60 lines
1.6 KiB
JavaScript
60 lines
1.6 KiB
JavaScript
'use strict';
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module.exports = partialSort;
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// Floyd-Rivest selection algorithm:
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// Rearrange items so that all items in the [left, k] range are smaller than all items in (k, right];
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// The k-th element will have the (k - left + 1)th smallest value in [left, right]
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function partialSort(arr, k, left, right, compare) {
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left = left || 0;
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right = right || (arr.length - 1);
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compare = compare || defaultCompare;
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while (right > left) {
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if (right - left > 600) {
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var n = right - left + 1;
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var m = k - left + 1;
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var z = Math.log(n);
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var s = 0.5 * Math.exp(2 * z / 3);
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var sd = 0.5 * Math.sqrt(z * s * (n - s) / n) * (m - n / 2 < 0 ? -1 : 1);
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var newLeft = Math.max(left, Math.floor(k - m * s / n + sd));
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var newRight = Math.min(right, Math.floor(k + (n - m) * s / n + sd));
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partialSort(arr, k, newLeft, newRight, compare);
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}
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var t = arr[k];
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var i = left;
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var j = right;
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swap(arr, left, k);
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if (compare(arr[right], t) > 0) swap(arr, left, right);
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while (i < j) {
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swap(arr, i, j);
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i++;
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j--;
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while (compare(arr[i], t) < 0) i++;
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while (compare(arr[j], t) > 0) j--;
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}
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if (compare(arr[left], t) === 0) swap(arr, left, j);
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else {
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j++;
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swap(arr, j, right);
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}
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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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}
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function swap(arr, i, j) {
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var tmp = arr[i];
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arr[i] = arr[j];
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arr[j] = tmp;
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}
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function defaultCompare(a, b) {
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return a < b ? -1 : a > b ? 1 : 0;
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}
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