day_18.rb

class Program
  def initialize(id, instructions, input, output)
    @id, @instructions, @input, @output = id, instructions, input, output
    @registers = Hash.new {|h,k|
      if k =~ /[a-z]/
        h[k] = 0
      else
        h[k] = k.to_i
      end
    }
    @registers["p"] = id
    @pc = 0
    @sent = 0
  end

  def run
    while instruction = @instructions[@pc]
      case instruction
      when /snd (\w)/
        @output << @registers[$1]
        p "#{@id} -> #{@registers[$1]}"
        @sent += 1
        p @sent if @id == 1
        @pc += 1
      when /set (\w) (.+)/
        @registers[$1] = @registers[$2]
        @pc += 1
      when /add (\w) (.+)/
        @registers[$1] += @registers[$2]
        @pc += 1
      when /mul (\w) (.+)/
        @registers[$1] *= @registers[$2]
        @pc += 1
      when /mod (\w) (.+)/
        @registers[$1] %= @registers[$2]
        @pc += 1
      when /rcv (\w)/
        until value = @input.shift
          Thread.pass
        end
        p "#{@id} <- #{value}"
        @registers[$1] = value
        @pc += 1
      when /jgz (\w) (.+)/
        @pc += if @registers[$1].zero?
                1
              else
                @registers[$2]
              end
      else
        raise instruction
      end
    end
  end
end

instructions = ARGF.read.strip.lines

instructions = <<EOF.strip.lines
snd 1
snd 2
snd p
rcv a
rcv b
rcv c
rcv d
EOF

queue_0 = []
queue_1 = []

program_0 = Program.new(0, instructions, queue_1, queue_0)
program_1 = Program.new(1, instructions, queue_0, queue_1)

threads = []
threads << Thread.new { program_0.run }
threads << Thread.new { program_1.run }
threads.each do |thread|
  thread.join
end