diff --git a/bin/rubocop-git b/bin/rubocop-git
index 7691b0f..dda0f2b 100755
--- a/bin/rubocop-git
+++ b/bin/rubocop-git
@@ -1,3 +1,5 @@
 #!/usr/bin/env ruby
 
-require 'rubocop/git'
+require 'rubocop/git/cli'
+
+RuboCop::Git::CLI.new.run
diff --git a/hound.yml b/hound.yml
new file mode 100644
index 0000000..dfbe0c5
--- /dev/null
+++ b/hound.yml
@@ -0,0 +1,235 @@
+AccessorMethodName:
+  Enabled: false
+
+Alias:
+  Enabled: false
+
+ArrayJoin:
+  Enabled: false
+
+AsciiComments:
+  Enabled: false
+
+AsciiIdentifiers:
+  Enabled: false
+
+Attr:
+  Enabled: false
+
+BlockNesting:
+  Enabled: false
+
+CaseEquality:
+  Enabled: false
+
+CharacterLiteral:
+  Enabled: false
+
+ClassLength:
+  Enabled: false
+
+ClassVars:
+  Enabled: false
+
+CollectionMethods:
+  PreferredMethods:
+    find: detect
+    reduce: inject
+    collect: map
+    find_all: select
+
+ColonMethodCall:
+  Enabled: false
+
+CommentAnnotation:
+  Enabled: false
+
+CyclomaticComplexity:
+  Enabled: false
+
+Delegate:
+  Enabled: false
+
+DeprecatedHashMethods:
+  Enabled: false
+
+Documentation:
+  Enabled: false
+
+DotPosition:
+  EnforcedStyle: trailing
+
+DoubleNegation:
+  Enabled: false
+
+EmptyLiteral:
+  Enabled: false
+
+Encoding:
+  Enabled: false
+
+EvenOdd:
+  Enabled: false
+
+FileName:
+  Enabled: false
+
+FlipFlop:
+  Enabled: false
+
+FormatString:
+  Enabled: false
+
+GlobalVars:
+  Enabled: false
+
+IfUnlessModifier:
+  Enabled: false
+
+IfWithSemicolon:
+  Enabled: false
+
+Lambda:
+  Enabled: false
+
+LambdaCall:
+  Enabled: false
+
+LineEndConcatenation:
+  Enabled: false
+
+LineLength:
+  Max: 80
+
+MethodLength:
+  Enabled: false
+
+ModuleFunction:
+  Enabled: false
+
+NegatedIf:
+  Enabled: false
+
+NegatedWhile:
+  Enabled: false
+
+NilComparison:
+  Enabled: false
+
+Not:
+  Enabled: false
+
+NumericLiterals:
+  Enabled: false
+
+OneLineConditional:
+  Enabled: false
+
+OpMethod:
+  Enabled: false
+
+ParameterLists:
+  Enabled: false
+
+PercentLiteralDelimiters:
+  PreferredDelimiters:
+    '%': '{}'
+
+PerlBackrefs:
+  Enabled: false
+
+PredicateName:
+  NamePrefixBlacklist:
+    - is_
+
+Proc:
+  Enabled: false
+
+RaiseArgs:
+  Enabled: false
+
+RegexpLiteral:
+  Enabled: false
+
+SelfAssignment:
+  Enabled: false
+
+SingleLineBlockParams:
+  Enabled: false
+
+SingleLineMethods:
+  Enabled: false
+
+SignalException:
+  Enabled: false
+
+SpecialGlobalVars:
+  Enabled: false
+
+VariableInterpolation:
+  Enabled: false
+
+TrailingComma:
+  Enabled: false
+
+TrivialAccessors:
+  Enabled: false
+
+VariableInterpolation:
+  Enabled: false
+
+WhenThen:
+  Enabled: false
+
+WhileUntilModifier:
+  Enabled: false
+
+WordArray:
+  Enabled: false
+
+# Lint
+
+AmbiguousOperator:
+  Enabled: false
+
+AmbiguousRegexpLiteral:
+  Enabled: false
+
+AssignmentInCondition:
+  Enabled: false
+
+ConditionPosition:
+  Enabled: false
+
+DeprecatedClassMethods:
+  Enabled: false
+
+ElseLayout:
+  Enabled: false
+
+HandleExceptions:
+  Enabled: false
+
+InvalidCharacterLiteral:
+  Enabled: false
+
+LiteralInCondition:
+  Enabled: false
+
+LiteralInInterpolation:
+  Enabled: false
+
+Loop:
+  Enabled: false
+
+ParenthesesAsGroupedExpression:
+  Enabled: false
+
+RequireParentheses:
+  Enabled: false
+
+UnderscorePrefixedVariableName:
+  Enabled: false
+
+Void:
+  Enabled: false
diff --git a/lib/rubocop/git.rb b/lib/rubocop/git.rb
index e4726cc..03b9409 100644
--- a/lib/rubocop/git.rb
+++ b/lib/rubocop/git.rb
@@ -1,7 +1,20 @@
 require 'rubocop/git/version'
+require 'rubocop'
+require 'active_support/core_ext/module/attribute_accessors'
 
 module RuboCop
   module Git
-    # Your code goes here...
