Simple anagram finder in Ruby

anagrams.rb

#!/usr/bin/env ruby

require 'set'

def usage
  puts 'usage: #{$PROGRAM_NAME} WORD...'
  exit(1)
end

usage if ARGV.length.zero?

def permutations(word)
  characters = word.downcase.split('')
  characters.permutation(word.length).map(&:join).to_set
end

def dictionary_words(dictionary_path)
  File.readlines(dictionary_path)
    .map(&:downcase)
    .map(&:chomp)
    .to_set
end

def anagrams(word, dictionary)
  permutations(word) & dictionary
end

def main
  dictionary = dictionary_words('/usr/share/dict/words')

  ARGV.each do |word|
    puts anagrams(word, dictionary).to_a
  end
end

main