A comparison of Ruby's Enumerable#inject and Enumerable#each_with_object

inject_vs_each_with_object.rb

require 'benchmark'

module InjectVsEachWithObject
  ARRAY_LENGTH = 10_000
  ARRAY = (1..ARRAY_LENGTH).to_a
  REPETITIONS = ARGV[0].nil? ? 100 : ARGV[0].to_i

  def self.immutable_inject
    REPETITIONS.times do
      ARRAY.inject(0) { |sum, n| sum + n }
    end
  end

  # DOES NOT WORK (returns 0)
  def self.immutable_each_with_object
    REPETITIONS.times do
      ARRAY.each_with_object(0) { |n, sum| sum += n }
    end
  end

  def self.mutable_inject_plus
    REPETITIONS.times do
      ARRAY.inject([]) { |new_array, n| new_array + [3 * n] }
    end
  end

  # DOES NOT WORK (returns [])
  def self.mutable_each_with_object_plus
    REPETITIONS.times do
      ARRAY.each_with_object([]) { |n, new_array| new_array += [3 * n]}
    end
  end

  def self.mutable_inject_shovel
    REPETITIONS.times do
      ARRAY.inject([]) { |new_array, n| new_array << 3 * n }
    end
  end

  def self.mutable_each_with_object_shovel
    REPETITIONS.times do
      ARRAY.each_with_object([]) { |n, new_array| new_array << 3 * n }
    end
  end

  def self.run
    puts 'Running benchmark...'

    puts "Summing an array of #{ ARRAY_LENGTH } integers (working with an immutable object) #{ REPETITIONS } times:"

    Benchmark.bm(30) do |x|
      x.report("inject:")   { immutable_inject }
    end

    puts "Populating an array from another array of #{ ARRAY_LENGTH } integers (working with a mutable object) #{ REPETITIONS } times:"

    Benchmark.bm(30) do |x|
      x.report("inject (+):")   { mutable_inject_plus }
      x.report("inject (<<):")   { mutable_inject_shovel }
      x.report("each_with_object (<<):") { mutable_each_with_object_shovel }
    end
  end
end

InjectVsEachWithObject.run