +1,39 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+namespace Recommender.Common
+{
+    public class FindOppositeReviews
+    {
+        private readonly Dictionary<string, double> FirstUserReviews;
+        private readonly Dictionary<string, double> SecondUserReviews;
+
+        public FindOppositeReviews(Dictionary<string, double> firstUserReviews, Dictionary<string, double> secondUserReviews)
+        {
+            FirstUserReviews = firstUserReviews;
+            SecondUserReviews = secondUserReviews;
+        }
+
+        public List<string> Calculate()
+        {
+            return (from r in SecondUserReviews where ShouldCountReviewAsOpposite(r) select r.Key).ToList();
+        }
+
+        private bool ShouldCountReviewAsOpposite(KeyValuePair<string, double> r)
+        {
+            return IsReviewUnique(r) && IsReviewValid(SecondUserReviews[r.Key]);
+        }
+
+        private static bool IsReviewValid(double value)
+        {
+            return value > 0.0;
+        }
+
+        private bool IsReviewUnique(KeyValuePair<string, double> r)
+        {
+            return !FirstUserReviews.ContainsKey(r.Key);
+        }
+    }
+}