change: Replace ArgumentError with custom errors

exercises/change/.meta/generator/change_case.rb

@@ -3,14 +3,23 @@
 class ChangeCase < Generator::ExerciseCase
   def workload
     if error_expected?
-      assert_raises(ArgumentError, subject_of_test)
+      handle_errors
     else
       assert_equal(expected, subject_of_test)
     end
   end
 
   private
 
+  def handle_errors
+    case test_name
+    when 'test_cannot_find_negative_change_values'
+      assert_raises('Change::NegativeTargetError', subject_of_test)
+    else
+      assert_raises('Change::ImpossibleCombinationError', subject_of_test)
+    end
+  end
+
   def subject_of_test
     "Change.generate(#{coins}, #{target})"
   end

exercises/change/.meta/solutions/change.rb

@@ -1,17 +1,21 @@
 class Change
   attr_reader :coins, :target
 
+  class NegativeTargetError < ArgumentError; end
+  class ImpossibleCombinationError < StandardError; end
+
   def initialize(coins, target)
     @coins  = coins.sort.reverse
     @target = target
     @total_change = []
   end
 
   def generate
+    raise NegativeTargetError if target.negative?
     return [] if target.zero?
 
     calculate_change(coins, [], target)
-    raise ArgumentError if total_change.none?
+    raise ImpossibleCombinationError if total_change.none?
 
     total_change.sort
   end

exercises/change/change_test.rb

@@ -45,21 +45,21 @@ def test_no_coins_make_0_change
 
   def test_error_testing_for_change_smaller_than_the_smallest_of_coins
     skip
-    assert_raises(ArgumentError) do
+    assert_raises(Change::ImpossibleCombinationError) do
       Change.generate([5, 10], 3)
     end
   end
 
   def test_error_if_no_combination_can_add_up_to_target
     skip
-    assert_raises(ArgumentError) do
+    assert_raises(Change::ImpossibleCombinationError) do
       Change.generate([5, 10], 94)
     end
   end
 
   def test_cannot_find_negative_change_values
     skip
-    assert_raises(ArgumentError) do
+    assert_raises(Change::NegativeTargetError) do
       Change.generate([1, 2, 5], -5)
     end
   end