Analysis of minimum heap implementation algorithm in libevent

libevent, a very good c network library, has recently begun to analyze.

Where to start? I choose to start from the simplest basic things, from simplicity to complexity.

Today, we will generate the minimum heap of libevent, and realize how the author of libevent realizes the minimum heap. The minimum heap is used in libevent's time management to calculate whether to time out.

Minimum heap: a sort of complete binary tree, in which the data value of any non terminal node is not greater than the value of its left and right child nodes.

1. Min? Heap? Shift? Up? Adjust up after inserting element

2. Min? Heap? Shift? Down? Element adjust down (delete element)

3. Code Notes

  1 /*
  2  * Copyright (c) 2006 Maxim Yegorushkin <maxim.yegorushkin@gmail.com>
  3  * All rights reserved.
  4  *
  5  * Redistribution and use in source and binary forms, with or without
  6  * modification, are permitted provided that the following conditions
  7  * are met:
  8  * 1. Redistributions of source code must retain the above copyright
  9  *    notice, this list of conditions and the following disclaimer.
 10  * 2. Redistributions in binary form must reproduce the above copyright
 11  *    notice, this list of conditions and the following disclaimer in the
 12  *    documentation and/or other materials provided with the distribution.
 13  * 3. The name of the author may not be used to endorse or promote products
 14  *    derived from this software without specific prior written permission.
 15  *
 16  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 17  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 18  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 19  * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 20  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 21  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 22  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 23  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 24  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 25  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 26  */
 27 #ifndef _MIN_HEAP_H_
 28 #define _MIN_HEAP_H_
 29 
 30 #include "event.h"
 31 #include "evutil.h"
 32 
 33 //The minimum heap is a sort of complete binary tree, in which the data value of any non terminal node is not greater than the value of its left and right child nodes.
 34 typedef struct min_heap
 35 {
 36     //Dynamically allocate memory to save points to*event Pointer to
 37     struct event** p;
 38     //n Is the number of elements,a Number capacity
 39     unsigned n, a;
 40 } min_heap_t;
 41 
 42 static inline void           min_heap_ctor(min_heap_t* s);
 43 static inline void           min_heap_dtor(min_heap_t* s);
 44 static inline void           min_heap_elem_init(struct event* e);
 45 static inline int            min_heap_elem_greater(struct event *a, struct event *b);
 46 static inline int            min_heap_empty(min_heap_t* s);
 47 static inline unsigned       min_heap_size(min_heap_t* s);
 48 static inline struct event*  min_heap_top(min_heap_t* s);
 49 static inline int            min_heap_reserve(min_heap_t* s, unsigned n);
 50 static inline int            min_heap_push(min_heap_t* s, struct event* e);
 51 static inline struct event*  min_heap_pop(min_heap_t* s);
 52 static inline int            min_heap_erase(min_heap_t* s, struct event* e);
 53 static inline void           min_heap_shift_up_(min_heap_t* s, unsigned hole_index, struct event* e);
 54 static inline void           min_heap_shift_down_(min_heap_t* s, unsigned hole_index, struct event* e);
 55 
 56 int min_heap_elem_greater(struct event *a, struct event *b)
 57 {
 58     return evutil_timercmp(&a->ev_timeout, &b->ev_timeout, >);
 59 }
 60 
 61 void min_heap_ctor(min_heap_t* s) { s->p = 0; s->n = 0; s->a = 0; }
 62 void min_heap_dtor(min_heap_t* s) { if(s->p) free(s->p); }
 63 void min_heap_elem_init(struct event* e) { e->min_heap_idx = -1; }
 64 int min_heap_empty(min_heap_t* s) { return 0u == s->n; }
 65 unsigned min_heap_size(min_heap_t* s) { return s->n; }
 66 struct event* min_heap_top(min_heap_t* s) { return s->n ? *s->p : 0; }
 67 
 68 //Insert element
 69 int min_heap_push(min_heap_t* s, struct event* e)
 70 {
 71     //Check memory
 72     if(min_heap_reserve(s, s->n + 1))
 73         return -1;
 74 
 75     //Insert element up adjustment
 76     min_heap_shift_up_(s, s->n++, e);
 77     return 0;
 78 }
 79 
 80 //pop Head element
 81 struct event* min_heap_pop(min_heap_t* s)
 82 {
 83     if(s->n)
 84     {
 85         //->Priority ratio*high
 86         //e Pointer to the head element
 87         struct event* e = *s->p;
 88         //Element down, 0 U Represents the head node index, s->p[--s->n]: The lowest and rightmost element, used to fill in the empty position after insertion
 89         min_heap_shift_down_(s, 0u, s->p[--s->n]);
 90 
 91         //The header element is indexed in the heap as-1,Pile out
 92         e->min_heap_idx = -1;
 93         return e;
 94     }
 95     return 0;
 96 }
 97 
