gem install ruby_indentation

ruby_indentation.rb

require 'coderay'
require 'set'

module RubyIndentation
  VERSION = '0.2.0'

  def self.[](buffer)
    opening_and_modifier_tokens = %w[if unless until while].to_set
    opening_tokens              = %w[begin case class def for module do {].to_set
    closing_tokens              = %w[end }].to_set
    separator                   = [';', :operator]
    indent                      = 0

    # parse each token
    buffer_tokens = [separator] + CodeRay.scan(buffer, :ruby).each_slice(2).select{|_, kind|
      kind != :space
    }

    buffer_tokens.each_cons(2){ |(*old_pair), (token, kind)|
      if kind == :keyword || kind == :operator
        # modifiers cause trouble, so
        #  fix it in 9/10 cases
        if opening_tokens.include?(token) ||
           opening_and_modifier_tokens.include?(token) &&
             ( old_pair == separator || old_pair == ['=', :operator ] )
          indent += 1
        elsif closing_tokens.include?(token)
          indent -= 1
        end
      end
    }
    # return a good value
    indent < 0 ? 0 : indent
  end
end

# J-_-L