nodejs simple offset helper

buffer-offset.ts

export class SmartCursor {

  private _cur: number;
  private _memorized: number;

  constructor(i: number = 0) {
    this._cur = i;
    this._memorized = i;
  }

  private check(i: number) {
    if (i < 0) {
      throw Error('Index should not be zero or negative number');
    }
  }

  jump(i: number) {
    this.check(i);
    this._cur = i;
    this._memorized = i;
    return this;
  }

  diff(sync = true) {
    const memorized = this._cur - this._memorized;
    if (sync) {
      this.jump(this._cur);
    }
    return memorized;
  }

  memorize() {
    this._memorized = this._cur;
    return this;
  }

  next(nextPos = 1) {
    this.check(nextPos);
    const clone = this._cur;
    this._cur += nextPos;
    return clone;
  }

  get memorized() {
    return this._memorized;
  }

  get cur() {
    return this._cur;
  }
}

const pos = new SmartCursor();
const buf = Buffer.from([0x01, 0x02, 0x03]);

buf.readUInt8(pos.next()); // => 0x01
buf.readUInt16BE(pos.next(2)); // => 0x0203

const readSize = pos.diff(); // => 3 bytes
const isBufferEnd = buf.length === pos.cur; // => true