prio_heap.c

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/*
 * Simple insertion-only static-sized priority heap containing
 * pointers, based on CLR, chapter 7
 */

#include <linux/slab.h>
#include <linux/prio_heap.h>

int heap_init(struct ptr_heap *heap, size_t size, gfp_t gfp_mask,
          int (*gt)(void *, void *))
{
    heap->ptrs = kmalloc(size, gfp_mask);
    if (!heap->ptrs)
        return -ENOMEM;
    heap->size = 0;
    heap->max = size / sizeof(void *);
    heap->gt = gt;
    return 0;
}

void heap_free(struct ptr_heap *heap)
{
    kfree(heap->ptrs);
}

void *heap_insert(struct ptr_heap *heap, void *p)
{
    void *res;
    void **ptrs = heap->ptrs;
    int pos;

    if (heap->size < heap->max) {
        /* Heap insertion */
        pos = heap->size++;
        while (pos > 0 && heap->gt(p, ptrs[(pos-1)/2])) {
            ptrs[pos] = ptrs[(pos-1)/2];
            pos = (pos-1)/2;
        }
        ptrs[pos] = p;
        return NULL;
    }

    /* The heap is full, so something will have to be dropped */

    /* If the new pointer is greater than the current max, drop it */
    if (heap->gt(p, ptrs[0]))
        return p;

    /* Replace the current max and heapify */
    res = ptrs[0];
    ptrs[0] = p;
    pos = 0;

    while (1) {
        int left = 2 * pos + 1;
        int right = 2 * pos + 2;
        int largest = pos;
        if (left < heap->size && heap->gt(ptrs[left], p))
            largest = left;
        if (right < heap->size && heap->gt(ptrs[right], ptrs[largest]))
            largest = right;
        if (largest == pos)
            break;
        /* Push p down the heap one level and bump one up */
        ptrs[pos] = ptrs[largest];
        ptrs[largest] = p;
        pos = largest;
    }
    return res;
}