iterable-iterator/tee

import assert from 'assert';

import {
    Node,
    _value,
    _setValue,
    _isLast,
    _shift,
    _setNext,
    single,
} from '@data-structure-algebra/singly-linked-list';

/**
 * Create a copy of an input iterator given a buffer cursor.
 *
 * @param {Iterator} iterator Input iterator.
 * @param {Node} cursor Buffer cursor.
 * @return {IterableIterator}
 */
const copy = (iterator, cursor) => {
    assert(cursor instanceof Node);

    return {
        [Symbol.iterator]() {
            return this;
        },
        next() {
            if (_isLast(cursor)) {
                assert(_value(cursor) === null);
                _setValue(cursor, iterator.next());
                _setNext(cursor, _value(cursor).done ? cursor : single(null));
            }

            assert(!_isLast(cursor));

            const returnValue = _value(cursor);

            cursor = _shift(cursor);

            return returnValue;
        },
    };
};

export default copy;