+    autoload :FileCollection,    'rubocop/git/file_collection'
+    autoload :FileViolation,     'rubocop/git/file_violation'
+    autoload :Line,              'rubocop/git/line'
+    autoload :ModifiedFile,      'rubocop/git/modified_file'
+    autoload :Patch,             'rubocop/git/patch'
+    autoload :PseudoPullRequest, 'rubocop/git/pseudo_pull_request'
+    autoload :PseudoResource,    'rubocop/git/pseudo_resource'
+    autoload :Runner,            'rubocop/git/runner'
+    autoload :StyleChecker,      'rubocop/git/style_checker'
+    autoload :StyleGuide,        'rubocop/git/style_guide'
+
+    mattr_accessor :config_path
   end
 end
diff --git a/lib/rubocop/git/cli.rb b/lib/rubocop/git/cli.rb
new file mode 100644
index 0000000..d271708
--- /dev/null
+++ b/lib/rubocop/git/cli.rb
@@ -0,0 +1,42 @@
+require 'rubocop/git'
+require 'optparse'
+
+module RuboCop
+  module Git
+    class CLI
+      def run(args = ARGV)
+        options = parse_arguments(args)
+        Runner.new.run(options)
+      end
+
+      private
+
+      def parse_arguments(args)
+        options = {}
+
+        OptionParser.new do |opt|
+          opt.on('-c', '--config FILE',
+                 'Specify configuration file') do |config|
+            options[:config] = config
+          end
+
+          opt.on('--cached', 'git diff --cached') do
+            options[:cached] = true
+          end
+
+          opt.on('--staged', 'synonym of --cached') do
+            options[:cached] = true
+          end
+
+          opt.on('--hound', 'Hound compatibility mode') do
+            options[:hound] = true
+          end
+
+          opt.parse(args)
+        end
+
+        options
+      end
+    end
+  end
+end
diff --git a/lib/rubocop/git/file_collection.rb b/lib/rubocop/git/file_collection.rb
new file mode 100644
index 0000000..db0ca01
--- /dev/null
+++ b/lib/rubocop/git/file_collection.rb
@@ -0,0 +1,18 @@
+module RuboCop::Git
+# copy from https://github.com/thoughtbot/hound/blob/a6a8d3f/app/models/file_collection.rb
+class FileCollection
+  IGNORED_FILES = ['db/schema.rb']
+
+  attr_reader :files
+
+  def initialize(files)
+    @files = files
+  end
+
+  def relevant_files
+    files.reject do |file|
+      file.removed? || !file.ruby? || IGNORED_FILES.include?(file.filename)
+    end
+  end
+end
+end
diff --git a/lib/rubocop/git/file_violation.rb b/lib/rubocop/git/file_violation.rb
new file mode 100644
index 0000000..10e01f9
--- /dev/null
+++ b/lib/rubocop/git/file_violation.rb
@@ -0,0 +1,5 @@
+module RuboCop::Git
+# copy from https://github.com/thoughtbot/hound/blob/a6a8d3f/app/models/file_violation.rb
+class FileViolation < Struct.new(:filename, :offenses)
+end
+end
diff --git a/lib/rubocop/git/line.rb b/lib/rubocop/git/line.rb
new file mode 100644
index 0000000..c733578
--- /dev/null
+++ b/lib/rubocop/git/line.rb
@@ -0,0 +1,8 @@
+module RuboCop::Git
+# copy from https://github.com/thoughtbot/hound/blob/a6a8d3f/app/models/line.rb
+class Line < Struct.new(:content, :line_number, :patch_position)
+  def ==(other_line)
+    content == other_line.content
+  end
+end
+end
diff --git a/lib/rubocop/git/modified_file.rb b/lib/rubocop/git/modified_file.rb
new file mode 100644
index 0000000..a976650
--- /dev/null
+++ b/lib/rubocop/git/modified_file.rb
@@ -0,0 +1,58 @@
+require 'active_support/core_ext/object/try.rb'
+require 'linguist'
+
+module RuboCop::Git
+# copy from https://github.com/thoughtbot/hound/blob/a6a8d3f/app/models/modified_file.rb
+class ModifiedFile
+  def initialize(file, pull_request)
+    @file = file
+    @pull_request = pull_request
+  end
+
+  def relevant_line?(line_number)
+    modified_lines.detect do |modified_line|
+      modified_line.line_number == line_number
+    end
+  end
+
+  def filename
+    @file.filename
+  end
+
+  def removed?
