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/* Barebones heap implementation supporting only insert and pop.
Copyright (C) 2010-2025 Free Software Foundation, Inc.
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <https://www.gnu.org/licenses/>. */
/* Full implementation: GDSL (http://gna.org/projects/gdsl/) by Nicolas
Darnis <ndarnis@free.fr>. */
#include <config.h>
#include "heap.h"
#include "stdlib--.h"
#include "xalloc.h"
static int heap_default_compare (void const *, void const *);
static void heapify_down (void **, idx_t, idx_t,
int (*) (void const *, void const *));
static void heapify_up (void **, idx_t,
int (*) (void const *, void const *));
struct heap
{
void **array; /* array[0] is not used */
idx_t capacity; /* Array size */
idx_t count; /* Used as index to last element. Also is num of items. */
int (*compare) (void const *, void const *);
};
/* Allocate memory for the heap. */
struct heap *
heap_alloc (int (*compare) (void const *, void const *), idx_t n_reserve)
{
struct heap *heap = xmalloc (sizeof *heap);
if (n_reserve == 0)
n_reserve = 1;
heap->array = xnmalloc (n_reserve, sizeof *(heap->array));
heap->array[0] = nullptr;
heap->capacity = n_reserve;
heap->count = 0;
heap->compare = compare ? compare : heap_default_compare;
return heap;
}
static int
heap_default_compare (void const *a, void const *b)
{
return 0;
}
void
heap_free (struct heap *heap)
{
free (heap->array);
free (heap);
}
/* Insert element into heap. */
int
heap_insert (struct heap *heap, void *item)
{
if (heap->capacity - 1 <= heap->count)
heap->array = xpalloc (heap->array, &heap->capacity, 1, -1,
sizeof heap->array[0]);
heap->array[++heap->count] = item;
heapify_up (heap->array, heap->count, heap->compare);
return 0;
}
/* Pop top element off heap. */
void *
heap_remove_top (struct heap *heap)
{
void *top;
if (heap->count == 0)
return nullptr;
top = heap->array[1];
heap->array[1] = heap->array[heap->count--];
heapify_down (heap->array, heap->count, 1, heap->compare);
return top;
}
/* Move element down into appropriate position in heap. */
static void
heapify_down (void **array, idx_t count, idx_t initial,
int (*compare) (void const *, void const *))
{
void *element = array[initial];
idx_t parent = initial;
while (parent <= count >> 1)
{
idx_t child = 2 * parent;
if (child < count && compare (array[child], array[child + 1]) < 0)
child++;
if (compare (array[child], element) <= 0)
break;
array[parent] = array[child];
parent = child;
}
array[parent] = element;
}
/* Move element up into appropriate position in heap. */
static void
heapify_up (void **array, idx_t count,
int (*compare) (void const *, void const *))
{
idx_t k = count;
void *new_element = array[k];
while (k != 1 && compare (array[k >> 1], new_element) <= 0)
{
array[k] = array[k >> 1];
k >>= 1;
}
array[k] = new_element;
}
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