New configurable tax calculator interface

core/app/models/spree/app_configuration.rb

@@ -309,6 +309,17 @@ def shipping_rate_taxer_class
       @shipping_rate_taxer_class ||= Spree::Tax::ShippingRateTaxer
     end
 
+    # Allows providing your own class for calculating taxes on a shipping rate.
+    #
+    # @!attribute [rw] shipping_rate_tax_calculator_class
+    # @return [Class] a class with the same public interfaces as
+    #   Spree::TaxCalculator::ShippingRate
+    # @api experimental
+    attr_writer :shipping_rate_tax_calculator_class
+    def shipping_rate_tax_calculator_class
+      @shipping_rate_tax_calculator_class ||= Spree::TaxCalculator::ShippingRate
+    end
+
     # Allows providing your own Mailer for shipped cartons.
     #
     # @!attribute [rw] carton_shipped_email_class
@@ -365,6 +376,17 @@ def tax_adjuster_class
       @tax_adjuster_class ||= Spree::Tax::OrderAdjuster
     end
 
+    # Allows providing your own class for calculating taxes on an order.
+    #
+    # @!attribute [rw] tax_calculator_class
+    # @return [Class] a class with the same public interfaces as
+    #   Spree::TaxCalculator::Default
+    # @api experimental
+    attr_writer :tax_calculator_class
+    def tax_calculator_class
+      @tax_calculator_class ||= Spree::TaxCalculator::Default
+    end
+
     def static_model_preferences
       @static_model_preferences ||= Spree::Preferences::StaticModelPreferences.new
     end

core/app/models/spree/order_taxation.rb

@@ -0,0 +1,79 @@
+module Spree
+  # Relatively simple class used to apply a {Spree::Tax::OrderTax} to a
+  # {Spree::Order}.
+  #
+  # This class will create or update adjustments on the taxed items and remove
+  # any now inapplicable tax adjustments from the order.
+  class OrderTaxation
+    # Create a new order taxation.
+    #
+    # @param [Spree::Order] order the order to apply taxes to
+    # @return [Spree::OrderTaxation] a {Spree::OrderTaxation} object
+    def initialize(order)
+      @order = order
+    end
+
+    # Apply taxes to the order.
+    #
+    # This method will create or update adjustments on all line items and
+    # shipments in the order to reflect the appropriate taxes passed in. It
+    # will also remove any now inapplicable tax adjustments.
+    #
+    # @param [Spree::Tax::OrderTax] taxes the taxes to apply to the order
+    # @return [void]
+    def apply(taxes)
+      @order.line_items.each do |item|
+        taxed_items = taxes.line_item_taxes.select { |i| i.item_id == item.id }
+        update_adjustments(item, taxed_items)
+      end
+
+      @order.shipments.each do |item|
+        taxed_items = taxes.shipment_taxes.select { |i| i.item_id == item.id }
+        update_adjustments(item, taxed_items)
+      end
+    end
+
+    private
+
+    # Walk through the taxes for an item and update adjustments for it. Once
+    # all of the taxes have been added as adjustments, remove any old tax
+    # adjustments that weren't touched.
+    #
+    # @private
+    # @param [#adjustments] item a {Spree::LineItem} or {Spree::Shipment}
+    # @param [Array<Spree::Tax::ItemTax>] taxed_items a list of calculated taxes for an item
+    # @return [void]
+    def update_adjustments(item, taxed_items)
+      tax_adjustments = item.adjustments.select(&:tax?)
+
+      active_adjustments = taxed_items.map do |tax_item|
+        update_adjustment(item, tax_item)
+      end
+
+      # Remove any tax adjustments tied to rates which no longer match.
+      unmatched_adjustments = tax_adjustments - active_adjustments
+      item.adjustments.destroy(unmatched_adjustments)
+    end
+
+    # Update or create a new tax adjustment on an item.
+    #
+    # @private
+    # @param [#adjustments] item a {Spree::LineItem} or {Spree::Shipment}
+    # @param [Spree::Tax::ItemTax] tax_item calculated taxes for an item
+    # @return [Spree::Adjustment] the created or updated tax adjustment
+    def update_adjustment(item, tax_item)
+      tax_adjustment = item.adjustments.detect do |adjustment|
+        adjustment.source == tax_item.tax_rate
+      end
+
+      tax_adjustment ||= item.adjustments.new(
+        source: tax_item.tax_rate,
+        order_id: item.order_id,
+        label: tax_item.label,
+        included: tax_item.included_in_price
+      )
+      tax_adjustment.update_attributes!(amount: tax_item.amount)
+      tax_adjustment
+    end
+  end
+end

core/app/models/spree/tax/item_tax.rb

@@ -0,0 +1,20 @@
+module Spree
+  module Tax
+    # Simple object used to hold tax data for an item.
+    #
+    # This generic object will hold the amount of tax that should be applied to
+    # an item. (Either a {Spree::LineItem} or a {Spree::Shipment}.)
+    #
+    # @attr_reader [Integer] item_id the {Spree::LineItem} or {Spree::Shipment} ID
+    # @attr_reader [String] label information about the taxes
+    # @attr_reader [Spree::TaxRate] tax_rate will be used as the source for tax
+    #   adjustments
+    # @attr_reader [BigDecimal] amount the amount of tax applied to the item
+    # @attr_reader [Boolean] included_in_price whether the amount is included
+    #   in the items price, or additional tax.
+    class ItemTax
+      include ActiveModel::Model
+      attr_accessor :item_id, :label, :tax_rate, :amount, :included_in_price
+    end
+  end
+end

