56 lines
1.1 KiB
C
56 lines
1.1 KiB
C
#include <string.h>
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#include <stdlib.h>
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#include "quick_select.h"
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static inline void swap_double(double *a, double *b) {
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double t = *a;
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*a = *b;
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*b = t;
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}
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// Lomuto partition
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static size_t partition(double *arr, size_t lo, size_t hi) {
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double pivot = arr[hi];
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size_t i = lo;
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for (size_t j = lo; j < hi; ++j) {
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if (arr[j] < pivot) {
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swap_double(&arr[i], &arr[j]);
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++i;
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}
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}
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swap_double(&arr[i], &arr[hi]);
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return i;
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}
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// 选择第 k 小的元素 (0-based)，平均 O(n)，最坏 O(n^2)
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static double select_k(double *arr, size_t n, size_t k) {
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size_t lo = 0;
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size_t hi = n - 1;
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for (;;) {
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size_t p = partition(arr, lo, hi);
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if (p == k) {
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return arr[p];
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} else if (k < p) {
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hi = p - 1;
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} else {
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lo = p + 1;
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}
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}
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return arr[k];
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}
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double percentile(const double *arr, size_t n, double perc) {
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if (n == 0) {
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return 0.0;
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}
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double *arr_copy = malloc(sizeof(double)*n);
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memcpy(arr_copy, arr, n);
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// nearest-rank: ceil(0.9*n) - 1
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size_t k = (size_t)(perc*n) - 1;
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double ans = select_k(arr_copy, n, k);
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free(arr_copy);
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return ans;
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}
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