Example custom metric for Square's Cane gem. Checks source code for `puts` and `print` left behind by developers.

puts_check.rb

require 'cane/file'
require 'cane/task_runner'

class PutsCheck < Struct.new(:opts)
  DESCRIPTION =
    "Lines output to console using `puts` or `print`"
  PUTS_REGEX = /^\s*p(uts|rint)?[\s\(]+(.+?)\s*[\)\s]*$/
  PUTS_GLOB = '{app,lib}/**/*.rb'

  def self.key; :puts; end
  def self.name; "puts output checking"; end
  def self.options
    {
      puts_glob:    ['Glob to run puts checks over',
                       default:   PUTS_GLOB,
                       variable:  'GLOB',
                       clobber:   :no_puts],
      puts_exclude:  ['Exclude file or glob from puts checking',
                       variable:  'GLOB',
                       type:      Array,
                       default:   [],
                       clobber:   :no_puts],
      no_puts:      ['Disable puts checking', cast: ->(x) { !x }]
    }
  end

  def violations
    return [] if opts[:no_puts]

    worker.map(file_names) do |file_name|
      find_violations(file_name)
    end.flatten
  end

  def find_violations(file_name)
    Cane::File.iterator(file_name).map.with_index do |line, number|
      puts_match = line.match(PUTS_REGEX)
      result = if !!puts_match
                 {
                   file: file_name,
                   line: number + 1,
                   label: "Line outputs '#{puts_match[2]}'",
                   description: DESCRIPTION
                 }
               end
    end.compact
  end

  def file_names
    Dir[opts.fetch(:puts_glob, PUTS_GLOB)].reject { |file| excluded?(file) }
  end

  def exclusions
    @exclusions ||= opts.fetch(:puts_exclude, []).flatten.map do |i|
      Dir[i]
    end.flatten.to_set
  end

  def excluded?(file)
    exclusions.include?(file)
  end

  def worker
    Cane.task_runner(opts)
  end
end