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.

3 files changed, 134 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
diff --git a/spec/super_good/solidus_taxjar/tax_rate_calculator_spec.rb b/spec/super_good/solidus_taxjar/tax_rate_calculator_spec.rb
new file mode 100644
index 0000000..9a4e2b8
--- /dev/null
+++ b/spec/super_good/solidus_taxjar/tax_rate_calculator_spec.rb
@@ -0,0 +1,97 @@
+require 'spec_helper'
+
+RSpec.describe ::SuperGood::SolidusTaxJar::TaxRateCalculator do
+  describe "#calculate" do
+    subject { calculator.calculate }
+
+    let(:calculator) { described_class.new(address, api: dummy_api) }
+
+    let(:dummy_api) do
+      instance_double ::SuperGood::SolidusTaxJar::API
+    end
+
+    let(:dummy_tax_rate) { BigDecimal(0) }
+
+    let(:incomplete_address) do
+      ::Spree::Address.new(
+        first_name: "Ronnie James",
+        zipcode: nil,
+        address1: nil,
+        city: "Beverly Hills",
+        state_name: "California",
+        country: ::Spree::Country.new(iso: "US")
+      )
+    end
+
+    let(:complete_address) do
+      incomplete_address.tap do |address|
+        address.zipcode = "90210"
+        address.address1 = "9900 Wilshire Blvd"
+      end
+    end
+
+    shared_examples "returns the dummy tax rate" do
+      it { expect(subject).to eq dummy_tax_rate }
+    end
+
+    context "when the address is not complete" do
+      let(:address) { incomplete_address }
+
+      it_behaves_like "returns the dummy tax rate"
+    end
+
+    context "when the address is complete" do
+      let(:address) { complete_address }
+
+      context "when the address is not taxable" do
+        before do
+          allow(SuperGood::SolidusTaxJar.taxable_address_check)
+            .to receive(:call).with(address)
+            .and_return(false)
+        end
+
+        it_behaves_like "returns the dummy tax rate"
+      end
+
+      context "when the address is taxable" do
+        let(:tax_rate) { 0.03 }
+
+        before do
+          allow(dummy_api).to receive(:tax_rate_for) { tax_rate }
+        end
+
+        it "returns the expected tax rate" do
+          expect(subject).to eq tax_rate
+        end
+      end
+    end
+
+    context "when the API encounters an error" do
+      let(:address) { complete_address }
+
+      before do
+        allow(dummy_api).to receive(:tax_rate_for).and_raise("A bad thing happened.")
+      end
+
+      it "calls the configured error handler" do
+        expect(SuperGood::SolidusTaxJar.exception_handler).to receive(:call) do |e|
+          expect(e).to be_a StandardError
+          expect(e.message).to eq "A bad thing happened."
+        end
+
+        subject
+      end
+
+      it_behaves_like "returns the dummy tax rate"
+    end
+
+    context "when test_mode is set" do
+      let(:address) { complete_address }
+
+      before { SuperGood::SolidusTaxJar.test_mode = true }
+      after { SuperGood::SolidusTaxJar.test_mode = false }
+
+      it_behaves_like "returns the dummy tax rate"
+    end
+  end
+end