thi-ng/umbrella

import type { Predicate } from "@thi.ng/api";
import type { Reducer, Transducer } from "./api.js";
import { compR } from "./compr.js";
import { __iter } from "./iterator.js";
import { reduced } from "./reduced.js";

/**
 * Transducer which applies predicate `pred` to each input and only
 * yields values as long as the predicate returned a truthy result. Once
 * the result is falsy, transformation is terminated (by emitting a
 * {@link reduced} value).
 *
 * @example
 * ```ts
 * import { range, takeWhile } from "@thi.ng/transducers";
 *
 * [...takeWhile((x) => x < 5, range(10))]
 * // [ 0, 1, 2, 3, 4 ]
 * ```
 *
 * @param pred -
 */
export function takeWhile<T>(pred?: Predicate<T>): Transducer<T, T>;
export function takeWhile<T>(src: Iterable<T>): IterableIterator<T>;
export function takeWhile<T>(
    pred: Predicate<T>,
    src: Iterable<T>
): IterableIterator<T>;
export function takeWhile<T>(...args: any[]): any {
    return (
        __iter(takeWhile, args) ||
        ((rfn: Reducer<T, any>) => {
            const r = rfn[2];
            const pred = args[0];
            let ok = true;
            return compR(rfn, (acc, x: T) =>
                (ok = ok && pred(x)) ? r(acc, x) : reduced(acc)
            );
        })
    );
}