fix: address code review feedback

- Preserve description through UnitSystem unit reconstruction
- Add public description reader to Unit
- Memoize DynamicConversionTableBuilder#to_h
- Add clarifying comment for inverse_conversion_amount ||= ordering
- Add test for description preservation in UnitSystem

Author: ntn

lib/measured/dynamic_conversion_table_builder.rb

@@ -14,7 +14,7 @@ def initialize(units, cache: nil)
   end
 
   def to_h
-    generate_table
+    @table ||= generate_table
   end
 
   def update_cache

lib/measured/unit.rb

@@ -2,13 +2,15 @@
 class Measured::Unit
   include Comparable
 
-  attr_reader :name, :names, :aliases, :conversion_amount, :conversion_unit, :unit_system, :inverse_conversion_amount
+  attr_reader :name, :names, :aliases, :conversion_amount, :conversion_unit, :unit_system, :inverse_conversion_amount, :description
 
   def initialize(name, aliases: [], value: nil, unit_system: nil)
     @name = name.to_s.freeze
     @aliases = aliases.map(&:to_s).map(&:freeze).freeze
     @names = ([@name] + @aliases).sort!.freeze
     @conversion_amount, @conversion_unit = parse_value(value) if value
+    # For dynamic units, parse_dynamic_value already sets @inverse_conversion_amount
+    # to the backward proc. The ||= ensures we don't overwrite it with compute_inverse.
     @inverse_conversion_amount ||= compute_inverse(@conversion_amount)
     @conversion_string = build_conversion_string
     @unit_system = unit_system

lib/measured/unit_system.rb

@@ -10,7 +10,7 @@ def initialize(units, cache: nil)
       next unit unless conversion_unit
       if unit.dynamic?
         unit.with(value: [
-          { forward: unit.conversion_amount, backward: unit.inverse_conversion_amount },
+          { forward: unit.conversion_amount, backward: unit.inverse_conversion_amount, description: unit.description },
           conversion_unit.name
         ])
       else

test/unit_system_test.rb

@@ -197,6 +197,19 @@ class Measured::UnitSystemTest < ActiveSupport::TestCase
     assert_equal BigDecimal("100"), system.convert(BigDecimal("100"), from: c_unit, to: c_unit)
   end
 
+  test "dynamic system preserves description through unit reconstruction" do
+    c = Measured::Unit.new(:C)
+    k = Measured::Unit.new(:K, value: [
+      { forward: ->(k) { k - BigDecimal("273.15") }, backward: ->(c) { c + BigDecimal("273.15") }, description: "celsius + 273.15" },
+      "C"
+    ])
+    system = Measured::UnitSystem.new([c, k])
+    k_unit = system.unit_for!(:K)
+
+    assert_equal "celsius + 273.15", k_unit.description
+    assert_equal "K (celsius + 273.15)", k_unit.to_s
+  end
+
   test "dynamic system #cached? returns false" do
     c = Measured::Unit.new(:C)
     k = Measured::Unit.new(:K, value: [