+    @file.status == 'removed'
+  end
+
+  def ruby?
+    language == 'Ruby'
+  end
+
+  def contents
+    @contents ||= begin
+      unless removed?
+        @pull_request.file_contents(filename)
+      end
+    end
+  end
+
+  def modified_lines
+    @modified_lines ||= patch.additions
+  end
+
+  def modified_line_at(line_number)
+    modified_lines.detect do |modified_line|
+      modified_line.line_number == line_number
+    end
+  end
+
+  private
+
+  def language
+    @language ||= Linguist::Language.detect(filename, contents).try(:name)
+  end
+
+  def patch
+    Patch.new(@file.patch)
+  end
+end
+end
diff --git a/lib/rubocop/git/patch.rb b/lib/rubocop/git/patch.rb
new file mode 100644
index 0000000..4f0de2f
--- /dev/null
+++ b/lib/rubocop/git/patch.rb
@@ -0,0 +1,36 @@
+module RuboCop::Git
+# copy from https://github.com/thoughtbot/hound/blob/a6a8d3f/app/models/patch.rb
+class Patch
+  RANGE_INFORMATION_LINE = /^@@ .+\+(?<line_number>\d+),/
+  MODIFIED_LINE = /^\+(?!\+|\+)/
+  NOT_REMOVED_LINE = /^[^-]/
+
+  def initialize(body)
+    @body = body || ''
+  end
+
+  def additions
+    line_number = 0
+
+    lines.each_with_index.inject([]) do |additions, (content, patch_position)|
+      case content
+      when RANGE_INFORMATION_LINE
+        line_number = Regexp.last_match[:line_number].to_i
+      when MODIFIED_LINE
+        additions << Line.new(content, line_number, patch_position)
+        line_number += 1
+      when NOT_REMOVED_LINE
+        line_number += 1
+      end
+
+      additions
+    end
+  end
+
+  private
+
+  def lines
+    @body.lines
+  end
+end
+end
diff --git a/lib/rubocop/git/pseudo_pull_request.rb b/lib/rubocop/git/pseudo_pull_request.rb
new file mode 100644
index 0000000..75039bb
--- /dev/null
+++ b/lib/rubocop/git/pseudo_pull_request.rb
@@ -0,0 +1,34 @@
+module RuboCop
+  module Git
+    # ref. https://github.com/thoughtbot/hound/blob/a6a8d3f/app/models/pull_request.rb
+    class PseudoPullRequest
+      HOUND_CONFIG_FILE = '.hound.yml'
+
+      def initialize(files, options)
+        @files = files
+        @options = options
+      end
+
+      def pull_request_files
+        @files.map do |file|
+          ModifiedFile.new(file, self)
+        end
+      end
+
+      def file_contents(filename)
+        if @options[:cached]
+          `git show ':#{filename}'`
+        else
+          File.read(filename)
+        end
+      end
+
+      def config
+        return unless @options[:hound]
+        File.read(HOUND_CONFIG_FILE)
+      rescue Errno::ENOENT
+        nil
+      end
+    end
+  end
+end
diff --git a/lib/rubocop/git/pseudo_resource.rb b/lib/rubocop/git/pseudo_resource.rb
new file mode 100644
index 0000000..ecfe617
--- /dev/null
+++ b/lib/rubocop/git/pseudo_resource.rb
@@ -0,0 +1,6 @@
+module RuboCop
+  module Git
+    class PseudoResource < Struct.new(:filename, :status, :patch)
+    end
+  end
+end
diff --git a/lib/rubocop/git/runner.rb b/lib/rubocop/git/runner.rb
new file mode 100644
index 0000000..f8a6532
--- /dev/null
+++ b/lib/rubocop/git/runner.rb
@@ -0,0 +1,87 @@
+module RuboCop
+  module Git
+    # ref. https://github.com/thoughtbot/hound/blob/a6a8d3f/app/services/build_runner.rb
+    class Runner
+      DEFAULT_CONFIG_FILE = '.rubocop.yml'
+      HOUND_DEFAULT_CONFIG_FILE =
+        File.expand_path('../../../../hound.yml', __FILE__)
+
+      def run(options)
+        @options = options
+        @files = parse_diff(git_diff(options[:cached]))
+
+        set_config_path(options[:config], options[:hound])
+        display_violations($stdout)
+      end
+
+      private
+
+      def violations
+        @violations ||= style_checker.violations
+      end
+
+      def style_checker
+        StyleChecker.new(modified_files, pull_request.config)
+      end
+
+      def modified_files
+        collection = FileCollection.new(pull_request.pull_request_files)
+        collection.relevant_files
+      end
+
+      def pull_request
+        @pull_request ||= PseudoPullRequest.new(@files, @options)
+      end
+
+      def git_diff(cached)
