/*
 *  This Quickselect routine is based on the algorithm described in
 *  "Numerical recipies in C", Second Edition,
 *  Cambridge University Press, 1992, Section 8.5, ISBN 0-521-43108-5
 */ 

#define ELEM_SWAP(a,b) { register int t=(a);(a)=(b);(b)=t; }

int quick_select(int arr[], int n)
{
    int low, high;
    int median;
    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
