feat: add gravatar hash calculation

OliverBooth/Data/Blog/Author.cs

@@ -1,10 +1,43 @@
-namespace OliverBooth.Data.Blog;
+using System.ComponentModel.DataAnnotations.Schema;
+using System.Security.Cryptography;
+using System.Text;
+
+namespace OliverBooth.Data.Blog;
 
 /// <summary>
 ///     Represents an author of a blog post.
 /// </summary>
 public sealed class Author : IEquatable<Author>
 {
+    [NotMapped]
+    public string AvatarHash
+    {
+        get
+        {
+            if (EmailAddress is null)
+            {
+                return string.Empty;
+            }
+
+            using var md5 = MD5.Create();
+            ReadOnlySpan<char> span = EmailAddress.AsSpan();
+            int byteCount = Encoding.UTF8.GetByteCount(span);
+            Span<byte> bytes = stackalloc byte[byteCount];
+            Encoding.UTF8.GetBytes(span, bytes);
+
+            Span<byte> hash = stackalloc byte[16];
+            md5.TryComputeHash(bytes, hash, out _);
+
+            var builder = new StringBuilder();
+            foreach (byte b in hash)
+            {
+                builder.Append(b.ToString("x2"));
+            }
+
+            return builder.ToString();
+        }
+    }
+
     /// <summary>
     ///     Gets or sets the email address of the author.
     /// </summary>