day8.rb

module Day8

  REGEXP = %r{
    ([[:alpha:]]+)
    \s
    (inc|dec)
    \s
    (-?\d+)
    \s
    if
    \s
    ([[:alpha:]]+)
    \s
    ([<>=!]+)
    \s
    (-?\d+)
  }x

  module_function

  def each
    File.readlines('./day8.txt').each do |line|
      if m = line.match(REGEXP)
        yield [m[1], m[2], m[3].to_i, m[4], m[5], m[6].to_i]
      end
    end
  end

  def solve
    registers = {}

    instructions = []

    max = 0

    each do |object_name, op1, mod, object_cond, op2, cond_val|
      registers[object_name] = 0
      registers[object_cond] = 0

      instructions << [object_name, op1, mod, object_cond, op2, cond_val]
    end

    instructions.each do |object_name, op1, mod, object_cond, op2, cond_val|
      object_cond = registers[object_cond]
      check_result = case op2
      when '>='
        object_cond >= cond_val
      when '>'
        object_cond > cond_val
      when '!='
        object_cond != cond_val
      when '=='
        object_cond == cond_val
      when '<='
        object_cond <= cond_val
      when '<'
        object_cond < cond_val
      else
        raise op2
      end

      if check_result
        if op1 == 'dec'
          registers[object_name] -= mod
        else
          registers[object_name] += mod
        end
      end

      max = registers[object_name] if registers[object_name] > max
    end

    [registers.values.max, max]
  end
end

p Day8.solve