module Waterfoul
  # The bootrom is a special program that is stored internally on the device
  # and is executed when the device is first turned on.
  #
  # The main purpose of the program is to bootstrap the device into a state
  # that is ready to run the game program. Some of the notable tasks the
  # program will perform are:
  # - zero all vram
  # - print and scroll a "nintendo" logo to the middle of the screen
  # - 2 notable sounds are made
  # - perform integrity check of the game program, basic form of DRM
  #
  # A video of the display showing the bootstrap process can be found here:
  # https://www.youtube.com/watch?v=-Fh9SpWFdn0
  #
  # This program is mapped to memory at 0x00 to 0xFF when the device is turned on.
  # The last instruction in the program will unmap the boot rom, making any read/write
  # between 0x00 and 0xFF be sent to the game program instead.
  #
  # A detailed blog post of the gameboy's boostrap process can be found here:
  # https://realboyemulator.wordpress.com/2013/01/03/a-look-at-the-game-boy-bootstrap-let-the-fun-begin/
  #
  # The source code of the Gameboy's boot rom can be found here:
  # http://gbdev.gg8.se/wiki/articles/Gameboy_Bootstrap_ROM
  class BootROM

    ROM = [
      0x31, 0xFE, 0xFF, 0xAF, 0x21, 0xFF, 0x9F, 0x32, 0xCB, 0x7C, 0x20, 0xFB,
      0x21, 0x26, 0xFF, 0x0E, 0x11, 0x3E, 0x80, 0x32, 0xE2, 0x0C, 0x3E, 0xF3,
      0xE2, 0x32, 0x3E, 0x77, 0x77, 0x3E, 0xFC, 0xE0, 0x47, 0x11, 0x04, 0x01,
      0x21, 0x10, 0x80, 0x1A, 0xCD, 0x95, 0x00, 0xCD, 0x96, 0x00, 0x13, 0x7B,
      0xFE, 0x34, 0x20, 0xF3, 0x11, 0xD8, 0x00, 0x06, 0x08, 0x1A, 0x13, 0x22,
      0x23, 0x05, 0x20, 0xF9, 0x3E, 0x19, 0xEA, 0x10, 0x99, 0x21, 0x2F, 0x99,
      0x0E, 0x0C, 0x3D, 0x28, 0x08, 0x32, 0x0D, 0x20, 0xF9, 0x2E, 0x0F, 0x18,
      0xF3, 0x67, 0x3E, 0x64, 0x57, 0xE0, 0x42, 0x3E, 0x91, 0xE0, 0x40, 0x04,
      0x1E, 0x02, 0x0E, 0x0C, 0xF0, 0x44, 0xFE, 0x90, 0x20, 0xFA, 0x0D, 0x20,
      0xF7, 0x1D, 0x20, 0xF2, 0x0E, 0x13, 0x24, 0x7C, 0x1E, 0x83, 0xFE, 0x62,
      0x28, 0x06, 0x1E, 0xC1, 0xFE, 0x64, 0x20, 0x06, 0x7B, 0xE2, 0x0C, 0x3E,
      0x87, 0xF2, 0xF0, 0x42, 0x90, 0xE0, 0x42, 0x15, 0x20, 0xD2, 0x05, 0x20,
      0x4F, 0x16, 0x20, 0x18, 0xCB, 0x4F, 0x06, 0x04, 0xC5, 0xCB, 0x11, 0x17,
      0xC1, 0xCB, 0x11, 0x17, 0x05, 0x20, 0xF5, 0x22, 0x23, 0x22, 0x23, 0xC9,
      0xCE, 0xED, 0x66, 0x66, 0xCC, 0x0D, 0x00, 0x0B, 0x03, 0x73, 0x00, 0x83,
      0x00, 0x0C, 0x00, 0x0D, 0x00, 0x08, 0x11, 0x1F, 0x88, 0x89, 0x00, 0x0E,
      0xDC, 0xCC, 0x6E, 0xE6, 0xDD, 0xDD, 0xD9, 0x99, 0xBB, 0xBB, 0x67, 0x63,
      0x6E, 0x0E, 0xEC, 0xCC, 0xDD, 0xDC, 0x99, 0x9F, 0xBB, 0xB9, 0x33, 0x3E,
      0x3c, 0x42, 0xB9, 0xA5, 0xB9, 0xA5, 0x42, 0x4C, 0x21, 0x04, 0x01, 0x11,
      0xA8, 0x00, 0x1A, 0x13, 0xBE, 0x20, 0xFE, 0x23, 0x7D, 0xFE, 0x34, 0x20,
      0xF5, 0x06, 0x19, 0x78, 0x86, 0x23, 0x05, 0x20, 0xFB, 0x86, 0x20, 0xFE,
      0x3E, 0x01, 0xE0, 0x50
    ].freeze

    attr_reader :cartridge

    def initialize(cartridge)
      @cartridge = cartridge
    end

    # Read bootstrap instruction given an index (memory location)
    def [](i)
      ROM[i] || @cartridge[i]
    end

    def []=(i, v)
      @cartridge[i] = v
    end
  end
end
