scrabble.rb

module Scrabble
  POINTS = {}
  {
    'EAIONRTLSU' => 1,
    'DG'         => 2,
    'BCMP'       => 3,
    'FHVWY'      => 4,
    'K'          => 5,
    'JX'         => 8,
    'QZ'         => 10,
  }.each do |letters, points|
    letters.downcase.chars.each do |letter|
      POINTS[letter] = points
    end
  end

  class Cheater
    attr_reader *%i[ word_map ]

    def initialize(words: [])
      words ||= File.read('/usr/share/dict/words').downcase.split("\n")
      @word_map = words.each
                       .with_object(Hash.new {|h,k| h[k] = Set.new }) {|word, map|
        key = word.chars.sort.join
        map[key] << word

        blanks(word).flat_map {|w| blanks(w) }.each do |blank|
          key = blank.chars.sort.join
          map[key] << word
        end
      }
    end

    def possible_words(tiles, required: '')
      tiles = tiles.downcase.chars
      words = (2..tiles.length).flat_map {|n|
        tiles.combination(n).flat_map {|combination|
          word_map[(combination << required).sort.join].to_a
        }
      }
      Hash[words.map {|word|
        points = word.chars.map {|c| POINTS[c] }.reduce(:+)
        [word, points]
      }]
    end

    private

    def blanks(word)
        word.length.times.map { |i|
          blank = word.chars
          blank[i] = ??
          blank.join
        }
    end
  end
end

require 'minitest/autorun'

class TestCheater < Minitest::Test
  include Scrabble

  def setup
    @cheater = Cheater.new(words: %w[ alice bob spot tops pasta ])
  end

  def test_word_map
    assert_equal Set.new(%w[ pasta ]), @cheater.word_map['aapst']
    assert_equal Set.new(%w[ alice ]), @cheater.word_map['aceil']
    assert_equal Set.new(%w[ bob ]), @cheater.word_map['bbo']
    assert_equal Set.new(%w[ spot tops ]), @cheater.word_map['opst']
    assert_equal Set.new(%w[ pasta ]), @cheater.word_map['?aapt']
    assert_equal Set.new(%w[ pasta ]), @cheater.word_map['??apt']
  end

  def test_possible_words
    assert_equal({ 'spot' => 6, 'tops' => 6 }, @cheater.possible_words('pots'))
    assert_equal({ 'spot' => 6, 'tops' => 6 }, @cheater.possible_words('potss'))
    assert_equal({ 'pasta' => 7 }, @cheater.possible_words('potsa', required: 'a'))
  end

  def test_blanks
    assert_equal({
      'spot' => 6,
      'tops' => 6,
      'pasta' => 7,
    }, @cheater.possible_words('pats?'))
  end
end

You own the input hash, so why do it in uppercase only to downcase it?