 98 //Delete in heap equal to e Elements of
 99 int min_heap_erase(min_heap_t* s, struct event* e)
100 {
101     if(((unsigned int)-1) != e->min_heap_idx)
102     {
103         struct event *last = s->p[--s->n];
104         //Parent node index
105         unsigned parent = (e->min_heap_idx - 1) / 2;
106     /* we replace e with the last element in the heap.  We might need to
107        shift it upward if it is less than its parent, or downward if it is
108        greater than one or both its children. Since the children are known
109        to be less than the parent, it can't need to shift both up and
110        down. */
111         //If e Not root element, current e The parent node value of is greater than last,Up adjustment required
112         if (e->min_heap_idx > 0 && min_heap_elem_greater(s->p[parent], last))
113              min_heap_shift_up_(s, e->min_heap_idx, last);
114         else
115         //If e Is the root element or e Parent node element value of is not greater than last,Element down, e->min_heap_idx Is the head node index, last: Bottom most right element
116         //,Used to fill the empty position after insertion
117              min_heap_shift_down_(s, e->min_heap_idx, last);
118         //take e Element out of pile
119         e->min_heap_idx = -1;
120         return 0;
121     }
122     return -1;
123 }
124 
125 //Adjust allocated memory
126 int min_heap_reserve(min_heap_t* s, unsigned n)
127 {
128     //If the capacity of an element is less than the number of elements, the memory needs to be reallocated
129     if(s->a < n)
130     {
131         struct event** p;
132         //a If the default value is 0, it will be 8. If there is a previous value (not the first adjustment), it will be doubled
133         unsigned a = s->a ? s->a * 2 : 8;
134         //If a It's not enough. Let's go straight a Be equal to n,Same number and capacity of elements
135         if(a < n)
136             a = n;
137         //Readjust memory, allocate continuously
138         if(!(p = (struct event**)realloc(s->p, a * sizeof *p)))
139             return -1;
140         //First address
141         s->p = p;
142         //capacity
143         s->a = a;
144     }
145     return 0;
146 }
147 
148 //Adjust up after inserting element
149 void min_heap_shift_up_(min_heap_t* s, unsigned hole_index, struct event* e)
150 {
151     //Index of the parent node
152     unsigned parent = (hole_index - 1) / 2;
153     //If hole_index Not equal to 0 and the parent node element is greater than the given element, continue to compare until it reaches hole_index Is the root element,
154     //Or the parent element is greater than e,Find where to insert
155     while(hole_index && min_heap_elem_greater(s->p[parent], e))
156     {
157         //The element value of the parent node is large. Put the parent node in the current hole_index Location on
158         (s->p[hole_index] = s->p[parent])->min_heap_idx = hole_index;
159 
160         //hole_index Index assigned as parent
161         hole_index = parent;
162 
163         //Find the current hole_index Parent node index of
164         parent = (hole_index - 1) / 2;
165     }
166 
167     //Jump out of the loop and find the location to insert. The index of the location is now hole_index
168     (s->p[hole_index] = e)->min_heap_idx = hole_index;
169 }
170 
171 //Element down (delete element)
172 void min_heap_shift_down_(min_heap_t* s, unsigned hole_index, struct event* e)
173 {
174     //Right child index
175     unsigned min_child = 2 * (hole_index + 1);
176     //There is a right child. If there is no right child tree, adjust it directly downward. Because there is a left child tree at most, and the value must be no less than the parent node, you can adjust it directly downward
177     while(min_child <= s->n)
178     {
179         //Select the index of the child with the lowest value of left and right children, and add it according to the priority()Better view
180         min_child -= ((min_child == s->n) || min_heap_elem_greater(s->p[min_child], s->p[min_child - 1]));
181         //If e The element is not greater than the smallest child element, so there is no need to continue, hole_index It's his position
182         if(!(min_heap_elem_greater(e, s->p[min_child])))
183             break;
184         //Put the little child element in hole_index Position
185         (s->p[hole_index] = s->p[min_child])->min_heap_idx = hole_index;
186         //hole_index Save the current child index
187         hole_index = min_child;
188         //The current small child's position is vacated. Continue the next cycle and compare the left and right children of the current small child
189         min_child = 2 * (hole_index + 1);
190     }
191     //take e Elements in hole_index,Then adjust up. commonly e Element is the lowest right node. Do not exclude the possibility of being more than the current position on the parent node
192     //So it needs to be adjusted up
193     min_heap_shift_up_(s, hole_index,  e);
194 }
195 
196 #endif /* _MIN_HEAP_H_ */

Tags: C less network

Posted on Tue, 05 May 2020 21:54:59 -0400 by kee2ka4