/** Bounds-checked access to the user's cartridge. No CPU or hardware state. */
export class RomReader {
  constructor(readonly bytes: Uint8Array) {}
  byte(offset: number): number {
    this.check(offset, 1);
    return this.bytes[offset];
  }
  word(offset: number): number {
    return this.byte(offset) | (this.byte(offset + 1) << 8);
  }
  slice(offset: number, length: number): Uint8Array {
    this.check(offset, length);
    return this.bytes.slice(offset, offset + length);
  }
  address(bank: number, address: number): number {
    if (
      !Number.isInteger(bank) ||
      bank < 0 ||
      bank >= this.bytes.length / 0x4000 ||
      !Number.isInteger(address) ||
      address < 0 ||
      address >= 0x8000
    ) {
      throw new Error("Invalid ROM bank or address.");
    }
    const offset =
      address < 0x4000 ? address : bank * 0x4000 + address - 0x4000;
    this.check(offset, 1);
    return offset;
  }
  near(pointer: number, bank: number): number {
    return this.address(bank, this.word(pointer));
  }
  far(pointer: number): number {
    return this.near(pointer + 1, this.byte(pointer));
  }
  bank(offset: number): number {
    this.check(offset, 1);
    return Math.floor(offset / 0x4000);
  }
  private check(offset: number, length: number) {
    if (
      !Number.isInteger(offset) ||
      !Number.isInteger(length) ||
      offset < 0 ||
      length < 0 ||
      offset + length > this.bytes.length
    ) {
      throw new Error(
        "Cartridge data is truncated or contains an invalid pointer.",
      );
    }
  }
}