thi-ng/umbrella

import type { Reducer } from "./api.js";
import { reduce } from "./reduce.js";
import { isReduced, reduced } from "./reduced.js";

export function reductions<A, B>(rfn: Reducer<A, B>): Reducer<A, B[]>;
export function reductions<A, B>(rfn: Reducer<A, B>, src: Iterable<A>): B[];
export function reductions<A, B>(rfn: Reducer<A, B>, src?: Iterable<A>): any {
    const [init, complete, _reduce] = rfn;
    return src
        ? reduce(reductions(rfn), src)
        : <Reducer<A, B[]>>[
                () => [init()],
                (acc) => (
                    (acc[acc.length - 1] = complete(acc[acc.length - 1])), acc
                ),
                (acc, x) => {
                    const res = _reduce(acc[acc.length - 1], x);
                    if (isReduced(res)) {
                        acc.push(res.deref());
                        return reduced(acc);
                    }
                    acc.push(res);
                    return acc;
                },
          ];
}