List unnecessary homebrew formulae.

brew-list-unnecessaries.rb

require 'formula'
require 'set'

DESCRIPTION = <<EOS
List unnecessary formulae except for specified and its dependant formulae
usage:
  brew list-unnecessaries FORMULA...

exmaple:
  brew list-unnecessaries git rbenv
EOS

if ARGV.include?('-h') || ARGV.include?('--help')
  puts DESCRIPTION
  exit 0
end

INSTALLED_FORMULA_MAP = Formula.installed.each_with_object({}) do |f, result|
  result[f.name] = f
end

required_formulae = Set.new
ARGV.each do |arg|
  if INSTALLED_FORMULA_MAP.key?(arg)
    required_formulae << INSTALLED_FORMULA_MAP[arg]
  else
    STDERR.puts "Uninstalled formula #{arg} is specified."
    exit 1
  end
end

# add dependant formulae
next_search = required_formulae
until next_search.empty?
  deps, reqs = [], []
  next_search.each do |f|
    deps << f.deps.default.map(&:to_formula)
    reqs << f.requirements.to_a.map(&:default_formula).map {|df| INSTALLED_FORMULA_MAP[df]}.compact
  end
  dependant = Set.new([deps, reqs].flatten)

  next_search = dependant - required_formulae
  required_formulae.add(dependant).flatten!
end

unnecessary_formulae = Set.new(Formula.installed) - required_formulae
unnecessary_formulae.sort.each do |f|
  puts f.name
end