Matcher to like perform_under but gets rid of results outside a standard deviation range

rspec_perform_under_variance.rb

it "maintains acceptable performance for the index action", skip_db_cleaner: true do # for some reason skip_db_cleaner: true when running the whole test suite wasn't being inherited, so I had to dup it :( . When running the describe, it was being inherited by the 'it' blocks, and once you run it and try to run the whole suite again, it starts passing.
  expect { get :index, params: params, format: :json }.to meet_performance_threshold(4.0)
    .sample(5)
    .under_standard_deviations(2)

# Also see rspec-benchmark. This matcher is like perform_under but it removes results outside a defined standard deviation range reduce high variance.
RSpec::Matchers.define :meet_performance_threshold do |threshold_seconds|
  chain :sample do |sample_size|
    @with_sample_size = sample_size
  end

  chain :under_standard_deviations do |deviations|
    @std_devs_for_outlier = deviations
  end

  chain :warmup do |warmup_count|
    @warmup_count = warmup_count
  end

  def supports_block_expectations?
    true
  end

  match do |action_to_benchmark|
    raise "expect must take a block or be passed a Proc or Lambda" unless action_to_benchmark.is_a?(Proc)

    @with_sample_size ||= 5
    @warmup_count ||= 1

    raise "under_standard_deviations must be provided and larger than 0 otherwise you might as well use perform_under from rspec-benchmark" if @std_devs_for_outlier.blank? || @std_devs_for_outlier <= 0
    raise "with_sample_size must at least be 1" if @with_sample_size < 1

    @times = (@warmup_count + @with_sample_size).times.map do
      Benchmark.measure { action_to_benchmark.call }.real
    end
    @times = @times.drop(@warmup_count)

    initial_avg = @times.sum / @times.size.to_f

    sum_of_squares = @times.map { |x| (x - initial_avg)**2 }.sum
    std_dev = Math.sqrt(sum_of_squares / @times.size)
    outlier_threshold = initial_avg + (@std_devs_for_outlier * std_dev)
    @filtered_times = @times.reject { |t| t > outlier_threshold }
    puts "filtered out #{(@times - @filtered_times).size} outlier(s) from #{@times}"

    # If all runs were outliers, fall back to the initial set of times.
    raise "All code execution times were outside of #{@std_devs_for_outlier} standard deviations. " if @filtered_times.empty?
    final_times = @filtered_times.empty? ? @times : @filtered_times
    @avg_time = final_times.sum / final_times.size.to_f

    @avg_time < threshold_seconds
  end

  failure_message do |_|
    "expected average time to be < #{threshold_seconds}s, but was #{@avg_time.round(4)}s.\n" \
    "Runs: #{@times.map { |t| t.round(4) }.join("s, ")}s"
  end

  failure_message_when_negated do |_|
    "expected average time not to be < #{threshold_seconds}s, but was #{@avg_time.round(4)}s"
  end

  description do
    "perform under #{threshold_seconds}s on average"
  end
end