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- /*
- * This Quickselect routine is based on the algorithm described in
- * "Numerical recipes in C", Second Edition,
- * Cambridge University Press, 1992, Section 8.5, ISBN 0-521-43108-5
- * This code by Nicolas Devillard - 1998. Public domain.
- */
-
- #define ELEM_SWAP(a, b) { register float t = (a); (a) = (b); (b) = t; }
- float quick_select(float arr[], int n) {
- int low, median, high;
- int middle, ll, hh;
-
- low = 0; high = n-1; median = (low + high) / 2;
- for (;;) {
- if (high <= low) /* One element only */
- return arr[median] ;
-
- if (high == low + 1) { /* Two elements only */
- if (arr[low] > arr[high])
- ELEM_SWAP(arr[low], arr[high]);
- return arr[median];
- }
-
- /* Find median of low, middle and high items; swap into position low */
- middle = (low + high) / 2;
- if (arr[middle] > arr[high]) ELEM_SWAP(arr[middle], arr[high]);
- if (arr[low] > arr[high]) ELEM_SWAP(arr[low], arr[high]);
- if (arr[middle] > arr[low]) ELEM_SWAP(arr[middle], arr[low]);
-
- /* Swap low item (now in position middle) into position (low+1) */
- ELEM_SWAP(arr[middle], arr[low+1]);
-
- /* Nibble from each end towards middle, swapping items when stuck */
- ll = low + 1;
- hh = high;
- for (;;) {
- do ll++; while (arr[low] > arr[ll]);
- do hh--; while (arr[hh] > arr[low]);
-
- if (hh < ll)
- break;
-
- ELEM_SWAP(arr[ll], arr[hh]);
- }
-
- /* Swap middle item (in position low) back into correct position */
- ELEM_SWAP(arr[low], arr[hh]);
-
- /* Re-set active partition */
- if (hh <= median)
- low = ll;
- if (hh >= median)
- high = hh - 1;
- }
- }
- #undef ELEM_SWAP
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