thi-ng/umbrella

import type { Maybe } from "@thi.ng/api";

/** @internal */
interface Cell<T> {
    x: T;
    n?: Cell<T>;
}

/**
 * Yields an iterator of all `src` values, followed by the same values in
 * reverse order. Efficiently builds the reversed order via an internal linked
 * list.
 *
 * @example
 * ```ts
 * import { symmetric } from "@thi.ng/transducers";
 *
 * [...symmetric([1, 2, 3])]
 * // [ 1, 2, 3, 3, 2, 1 ]
 * ```
 *
 * @param src -
 */
export function* symmetric<T>(src: Iterable<T>): IterableIterator<T> {
    let head: Maybe<Cell<T>> = undefined;
    for (let x of src) {
        head = { x, n: head };
        yield x;
    }
    while (head) {
        yield head.x;
        head = head.n;
    }
}