diff --git a/.rubocop.yml b/.rubocop.yml
new file mode 100644
index 0000000..f96ec5e
--- /dev/null
+++ b/.rubocop.yml
@@ -0,0 +1,10 @@
+  AllCops:
+    NewCops: enable
+
+  Metrics/BlockLength:
+    Enabled: true
+    Exclude:
+      - spec/**/*
+
+  Style/Documentation:
+    Enabled: false
diff --git a/.tool-versions b/.tool-versions
new file mode 100644
index 0000000..0b2d858
--- /dev/null
+++ b/.tool-versions
@@ -0,0 +1 @@
+ruby 3.1.2
diff --git a/.travis.yml b/.travis.yml
index dbe5154..7969780 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,3 @@
 language: ruby
 rvm:
- - 2.7
\ No newline at end of file
+ - 3.1.2
diff --git a/Gemfile b/Gemfile
index 557766f..57a7fcb 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,10 +1,16 @@
 # frozen_string_literal: true
 
-source "https://rubygems.org"
+source 'https://rubygems.org'
 
-git_source(:github) {|repo_name| "https://github.com/#{repo_name}" }
+git_source(:github) { |repo_name| "https://github.com/#{repo_name}" }
 
-gem "rspec"
 gem 'guard'
 gem 'guard-shell'
-gem 'rake'
\ No newline at end of file
+gem 'rake'
+gem 'rspec'
+
+group :development do
+  gem 'rubocop'
+  gem 'rubocop-shopify', require: false
+  gem 'ruby-lsp'
+end
diff --git a/Gemfile.lock b/Gemfile.lock
index d1fea3e..4386bf1 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -1,24 +1,27 @@
 GEM
   remote: https://rubygems.org/
   specs:
+    ast (2.4.2)
     coderay (1.1.3)
-    diff-lcs (1.4.4)
-    ffi (1.13.1)
-    formatador (0.2.5)
-    guard (2.16.2)
+    diff-lcs (1.5.0)
+    ffi (1.15.5)
+    formatador (1.1.0)
+    guard (2.18.0)
       formatador (>= 0.2.4)
       listen (>= 2.7, < 4.0)
       lumberjack (>= 1.0.12, < 2.0)
       nenv (~> 0.1)
       notiffany (~> 0.0)
-      pry (>= 0.9.12)
+      pry (>= 0.13.0)
       shellany (~> 0.0)
       thor (>= 0.18.1)
     guard-compat (1.2.1)
-    guard-shell (0.7.1)
+    guard-shell (0.7.2)
       guard (>= 2.0.0)
       guard-compat (~> 1.0)
-    listen (3.2.1)
+    json (2.6.2)
+    language_server-protocol (3.17.0.1)
+    listen (3.7.1)
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)
     lumberjack (1.2.8)
@@ -27,37 +30,70 @@ GEM
     notiffany (0.1.3)
       nenv (~> 0.1)
       shellany (~> 0.0)
-    pry (0.13.1)
+    parallel (1.22.1)
+    parser (3.1.2.1)
+      ast (~> 2.4.1)
+    prettier_print (0.1.0)
+    pry (0.14.1)
       coderay (~> 1.1)
       method_source (~> 1.0)
-    rake (13.0.1)
-    rb-fsevent (0.10.4)
+    rainbow (3.1.1)
+    rake (13.0.6)
+    rb-fsevent (0.11.2)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    rspec (3.9.0)
-      rspec-core (~> 3.9.0)
-      rspec-expectations (~> 3.9.0)
-      rspec-mocks (~> 3.9.0)
-    rspec-core (3.9.3)
-      rspec-support (~> 3.9.3)
-    rspec-expectations (3.9.2)
+    regexp_parser (2.5.0)
+    rexml (3.2.5)
+    rspec (3.11.0)
+      rspec-core (~> 3.11.0)
+      rspec-expectations (~> 3.11.0)
+      rspec-mocks (~> 3.11.0)
+    rspec-core (3.11.0)
+      rspec-support (~> 3.11.0)
+    rspec-expectations (3.11.0)
       diff-lcs (>= 1.2.0, < 2.0)
-      rspec-support (~> 3.9.0)
-    rspec-mocks (3.9.1)
+      rspec-support (~> 3.11.0)
+    rspec-mocks (3.11.1)
       diff-lcs (>= 1.2.0, < 2.0)
-      rspec-support (~> 3.9.0)
-    rspec-support (3.9.3)
+      rspec-support (~> 3.11.0)
+    rspec-support (3.11.0)
+    rubocop (1.36.0)
+      json (~> 2.3)
+      parallel (~> 1.10)
+      parser (>= 3.1.2.1)
+      rainbow (>= 2.2.2, < 4.0)
+      regexp_parser (>= 1.8, < 3.0)
+      rexml (>= 3.2.5, < 4.0)
+      rubocop-ast (>= 1.20.1, < 2.0)
+      ruby-progressbar (~> 1.7)
+      unicode-display_width (>= 1.4.0, < 3.0)
+    rubocop-ast (1.21.0)
+      parser (>= 3.1.1.0)
+    rubocop-shopify (2.9.0)
+      rubocop (~> 1.33)
+    ruby-lsp (0.3.2)
+      language_server-protocol (~> 3.17.0)
+      sorbet-runtime
+      syntax_tree (>= 3.4)
+    ruby-progressbar (1.11.0)
     shellany (0.0.1)
-    thor (1.0.1)
+    sorbet-runtime (0.5.10415)
+    syntax_tree (3.5.0)
+      prettier_print
+    thor (1.2.1)
+    unicode-display_width (2.2.0)
 
