thi-ng/umbrella

import type { Fn0 } from "@thi.ng/api";
import { isIterable } from "@thi.ng/checks/is-iterable";
import type { Reducer, Transducer } from "./api.js";
import { compR } from "./compr.js";
import { iterator } from "./iterator.js";
import { isReduced } from "./reduced.js";

export function interpose<A, B>(sep: B | Fn0<B>): Transducer<A, A | B>;
export function interpose<A, B>(
    sep: B | Fn0<B>,
    src: Iterable<A>
): IterableIterator<A | B>;
export function interpose<A, B>(sep: any, src?: Iterable<A>): any {
    return isIterable(src)
        ? iterator(interpose(sep), src)
        : (rfn: Reducer<A | B, any>) => {
                const r = rfn[2];
                const _sep: Fn0<B> =
                    typeof sep === "function" ? sep : () => sep;
                let first = true;
                return compR(rfn, (acc, x: A) => {
                    if (first) {
                        first = false;
                        return r(acc, x);
                    }
                    acc = r(acc, _sep());
                    return isReduced(acc) ? acc : r(acc, x);
                });
          };
}