Indexer

public struct DocIdLookup
{
    public List<string> DocIds;
    public int Index;
}

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

    private readonly Dictionary<string, List<DocIdLookup>> _docIds =
        new Dictionary<string, List<DocIdLookup>>();

    public void Index(string docId, string text)
    {
        List<DocIdLookup> indexes;
        if (_docIds.TryGetValue(docId, out indexes))
        {
            foreach (var index in indexes)
                index.DocIds.RemoveAt(index.Index);
        }
        else
        {
            indexes = new List<DocIdLookup>();
        }

        var words = text.Split();
        foreach (var term in words)
        {
            List<string> val;
            if (_terms.TryGetValue(term, out val) == false)
            {
                val = new List<string>();
                _terms[term] = val;
            }
            val.Add(docId);

            indexes.Add(new DocIdLookup
            {
                DocIds = val,
                Index = val.Count - 1
            });
        }

        _docIds[docId] = indexes;
    }

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

    public void Delete(string docId)
    {
        List<DocIdLookup> indexes;
        if (!_docIds.TryGetValue(docId, out indexes)) 
            return;

        foreach (var index in indexes)
            index.DocIds.RemoveAt(index.Index);

        _docIds.Remove(docId);
    }
}

[TestMethod]
public void CanIndexAndQuery()
{
    var index = new Indexer();
    index.Index("users/1", "Oren Eini");
    index.Index("users/2", "Hibernating Rhinos");
    index.Index("users/3", "Oren is Anyende");

    Assert.IsTrue(index.Query("eini").Contains("users/1"));
    Assert.IsTrue(index.Query("rhinos").Contains("users/2"));
    Assert.IsTrue(index.Query("Oren").Contains("users/1"));
    Assert.IsTrue(index.Query("Oren").Contains("users/3"));
}

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

    Assert.IsTrue(index.Query("Rahien").Contains("users/1"));
    Assert.IsTrue(index.Query("eini").Count == 0);
}

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

    var query = index.Query("Oren");
    Assert.IsTrue(query.Contains("users/1"));

    // deleting
    index.Delete("users/1");
    Assert.IsTrue(index.Query("Oren").Count == 0);
}