 PLATFORMS
-  ruby
+  arm64-darwin-21
 
 DEPENDENCIES
   guard
   guard-shell
   rake
   rspec
+  rubocop
+  rubocop-shopify
+  ruby-lsp
 
 BUNDLED WITH
-   2.1.2
+   2.3.19
diff --git a/Guardfile b/Guardfile
index 4e66be6..65972ea 100644
--- a/Guardfile
+++ b/Guardfile
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # A sample Guardfile
 # More info at https://github.com/guard/guard#readme
 
@@ -19,5 +21,5 @@
 #   watch(%r{file/path}) { `command(s)` }
 #
 guard :shell do
-  watch(%r{^*\.rb}) { `bundle exec rspec --force-color spec/` }
+  watch(/^*\.rb/) { `bundle exec rspec --force-color spec/` }
 end
diff --git a/Rakefile b/Rakefile
index a2cb1a5..a6cd24a 100644
--- a/Rakefile
+++ b/Rakefile
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 begin
   require 'rspec/core/rake_task'
 
@@ -5,7 +7,7 @@ begin
     t.fail_on_error = false
   end
 
-  task :default => :spec
+  task default: :spec
 rescue LoadError
   # no rspec available
-end
\ No newline at end of file
+end
diff --git a/lib/checkout.rb b/lib/checkout.rb
index 0e95bb7..dbe243d 100644
--- a/lib/checkout.rb
+++ b/lib/checkout.rb
@@ -22,7 +22,7 @@ def scan(barcode)
   def total
     @cart.reduce(0) do |total, (item, quantity)|
       rule = @rules.find(item)
-      total + rule.apply(quantity: quantity)
+      total + rule.apply(quantity:)
     end.floor(2)
   end
 end
diff --git a/lib/product.rb b/lib/product.rb
index 94c28ff..158d155 100644
--- a/lib/product.rb
+++ b/lib/product.rb
@@ -4,8 +4,8 @@ class Product
   attr_accessor :code, :price
 
   def initialize(code, price = 0.00)
-    raise ArgumentError.new('product needs a valid code') if code.empty?
-    raise ArgumentError.new('price needs to be positive or 0') if price.negative?
+    raise ArgumentError, 'product needs a valid code' if code.empty?
+    raise ArgumentError, 'price needs to be positive or 0' if price.negative?
 
     self.code = code
     self.price = price
diff --git a/lib/rule.rb b/lib/rule.rb
index 1cef1b2..2b04dad 100644
--- a/lib/rule.rb
+++ b/lib/rule.rb
@@ -7,8 +7,8 @@ def initialize(product, minimun: 0, per: 1, price: nil)
     # the price of the rule should be the same of
     # the product in case it wasn't informed
     self.price = price || product.price
-    raise ArgumentError.new('price should be a positive number') if self.price.negative?
-    
+    raise ArgumentError, 'price should be a positive number' if self.price.negative?
+
     self.product = product
     self.minimun = minimun
     self.per = per
@@ -30,6 +30,6 @@ def apply(quantity: 0)
       quantity = quantity % per
     end
 
