internal GroupEnumerator(LocalList <T> list, Func <T, TKey> keySelector, EqualityComparer <TKey> comparer)
            {
                var values     = new LocalList <Row <TKey, T> >(list.Length);
                var uniqueKeys = new LocalList <TKey>();

                for (var i = 0; i < list.Length; i++)
                {
                    var element = list.Get(i);
                    var key     = keySelector(element);

                    values.Add(new Row <TKey, T>(key, element));

                    if (!uniqueKeys.Contains(key, comparer))
                    {
                        uniqueKeys.Add(key);
                    }
                }

                _comparer   = comparer;
                _current    = default;
                _position   = 0;
                _values     = values;
                _uniqueKeys = uniqueKeys;
            }