LINQPad runnable resolution to the Fix the Index interview problem (http://ayende.com/blog/170402/interview-question-fix-the-index)

gistfile1.cs

void Main()
{
    var tests = new IndexTests();
    
    tests.CanIndexAndQuery();
    tests.CanUpdate();
}

// Internal doc class
class Document
{
    public readonly string Id; // Let's pretend there is a constructor that initializes this value...
    public List<string> Contents; // Probably turn this into a property later...
       
    public Document(string id)
    {
        Id = id;
    }
    
    public override int GetHashCode()
    {
        return Id.GetHashCode();
    }
   
       public override bool Equals(object other)
    {
        if(ReferenceEquals(other, this)) return true;
        if(ReferenceEquals(other, null)) return false;
        if(other is Document) return Equals((Document)other);
        
        return false;
    }
    
    public bool Equals(Document other)
    {
        return other.Id == Id;
    }
}

// Interface for indexers
interface IIndexer
{
    void Index(string docId, string text);
    List<string> Query(string term);
    void Delete(string docId);
}

// Iterative-implemented hashset indexer, should be clearer than the Linq implementation
class IterativeHashIndexer : IIndexer
{
    private HashSet<Document> docs = new HashSet<Document>();
    
    public void Index(string docId, string text)
    {
        Document doc = new Document(docId);
        doc.Contents = new List<string>(text.Split());
        
        docs.Remove(doc);
        docs.Add(doc);
    }
    
    public List<string> Query(string term)
    {
        var ret = new List<string>();
        
        foreach(var doc in docs)
        {
            if(doc.Contents.Any(s => s.Equals(term, StringComparison.OrdinalIgnoreCase)))
            {
                ret.Add(doc.Id);
            }
        }
        
        return ret;
    }
    
    public void Delete(string docId)
    {
        docs.RemoveWhere(d => d.Id == docId);
    }
}

class LinqHashIndexer : IIndexer
{
    private HashSet<Document> docs = new HashSet<Document>();
    
    public void Index(string docId, string text)
    {
        Document doc = new Document(docId);
        doc.Contents = new List<string>(text.Split());
        
        docs.Remove(doc);
        docs.Add(doc);
    }
    
    public List<string> Query(string term)
    {
        // This gets a little convoluted, but you can always transform this into a loop
        return docs.Where
                    (
                          d => d.Contents
                        .Any
                        (
                            s => s.Equals(term, StringComparison.OrdinalIgnoreCase)
                        )
                    )
                    .Select(d => d.Id)
                    .ToList();
    }
    
    public void Delete(string docId)
    {
        docs.RemoveWhere(d => d.Id == docId);
    }
}

class DictionaryIndexer : IIndexer
{
    private Dictionary<string, Document> docs = new Dictionary<string, Document>();
    
    public void Index(string docId, string text)
    {
        Document doc = new Document(docId);
        doc.Contents = new List<string>(text.Split());
        
        docs[docId] = doc;
    }
    
    public List<string> Query(string term)
    {
        var ret = new List<string>();
        
        foreach(var kv in docs)
        {
            if(kv.Value.Contents.Any(s => s.Equals(term, StringComparison.OrdinalIgnoreCase)))
            {
                ret.Add(kv.Key);
            }
        }
        
        return ret;
    }
    
    public void Delete(string docId)
    {
        docs.Remove(docId);
    }
}

public class IndexTests
{
    IIndexer CreateIndexer()
    {
        // Change with any of the implementations above
        return new DictionaryIndexer();
    }
    
    [Fact]
    public void CanIndexAndQuery()
    {
        var index = CreateIndexer();
        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 = CreateIndexer();
        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 = CreateIndexer();
        index.Index("users/1", "Oren Eini");
        index.Index("users/2", "Hibernating Rhinos");
        
        index.Delete("users/1");
        
        Assert.NotContains("users/1", index.Query("eini"));
        Assert.Contains("users/2", index.Query("rhinos"));
    }
}

// Dummy implementation of the Assert class above, so the code can be executed on LinqPad
static class Assert
{
    public static void Contains<T>(T value, IEnumerable<T> values)
    {
        if(!values.Contains(value))
        {
            throw new Exception();
        }
    }
    
    public static void NotContains<T>(T value, IEnumerable<T> values)
    {
        if(values.Contains(value))
        {
            throw new Exception();
        }
    }
    
    public static void Empty<T>(IEnumerable<T> contents)
    {
        if(contents.Any())
        {
            throw new Exception();
        }
    }
}

// Implementing this attribute so the code can run by just removing this FactAttribute and dummy Assert implementation above and running on VisualStudio
class FactAttribute : Attribute { }