-    total + product.price * quantity
+    total + (product.price * quantity)
   end
 end
diff --git a/lib/rules.rb b/lib/rules.rb
index b8a17cd..f26e67e 100644
--- a/lib/rules.rb
+++ b/lib/rules.rb
@@ -1,32 +1,34 @@
 # frozen_string_literal: true
 
 class Rules
-  def initialize(*rules)
+  attr_reader :rules
+
+  def initialize(*received_rules)
     @rules = []
-    rules.each { |rule| add(rule) }
+    received_rules.each { |rule| add(rule) }
   end
 
   def count
-    @rules.count
+    rules.count
   end
 
-  def each
-    @rules.each { |p| yield p }
+  def each(&)
+    rules.each(&)
   end
 
   def add(rule)
-    raise StandardError.new('only one rule per product') if self.exists?(rule)
-    
-    @rules << rule
+    raise StandardError, 'only one rule per product' if exists?(rule)
+
+    rules << rule
   end
 
   def find(code)
-    @rules.find { |rule| rule.code == code }
+    rules.find { |rule| rule.code == code }
   end
 
   private
 
   def exists?(rule)
-    @rules.map(&:code).include?(rule.code)
+    rules.map(&:code).include?(rule.code)
   end
 end
diff --git a/spec/checkout_spec.rb b/spec/checkout_spec.rb
index 9d64b79..9cf479b 100644
--- a/spec/checkout_spec.rb
+++ b/spec/checkout_spec.rb
@@ -1,10 +1,10 @@
 # frozen_string_literal: true
 
-require 'rule.rb'
-require 'rules.rb'
-require 'product.rb'
-require 'errors.rb'
-require 'checkout.rb'
+require 'rule'
+require 'rules'
+require 'product'
+require 'errors'
+require 'checkout'
 
 RSpec.describe Checkout do
   before(:each) do
@@ -17,7 +17,7 @@
       Rule.new(gr1, minimun: 2, per: 2, price: 3.11),
       Rule.new(sr1, minimun: 3, per: 1, price: 4.50),
       Rule.new(cf1, minimun: 3, per: 1, price: 11.23 / 3 * 2),
-      Rule.new(gr2),
+      Rule.new(gr2)
     ].each { |r| pricing_rules.add(r) }
     @checkout = Checkout.new(pricing_rules)
   end
diff --git a/spec/product_spec.rb b/spec/product_spec.rb
index f2b6a65..de43f21 100644
--- a/spec/product_spec.rb
+++ b/spec/product_spec.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require 'product.rb'
+require 'product'
 
 RSpec.describe Product do
   describe '#new' do
diff --git a/spec/rule_spec.rb b/spec/rule_spec.rb
index a69711e..6960bac 100644
--- a/spec/rule_spec.rb
+++ b/spec/rule_spec.rb
@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
-require 'product.rb'
-require 'rule.rb'
+require 'product'
+require 'rule'
 
 RSpec.describe Rule do
   before(:each) do
diff --git a/spec/rules_spec.rb b/spec/rules_spec.rb
index 6f60400..2699fe4 100644
--- a/spec/rules_spec.rb
+++ b/spec/rules_spec.rb
@@ -1,8 +1,8 @@
 # frozen_string_literal: true
 
-require 'product.rb'
-require 'rule.rb'
-require 'rules.rb'
+require 'product'
+require 'rule'
+require 'rules'
 
 RSpec.describe Rules do
   before(:each) do
@@ -33,7 +33,7 @@
     end
   end
 
-  describe '#find' do 
+  describe '#find' do
     before(:each) do
       @rules = Rules.new
       @rules.add(@rule)
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 251aa51..5681bb3 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # This file was generated by the `rspec --init` command. Conventionally, all
 # specs live under a `spec` directory, which RSpec adds to the `$LOAD_PATH`.
 # The generated `.rspec` file contains `--require spec_helper` which will cause
@@ -44,57 +46,55 @@
   # triggering implicit auto-inclusion in groups with matching metadata.
   config.shared_context_metadata_behavior = :apply_to_host_groups
 
