Add DistributedAmount calculator for line item adjustments

This calculator allows us to use order level promotions ("Save $50 on
your order") and spread that $50 over all of the line items. This
calculator shoots logic off to another class to handle the calculations.

Each line item needs to call out to the DistributedAmountsHandler
separately which is annoying but each line item needs to be aware of the
order as a whole.

The calculator spreads the discount amount among each of the line items
relative to their price. This means that more expensive items will
receive a greater share of the discount.

backend/app/views/spree/admin/promotions/calculators/weighted_rate/_fields.html.erb

@@ -0,0 +1,54 @@
+<%= fields_for "#{prefix}[calculator_attributes]", calculator do |f| %>
+  <%= f.label :preferred_amount %>
+  <%= render "spree/admin/shared/number_with_currency", f: f, amount_attr: :preferred_amount, currency_attr: :preferred_currency %>
+<% end %>
+
+<div class="field">
+  <p>
+    <%= admin_hint(
+      calculator.model_name.human,
+      """
+        <p>
+          This amount will distributed to line items weighted relative to their
+          price. More expensive line items will receive a greater share of the
+          adjustment.
+        </p>
+
+        <p>
+          For example, with three line items and a preferred amount of $15 we
+          would end up with the following distribution:
+        </p>
+
+        <table>
+          <thead>
+            <tr>
+              <th></th>
+              <th>Price</th>
+              <th>Weighted Adj.</th>
+            </tr>
+          </thead>
+          <tbody>
+            <tr>
+              <td>Socks</td>
+              <td>$5</td>
+              <td>-$1.5</td>
+            </tr>
+            <tr>
+              <td>Shoes</td>
+              <td>$30</td>
+              <td>-$9</td>
+            </tr>
+            <tr>
+              <td>Slippers</td>
+              <td>$15</td>
+              <td>-$4.5</td>
+            </tr>
+          </tbody>
+        </table>
+      """
+    ) %>
+
+    This amount will be the <strong>total</strong> discount spread amongst all
+    of the line items.
+  </p>
+</div>

core/app/models/spree/calculator/distributed_amount.rb

@@ -0,0 +1,24 @@
+require_dependency 'spree/calculator'
+
+# This is a calculator for line item adjustment actions. It accepts a line item
+# and calculates its weighted adjustment amount based on the value of the
+# preferred amount and the price of the other line items. More expensive line
+# items will receive a greater share of the preferred amount.
+
+module Spree
+  class Calculator::DistributedAmount < Calculator
+    preference :amount, :decimal, default: 0
+    preference :currency, :string, default: -> { Spree::Config[:currency] }
+
+    def compute_line_item(line_item)
+      if line_item && preferred_currency.casecmp(line_item.currency).zero?
+        Spree::DistributedAmountsHandler.new(
+          line_item,
+          preferred_amount
+        ).amount
+      else
+        0
+      end
+    end
+  end
+end

core/config/locales/en.yml

@@ -401,6 +401,9 @@ en:
       spree/calculator/default_tax:
         one: Default Tax
         other: Default Tax
+      spree/calculator/distributed_amount:
+        one: Distributed Amount
+        other: Distributed Amount
       spree/calculator/flat_percent_item_total:
         one: Flat Percent
         other: Flat Percent

core/lib/spree/core/engine.rb

@@ -67,9 +67,10 @@ class Engine < ::Rails::Engine
         ]
 
         app.config.spree.calculators.promotion_actions_create_item_adjustments = %w[
-          Spree::Calculator::PercentOnLineItem
+          Spree::Calculator::DistributedAmount
           Spree::Calculator::FlatRate
           Spree::Calculator::FlexiRate
+          Spree::Calculator::PercentOnLineItem
           Spree::Calculator::TieredPercent
         ]
 

core/spec/models/spree/calculator/distributed_amount_spec.rb

@@ -0,0 +1,32 @@
+require 'spec_helper'
+require 'shared_examples/calculator_shared_examples'
+
+describe Spree::Calculator::DistributedAmount, type: :model do
+  describe "#compute_line_item" do
+    subject { calculator.compute_line_item(order.line_items.first) }
+
+    let(:calculator) { Spree::Calculator::DistributedAmount.new }
+
+    let(:order) do
+      FactoryGirl.create(
+        :order_with_line_items,
+        line_items_attributes: [{ price: 50 }, { price: 50 }, { price: 50 }]
+      )
+    end
+
+    before do
+      calculator.preferred_amount = 15
+      calculator.preferred_currency = currency
+    end
+
+    context "when the order currency matches the store's currency" do
+      let(:currency) { "USD" }
+      it { is_expected.to eq 5 }
+    end
+
+    context "when the order currency does not match the store's currency" do
+      let(:currency) { "CAD" }
+      it { is_expected.to eq 0 }
+    end
+  end
+end