class Memory{
  constructor(size, ro){
    this.__ro = (ro === true);
    this.__map = null;
    this.__addr = 0;
    if (size > 0){
      this.__map = new Uint8Array(size);
    }
    this.__listeners = {};
  }

  get size(){return (this.__map) ? this.__map.length : 0;}

  get address(){return this.__addr;}
  set address(a){
    if (this.__map)
      this.__addr = Math.min(this.__map.length, Math.max(0, a));
  }

  get byte(){return (this.__map) ? this.__map[this.__addr] : -1;}
  set byte(b){
    if (!this.__ro && this.__map){
      this.__map[this.__addr] = b;
      if (this.__addr in this.__listeners)
        this.__listeners[this.__addr].forEach((fn)=>{fn(b);});
    }
  }

  onAddressWrite(addr, fn){
    if (addr < 0 || addr >= this.size)
      throw new RangeError("Memory address is out of bounds.");
    if (typeof(fn) !== 'function')
      throw new TypeErrpr("Expected a callback function.");
    if (!(addr in this.__listeners))
      this.__listeners[addr] = [];
    // WARNING: Not testing to see if using the same function more than once.
    this.__listeners[addr].push(fn);
  }

  // This method is intended for the emulator to fill a Read-Only memory module with data prior to
  // the start of execution. This method should never be used in a running system.
  sysStore(address, data){
    if (this.__map){
      let dc = data;
      if (address < 0 || address >= this.__map.length)
        throw new RangeError("Memory address out of range.");
      if (this.__map.length - address < dc.length)
        dc = dc.slice(0, this.__map.length - address);
      this.__map.set(address, dc);
      return dc.length;
    }
  }
}


module.exports = Memory;