Adding a Qa::Authorities::Loc.type_for

Prior to this commit, we always assumed that each "find" would request
from `https://id.loc.gov/authorities`.

With this commit, we use the "subauthority" to inform what the base URL
is for the `https://id.loc.gov/` host.  In some cases this is
`authorities` and in others it's as the code indicates.

Hopefully works to close the following:

- notch8/utk-hyku#267

lib/qa/authorities/loc/generic_authority.rb

@@ -35,7 +35,8 @@ def find(id)
     end
 
     def find_url(id)
-      "https://id.loc.gov/authorities/#{@subauthority}/#{id}.json"
+      root_fetch_slug = Loc.root_fetch_slug_for(@subauthority)
+      File.join("https://id.loc.gov/", root_fetch_slug, "/#{@subauthority}/#{id}.json")
     end
 
     private

lib/qa/authorities/loc_subauthority.rb

@@ -1,4 +1,5 @@
 module Qa::Authorities::LocSubauthority
+  # @todo Rename to reflect that this is a URI encoded url fragement used only for searching.
   def get_url_for_authority(authority)
     if authorities.include?(authority) then authority_base_url
     elsif vocabularies.include?(authority) then vocab_base_url
@@ -7,6 +8,23 @@ def get_url_for_authority(authority)
     end
   end
 
+  # @note The returned value is the root directory of the URL.  The graphicMaterials sub-authority
+  #       has a "type" of vocabulary.  https://id.loc.gov/vocabulary/graphicMaterials/tgm008083.html
+  #       In some cases, this is plural and in others this is singular.
+  #
+  # @param authority [String] the LOC authority that matches one of the types
+  # @return [String]
+  #
+  # @note there is a relationship between the returned value and the encoded URLs returned by
+  #       {#get_url_for_authority}.
+  def root_fetch_slug_for(authority)
+    validate_subauthority!(authority)
+    return "authorities" if authorities.include?(authority)
+    return "vocabulary" if vocabularies.include?(authority)
+    return "datatype" if datatypes.include?(authority)
+    return "vocabulary/preservation" if preservation.include?(authority)
+  end
+
   def authorities
     [
       "subjects",

spec/lib/authorities/loc_spec.rb

@@ -42,6 +42,18 @@
     end
   end
 
+  describe ".root_fetch_slug_for" do
+    it "raises an error for an invalid subauthority" do
+      expect do
+        described_class.root_fetch_slug_for("no-one-would-ever-have-this-one")
+      end.to raise_error Qa::InvalidSubAuthority
+    end
+
+    it "returns the corresponding type for the given subauthority" do
+      expect(described_class.root_fetch_slug_for("graphicMaterials")).to eq("vocabulary")
+    end
+  end
+
   describe "#response" do
     subject { authority.response(url) }
     let :authority do