Naive implementation of http://ayende.com/blog/170402/interview-question-fix-the-index

Indexer.cs

public class Indexer
    {
        private Dictionary<string, List<string>> termToIds =
            new Dictionary<string, List<string>>(StringComparer.OrdinalIgnoreCase);

        private Dictionary<string, List<string>> idToTerms =  
            new Dictionary<string, List<string>>(StringComparer.OrdinalIgnoreCase);

        public void Index(string docId, string text)
        {
            //could just call Delete(docId) here for brevity, but would involve making another lookup
            List<string> docTerms;

            if (idToTerms.TryGetValue(docId, out docTerms))
            {
                foreach (var existingTerm in docTerms)
                {
                    termToIds[existingTerm].Remove(docId);
                }

                docTerms = new List<string>();
            }
            else
            {
                docTerms = new List<string>();
            }

            var words = text.Split();
            foreach (var term in words)
            {
                List<string> termDocs;

                if (termToIds.TryGetValue(term, out termDocs) == false)
                {
                    termDocs = new List<string>();
                    termToIds[term] = termDocs;
                }

                termDocs.Add(docId);
                docTerms.Add(term);
            }

            idToTerms[docId] = docTerms;
        }

        public List<string> Query(string term)
        {
            List<string> val;
            termToIds.TryGetValue(term, out val);
            return val ?? new List<string>();
        }

        public void Delete(string docId)
        {
            List<string> existingTermsForDocument;

            if (idToTerms.TryGetValue(docId, out existingTermsForDocument))
            {
                foreach (var existingTerm in existingTermsForDocument)
                {
                    termToIds[existingTerm].Remove(docId);
                }

                idToTerms.Remove(docId);
            }
        }
    }

    public class IndexTests
    {
        [Fact]
        public void CanIndexAndQuery()
        {
            var index = new Indexer();
            index.Index("users/1", "Oren Eini");
            index.Index("users/2", "Hibernating Rhinos");

            Assert.Contains("users/1", index.Query("eini"));
            Assert.Contains("users/2", index.Query("rhinos"));
        }

        [Fact]
        public void CanUpdate()
        {
            var index = new Indexer();
            index.Index("users/1", "Oren Eini");
            //updating
            index.Index("users/1", "Ayende Rahien");

            Assert.Contains("users/1", index.Query("Rahien"));
            Assert.Empty(index.Query("eini"));
        }

        [Fact]
        public void CanDelete()
        {
            var index = new Indexer();
            index.Index("users/1", "Oren Eini");

            index.Delete("users/1");
            Assert.Empty(index.Query("Oren"));
        }

        [Fact]
        public void UpdatingShouldNotAffectDocumentsThatShareTerms()
        {
            var index = new Indexer();
            index.Index("docs/1", "Apples are green");
            index.Index("docs/2", "Trees are green");

            index.Index("docs/1", "My car is grey");

            Assert.Empty(index.Query("apples"));
            Assert.False(index.Query("green").Contains("docs/1"));
            Assert.True(index.Query("green").Contains("docs/2"));
        }

        [Fact]
        public void DeletingShouldNotRemoveDocumentsThatShareTerms()
        {
            var index = new Indexer();
            index.Index("docs/1", "Apples are green");
            index.Index("docs/2", "Trees are green");

            index.Delete("docs/1");

            Assert.Empty(index.Query("apples"));
            Assert.False(index.Query("green").Contains("docs/1"));
            Assert.True(index.Query("green").Contains("docs/2"));
        }
        
        
    }