/**
 * Reduce function for iterables.
 *
 * @see https://github.com/jb55/reduce-iterator
 *
 * @param xs
 * @param reducer
 * @param acc
 * @returns {*}
 */
function reduceIterator (xs, reducer, acc) {
    let first = true;
    let prev = acc;

    for (let x of xs) {
        if (first) {
            first = false;
            if (arguments.length < 3) {
                prev = x;
                continue;
            }
        }
        prev = reducer(prev, x)
    }

    return prev;
}

/**
 * Pipes an iterable through a variadric functions array.
 *
 * @param fns
 * @returns {function(*=): *}
 */
export const pipe = (...fns) => function (iterable) {
    return reduceIterator(fns, (result, fn) => fn(result), iterable);
};

/**
 * Takes a number of elements from an iterable.
 *
 * @param {Number} n Number of elements
 * @returns {Function} Generator
 */
export const take = n => function* (iterable) {
    let i = 0;

    for (let item of iterable) {
        if (i < n) {
            yield item;
        }

        i++;
    }
};

/**
 * Takes elements from an iterable that match the given predicate.
 *
 * @param {Function} predicate
 * @returns {Function}
 */
export const takeIf = predicate => function* (iterable) {
    for (let item of iterable) {
        if (predicate(item)) {
            yield item;
        }
    }
};

/**
 * Skips a number of elements in an iterable and yields the rest.
 *
 * @param {Number} n Number of elements
 * @returns {Function} Generator
 */
export const skip = n => function* (iterable) {
    let i = 0;

    for (let item of iterable) {
        if (i >= n) {
            yield item;
        }

        i++;
    }
};

/**
 * Skips items of an iterable that match the given predicate.
 *
 * @param {Function} predicate
 * @returns {Function} Generator
 */
export const skipIf = predicate => function* (iterable) {
    for (let item of iterable) {
        if (!predicate(item)) {
            yield item;
        }
    }
};