index.js

/**
 * Rearranges items so that all items in the [left, k] are the smallest.
 * The k-th element will have the (k - left + 1)-th smallest value in [left, right].
 *
 * @template T
 * @param {T[]} arr the array to partially sort (in place)
 * @param {number} k middle index for partial sorting (as defined above)
 * @param {number} [left=0] left index of the range to sort
 * @param {number} [right=arr.length-1] right index
 * @param {(a: T, b: T) => number} [compare = (a, b) => a - b] compare function
 */
export default function quickselect(arr, k, left = 0, right = arr.length - 1, compare = defaultCompare) {

    while (right > left) {
        if (right - left > 600) {
            const n = right - left + 1;
            const m = k - left + 1;
            const z = Math.log(n);
            const s = 0.5 * Math.exp(2 * z / 3);
            const sd = 0.5 * Math.sqrt(z * s * (n - s) / n) * (m - n / 2 < 0 ? -1 : 1);
            const newLeft = Math.max(left, Math.floor(k - m * s / n + sd));
            const newRight = Math.min(right, Math.floor(k + (n - m) * s / n + sd));
            quickselect(arr, k, newLeft, newRight, compare);
        }

        const t = arr[k];
        let i = left;
        /** @type {number} */
        let j = right;

        swap(arr, left, k);
        if (compare(arr[right], t) > 0) swap(arr, left, right);

        while (i < j) {
            swap(arr, i, j);
            i++;
            j--;
            while (compare(arr[i], t) < 0) i++;
            while (compare(arr[j], t) > 0) j--;
        }

        if (compare(arr[left], t) === 0) swap(arr, left, j);
        else {
            j++;
            swap(arr, j, right);
        }

        if (j <= k) left = j + 1;
        if (k <= j) right = j - 1;
    }
}

/**
 * @template T
 * @param {T[]} arr
 * @param {number} i
 * @param {number} j
 */
function swap(arr, i, j) {
    const tmp = arr[i];
    arr[i] = arr[j];
    arr[j] = tmp;
}

/**
 * @template T
 * @param {T} a
 * @param {T} b
 * @returns {number}
 */
function defaultCompare(a, b) {
    return a < b ? -1 : a > b ? 1 : 0;
}