adds methods to add and subtract priorities without type coercion

Gemfile.lock

@@ -1,7 +1,7 @@
 PATH
   remote: .
   specs:
-    delayed (0.5.3)
+    delayed (0.5.4)
       activerecord (>= 5.2)
       concurrent-ruby
 

lib/delayed/priority.rb

@@ -117,11 +117,11 @@ def default_alerts
       def names_to_midpoint_priority
         names.each_cons(2).to_h { |(name, priority_value), (_, next_priority_value)|
           [name, new(midpoint(priority_value, next_priority_value))]
-        }.merge(names.keys.last => new(names.values.last.to_i + 5))
+        }.merge(names.keys.last => new(names.values.last + 5))
       end
 
       def midpoint(low, high)
-        low.to_i + ((high.to_i - low.to_i).to_d / 2).ceil
+        low + ((high - low).to_d / 2).ceil
       end
 
       def respond_to_missing?(method_name, include_private = false)
@@ -173,6 +173,14 @@ def <=>(other)
       to_i <=> other
     end
 
+    def -(other)
+      to_i - other.to_i
+    end
+
+    def +(other)
+      to_i + other.to_i
+    end
+
     private
 
     def respond_to_missing?(method_name, include_private = false)

spec/delayed/priority_spec.rb

@@ -226,4 +226,13 @@
       ].sort,
     ).to eq [-13, 3, 5, 40]
   end
+
+  it 'supports addition and subtraction' do
+    expect(described_class.new(0) + 10).to eq(10)
+    expect(10 + described_class.new(5)).to eq(15)
+    expect(described_class.new(0) + described_class.new(33)).to eq(33)
+    expect(described_class.new(10) - 5).to eq(5)
+    expect(15 - described_class.new(10)).to eq(5)
+    expect(described_class.new(5) - described_class.new(15)).to eq(-10)
+  end
 end