Webfinger resource to extract username from resource string (#1607)

* Add WebfingerResource class to extract usernames

* Use WebfingerResource in xrd#webfinger

app/lib/webfinger_resource.rb

@@ -0,0 +1,66 @@
+# frozen_string_literal: true
+
+class WebfingerResource
+  attr_reader :resource
+
+  def initialize(resource)
+    @resource = resource
+  end
+
+  def username
+    case resource
+    when /\Ahttps?/i
+      username_from_url
+    when /\@/
+      username_from_acct
+    else
+      raise(ActiveRecord::RecordNotFound)
+    end
+  end
+
+  private
+
+  def username_from_url
+    if account_show_page?
+      path_params[:username]
+    else
+      raise ActiveRecord::RecordNotFound
+    end
+  end
+
+  def account_show_page?
+    path_params[:controller] == 'accounts' && path_params[:action] == 'show'
+  end
+
+  def path_params
+    Rails.application.routes.recognize_path(resource)
+  end
+
+  def username_from_acct
+    if domain_matches_local?
+      local_username
+    else
+      raise ActiveRecord::RecordNotFound
+    end
+  end
+
+  def split_acct
+    resource_without_acct_string.split('@')
+  end
+
+  def resource_without_acct_string
+    resource.gsub(/\Aacct:/, '')
+  end
+
+  def local_username
+    split_acct.first
+  end
+
+  def local_domain
+    split_acct.last
+  end
+
+  def domain_matches_local?
+    TagManager.instance.local_domain?(local_domain)
+  end
+end