fix notation for index for SlidingWindow and edge case for empty array

Author: abishanan-shopify

lib/semian.rb

@@ -310,6 +310,11 @@ def create_circuit_breaker(name, **options)
                                        else
                                          options[:error_threshold_timeout_enabled]
                                        end,
+      lumping_interval: if options[:lumping_interval].nil?
+                          0
+                        else
+                          options[:lumping_interval]
+                        end,
       exceptions: Array(exceptions) + [::Semian::BaseError],
       half_open_resource_timeout: options[:half_open_resource_timeout],
       implementation: implementation(**options),

lib/semian/circuit_breaker.rb

@@ -29,7 +29,7 @@ def initialize(name, exceptions:, success_threshold:, error_threshold:,
       @half_open_resource_timeout = half_open_resource_timeout
       @lumping_interval = lumping_interval
 
-      if @lumping_interval >= @error_threshold_timeout
+      if @lumping_interval >= @error_threshold_timeout && @lumping_interval > 0
         raise ArgumentError, "lumping_interval (#{@lumping_interval}) must be less than error_threshold_timeout (#{@error_threshold_timeout})"
       end
 
@@ -148,7 +148,7 @@ def push_time
         @errors.reject! { |err_time| err_time + @error_threshold_timeout < time }
       end
 
-      if @errors[-1] < time - @lumping_interval
+      if @lumping_interval == 0 || @errors.empty? || @errors.last < time - @lumping_interval
         @errors << time
       end
     end

test/circuit_breaker_test.rb

@@ -628,9 +628,9 @@ def test_lumping_interval_prevents_rapid_error_accumulation
       assert_circuit_closed(resource)
 
       time_travel(3) do
-        6.times do
-          trigger_error!(resource)
-        end
+        # 6.times do
+        trigger_error!(resource)
+        # end
 
         assert_circuit_opened(resource)
       end