Add rule implementation

lib/planet_express_express.rb

@@ -3,6 +3,7 @@
 require_relative "planet_express_express/version"
 require_relative "planet_express_express/constants"
 require_relative "planet_express_express/shipment_reader"
+require_relative "planet_express_express/rules"
 
 module PlanetExpressExpress
   class Error < StandardError; end

lib/planet_express_express/rules.rb

@@ -0,0 +1,40 @@
+module PlanetExpressExpress
+  class Rules
+    def initialize(prices)
+      @lowest_s_price = prices.select { |k,v| k[1] == :S }.values.min
+    end
+
+    def apply_rules(shipments)
+      s_rule(shipments[:S])
+      l_rule(shipments[:L])
+
+      shipments
+    end
+
+    private
+
+    attr_reader :lowest_s_price
+
+    def l_rule(monthly_shipments)
+      monthly_shipments.each do |month|
+        lp_count = 0
+        month.each do |shipment|
+          next if shipment[:provider] != :LP
+
+          lp_count += 1
+          next if lp_count != 3
+
+          shipment[:discount] = shipment[:price]
+        end
+      end
+    end
+
+    def s_rule(monthly_shipments)
+      monthly_shipments.each do |month|
+        month.each do |shipment|
+          shipment[:discount] += shipment[:price] - lowest_s_price
+        end
+      end
+    end
+  end
+end