dis.rb

# frozen_string_literal: true

rom_name = ARGV[0]
if ARGV.empty?
  puts 'emu.rb <name of ROM>'
  exit
end
f = File.open("ROMS/#{rom_name}", 'rb')

rom = f.read

$address    = 0x200
$current_op = 0x0

def h(n)
  n.to_s(16)
end

def pretty_print(arg)
  puts "0x#{h($address)} : #{h($current_op).rjust(4, '0')}:  #{arg}"
end

def cls
  pretty_print 'CLS'
end

def ret
  pretty_print 'RET'
end

def jmp(address)
  pretty_print "JMP #{h(address)}"
end

def call(address)
  pretty_print "CALL #{h(address)}"
end

def set_i(address)
  pretty_print "I = #{h(address)}"
end

def jmp_v0(address)
  pretty_print "JMP #{h(address)} + V0"
end

def skip_next_if_vx_equal(vx, val)
  pretty_print "SKP IF V#{h(vx)} == #{h(val)}"
end

def skip_next_if_vx_not_equal(vx, val)
  pretty_print "SKP IF V#{h(vx)} != #{h(val)}"
end

def skip_next_if_vx_equal_vy(vx, vy)
  pretty_print "SKP IF V#{h(vx)} == V#{h(vy)}"
end

def skip_next_if_vx_not_equal_vy(vx, vy)
  pretty_print "SKP IF V#{h(vx)} != V#{h(vy)}"
end

def set_vx(vx, val)
  pretty_print "SET V#{h(vx)} = #{h(val)}"
end

def add_vx(vx, val)
  pretty_print "SET V#{h(vx)} = V#{h(vx)} + #{h(val)}"
end

def set_vx_rand(vx, val)
  pretty_print "SET V#{h(vx)} = RAND() & #{h(val)}"
end

def set_vx_to_vy(vx, vy)
  pretty_print "SET V#{h(vx)} = V#{h(vy)}"
end

def set_vx_to_vx_or_vy(vx, vy)
  pretty_print "SET V#{h(vx)} = V#{h(vx)} | V#{h(vy)}"
end

def set_vx_to_vx_and_vy(vx, vy)
  pretty_print "SET V#{h(vx)} = V#{h(vx)} & V#{h(vy)}"
end

def set_vx_to_vx_xor_vy(vx, vy)
  pretty_print "SET V#{h(vx)} = V#{h(vx)} ^ V#{h(vy)}"
end

def add_vy_to_vx(vx, vy)
  # 8XY4: VX += VY (VF set to 1 on carry)
  pretty_print "SET V#{h(vx)} = V#{h(vx)} + V#{h(vy)}"
end

def subt_vy_from_vx(vx, vy)
  # 8XY5: VX = VX - VY (VF set to 0 on borrow)
  pretty_print "SET V#{h(vx)} = V#{h(vx)} - V#{h(vy)}"
end

def sub_vx_from_vy(vx, vy)
  # 8XY7: VX = VY - VX (VF set to 0 on borrow)
  pretty_print "SET V#{h(vx)} = V#{h(vy)} - V#{h(vx)}"
end

def right_shift_vx(vx)
  pretty_print "SET V#{h(vx)} = V#{h(vx)} >> 1"
end

def left_shift_vx(vx)
  pretty_print "SET V#{h(vx)} = V#{h(vx)} << 1"
end

def skip_if_pressed(vx)
  pretty_print "SKP IF KEY IN #{h(vx)} IS DOWN"
end

def skip_if_not_pressed(vx)
  pretty_print "SKP IF KEY IN #{h(vx)} IS NOT DOWN"
end

def draw(vx, vy, n)
  pretty_print "DRAW AT (#{h(vx)}, #{h(vy)}) Height: #{h(n)}"
end

def wait_for_keypress(vx)
  # FX0A: Wait for key press, store in VX
  pretty_print "WAIT -> #{h(vx)}"
end

def set_vx_to_delay_timer(vx)
  # FX07: VX = Delay Timer value
  pretty_print "SET V#{h(vx)} to DELAY TIMER"
end

def set_delay_timer_to_vx(vx)
  # FX15: Delay Timer = VX
  pretty_print "SET DELAY TIMER TO V#{h(vx)}"
end

def set_sound_timer_to_vx(vx)
  # FX18: Sound Timer = VX
  pretty_print "SET SOUND TIMER TO V#{h(vx)}"
end

def increment_index_register(vx)
  # FX1E: I += VX
  pretty_print "I += V#{h(vx)}"
end

def set_index_register_to_sprite(vx)
  # FX29: Sets I to location of character sprite for VX
  pretty_print "I = SPRITE(V#{h(vx)})"
end

def set_bcd_of_vx(vx)
  # FX33: Sets *I - *I+2 to BCD value of VX
  pretty_print "I = BCD(V#{h(vx)})"
end

def copy_vx_to_memory(vx)
  # FX55: Stores V0-VX in memory at address I. I += N + 1
  pretty_print "COPY V0-V#{h(vx)} TO *I"
end

def copy_memory_to_vx(vx)
  # FX65: Fills V0-VX with memory from address I. I += N + 1
  pretty_print "COPY *I TO V0-V#{h(vx)}"
end

rom.split('').each_slice(2) do |op_arr|
  op = op_arr.join.unpack1('n')
  $current_op = op
  # pretty_print op.to_s(16)

