Benchmarking a reimplementation of Hash#deep_merge from the ActiveSupport library

deep_merge_benchmark.rb

require 'active_support'
require 'benchmark/ips'

class Hash
  def new_deep_merge(other_hash, &block)
    dup.new_deep_merge!(other_hash, &block)
  end

  def new_deep_merge!(other, &block)
    merge!(other) do |key, old_val, new_val|
      if old_val.is_a?(Hash) && new_val.is_a?(Hash)
        old_val.new_deep_merge(new_val, &block)
      elsif block_given?
        block.call(key, old_val, new_val)
      else
        new_val
      end
    end
  end
end

h1 = { a: true, b: { c: [1, 2, 3] }, d: 1 }
h2 = { a: false, b: { c: [3, 4, 5] }, d: 2 }
h3 = { a: false, b: { x: [3, 4, 5] }, e: 2, f: { y: false } }
h4 = { e: 2, f: { y: false }, g: 'hi', h: true }

Benchmark.ips do |bm|
  bm.report('old_deep_merge_all_dupes') { h1.deep_merge(h2) }
  bm.report('new_deep_merge_all_dupes') { h1.new_deep_merge(h2) }
  bm.compare!
end

Benchmark.ips do |bm|
  bm.report('old_deep_merge_some_dupes') { h1.deep_merge(h3) }
  bm.report('new_deep_merge_some_dupes') { h1.new_deep_merge(h3) }
  bm.compare!
end

Benchmark.ips do |bm|
  bm.report('old_deep_merge_no_dupes') { h1.deep_merge(h4) }
  bm.report('new_deep_merge_no_dupes') { h1.new_deep_merge(h4) }
  bm.compare!
end