using System;
using System.Security.Cryptography;
using System.Text;
using AutoMapper;

public class GravatarUrlResolver : IValueResolver
{
    const string GravatarUrlFormat =
        "http://www.gravatar.com/avatar/{0}";

    static readonly Uri DefaultUrl = 
        new Uri("http://www.gravatar.com/avatar/?d=identicon");

    public ResolutionResult Resolve(ResolutionResult source)
    {
        if (source == null)
            throw new ArgumentNullException("source");

        var url = GetGravatarUrl(source.Value as string);

        return new ResolutionResult(url);
    }

    public Uri GetGravatarUrl(string email)
    {
        if (String.IsNullOrEmpty(email))
            return DefaultUrl;

        string emailHash = GetHash(email);
        return new Uri(String.Format(GravatarUrlFormat, emailHash));
    }

    string GetHash(string value)
    {
        byte[] bytes = Encoding.ASCII.GetBytes(value.ToLower().Trim());
        byte[] hash;
        using (HashAlgorithm md5 = new MD5CryptoServiceProvider())
            hash = md5.ComputeHash(bytes);
        var hashAsHex = new StringBuilder();
        Array.ForEach(hash, b => hashAsHex.Append(b.ToString("X2")));
        return hashAsHex.ToString().ToLower();
    }
}