class PeekableCursor {
    constructor(cursor) {
        this.cursor = cursor;
        this.stashedDoc = undefined;
    }

    hasNext() {
        return this.cursor.hasNext();
    }

    peekNext() {
        if (this.stashedDoc === undefined) {
            this.stashedDoc = this.cursor.next();
        }
        return this.stashedDoc;
    }

    next() {
        const result = (this.stashedDoc === undefined) ? this.cursor.next() : this.stashedDoc;
        this.stashedDoc = undefined;
        return result;
    }
}