Add YARD docs to TieredFlatRate promo calculator

promotions/app/models/solidus_promotions/calculators/tiered_flat_rate.rb

@@ -4,6 +4,37 @@
 
 module SolidusPromotions
   module Calculators
+    # A calculator that applies tiered flat-rate discounts based on discountable amount thresholds.
+    #
+    # This calculator allows defining multiple discount tiers where each tier specifies a minimum
+    # discountable amount threshold and the corresponding discount amount to apply. The calculator
+    # selects the highest tier that the item qualifies for based on its discountable amount.
+    #
+    # If the item doesn't meet any tier threshold, the base amount is used. The discount is only
+    # applied if the currency matches the preferred currency.
+    #
+    # @example Use case: Volume-based shipping discounts
+    #   # Free shipping on orders over $100, $5 off on orders over $50
+    #   calculator = TieredFlatRate.new(
+    #     preferred_base_amount: 0,
+    #     preferred_tiers: {
+    #       50 => 5,   # $5 discount when amount >= $50
+    #       100 => 15  # $15 discount when amount >= $100
+    #     },
+    #     preferred_currency: 'USD'
+    #   )
+    #
+    # @example Use case: Bulk purchase incentives
+    #   # Tiered discounts for line items based on total line value
+    #   calculator = TieredFlatRate.new(
+    #     preferred_base_amount: 2,
+    #     preferred_tiers: {
+    #       25 => 5,    # $5 off when line total >= $25
+    #       50 => 12,   # $12 off when line total >= $50
+    #       100 => 30   # $30 off when line total >= $100
+    #     },
+    #     preferred_currency: 'USD'
+    #   )
     class TieredFlatRate < Spree::Calculator
       include PromotionCalculator
 
@@ -22,6 +53,40 @@ class TieredFlatRate < Spree::Calculator
 
       validate :preferred_tiers_content
 
+      # Computes the tiered flat-rate discount for an item.
+      #
+      # Evaluates the item's discountable amount against all defined tiers and selects
+      # the highest tier threshold that the item meets or exceeds. Returns the discount
+      # amount associated with that tier, or the base amount if no tier threshold is met.
+      # Returns 0 if the currency doesn't match.
+      #
+      # @param object [Object] The object to calculate the discount for (e.g., LineItem, Shipment)
+      #
+      # @return [BigDecimal] The discount amount from the matching tier, base amount, or 0
+      #
+      # @example Computing discount with tier matching
+      #   calculator = TieredFlatRate.new(
+      #     preferred_base_amount: 2,
+      #     preferred_tiers: { 25 => 5, 50 => 10, 100 => 20 }
+      #   )
+      #   line_item.discountable_amount # => 75.00
+      #   calculator.compute_item(line_item) # => 10.00 (matches $50 tier)
+      #
+      # @example Computing discount below all tiers
+      #   calculator = TieredFlatRate.new(
+      #     preferred_base_amount: 2,
+      #     preferred_tiers: { 25 => 5, 50 => 10 }
+      #   )
+      #   line_item.discountable_amount # => 15.00
+      #   calculator.compute_item(line_item) # => 2.00 (base amount)
+      #
+      # @example Computing discount with currency mismatch
+      #   calculator = TieredFlatRate.new(
+      #     preferred_currency: 'USD',
+      #     preferred_tiers: { 50 => 10 }
+      #   )
+      #   line_item.currency # => 'EUR'
+      #   calculator.compute_item(line_item) # => 0
       def compute_item(object)
         _base, amount = preferred_tiers.sort.reverse.detect do |value, _|
           object.discountable_amount >= value
@@ -38,10 +103,17 @@ def compute_item(object)
 
       private
 
+      # Converts a value to a BigDecimal.
+      #
+      # @param value [String, Numeric] The value to convert
+      # @return [BigDecimal] The converted decimal value
       def cast_to_d(value)
         value.to_s.to_d
       end
 
+      # Validates that preferred_tiers is a hash with positive numeric keys.
+      #
+      # Ensures the tiers preference is properly formatted for tier-based calculations.
       def preferred_tiers_content
         if preferred_tiers.is_a? Hash
           unless preferred_tiers.keys.all? { |key| key.is_a?(Numeric) && key > 0 }