anagram.rb

class String
  def char_counts_hash
    char_hash = Hash.new(0)
    each_char {|char| char_hash[char] += 1 }
    char_hash
  end

  def anagram_of?(word)
    word.char_counts_hash == char_counts_hash
  end
end

class Anagram
  def initialize(word)
    @word = word
  end

  attr_reader :word

  def match(words)
    words.find_all {|anagram| word.anagram_of?(anagram) }
  end

end

test.rb

require 'minitest/autorun'
require 'minitest/pride'
require_relative './anagram'

class AnagramTest < MiniTest::Unit::TestCase

  def test_no_matches
    detector = Anagram.new('diaper')
    assert_equal [], detector.match(%w(hello world zombies pants))
  end

  def test_detect_simple_anagram
    detector = Anagram.new('ba')
    anagrams = detector.match(['ab', 'abc', 'bac'])
    assert_equal ['ab'], anagrams
  end

  def test_detect_anagram
    detector = Anagram.new('listen')
    anagrams = detector.match %w(enlists google inlets banana)
    assert_equal ['inlets'], anagrams
  end

  def test_multiple_anagrams
    detector = Anagram.new('allergy')
    anagrams = detector.match %w(gallery ballerina regally clergy largely leading)
    assert_equal ['gallery', 'regally', 'largely'], anagrams
  end
end