54 lines
1.5 KiB
JavaScript
54 lines
1.5 KiB
JavaScript
import {ascendingDefined, compareDefined} from "./sort.js";
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// Based on https://github.com/mourner/quickselect
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// ISC license, Copyright 2018 Vladimir Agafonkin.
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export default function quickselect(array, k, left = 0, right = Infinity, compare) {
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k = Math.floor(k);
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left = Math.floor(Math.max(0, left));
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right = Math.floor(Math.min(array.length - 1, right));
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if (!(left <= k && k <= right)) return array;
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compare = compare === undefined ? ascendingDefined : compareDefined(compare);
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while (right > left) {
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if (right - left > 600) {
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const n = right - left + 1;
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const m = k - left + 1;
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const z = Math.log(n);
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const s = 0.5 * Math.exp(2 * z / 3);
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const sd = 0.5 * Math.sqrt(z * s * (n - s) / n) * (m - n / 2 < 0 ? -1 : 1);
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const newLeft = Math.max(left, Math.floor(k - m * s / n + sd));
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const newRight = Math.min(right, Math.floor(k + (n - m) * s / n + sd));
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quickselect(array, k, newLeft, newRight, compare);
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}
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const t = array[k];
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let i = left;
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let j = right;
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swap(array, left, k);
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if (compare(array[right], t) > 0) swap(array, left, right);
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while (i < j) {
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swap(array, i, j), ++i, --j;
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while (compare(array[i], t) < 0) ++i;
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while (compare(array[j], t) > 0) --j;
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}
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if (compare(array[left], t) === 0) swap(array, left, j);
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else ++j, swap(array, j, right);
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if (j <= k) left = j + 1;
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if (k <= j) right = j - 1;
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}
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return array;
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}
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function swap(array, i, j) {
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const t = array[i];
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array[i] = array[j];
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array[j] = t;
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}
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