  # Exact Match OP-codes
  if op == 0x00E0          # 00E0 - clear screen
    cls
  elsif op == 0x00EE       # 00EE - return
    ret
  elsif (op >> 12) == 0x1  # 1NNN - jump to
    jmp(op & 0xFFF)
  elsif (op >> 12) == 0x2  # 2NNN - call
    call(op & 0xFFF)
  elsif (op >> 12) == 0x3  # 3XNN - skip if VX == NN
    skip_next_if_vx_equal((op >> 8) & 0xF, op & 0xFF)
  elsif (op >> 12) == 0x4  # 4XNN - skip if VX != NN
    skip_next_if_vx_not_equal((op >> 8) & 0xF, op & 0xFF)
  elsif (op >> 12) == 0x6  # 6XNN - set VX = NN
    set_vx((op >> 8) & 0xF, op & 0xFF)
  elsif (op >> 12) == 0x7  # 7XNN - set VX += NN
    add_vx((op >> 8) & 0xF, op & 0xFF)
  elsif (op >> 12) == 0xA  # ANNN - set I to
    set_i(op & 0xFFF)
  elsif (op >> 12) == 0xB  # BNNN - jmp to + v0
    jmp_v0(op & 0xFFF)
  elsif (op >> 12) == 0xC  # CXNN - jvx = rand & NN
    set_vx_rand((op >> 8) & 0xF, op & 0xFF)

  ## Ops involving two registers
  elsif (op >> 12) == 0x5  # 5XY0 - skip if vx == vy
    skip_next_if_vx_equal_vy((op >> 8) & 0xF, (op >> 4) & 0xF)
  elsif (op >> 12) == 0x9  # 9XY0 - skip if vx != vy
    skip_next_if_vx_not_equal_vy((op >> 8) & 0xF, (op >> 4) & 0xF)

  elsif (op >> 12) == 0x8  # A whole bunch of ops start with 0x8NNN
    cmd = op & 0xF # And it's all based on the last 4-bits
    a   = (op >> 8) & 0xF
    b   = (op >> 4) & 0xF
    case cmd
    when 0x0 # 8XYO - VX = VY
      set_vx_to_vy(a, b)
    when 0x1     # 8XY1 - VX = VX | VY
      set_vx_to_vx_or_vy(a, b)
    when 0x2     # 8XY2: VX = VX & VY
      set_vx_to_vx_and_vy(a, b)
    when 0x3     # 8XY3: VX = VX xor VY
      set_vx_to_vx_xor_vy(a, b)
    when 0x4     # 8XY4: VX += VY (VF set to 1 on carry)
      add_vy_to_vx(a, b)
    when 0x5     # 8XY5: VX = VX - VY (VF set to 0 on borrow)
      subt_vy_from_vx(a, b)
    when 0x7     # 8XY7: VX = VY - VX (VF set to 0 on borrow)
      sub_vx_from_vy(a, b)
    when 0x6     # 8X06: VX = VX >> 1 (VF set to bit shifted out)
      right_shift_vx(a)
    when 0xE     # 8X0E: VX = VX << 1 (VF set to bit shifted out)
      left_shift_vx(a)
    else
      pretty_print 'Unhandled 0x8 op code?!'
    end
  elsif (op >> 12) == 0XE # Hardware operations
    if (op & 0xFF) == 0x9E # EX9E: Skip next instruction if key stored in VX is pressed
      skip_if_pressed((op >> 8) & 0xF)
    elsif (op & 0xFF) == 0xA1 # EXA1: Skip next instruction if key stored in VX is not pressed
      skip_if_not_pressed((op >> 8) & 0xF)
    else
      pretty_print 'Unhandled 0xE op code'
    end
  elsif (op >> 12) == 0xF # Lots of various commands in "F"
    cmd = op & 0xFF # Stored in the last 8-bits
    arg = (op >> 8) & 0xF
    case cmd
    when 0x0A # FX0A: Wait for key press, store in VX
      wait_for_keypress(arg)
    when 0x07 # FX07: VX = Delay Timer value
      set_vx_to_delay_timer(arg)
    when 0x15 # FX15: Delay Timer = VX
      set_delay_timer_to_vx(arg)
    when 0x18 # FX18: Sound Timer = VX
      set_sound_timer_to_vx(arg)
    when 0x1E # FX1E: I += VX
      increment_index_register(arg)
    when 0x29 # FX29: Sets I to location of character sprite for VX
      set_index_register_to_sprite(arg)
    when 0x33 # FX33: Sets *I - *I+2 to BCD value of VX
      set_bcd_of_vx(arg)
    when 0x55 # FX55: Stores V0-VX in memory at address I. I += N + 1
      copy_vx_to_memory(arg)
    when 0x65 # FX65: Fills V0-VX with memory from address I. I += N + 1
      copy_memory_to_vx(arg)
    else
      pretty_print "Unrecognized 0XF op-code (#{h(op)})"
    end
  elsif (op >> 12) == 0xD # DXYN - Draw sprite at vx, vy, height n, sprite at I
    draw((op >> 8) & 0xF, (op >> 4) & 0xF, op & 0xF)
  else
    pretty_print "Unrecognized op-code (#{h(op)})"
  end

  $address += 2
end