Add new Auth module, method to construct host app URL

Use Base64.strict_encode64 (to avoid adding newlines), but
Base64.decode64 (using strict_decode64 breaks with what's provided by
Shopify)

CHANGELOG.md

@@ -4,7 +4,7 @@ Note: For changes to the API, see https://shopify.dev/changelog?filter=api
 
 ## Unreleased
 
-N/A
+- [#1002](https://github.com/Shopify/shopify-api-ruby/pull/1002) Add new method to construct the host app URL
 
 ## Version 11.0.1
 

lib/shopify_api/auth.rb

@@ -0,0 +1,26 @@
+# typed: strict
+# frozen_string_literal: true
+
+module ShopifyAPI
+  module Auth
+    extend T::Sig
+
+    class << self
+      extend T::Sig
+
+      sig { params(host: T.nilable(String)).returns(String) }
+      def embedded_app_url(host)
+        unless Context.setup?
+          raise Errors::ContextNotSetupError, "ShopifyAPI::Context not setup, please call ShopifyAPI::Context.setup"
+        end
+
+        unless host
+          raise Errors::MissingRequiredArgumentError, "host argument is required"
+        end
+
+        decoded_host = Base64.decode64(host)
+        "https://#{decoded_host}/apps/#{Context.api_key}"
+      end
+    end
+  end
+end

lib/shopify_api/errors/missing_required_argument_error.rb

@@ -0,0 +1,9 @@
+# typed: strict
+# frozen_string_literal: true
+
+module ShopifyAPI
+  module Errors
+    class MissingRequiredArgumentError < StandardError
+    end
+  end
+end

test/auth_test.rb

@@ -0,0 +1,35 @@
+# typed: false
+# frozen_string_literal: true
+
+require_relative "./test_helper"
+
+module ShopifyAPITest
+  class AuthTest < Test::Unit::TestCase
+    def setup
+      super
+      @host = "some-shopify-store.myshopify.com/admin"
+      @encoded_host = Base64.strict_encode64(@host)
+    end
+
+    def test_valid_host
+      assert_equal(
+        ShopifyAPI::Auth.embedded_app_url(@encoded_host),
+        "https://#{@host}/apps/#{ShopifyAPI::Context.api_key}"
+      )
+    end
+
+    def test_no_host
+      assert_raises(ShopifyAPI::Errors::MissingRequiredArgumentError) do
+        ShopifyAPI::Auth.embedded_app_url(nil)
+      end
+    end
+
+    def test_context_not_setup
+      modify_context(api_key: "", api_secret_key: "", host_name: "")
+
+      assert_raises(ShopifyAPI::Errors::ContextNotSetupError) do
+        ShopifyAPI::Auth.embedded_app_url(@encoded_host)
+      end
+    end
+  end
+end