Memoize config hash on finalize (#151)

Freezing the hash is beneficial at this point because it saves repeated expensive
computation if that hash is to be used later in performance-sensitive situations, such as
when serving as a cache key or similar.

Gemfile

@@ -19,4 +19,5 @@ end
 group :tools do
   gem "hotch", platform: :mri
   gem "pry-byebug", platform: :mri
+  gem "rspec-benchmark"
 end

lib/dry/configurable/config.rb

@@ -146,6 +146,16 @@ module Dry
         values.to_h { |key, value| [key, value.is_a?(self.class) ? value.to_h : value] }
       end
 
+      # @api private
+      alias_method :_dry_equalizer_hash, :hash
+
+      # @api public
+      def hash
+        return @__hash__ if instance_variable_defined?(:@__hash__)
+
+        _dry_equalizer_hash
+      end
+
       # @api private
       def finalize!(freeze_values: false)
         values.each_value do |value|
@@ -156,6 +166,13 @@ module Dry
           end
         end
 
+        # Memoize the hash for the object when finalizing (regardless of whether values themselves
+        # are to be frozen; the intention of finalization is that no further changes should be
+        # made). The benefit of freezing the hash at this point is that it saves repeated expensive
+        # computation (through Dry::Equalizer's hash implementation) if that hash is to be used
+        # later in performance-sensitive situations, such as when serving as a cache key or similar.
+        @__hash__ = _dry_equalizer_hash unless frozen?
+
         freeze
       end
 

spec/integration/dry/configurable/config_spec.rb

@@ -239,6 +239,28 @@ RSpec.describe Dry::Configurable::Config do
     end
   end
 
+  describe "#hash" do
+    it "returns the integer hash value for the convig based on its values" do
+      klass.setting :db
+
+      expect(klass.config.hash).to be_an_instance_of(Integer)
+      expect { klass.config.db = "sqlite" }.to change { klass.config.hash }
+    end
+
+    it "is memoized when the config is finalized", :performance do
+      klass.setting :a
+      klass.setting :b
+      klass.setting :c
+      klass.setting :d
+      klass.setting :e
+
+      finalized_config = klass.config.dup.finalize!
+
+      expect(finalized_config.hash).to eq klass.config.hash
+      expect { finalized_config.hash }.to perform_faster_than { klass.config.hash }.at_least(50).times
+    end
+  end
+
   describe "#method_missing" do
     it "provides access to reader methods" do
       klass.setting :hello

spec/spec_helper.rb

@@ -2,6 +2,7 @@
 
 require_relative "support/coverage"
 require "pathname"
+require "rspec-benchmark"
 
 SPEC_ROOT = Pathname(__FILE__).dirname
 
@@ -22,6 +23,8 @@ RSpec.configure do |config|
   config.disable_monkey_patching!
   config.filter_run_when_matching :focus
 
+  config.include RSpec::Benchmark::Matchers
+
   config.around do |example|
     module Test
     end