aptdec/median.c

/*
 *  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