Add Application::Urn

This service will retreive the associated urn for an application that
exists in the database.

When the availabe urn set is exhausted it will increase its size automatically.

app/models/application/urn.rb

@@ -0,0 +1,81 @@
+#
+# build a pseudo random urns list with the existing urns removed from that list
+# and we calculate the global index of the current application based on list sorted by created_at
+# then we get the urn located at the determined application index
+#
+
+class Application::Urn
+  class << self
+    def reset_urns
+      const_set(:URNS, build_urns)
+      nil
+    end
+
+    def build_urns
+      {
+        "teacher" => build_list("TE"),
+        "salaried_trainee" => build_list("ST"),
+      }.freeze
+    end
+
+    def build_list(code)
+      su_size = LENGTH.to_s.size
+      Array
+        .new(LENGTH) { [PREFIX, code, sprintf("%0##{su_size}d", _1)].join }
+        .drop(1)
+        .shuffle!
+    end
+  end
+
+  LENGTH = 99_999
+  PREFIX = "IRP".freeze
+  URNS = build_urns
+
+  def initialize(application)
+    @application = application
+  end
+
+  def urn
+    MUTEX.synchronize do
+      increase_suffix if urns_exhausted?
+      available_urns[application_index]
+    end
+  end
+
+private
+
+  def increase_suffix
+    self.class.const_set(:LENGTH, "#{self.class::LENGTH}9".to_i)
+    self.class.reset_urns
+  end
+
+  def urns_exhausted?
+    application_index > available_urns.size
+  end
+
+  def application_index
+    return @application_index if @application_index
+
+    @application_index = FindApplicationIndexQuery
+      .new(application_id: @application.id)
+      .execute
+      .to_a
+      .dig(0, "application_index")
+
+    raise(ArgumentError, "application not found") unless @application_index
+
+    @application_index -= 1 # FindApplicationIndexQuery returns a index starting at 1
+    @application_index -= used_urns.size # removes the existing applications to have a correct index
+    @application_index
+  end
+
+  def available_urns
+    @available_urns ||= URNS.fetch(@application.application_route) - used_urns
+  end
+
+  def used_urns
+    @used_urns ||= Application.where(application_route: @application.application_route).pluck(:urn)
+  end
+
+  MUTEX = Mutex.new
+end

spec/models/application/urn_spec.rb

@@ -0,0 +1,35 @@
+require "rails_helper"
+
+RSpec.describe Application::Urn do
+  describe "constants" do
+    it { expect(described_class::LENGTH).to eq(99_999) }
+    it { expect(described_class::PREFIX).to eq("IRP") }
+    it { expect(described_class::URNS.keys).to contain_exactly("teacher", "salaried_trainee") }
+    it { expect(described_class::URNS.dig("teacher", 1)).to include("IRPTE") }
+    it { expect(described_class::URNS.fetch("teacher").size).to eq(99_998) }
+    it { expect(described_class::URNS.dig("salaried_trainee", 1)).to include("IRPST") }
+    it { expect(described_class::URNS.fetch("salaried_trainee").size).to eq(99_998) }
+  end
+
+  describe "urn" do
+    subject(:service) { described_class.new(application) }
+
+    context "teacher" do
+      let(:application) { create(:teacher_application) }
+
+      it { expect(service.urn).to include("IRPTE") }
+    end
+
+    context "salaried_trainee" do
+      let(:application) { create(:salaried_trainee_application) }
+
+      it { expect(service.urn).to include("IRPST") }
+    end
+
+    context "missing application" do
+      let(:application) { build(:salaried_trainee_application) }
+
+      it { expect { service.urn }.to raise_error(ArgumentError, "application not found") }
+    end
+  end
+end