core/app/models/spree/tax/order_adjuster.rb

@@ -4,8 +4,6 @@ module Tax
     class OrderAdjuster
       attr_reader :order
 
-      include TaxHelpers
-
       # @param [Spree::Order] order to be adjusted
       def initialize(order)
         @order = order
@@ -14,18 +12,8 @@ def initialize(order)
       # Creates tax adjustments for all taxable items (shipments and line items)
       # in the given order.
       def adjust!
-        (order.line_items + order.shipments).each do |item|
-          ItemAdjuster.new(item, order_wide_options).adjust!
-        end
-      end
-
-      private
-
-      def order_wide_options
-        {
-          rates_for_order: rates_for_order(order),
-          rates_for_default_zone: rates_for_default_zone
-        }
+        taxes = Spree::Config.tax_calculator_class.new(order).calculate
+        Spree::OrderTaxation.new(order).apply(taxes)
       end
     end
   end

core/app/models/spree/tax/order_tax.rb

@@ -0,0 +1,18 @@
+module Spree
+  module Tax
+    # Simple object to pass back tax data from a calculator.
+    #
+    # Will be used by {Spree::OrderTaxation} to create or update tax
+    # adjustments on an order.
+    #
+    # @attr_reader [Integer] order_id the {Spree::Order} these taxes apply to
+    # @attr_reader [Array<Spree::Tax::ItemTax>] line_item_taxes an array of
+    #   tax data for order's line items
+    # @attr_reader [Array<Spree::Tax::ItemTax>] shipment_taxes an array of
+    #   tax data for the order's shipments
+    class OrderTax
+      include ActiveModel::Model
+      attr_accessor :order_id, :line_item_taxes, :shipment_taxes
+    end
+  end
+end

core/app/models/spree/tax/shipping_rate_taxer.rb

@@ -2,30 +2,21 @@ module Spree
   module Tax
     # Used to build shipping rate taxes
     class ShippingRateTaxer
-      include TaxHelpers
-
       # Build shipping rate taxes for a shipping rate
       # Modifies the passed-in shipping rate with associated shipping rate taxes.
       # @param [Spree::ShippingRate] shipping_rate The shipping rate to add taxes to.
       #   This parameter will be modified.
       # @return [Spree::ShippingRate] The shipping rate with associated tax objects
       def tax(shipping_rate)
-        tax_rates_for_shipping_rate(shipping_rate).each do |tax_rate|
+        taxes = Spree::Config.shipping_rate_tax_calculator_class.new(shipping_rate).calculate
+        taxes.each do |tax|
           shipping_rate.taxes.build(
-            amount: tax_rate.compute_amount(shipping_rate),
-            tax_rate: tax_rate
+            amount: tax.amount,
+            tax_rate: tax.tax_rate
           )
         end
         shipping_rate
       end
-
-      private
-
-      def tax_rates_for_shipping_rate(shipping_rate)
-        applicable_rates(shipping_rate.order).select do |tax_rate|
-          tax_rate.tax_categories.include?(shipping_rate.tax_category)
-        end
-      end
     end
   end
 end

core/app/models/spree/tax_calculator/default.rb

@@ -0,0 +1,83 @@
+module Spree
+  module TaxCalculator
+    # Default implementation for tax calculations. Will go through all line
+    # items and shipments and calculate their tax based on tax rates in the DB.
+    #
+    # The class used for tax calculation is configurable, so that the
+    # calculation can easily be pushed to third-party services. Users looking
+    # to provide their own calculator should adhere to the API of this class.
+    #
+    # @api experimental
+    # @note This API is currently in development and likely to change.
+    #   Specifically, the input format is not yet finalized.
+    class Default
+      include Spree::Tax::TaxHelpers
+
+      # Create a new tax calculator.
+      #
+      # @param [Spree::Order] order the order to calculator taxes on
+      # @return [Spree::TaxCalculator::Default] a Spree::TaxCalculator::Default object
+      def initialize(order)
+        @order = order
+      end
+
+      # Calculate taxes for an order.
+      #
+      # @return [Spree::Tax::OrderTax] the calculated taxes for the order
+      def calculate
+        Spree::Tax::OrderTax.new(
+          order_id: order.id,
+          line_item_taxes: line_item_rates,
+          shipment_taxes: shipment_rates
+        )
+      end
+
+      private
+
+      attr_reader :order
+
+      # Calculate the taxes for line items.
+      #
+      # @private
+      # @return [Array<Spree::Tax::ItemTax>] calculated taxes for the line items
+      def line_item_rates
+        order.line_items.flat_map do |line_item|
+          calculate_rates(line_item)
+        end
+      end
+
+      # Calculate the taxes for shipments.
+      #
+      # @private
+      # @return [Array<Spree::Tax::ItemTax>] calculated taxes for the shipments
+      def shipment_rates
+        order.shipments.flat_map do |shipment|
+          calculate_rates(shipment)
+        end
+      end
+
+      # Calculate the taxes for a single item.
+      #
+      # The item could be either a {Spree::LineItem} or a {Spree::Shipment}.
+      #
+      # Will go through all applicable rates for an item and create a new
+      # {Spree::Tax::ItemTax} containing the calculated taxes for the item.
+      #
+      # @private
+      # @return [Array<Spree::Tax::ItemTax>] calculated taxes for the item
+      def calculate_rates(item)
+        rates_for_item(item).map do |rate|
+          amount = rate.compute_amount(item)
+
+          Spree::Tax::ItemTax.new(
+            item_id: item.id,
+            label: rate.adjustment_label(amount),
+            tax_rate: rate,
+            amount: amount,
+            included_in_price: rate.included_in_price
+          )
+        end
+      end
+    end
+  end
+end