SRC/heapsort.c

Go to the documentation of this file.

#include "heapsort.h"

/******************* Subroutines local to heapsort.c ************************/

static void add_to_sort_heap(int *heap,
                             float *sort_values,
                             int index,
                             int heap_tail);

static int get_top_of_heap_index(int *heap,
                                 float *sort_values,
                                 int heap_tail);


/********************* Subroutine definitions *******************************/


void
heapsort(int *sort_index,
         float *sort_values,
         int nelem)
{

/* This routine loads sort_index [1..nelem] with nelem values:  the indices  *
 * of the sort_values [1..nelem] array that lead to sort_values[index] being *
 * decreasing as one goes through sort_index.  Sort_values is not changed.   */

    int i;

/* Build a heap with the *smallest* element at the top. */

    for(i = 1; i <= nelem; i++)
        add_to_sort_heap(sort_index, sort_values, i, i);

/* Now pull items off the heap, smallest first.  As the heap (in sort_index) *
 * shrinks, I store the item pulled off (the smallest) in sort_index,        *
 * starting from the end. The heap and the stored smallest values never      *
 * overlap, so I get a nice sort-in-place.                                   */

    for(i = nelem; i >= 1; i--)
        sort_index[i] = get_top_of_heap_index(sort_index, sort_values, i);
}


static void
add_to_sort_heap(int *heap,
                 float *sort_values,
                 int index,
                 int heap_tail)
{

/* Adds element index, with value = sort_values[index] to the heap.          *
 * Heap_tail is the number of elements in the heap *after* the insert.       */

    unsigned int ifrom, ito;    /* Making these unsigned helps compiler opts. */
    int temp;

    heap[heap_tail] = index;
    ifrom = heap_tail;
    ito = ifrom / 2;

    while(ito >= 1 && sort_values[heap[ifrom]] < sort_values[heap[ito]])
        {
            temp = heap[ito];
            heap[ito] = heap[ifrom];
            heap[ifrom] = temp;
            ifrom = ito;
            ito = ifrom / 2;
        }
}


static int
get_top_of_heap_index(int *heap,
                      float *sort_values,
                      int heap_tail)
{

/* Returns the index of the item at the top of the heap (the smallest one).  *
 * It then rebuilds the heap.  Heap_tail is the number of items on the heap  *
 * before the top item is deleted.                                           */

    int index_of_smallest, temp;
    unsigned int ifrom, ito, heap_end;

    index_of_smallest = heap[1];
    heap[1] = heap[heap_tail];
    heap_end = heap_tail - 1;   /* One item deleted. */
    ifrom = 1;
    ito = 2 * ifrom;

    while(ito <= heap_end)
        {
            if(sort_values[heap[ito + 1]] < sort_values[heap[ito]])
                ito++;

            if(sort_values[heap[ito]] > sort_values[heap[ifrom]])
                break;

            temp = heap[ito];
            heap[ito] = heap[ifrom];
            heap[ifrom] = temp;
            ifrom = ito;
            ito = 2 * ifrom;
        }

    return (index_of_smallest);
}

---