require 'benchmark/ips'

# hash.size # 1000
# hash.values.flatten.size # 500500
def a_big_hash
  {}.tap do |h|
    1.upto(1000) { |i| h[i.to_s.to_sym] = 1.upto(i).to_a }
  end
end

class Hash
  # map to_h
  def hmap1 &block
    self.map(&block).to_h
  end

  # tap each
  def hmap2 &block
    {}.tap do |h|
      each do |i|
        k, v = block.call(i)
        h[k] = v
      end
    end
  end

  # reduce
  def hmap3 &block
    reduce({}) do |acc, i|
      pair = block.call(i)
      acc.merge({pair[0] => pair[1].size})
    end
  end

  # map reduce
  def hmap4 &block
    map { |i|
      k, v = block.call(i)
      {k => v}
    }.reduce(:merge)
  end

  # brackets splat flat_map
  def hmap5 &block
    Hash[*flat_map(&block)]
  end
end


Benchmark.ips do |bm|
  bm.time = 10
  bm.warmup = 5

  hash = a_big_hash.freeze # nothing should attempt to mutate hash
  counter = proc { |k, v| [k, v.size] }

  bm.report 'map to_h' do
    hash.hmap1 &counter
  end

  bm.report 'tap each' do
    hash.hmap2 &counter
  end

  bm.report 'reduce' do
    hash.hmap3 &counter
  end

  bm.report 'map reduce' do
    hash.hmap4 &counter
  end

  bm.report '[] splat flat_map' do
    hash.hmap5 &counter
  end

  bm.compare!
end


# Calculating -------------------------------------
#             map to_h   110.000  i/100ms
#             tap each    88.000  i/100ms
#               reduce     1.000  i/100ms
#           map reduce     1.000  i/100ms
#    [] splat flat_map   116.000  i/100ms
# -------------------------------------------------
#             map to_h      1.374k (±19.7%) i/s -     13.310k
#             tap each    926.439  (±25.7%) i/s -      8.536k
#               reduce      3.397  (±29.4%) i/s -     33.000
#           map reduce      3.596  (±27.8%) i/s -     35.000
#    [] splat flat_map      1.246k (±19.5%) i/s -     11.948k

# Comparison:
#             map to_h:     1373.5 i/s
#    [] splat flat_map:     1245.5 i/s - 1.10x slower
#             tap each:      926.4 i/s - 1.48x slower
#           map reduce:        3.6 i/s - 381.94x slower
#               reduce:        3.4 i/s - 404.28x slower