Refactor ChoosePromotions service class

This class suffered of unnecessary variables, and can also live with the
service classes for the FriendlyPromotionAdjuster.

It also could use a spec.

app/models/solidus_friendly_promotions/friendly_promotion_adjuster/choose_discounts.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+module SolidusFriendlyPromotions
+  class FriendlyPromotionAdjuster
+    class ChooseDiscounts
+      attr_reader :discounts
+
+      def initialize(discounts)
+        @discounts = discounts
+      end
+
+      def call
+        Array.wrap(
+          discounts.min_by do |discount|
+            [discount.amount, -discount.source&.id.to_i]
+          end
+        )
+      end
+    end
+  end
+end

app/models/solidus_friendly_promotions/friendly_promotion_adjuster/discount_order.rb

@@ -34,23 +34,23 @@ def adjust_line_items(promotions)
           line_item.variant.product.promotionable?
         end.map do |line_item|
           discounts = generate_discounts(promotions, line_item)
-          chosen_item_discounts = SolidusFriendlyPromotions.config.discount_chooser_class.new(line_item).call(discounts)
+          chosen_item_discounts = SolidusFriendlyPromotions.config.discount_chooser_class.new(discounts).call
           [line_item, chosen_item_discounts]
         end
       end
 
       def adjust_shipments(promotions)
         order.shipments.map do |shipment|
           discounts = generate_discounts(promotions, shipment)
-          chosen_item_discounts = SolidusFriendlyPromotions.config.discount_chooser_class.new(shipment).call(discounts)
+          chosen_item_discounts = SolidusFriendlyPromotions.config.discount_chooser_class.new(discounts).call
           [shipment, chosen_item_discounts]
         end
       end
 
       def adjust_shipping_rates(promotions)
         order.shipments.flat_map(&:shipping_rates).select(&:cost).map do |rate|
           discounts = generate_discounts(promotions, rate)
-          chosen_item_discounts = SolidusFriendlyPromotions.config.discount_chooser_class.new(rate).call(discounts)
+          chosen_item_discounts = SolidusFriendlyPromotions.config.discount_chooser_class.new(discounts).call
           [rate, chosen_item_discounts]
         end
       end

lib/generators/solidus_friendly_promotions/install/templates/initializer.rb

@@ -58,7 +58,7 @@
 SolidusFriendlyPromotions.configure do |config|
   # This class chooses which promotion should apply to a line item in case
   # that more than one promotion is eligible.
-  config.discount_chooser_class = "SolidusFriendlyPromotions::DiscountChooser"
+  config.discount_chooser_class = "SolidusFriendlyPromotions::FriendlyPromotionAdjuster::ChooseDiscounts"
 
   # How many promotions should be displayed on the index page in the admin.
   config.promotions_per_page = 25

spec/lib/solidus_friendly_promotions/configuration_spec.rb

@@ -15,7 +15,7 @@
 
   describe ".promotion_chooser_class" do
     it "is the promotion chooser" do
-      expect(subject.discount_chooser_class).to eq(SolidusFriendlyPromotions::DiscountChooser)
+      expect(subject.discount_chooser_class).to eq(SolidusFriendlyPromotions::FriendlyPromotionAdjuster::ChooseDiscounts)
     end
   end
 

spec/models/solidus_friendly_promotions/friendly_promotion_adjuster/choose_discounts_spec.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+require "spec_helper"
+
+RSpec.describe SolidusFriendlyPromotions::FriendlyPromotionAdjuster::ChooseDiscounts do
+  subject { described_class.new(discounts).call }
+
+  let(:source_1) { create(:friendly_promotion, :with_adjustable_action).actions.first }
+  let(:source_2) { create(:friendly_promotion, :with_adjustable_action).actions.first }
+  let(:good_discount) { SolidusFriendlyPromotions::ItemDiscount.new(amount: -2, source: source_1) }
+  let(:bad_discount) { SolidusFriendlyPromotions::ItemDiscount.new(amount: -1, source: source_2) }
+
+  let(:discounts) do
+    [
+      good_discount,
+      bad_discount
+    ]
+  end
+
+  it { is_expected.to contain_exactly(good_discount) }
+end