registers, program = DATA.read.split("\n\n")

a, b, c = registers.scan(/\d+/).map(&:to_i)
program = program.scan(/\d+/).map(&:to_i)

combo = ->(operand) {
  case operand
  when 0, 1, 2, 3 then operand
  when 4 then a
  when 5 then b
  when 6 then c
  else
    fail
  end
}

ip = 0
out = []
while (0...program.size).cover?(ip)
  opcode = program.fetch(ip)
  operand = program.fetch(ip+1)
  case opcode
  when 0
    a = (a / 2**combo[operand]).to_i
    ip += 2
  when 1
    b  = b ^ operand
    ip += 2
  when 2
    b = combo[operand] % 8
    ip += 2
  when 3
    if a.zero?
      ip += 2
    else
      ip = operand
    end
  when 4
    b  = b ^ c
    ip += 2
  when 5
    out << combo[operand] % 8
    ip += 2
  when 6
    b = (a / 2**combo[operand]).to_i
    ip += 2
  when 7
    c = (a / 2**combo[operand]).to_i
    ip += 2
  else
    fail
  end
end

puts out.join(?,)

__END__
Register A: 729
Register B: 0
Register C: 0

Program: 0,1,5,4,3,0