include/linux/min_heap.h

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2 #ifndef _LINUX_MIN_HEAP_H
   3 #define _LINUX_MIN_HEAP_H
   4
   5 #include <linux/bug.h>
   6 #include <linux/string.h>
   7 #include <linux/types.h>
   8
   9 /**
  10  * struct min_heap - Data structure to hold a min-heap.
  11  * @data: Start of array holding the heap elements.
  12  * @nr: Number of elements currently in the heap.
  13  * @size: Maximum number of elements that can be held in current storage.
  14  */
  15 struct min_heap {
  16         void *data;
  17         int nr;
  18         int size;
  19 };
  20
  21 /**
  22  * struct min_heap_callbacks - Data/functions to customise the min_heap.
  23  * @elem_size: The nr of each element in bytes.
  24  * @less: Partial order function for this heap.
  25  * @swp: Swap elements function.
  26  */
  27 struct min_heap_callbacks {
  28         int elem_size;
  29         bool (*less)(const void *lhs, const void *rhs);
  30         void (*swp)(void *lhs, void *rhs);
  31 };
  32
  33 /* Sift the element at pos down the heap. */
  34 static __always_inline
  35 void min_heapify(struct min_heap *heap, int pos,
  36                 const struct min_heap_callbacks *func)
  37 {
  38         void *left, *right, *parent, *smallest;
  39         void *data = heap->data;
  40
  41         for (;;) {
  42                 if (pos * 2 + 1 >= heap->nr)
  43                         break;
  44
  45                 left = data + ((pos * 2 + 1) * func->elem_size);
  46                 parent = data + (pos * func->elem_size);
  47                 smallest = parent;
  48                 if (func->less(left, smallest))
  49                         smallest = left;
  50
  51                 if (pos * 2 + 2 < heap->nr) {
  52                         right = data + ((pos * 2 + 2) * func->elem_size);
  53                         if (func->less(right, smallest))
  54                                 smallest = right;
  55                 }
  56                 if (smallest == parent)
  57                         break;
  58                 func->swp(smallest, parent);
  59                 if (smallest == left)
  60                         pos = (pos * 2) + 1;
  61                 else
  62                         pos = (pos * 2) + 2;
  63         }
  64 }
  65
  66 /* Floyd's approach to heapification that is O(nr). */
  67 static __always_inline
  68 void min_heapify_all(struct min_heap *heap,
  69                 const struct min_heap_callbacks *func)
  70 {
  71         int i;
  72
  73         for (i = heap->nr / 2; i >= 0; i--)
  74                 min_heapify(heap, i, func);
  75 }
  76
  77 /* Remove minimum element from the heap, O(log2(nr)). */
  78 static __always_inline
  79 void min_heap_pop(struct min_heap *heap,
  80                 const struct min_heap_callbacks *func)
  81 {
  82         void *data = heap->data;
  83
  84         if (WARN_ONCE(heap->nr <= 0, "Popping an empty heap"))
  85                 return;
  86
  87         /* Place last element at the root (position 0) and then sift down. */
  88         heap->nr--;
  89         memcpy(data, data + (heap->nr * func->elem_size), func->elem_size);
  90         min_heapify(heap, 0, func);
  91 }
  92
  93 /*
  94  * Remove the minimum element and then push the given element. The
  95  * implementation performs 1 sift (O(log2(nr))) and is therefore more
  96  * efficient than a pop followed by a push that does 2.
  97  */
  98 static __always_inline
  99 void min_heap_pop_push(struct min_heap *heap,
 100                 const void *element,
 101                 const struct min_heap_callbacks *func)
 102 {
 103         memcpy(heap->data, element, func->elem_size);
 104         min_heapify(heap, 0, func);
 105 }
 106
 107 /* Push an element on to the heap, O(log2(nr)). */
 108 static __always_inline
 109 void min_heap_push(struct min_heap *heap, const void *element,
 110                 const struct min_heap_callbacks *func)
 111 {
 112         void *data = heap->data;
 113         void *child, *parent;
 114         int pos;
 115
 116         if (WARN_ONCE(heap->nr >= heap->size, "Pushing on a full heap"))
 117                 return;
 118
 119         /* Place at the end of data. */
 120         pos = heap->nr;
 121         memcpy(data + (pos * func->elem_size), element, func->elem_size);
 122         heap->nr++;
 123
 124         /* Sift child at pos up. */
 125         for (; pos > 0; pos = (pos - 1) / 2) {
 126                 child = data + (pos * func->elem_size);
 127                 parent = data + ((pos - 1) / 2) * func->elem_size;
 128                 if (func->less(parent, child))
 129                         break;
 130                 func->swp(parent, child);
 131         }
 132 }
 133
 134 #endif /* _LINUX_MIN_HEAP_H */