Advent of Code 2022 (parts 1 & 2) using matrix multiplication

program_02.rb

#! /usr/bin/env ruby

require 'matrix'

class RPS
  attr_accessor :scoring, :choices

  def initialize
    @scoring = Matrix[
      [1, 0, 2],
      [2, 1, 0],
      [0, 2, 1]
    ]
    @choices = scoring.transpose.rotate_entries
  end

  def read_letter(letter)
    case letter
    when 'A', 'X' then 0
    when 'B', 'Y' then 1
    when 'C', 'Z' then 2
    else raise("Bad letter #{letter}")
    end
  end

  def score(opponent, player)
    o = read_letter(opponent)
    p = read_letter(player)
    (scoring * 3)[o, p] + (p + 1)
  end

  def score2(opponent, player)
    o = read_letter(opponent)
    p = read_letter(player)
    choice = choices[o, p]
    (scoring * 3)[o, choice] + (choice + 1)
  end
end

rps = RPS.new

result = STDIN
  .readlines
  .map(&:split)
  .map { |o, p| rps.score2(o, p) }
  .sum

puts result