iterable-iterator/slice

import assert from 'assert';

import {deque} from '@data-structure/deque';
import {iter} from '@iterable-iterator/iter';

/**
 * Yields all elements of the iterable except the last <code>n</code> ones.
 *
 * @example
 * // returns [ 0 , 1 , 2 ]
 * list( _trunc( range( 5 ) , 2 ) ) ;
 *
 * @param {Iterable} iterable - The input iterable.
 * @param {Number} n - The nonnegative number of elements to exclude from the output.
 * @returns {IterableIterator}
 */
export default function* _trunc(iterable, n) {
    assert(Number.isInteger(n) && n >= 0);

    if (n === 0) {
        yield* iterable;
        return;
    }

    const iterator = iter(iterable);

    const buffer = deque(null, n);

    while (n-- > 0) {
        const event = iterator.next();
        if (event.done) {
            return;
        }

        buffer.append(event.value);
    }

    for (const value of iterator) {
        yield buffer.popleft();
        buffer.append(value);
    }
}