-# The settings below are suggested to provide a good initial experience
-# with RSpec, but feel free to customize to your heart's content.
-=begin
-  # This allows you to limit a spec run to individual examples or groups
-  # you care about by tagging them with `:focus` metadata. When nothing
-  # is tagged with `:focus`, all examples get run. RSpec also provides
-  # aliases for `it`, `describe`, and `context` that include `:focus`
-  # metadata: `fit`, `fdescribe` and `fcontext`, respectively.
-  config.filter_run_when_matching :focus
-
-  # Allows RSpec to persist some state between runs in order to support
-  # the `--only-failures` and `--next-failure` CLI options. We recommend
-  # you configure your source control system to ignore this file.
-  config.example_status_persistence_file_path = "spec/examples.txt"
-
-  # Limits the available syntax to the non-monkey patched syntax that is
-  # recommended. For more details, see:
-  #   - http://rspec.info/blog/2012/06/rspecs-new-expectation-syntax/
-  #   - http://www.teaisaweso.me/blog/2013/05/27/rspecs-new-message-expectation-syntax/
-  #   - http://rspec.info/blog/2014/05/notable-changes-in-rspec-3/#zero-monkey-patching-mode
-  config.disable_monkey_patching!
-
-  # This setting enables warnings. It's recommended, but in some cases may
-  # be too noisy due to issues in dependencies.
-  config.warnings = true
-
-  # Many RSpec users commonly either run the entire suite or an individual
-  # file, and it's useful to allow more verbose output when running an
-  # individual spec file.
-  if config.files_to_run.one?
-    # Use the documentation formatter for detailed output,
-    # unless a formatter has already been configured
-    # (e.g. via a command-line flag).
-    config.default_formatter = "doc"
-  end
-
-  # Print the 10 slowest examples and example groups at the
-  # end of the spec run, to help surface which specs are running
-  # particularly slow.
-  config.profile_examples = 10
-
-  # Run specs in random order to surface order dependencies. If you find an
-  # order dependency and want to debug it, you can fix the order by providing
-  # the seed, which is printed after each run.
-  #     --seed 1234
-  config.order = :random
-
-  # Seed global randomization in this process using the `--seed` CLI option.
-  # Setting this allows you to use `--seed` to deterministically reproduce
-  # test failures related to randomization by passing the same `--seed` value
-  # as the one that triggered the failure.
-  Kernel.srand config.seed
-=end
+  # The settings below are suggested to provide a good initial experience
+  # with RSpec, but feel free to customize to your heart's content.
+  #   # This allows you to limit a spec run to individual examples or groups
+  #   # you care about by tagging them with `:focus` metadata. When nothing
+  #   # is tagged with `:focus`, all examples get run. RSpec also provides
+  #   # aliases for `it`, `describe`, and `context` that include `:focus`
+  #   # metadata: `fit`, `fdescribe` and `fcontext`, respectively.
+  #   config.filter_run_when_matching :focus
+  #
+  #   # Allows RSpec to persist some state between runs in order to support
+  #   # the `--only-failures` and `--next-failure` CLI options. We recommend
+  #   # you configure your source control system to ignore this file.
+  #   config.example_status_persistence_file_path = "spec/examples.txt"
+  #
+  #   # Limits the available syntax to the non-monkey patched syntax that is
+  #   # recommended. For more details, see:
+  #   #   - http://rspec.info/blog/2012/06/rspecs-new-expectation-syntax/
+  #   #   - http://www.teaisaweso.me/blog/2013/05/27/rspecs-new-message-expectation-syntax/
+  #   #   - http://rspec.info/blog/2014/05/notable-changes-in-rspec-3/#zero-monkey-patching-mode
+  #   config.disable_monkey_patching!
+  #
+  #   # This setting enables warnings. It's recommended, but in some cases may
+  #   # be too noisy due to issues in dependencies.
+  #   config.warnings = true
+  #
+  #   # Many RSpec users commonly either run the entire suite or an individual
+  #   # file, and it's useful to allow more verbose output when running an
+  #   # individual spec file.
+  #   if config.files_to_run.one?
+  #     # Use the documentation formatter for detailed output,
+  #     # unless a formatter has already been configured
+  #     # (e.g. via a command-line flag).
+  #     config.default_formatter = "doc"
+  #   end
+  #
+  #   # Print the 10 slowest examples and example groups at the
+  #   # end of the spec run, to help surface which specs are running
+  #   # particularly slow.
+  #   config.profile_examples = 10
+  #
+  #   # Run specs in random order to surface order dependencies. If you find an
+  #   # order dependency and want to debug it, you can fix the order by providing
+  #   # the seed, which is printed after each run.
+  #   #     --seed 1234
+  #   config.order = :random
+  #
+  #   # Seed global randomization in this process using the `--seed` CLI option.
+  #   # Setting this allows you to use `--seed` to deterministically reproduce
+  #   # test failures related to randomization by passing the same `--seed` value
+  #   # as the one that triggered the failure.
+  #   Kernel.srand config.seed
 end