From 716d79739ca9d5f3155e5ac560537a3ceed7fb68 Mon Sep 17 00:00:00 2001
From: Stevan Freeborn <stevan.freeborn@gmail.com>
Date: Fri, 28 Aug 2026 09:14:52 -0500
Subject: [PATCH] feat: introduce models to represent atproto id

---
 .../AtprotoId.cs                              | 132 ++++++++++++++++++
 .../AtprotoIdTests.cs                         |  19 +++
 2 files changed, 151 insertions(+)
 create mode 100644 src/StevanFreeborn.Atproto.IdentityResolver/AtprotoId.cs
 create mode 100644 tests/StevanFreeborn.Atproto.IdentityResolver.Tests/AtprotoIdTests.cs

diff --git a/src/StevanFreeborn.Atproto.IdentityResolver/AtprotoId.cs b/src/StevanFreeborn.Atproto.IdentityResolver/AtprotoId.cs
new file mode 100644
index 0000000..a7dd472
--- /dev/null
+++ b/src/StevanFreeborn.Atproto.IdentityResolver/AtprotoId.cs
@@ -0,0 +1,132 @@
+namespace StevanFreeborn.Atproto.IdentityResolver;
+
+/// <summary>
+///
+/// </summary>
+public abstract class AtprotoId
+{
+  /// <summary>
+  ///
+  /// </summary>
+  public string Value { get; protected set; } = string.Empty;
+
+  /// <summary>
+  ///
+  /// </summary>
+  public static AtprotoId From(string atprotoId)
+  {
+    if (string.IsNullOrEmpty(atprotoId))
+    {
+      throw new ArgumentException("Cannot be null or empty", nameof(atprotoId));
+    }
+
+    if (IsDomain(atprotoId))
+    {
+      return Handle.From(atprotoId);
+    }
+
+    return Did.From(atprotoId);
+  }
+
+  private static bool IsDomain(string did)
+  {
+    var hostType = Uri.CheckHostName(did);
+    return hostType == UriHostNameType.Dns && did.Contains('.', StringComparison.OrdinalIgnoreCase);
+  }
+
+}
+
+/// <summary>
+///
+/// </summary>
+public class Did : AtprotoId
+{
+  private static readonly string[] ValidPrefixes = ["did:web:", "did:plc:"];
+
+  /// <summary>
+  ///
+  /// </summary>
+  public string Method { get; private set; }
+
+  private Did(string did)
+  {
+    if (string.IsNullOrEmpty(did))
+    {
+      throw new ArgumentException("Cannot be null or empty", nameof(did));
+    }
+
+    if (TryParseWithoutPrefix(did, out var prefix, out var didWithoutPrefix) is false)
+    {
+      throw new ArgumentException($"Must be did with one of these methods: {string.Join(',', ValidPrefixes)}", nameof(did));
+    }
+
+    if (string.IsNullOrEmpty(didWithoutPrefix))
+    {
+      throw new ArgumentException("Must not be null or empty without did method prefix", nameof(did));
+    }
+
+    // Method should probably be a strongly
+    // typed object
+    Method = prefix.Contains("web", StringComparison.OrdinalIgnoreCase) ? "web" : "plc";
+    Value = did;
+  }
+
+  /// <summary>
+  ///
+  /// </summary>
+  public static new Did From(string did)
+  {
+    return new(did);
+  }
+
+  private static bool TryParseWithoutPrefix(
+    string did,
+    out string prefix,
+    out string didWithoutPrefix
+  )
+  {
+    prefix = string.Empty;
+    didWithoutPrefix = string.Empty;
+
+    foreach (var validPrefix in ValidPrefixes)
+    {
+      if (did.StartsWith(validPrefix, StringComparison.OrdinalIgnoreCase))
+      {
+        prefix = did[..validPrefix.Length];
+        didWithoutPrefix = did[validPrefix.Length..];
+        return true;
+      }
+    }
+
+    return false;
+  }
+}
+
+/// <summary>
+///
+/// </summary>
+public class Handle : AtprotoId
+{
+  private Handle(string handle)
+  {
+    if (string.IsNullOrEmpty(handle))
+    {
+      throw new ArgumentException("Cannot be null or empty", nameof(handle));
+    }
+
+    if (handle.StartsWith('@'))
+    {
+      throw new ArgumentException("Cannot start with @", nameof(handle));
+    }
+
+    Value = handle;
+  }
+
+  /// <summary>
+  ///
+  /// </summary>
+  public static new Handle From(string handle)
+  {
+    return new(handle);
+  }
+}
\ No newline at end of file
diff --git a/tests/StevanFreeborn.Atproto.IdentityResolver.Tests/AtprotoIdTests.cs b/tests/StevanFreeborn.Atproto.IdentityResolver.Tests/AtprotoIdTests.cs
new file mode 100644
index 0000000..3cf3cf6
--- /dev/null
+++ b/tests/StevanFreeborn.Atproto.IdentityResolver.Tests/AtprotoIdTests.cs
@@ -0,0 +1,19 @@
+namespace StevanFreeborn.Atproto.IdentityResolver.Tests;
+
+public class AtprotoIdTests
+{
+  [Theory]
+  [InlineData("")]
+  [InlineData("localhost")]
+  [InlineData("did:web:")]
+  [InlineData("did:plc:")]
+  [InlineData("did:mu:dxbmien5xsrkov5yhywgc4zu")]
+  [InlineData("@stevanfreeborn.com")]
+  [InlineData(null)]
+  public void From_WhenCalledWithInvalidDid_ItShouldThrowInvalidArgumentException(string? atprotoId)
+  {
+    var act = () => AtprotoId.From(atprotoId!);
+
+    act.Should().Throw<ArgumentException>();
+  }
+}
\ No newline at end of file