Introduce DiscountCalculator

This extracts the functionality of calculating the discount for a line
item in the interest of making this behaviour configurable for stores
that would like to factor in order level adjustments in some way.

2 files changed, 30 insertions, 0 deletions

diff --git a/lib/super_good/solidus_taxjar/discount_calculator.rb b/lib/super_good/solidus_taxjar/discount_calculator.rb
new file mode 100644
index 0000000..747a9d0
--- /dev/null
+++ b/lib/super_good/solidus_taxjar/discount_calculator.rb
@@ -0,0 +1,17 @@
+module SuperGood
+  module SolidusTaxJar
+    class DiscountCalculator
+      def initialize(line_item)
+        @line_item = line_item
+      end
+
+      def discount
+        -line_item.promo_total
+      end
+
+      private
+
+      attr_reader :line_item
+    end
+  end
+end
diff --git a/spec/super_good/solidus_taxjar/discount_calculator_spec.rb b/spec/super_good/solidus_taxjar/discount_calculator_spec.rb
new file mode 100644
index 0000000..b89b4dc
--- /dev/null
+++ b/spec/super_good/solidus_taxjar/discount_calculator_spec.rb
@@ -0,0 +1,13 @@
+require 'spec_helper'
+
+RSpec.describe SuperGood::SolidusTaxJar::DiscountCalculator do
+  describe "#discount" do
+    subject { calculator.discount }
+
+    let(:calculator) { described_class.new line_item }
+
+    let(:line_item) { ::Spree::LineItem.new(promo_total: 12.34) }
+
+    it { is_expected.to eq(-12.34) }
+  end
+end