Add tax rate calculator class

This new calculator allows to retrieve tax rate information starting
from a `Spree::Address` model.

It shares some logic with the tax calculator, for example the ability
to handle exceptions using `SolidusTaxJar.exception_handler` lambda.

2 files changed, 37 insertions, 0 deletions

diff --git a/lib/super_good/solidus_taxjar.rb b/lib/super_good/solidus_taxjar.rb
index 35d9155..1b1a4ac 100644
--- a/lib/super_good/solidus_taxjar.rb
+++ b/lib/super_good/solidus_taxjar.rb
@@ -7,6 +7,7 @@ require "super_good/solidus_taxjar/api_params"
 require "super_good/solidus_taxjar/api"
 require "super_good/solidus_taxjar/calculator_helper"
 require "super_good/solidus_taxjar/tax_calculator"
+require "super_good/solidus_taxjar/tax_rate_calculator"
 require "super_good/solidus_taxjar/discount_calculator"
 
 module SuperGood
diff --git a/lib/super_good/solidus_taxjar/tax_rate_calculator.rb b/lib/super_good/solidus_taxjar/tax_rate_calculator.rb
new file mode 100644
index 0000000..dc3d771
--- /dev/null
+++ b/lib/super_good/solidus_taxjar/tax_rate_calculator.rb
@@ -0,0 +1,36 @@
+module SuperGood
+  module SolidusTaxJar
+    class TaxRateCalculator
+      include CalculatorHelper
+      def initialize(address, api: self.class.default_api)
+        @address = address
+        @api = api
+      end
+
+      def calculate
+        return no_rate if SuperGood::SolidusTaxJar.test_mode
+        return no_rate if incomplete_address?(address)
+        return no_rate unless taxable_address?(address)
+        cache do
+          api.tax_rate_for(address).to_d
+        end
+
+      rescue StandardError => e
+        exception_handler.(e)
+        no_rate
+      end
+
+      private
+
+      attr_reader :address, :api
+
+      def no_rate
+        BigDecimal(0)
+      end
+
+      def cache_key
+        SuperGood::SolidusTaxJar.cache_key.(address)
+      end
+    end
+  end
+end