Add handler to create weighted adjustments

We'll be introducing a new calculator for promotions that will allow
order-level promotion values to be applied through line item
adjustments.

This class keeps some of the more complicated logic out of the
calculator as it has to reach up to the order and calculate values on
multiple line items.

core/app/models/spree/distributed_amounts_handler.rb

@@ -0,0 +1,43 @@
+module Spree
+  class DistributedAmountsHandler
+    attr_reader :line_item, :order, :total_amount
+
+    def initialize(line_item, total_amount)
+      @line_item = line_item
+      @order = line_item.order
+      @total_amount = total_amount
+    end
+
+    # @return [Float] the weighted adjustment for the initialized line item
+    def amount
+      distributed_amounts[@line_item.id].to_f
+    end
+
+    private
+
+    # @private
+    # @return [Hash<Integer, BigDecimal>] a hash of line item IDs and their
+    #   corresponding weighted adjustments
+    def distributed_amounts
+      remaining_amount = @total_amount
+
+      @order.line_items.each_with_index.map do |line_item, i|
+        if i == @order.line_items.length - 1
+          # If this is the last line item on the order we want to use the
+          # remaining preferred amount to ensure our total adjustment is what
+          # has been set as the preferred amount.
+          [line_item.id, remaining_amount]
+        else
+          # Calculate the weighted amount by getting this line item's share of
+          # the order's total and multiplying it with the preferred amount.
+          weighted_amount = ((line_item.amount / @order.item_total) * total_amount).round(2)
+
+          # Subtract this line item's weighted amount from the total.
+          remaining_amount -= weighted_amount
+
+          [line_item.id, weighted_amount]
+        end
+      end.to_h
+    end
+  end
+end

core/spec/models/spree/distributed_amounts_handler_spec.rb

@@ -0,0 +1,79 @@
+require "spec_helper"
+
+RSpec.describe Spree::DistributedAmountsHandler, type: :model do
+  let(:order) do
+    FactoryGirl.create(
+      :order_with_line_items,
+      line_items_attributes: line_items_attributes
+    )
+  end
+
+  describe "#amount" do
+    let(:total_amount) { 15 }
+
+    subject { described_class.new(line_item, total_amount).amount }
+
+    context "when there is only one line item" do
+      let(:line_items_attributes) { [{ price: 100 }] }
+      let(:line_item) { order.line_items.first }
+
+      it "applies the entire amount to the line item" do
+        expect(subject).to eq(15)
+      end
+    end
+
+    context "when there are multiple line items" do
+      let(:line_items_attributes) do
+        [{ price: 50 }, { price: 50 }, { price: 50 }]
+      end
+
+      context "and the line items are equally priced" do
+        it "evenly distributes the total amount" do
+          expect(
+            [
+              described_class.new(order.line_items[0], total_amount).amount,
+              described_class.new(order.line_items[1], total_amount).amount,
+              described_class.new(order.line_items[2], total_amount).amount
+            ]
+          ).to eq(
+            [5, 5, 5]
+          )
+        end
+
+        context "and the total amount cannot be equally distributed" do
+          let(:total_amount) { 10 }
+
+          it "applies the remainder of the total amount to the last item" do
+            expect(
+              [
+                described_class.new(order.line_items[0], total_amount).amount,
+                described_class.new(order.line_items[1], total_amount).amount,
+                described_class.new(order.line_items[2], total_amount).amount
+              ]
+            ).to eq(
+              [3.33, 3.33, 3.34]
+            )
+          end
+        end
+      end
+
+      context "and the line items are not equally priced" do
+        let(:line_items_attributes) do
+          [{ price: 150 }, { price: 50 }, { price: 100 }]
+        end
+
+        it "distributes the total amount relative to the item's price" do
+          expect(
+            [
+              described_class.new(order.line_items[0], total_amount).amount,
+              described_class.new(order.line_items[1], total_amount).amount,
+              described_class.new(order.line_items[2], total_amount).amount
+            ]
+          ).to eq(
+            [7.5, 2.5, 5]
+          )
+        end
+      end
+    end
+  end
+end