thi-ng/umbrella

import { isIterable } from "@thi.ng/checks/is-iterable";
import type { Reducer, Transducer } from "./api.js";
import { __drain } from "./internal/drain.js";
import { iterator } from "./iterator.js";

/**
 * Transducer which only yields the last `n` values. Assumes
 * input source is finite (of course).
 *
 * @example
 * ```ts
 * import { range, takeLast } from "@thi.ng/transducers";
 *
 * [...takeLast(3, range(10))]
 * // [ 7, 8, 9 ]
 * ```
 *
 * @param n -
 */
export function takeLast<T>(n: number): Transducer<T, T>;
export function takeLast<T>(n: number, src: Iterable<T>): IterableIterator<T>;
export function takeLast<T>(n: number, src?: Iterable<T>): any {
    return isIterable(src)
        ? iterator(takeLast(n), src)
        : ([init, complete, reduce]: Reducer<T, any>) => {
                const buf: T[] = [];
                return <Reducer<T, any>>[
                    init,
                    __drain(buf, complete, reduce),
                    (acc, x) => {
                        if (buf.length === n) {
                            buf.shift();
                        }
                        buf.push(x);
                        return acc;
                    },
                ];
          };
}