+        if cached
+          `git diff --cached --diff-filter=AM`
+        else
+          `git diff --diff-filter=AM`
+        end
+      end
+
+      def parse_diff(diff)
+        files    = []
+        in_patch = false
+
+        diff.each_line do |line|
+          case line
+          when %r{^diff --git a/(.*) b/\1$}
+            files << PseudoResource.new($1, 'modified', '')
+            in_patch = false
+          when /^@@/
+            in_patch = true
+          end
+
+          files.last.patch << line if in_patch
+        end
+
+        files
+      end
+
+      def set_config_path(config, hound)
+        RuboCop::Git.config_path =
+          if hound
+            HOUND_DEFAULT_CONFIG_FILE
+          else
+            config || DEFAULT_CONFIG_FILE
+          end
+      end
+
+      def display_violations(io)
+        formatter = Rubocop::Formatter::ClangStyleFormatter.new(io)
+        formatter.started(nil)
+
+        violations.map do |violation|
+          formatter.file_finished(
+            violation.filename,
+            violation.offenses.compact.sort.freeze
+          )
+        end
+
+        formatter.finished(@files.map(&:filename).freeze)
+      end
+    end
+  end
+end
diff --git a/lib/rubocop/git/style_checker.rb b/lib/rubocop/git/style_checker.rb
new file mode 100644
index 0000000..0e22e69
--- /dev/null
+++ b/lib/rubocop/git/style_checker.rb
@@ -0,0 +1,36 @@
+module RuboCop::Git
+# ref. https://github.com/thoughtbot/hound/blob/a6a8d3f/app/models/style_checker.rb
+class StyleChecker
+  def initialize(modified_files, custom_config = nil)
+    @modified_files = modified_files
+    @custom_config = custom_config
+  end
+
+  def violations
+    file_violations = @modified_files.map do |modified_file|
+      FileViolation.new(modified_file.filename, offenses(modified_file))
+    end
+
+    file_violations.select do |file_violation|
+      file_violation.offenses.any?
+    end
+  end
+
+  private
+
+  def offenses(modified_file)
+    violations = style_guide.violations(modified_file)
+    violations_on_changed_lines(modified_file, violations)
+  end
+
+  def violations_on_changed_lines(modified_file, violations)
+    violations.select do |violation|
+      modified_file.relevant_line?(violation.line)
+    end
+  end
+
+  def style_guide
+    @style_guide ||= StyleGuide.new(@custom_config)
+  end
+end
+end
diff --git a/lib/rubocop/git/style_guide.rb b/lib/rubocop/git/style_guide.rb
new file mode 100644
index 0000000..9c38d69
--- /dev/null
+++ b/lib/rubocop/git/style_guide.rb
@@ -0,0 +1,41 @@
+module RuboCop::Git
+# ref. https://github.com/thoughtbot/hound/blob/a6a8d3f/app/models/style_guide.rb
+class StyleGuide
+  def initialize(override_config_content = nil)
+    @override_config_content = override_config_content
+  end
+
+  def violations(file)
+    parsed_source = parse_source(file)
+    team = Rubocop::Cop::Team.new(Rubocop::Cop::Cop.all, configuration)
+    commissioner = Rubocop::Cop::Commissioner.new(team.cops, [])
+    commissioner.investigate(parsed_source)
+  end
+
+  private
+
+  def parse_source(file)
+    Rubocop::SourceParser.parse(file.contents, file.filename)
+  end
+
+  def configuration
+    config = Rubocop::ConfigLoader
+               .configuration_from_file(RuboCop::Git.config_path)
+
+    if override_config
+      config = Rubocop::Config.new(
+        Rubocop::ConfigLoader.merge(config, override_config),
+        ''
+      )
+    end
+
+    config
+  end
+
+  def override_config
+    if @override_config_content
+      Rubocop::Config.new(YAML.load(@override_config_content))
+    end
+  end
+end
+end
diff --git a/rubocop-git.gemspec b/rubocop-git.gemspec
index d4cc925..32d3b53 100644
--- a/rubocop-git.gemspec
+++ b/rubocop-git.gemspec
@@ -20,4 +20,8 @@ Gem::Specification.new do |spec|
 
   spec.add_development_dependency 'bundler', '~> 1.6'
   spec.add_development_dependency 'rake'
+
+  spec.add_dependency 'activesupport',   '>= 3.0.0'
+  spec.add_dependency 'github-linguist', '~> 2.12.0'
+  spec.add_dependency 'rubocop',         '0.22.0'
 end