immutable-vs-mutable? Both!

code.rb

CAT_TO_ITEM_MAP = {
  'numbers' => %w[0 1 2 3],
  'letters' => %w[a b c d],
}

def items_to_categories_map_immutable
  reducer = -> (acc, (cat, items)) do
    items.reduce(acc) { |acc, item| acc.merge(item => cat) }
  end

  CAT_TO_ITEM_MAP.reduce({}, &reducer)
end

def items_to_categories_map_mutable
  acc = {}

  reducer = -> (cat, items) do
    items.each { |item| acc[item] = cat }
  end

  CAT_TO_ITEM_MAP.each(&reducer)

  acc
end


if __FILE__ == $0
  require 'benchmark'
  require 'test/unit'

  include Test::Unit::Assertions

  val = {
    "0"=>"numbers",
    "1"=>"numbers",
    "2"=>"numbers",
    "3"=>"numbers",
    "a"=>"letters",
    "b"=>"letters",
    "c"=>"letters",
    "d"=>"letters"
  }

  assert_equal val, items_to_categories_map_immutable
  assert_equal val, items_to_categories_map_mutable

  n = 500000

  Benchmark.bm(10) do |x|
    x.report("mutable") { n.times { items_to_categories_map_mutable } }
    x.report("immutable") { n.times { items_to_categories_map_immutable } }
  end
end

# $ ruby test.rb 
#                 user     system      total        real
# mutable      2.550000   0.390000   2.940000 (  2.946557)
# immutable    7.900000   0.920000   8.820000 (  8.842163)