EuclideanDistance.cs

public class EuclideanDistance : SimilarityScore
{
  private readonly Reviewer CompareTo;
  private readonly Reviewer CompareWith;

  public EuclideanDistance(Reviewer compareTo, Reviewer compareWith)
  {
    CompareTo = compareTo;
    CompareWith = compareWith;
  }

  public double Score()
  {
    var similarTitles = FindSharedItems();
    if (similarTitles.Count == 0)
    {
      return 0.0;
    }

    double sumOfSquares = similarTitles.Sum(title => 
      Math.Pow(CompareTo.Reviews[title] - CompareWith.Reviews[title], 2));

    return Math.Round(1 / (1 + sumOfSquares), 3);
  }

  public List<string> FindSharedItems()
  {
    return (from r in CompareTo.Reviews where 
      CompareWith.Reviews.ContainsKey(r.Key) select r.Key).ToList();
  }
}