enum DiffOp {
  Add = 'ADD',
  Remove = 'REMOVE',
}

class DiffNode {
  op: DiffOp;

  item: string;

  constructor(op: DiffOp, item: string) {
    this.op = op;
    this.item = item;
  }
}

export default function diffStringList(
  a: Array<string>,
  b: Array<string>,
): Array<DiffNode> {
  const diff: Array<DiffNode> = [];
  const set = new Set(a);
  for (const item of b) {
    if (set.has(item)) {
      set.delete(item);
    } else {
      diff.push(new DiffNode(DiffOp.Add, item));
    }
  }
  for (const item of set) {
    diff.push(new DiffNode(DiffOp.Remove, item));
  }
  return diff;
}