gl/lib/heap.c

   1 /* Barebones heap implementation supporting only insert and pop.
   2
   3    Copyright (C) 2010-2023 Free Software Foundation, Inc.
   4
   5    This program is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  17
  18 /* Full implementation: GDSL (http://gna.org/projects/gdsl/) by Nicolas
  19    Darnis <ndarnis@free.fr>. */
  20
  21 #include <config.h>
  22
  23 #include "heap.h"
  24 #include "stdlib--.h"
  25 #include "xalloc.h"
  26
  27 static int heap_default_compare (void const *, void const *);
  28 static size_t heapify_down (void **, size_t, size_t,
  29                             int (*) (void const *, void const *));
  30 static void heapify_up (void **, size_t,
  31                         int (*) (void const *, void const *));
  32
  33 struct heap
  34 {
  35   void **array;     /* array[0] is not used */
  36   size_t capacity;  /* Array size */
  37   size_t count;     /* Used as index to last element. Also is num of items. */
  38   int (*compare) (void const *, void const *);
  39 };
  40
  41 /* Allocate memory for the heap. */
  42
  43 struct heap *
  44 heap_alloc (int (*compare) (void const *, void const *), size_t n_reserve)
  45 {
  46   struct heap *heap = xmalloc (sizeof *heap);
  47
  48   if (n_reserve == 0)
  49     n_reserve = 1;
  50
  51   heap->array = xnmalloc (n_reserve, sizeof *(heap->array));
  52
  53   heap->array[0] = nullptr;
  54   heap->capacity = n_reserve;
  55   heap->count = 0;
  56   heap->compare = compare ? compare : heap_default_compare;
  57
  58   return heap;
  59 }
  60
  61
  62 static int
  63 heap_default_compare (void const *a, void const *b)
  64 {
  65   return 0;
  66 }
  67
  68
  69 void
  70 heap_free (struct heap *heap)
  71 {
  72   free (heap->array);
  73   free (heap);
  74 }
  75
  76 /* Insert element into heap. */
  77
  78 int
  79 heap_insert (struct heap *heap, void *item)
  80 {
  81   if (heap->capacity - 1 <= heap->count)
  82     heap->array = x2nrealloc (heap->array, &heap->capacity,
  83                               sizeof *(heap->array));
  84
  85   heap->array[++heap->count] = item;
  86   heapify_up (heap->array, heap->count, heap->compare);
  87
  88   return 0;
  89 }
  90
  91 /* Pop top element off heap. */
  92
  93 void *
  94 heap_remove_top (struct heap *heap)
  95 {
  96   void *top;
  97
  98   if (heap->count == 0)
  99     return nullptr;
 100
 101   top = heap->array[1];
 102   heap->array[1] = heap->array[heap->count--];
 103   heapify_down (heap->array, heap->count, 1, heap->compare);
 104
 105   return top;
 106 }
 107
 108 /* Move element down into appropriate position in heap. */
 109
 110 static size_t
 111 heapify_down (void **array, size_t count, size_t initial,
 112               int (*compare) (void const *, void const *))
 113 {
 114   void *element = array[initial];
 115
 116   size_t parent = initial;
 117   while (parent <= count / 2)
 118     {
 119       size_t child = 2 * parent;
 120
 121       if (child < count && compare (array[child], array[child + 1]) < 0)
 122         child++;
 123
 124       if (compare (array[child], element) <= 0)
 125         break;
 126
 127       array[parent] = array[child];
 128       parent = child;
 129     }
 130
 131   array[parent] = element;
 132   return parent;
 133 }
 134
 135 /* Move element up into appropriate position in heap. */
 136
 137 static void
 138 heapify_up (void **array, size_t count,
 139             int (*compare) (void const *, void const *))
 140 {
 141   size_t k = count;
 142   void *new_element = array[k];
 143
 144   while (k != 1 && compare (array[k / 2], new_element) <= 0)
 145     {
 146       array[k] = array[k / 2];
 147       k /= 2;
 148     }
 149
 150   array[k] = new_element;
 151 }