Hacking Ruby Hash: Fastest #to_struct method I am trying to make the fastest #to_struct method in Ruby's Hash.

I am including a use case and benchmark so you can run and see if you have really improved the code.

This is my implementation and the benchmark is included. The time at the bottom is the time it takes on my machine. How can I make this faster?

require "json"
require 'benchmark'
require 'bigdecimal/math'

class Hash
  def to_struct
    k = self.keys
    klass = k.map(&amp;:to_s).sort_by {|word| word.downcase}.join.capitalize
    begin
      Kernel.const_get("Struct::" + klass).new(*self.values_at(*k))
    rescue NameError
      Struct.new(klass, *(k)).new(*self.values_at(*k))
    end
  end
end

# You have a hash that you have built in your app
sample_hash = {
  foo_key: "foo_val",
  bar_key: "bar_val",
  baz_key: "baz_val",
  foo1_key: "foo_val",
  bar1_key: "bar_val",
  baz1_key: "baz_val",
  foo2_key: "foo_val",
  bar2_key: "bar_val",
  baz2_key: "baz_val",
  foo3_key: "foo_val",
  bar3_key: "bar_val",
  baz3_key: "baz_val",
  foo4_key: "foo_val",
  bar4_key: "bar_val",
  baz4_key: "baz_val",
  foo5_key: "foo_val",
  bar5_key: "bar_val",
  baz5_key: "baz_val",
  foo6_key: "foo_val",
  bar6_key: "bar_val",
  baz6_key: "baz_val",
  foo7_key: "foo_val",
  bar7_key: "bar_val",
  baz7_key: "baz_val",
}

# Then you have JSON coming from some external api
json_response = "{\"qux_key\":\"qux_val\",\"quux_key\":\"quux_val\",\"corge_key\":\"corge_val\"}"
hash_with_unknown_keys = JSON.parse(json_response)

# Merge these two together
sample_hash.merge!(hash_with_unknown_keys)

iterations = 100_000

Benchmark.bm do |bm|
  bm.report "#to_struct" do
    iterations.times do
      # Would be super nice if I could convert this to a struct with a method
      # Somehow a bit faster than the explicit example below and much faster than open struct
      sample_struct = sample_hash.to_struct
      unless sample_struct.foo_key == "foo_val"
        raise "Wrong value"
      end
    end
  end

  bm.report "Struct" do
    iterations.times do
      sample_struct = Struct.new(*sample_hash.keys)
        .new(*sample_hash.values)
      unless sample_struct.foo_key == "foo_val"
        raise "Wrong value"
      end
    end
  end

  bm.report "OpenStruct" do
    iterations.times do
      sample_open_struct = OpenStruct.new(sample_hash)
      unless sample_open_struct.foo_key  == "foo_val"
        raise "Wrong value"
      end
    end
  end
end



#       user     system      total        real
# #to_struct  4.030000   0.010000   4.040000 (  4.072031)
#     Struct  6.870000   0.290000   7.160000 (  7.320459)
# OpenStruct 23.550000   0.210000  23.760000 ( 23.895187)
Use OpenHash and Ruby >= 2.3.0

Starting with MRI 2.3.0, your benchmark using OpenHash gets fast.  Very fast:

ruby-2.2.5: ruby 2.2.5p319 (2016-04-26 revision 54774) [x86_64-linux]
       user     system      total        real
#to_struct  1.780000   0.000000   1.780000 (  1.774490)
Struct  9.100000   0.000000   9.100000 (  9.099619)
OpenStruct  7.910000   0.000000   7.910000 (  7.911342)

ruby-2.3.0: ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-linux]
       user     system      total        real
#to_struct  1.700000   0.000000   1.700000 (  1.695587)
Struct  7.660000   0.000000   7.660000 (  7.660869)
OpenStruct  0.650000   0.000000   0.650000 (  0.658817)


With the latest MRI, Your #to_struct method gets a bit of a speed boost as well.

ruby-2.4.1: ruby 2.4.1p111 (2017-03-22 revision 58053) [x86_64-linux]
       user     system      total        real
#to_struct  1.460000   0.000000   1.460000 (  1.459063)
Struct  7.420000   0.000000   7.420000 (  7.416505)
OpenStruct  0.660000   0.000000   0.660000 (  0.658009)


So if you can, use Ruby >= ruby 2.3.0, and use OpenHash.

How to make #to_struct faster

I made the following changes for performance:


Eliminate the mapping of hash keys using #downcase.
Use #values instead of #values_at (values are always the same order as keys).  See https://stackoverflow.com/a/31425274/238886


and these for clarity:


Eliminate the temporary for self.keys
DRY the creation of the struct instance
Removed self references.


With these changes, the code is:

class Hash
  def new_to_struct
    klass_name = keys.map(&amp;:to_s).sort.join.capitalize
    klass = begin
              Kernel.const_get("Struct::" + klass_name)
            rescue NameError
              Struct.new(klass_name, *keys)
            end
    klass.new(*values)
  end
end


and the benchmark (run against ruby-2.4.1):

       user     system      total        real
#to_struct  1.410000   0.000000   1.410000 (  1.403908)
#new_to_struct  0.760000   0.000000   0.760000 (  0.757548)
Struct  7.060000   0.010000   7.070000 (  7.075619)
OpenStruct  0.650000   0.000000   0.650000 (  0.649057)


These changes get to_struct close to OpenStruct, but still not as fast.