letters.rb

#! /usr/bin/env ruby

require 'minitest/autorun'
require 'minitest/spec'

class Letters
  WORDS = %w.cat bat bath baths.

  attr_accessor :letters

  def initialize(letters)
    @letters = letters
  end

  def find_longest
    matches.group_by(&:size).max.last[0]
  end

  def matches
    WORDS.select { |word| word if contains_letters?(word) }
  end

  def contains_letters?(word)
    (sorted_word(word) - sorted_letters).empty?
  end

  def sorted_word(word)
    word.chars.to_a.sort
  end

  def sorted_letters
    letters.chars.to_a.sort
  end
end

describe Letters do
  before { @result = Letters.new('abhtg').find_longest }

  it 'finds bath' do
    assert_equal 'bath', @result
  end

  %w.cat bat baths..each do |word|
    it "does not find #{word}" do
      refute_equal word, @result
    end
  end
end