/** Original test cartridge: no Nintendo logo, boot ROM, graphics, or game code.
 * A tiny CPU loop draws a striped tile and writes joypad reads to battery RAM.
 * This intentionally bypasses the product ROM gate ONLY in the test harness.
 */
export function syntheticRom(): Uint8Array {
  const rom = new Uint8Array(32768);
  rom.set([0xc3, 0x50, 0x01], 0x100); // JP $0150
  rom.set(new TextEncoder().encode("CRYSTALTEST"), 0x134);
  rom[0x143] = 0x80; // CGB-capable
  rom[0x147] = 0x10; // MBC3 + RTC + RAM + battery
  rom[0x148] = 0;
  rom[0x149] = 3; // 32 KiB RAM
  const code = [
    0xf3, // DI
    0x31,
    0xfe,
    0xff, // LD SP,$fffe
    0xaf,
    0xe0,
    0x40, // LCD off
    0x3e,
    0x0a,
    0xea,
    0,
    0, // enable cartridge RAM
    0x3e,
    0x80,
    0xe0,
    0x68, // CGB background palette auto-increment
    0x3e,
    0xff,
    0xe0,
    0x69,
    0x3e,
    0x7f,
    0xe0,
    0x69, // white
    0xaf,
    0xe0,
    0x69,
    0xe0,
    0x69, // black
    0x21,
    0,
    0x80,
    0x06,
    0x10,
    0x3e,
    0xaa, // HL=$8000, B=16, A=$aa
    0x22,
    0x05,
    0x20,
    0xfc, // tile pattern
    0x3e,
    0xe4,
    0xe0,
    0x47, // DMG palette
    0x3e,
    0x91,
    0xe0,
    0x40, // LCD on, background tile 0
  ];
  const loop = 0x150 + code.length;
  code.push(
    0x3e,
    0x10,
    0xe0,
    0,
    0xf0,
    0,
    0xea,
    0,
    0xa0,
    0xc3,
    loop & 255,
    loop >> 8,
  );
  rom.set(code, 0x150);
  let checksum = 0;
  for (let i = 0x134; i <= 0x14c; i++) checksum = (checksum - rom[i] - 1) & 255;
  rom[0x14d] = checksum;
  return rom;
}