memoization_with_nil_assignment.rb

require 'benchmark'

class Measurement
  LEAN_BODY_MASS_PARAM = 0.01 # dummy value
  
  attr_reader :weight
  
  def lean_body_mass
    weight * LEAN_BODY_MASS_PARAM unless weight.nil? # returns nil when no weight
  end
end

class Calculator
  def lean_body_mass
    @lean_body_mass ||= measurements.sort_by(&:lean_body_mass).first.lean_body_mass # simple memoization
  end

  private

  def measurements
    a = []
    1_000.times do
      a << Measurement.new
    end
    a
  end
end

calculator = Calculator.new

Benchmark.bm do |bm|
  bm.report do
    5_000.times { calculator.lean_body_mass } # will sort every single time due to nil assignment!
  end
  
  bm.report do
    def calculator.lean_body_mass # fix memoization
      return @lean_body_mass if instance_variable_defined?(:@lean_body_mass)
      @lean_body_mass ||= measurements.sort_by(&:lean_body_mass).first.lean_body_mass # simple memoization
    end
    
    5_000.times { calculator.lean_body_mass } # returns correct memoized value
  end
end

####################################################################
#       user     system      total        real
#   1.760000   0.010000   1.770000 (  1.776173) memoization with nil
#   0.000000   0.000000   0.000000 (  0.000581) fixed memoization
####################################################################