heapsort.c

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#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,
        int start_index);

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

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

    /* 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;

    sort_index -= start_index;
    sort_values -= start_index;

    /* 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,
                start_index);

    sort_index += start_index;
    sort_values += start_index;
}

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,
        int start_index) {

    /* 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;
    }

    /*index must have the start_index subracted to be consistent with the original array*/
    return (index_of_smallest - start_index);
}