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优先队列（priority_queue)和一般队列(queue)的函数接口一致，不同的是，优先队列每次出列的是整个队列中
最小（或者最大）的元素。
本文简要介绍一种基于数组二叉堆实现的优先队列，定义的数据结构和实现的函数接口说明如下：
一、键值对结构体：KeyValue
// =============KeyValue Struct==================================
typedef struct key_value_struct KeyValue;
struct key_value_struct
{
int _key;
void *_value;
};
KeyValue *key_value_new(int key, void *value);
void key_value_free(KeyValue *kv, void (*freevalue)(void *));
键值对作为优先队列的中数据的保存形式，其中key用于保存优先级，_value用于指向实际的数据。
key_value_new用于创建一个KeyValue结构体；key_value_free用于释放一个KeyValue结构体的内存，
参数freevalue用于释放数据指针_value指向的内存。
二、优先队列结构体：PriorityQueue
//=============PriorityQueueStruct==============================
#define PRIORITY_MAX 1
#define PRIORITY_MIN 2
typedef struct priority_queue_struct PriorityQueue;
struct priority_queue_struct
{
KeyValue **_nodes;
int _size;
int _capacity;
int _priority;
};
PriorityQueue *priority_queue_new(int priority);
void priority_queue_free(PriorityQueue *pq, void (*freevalue)(void *));
const KeyValue *priority_queue_top(PriorityQueue *pq);
KeyValue *priority_queue_dequeue(PriorityQueue *pq);
void priority_queue_enqueue(PriorityQueue *pq, KeyValue *kv);
int priority_queue_size(PriorityQueue *pq);
int priority_queue_empty(PriorityQueue *pq);
void priority_queue_print(PriorityQueue *pq);
1) 其中nodes字段是二叉堆数组，_capacity是nodes指向的KeyValue*指针的个数，_size是nodes中实际存储的元素个数。
_priority可以是PRIORITY_MAX或PRIORITY_MIN，分别表示最大元素优先和最小元素优先。
2) priority_queue_new和priority_queue_free分别用于创建和释放优先队列。
3) priority_queue_top用于取得队列头部元素，
4）priority_queue_dequeue用于取得队列头部元素并将元素出列。
其实现的基本思路，以最大优先队列说明如下：
①将队列首部nodes[0]保存作为返回值
②将队列尾部nodes[_size-1]置于nodes[0]位置，并令_size=_size-1
③令当前父节点parent(nodes[i])等于新的队列首部(i=0)元素，
parent指向元素的儿子节点为left = nodes[2 * i + 1]和rigth = nodes[2 * i + 2]，
比较left和right得到优先级高的儿子节点,设为nodes[j](j = 2 *i + 1或2 *i + 2)，
④如果当前父节点parent的优先级高于nodes[j]，交换nodes[i]和nodes[j]，并更新当前父节点,
即令i=j,并循环 ③；
如果当前父节点的优先级低于nodes[j],处理结束。
5）priority_queue_enqueue用于将KeyValue入列
其实现的基本思路，以最大优先队列说明如下：
①设置nodes[_size] 为新的KeyValue,并令_size++
②令当前儿子节点child(nodes[i])为新的队列尾部节点（i=_size-1)，child的父节点parent为nodes[j],
其中j= (i – 1) / 2
③如果当前儿子节点child的优先级高于parent, 交换nodes[i]和nodes[j]，并更新当前儿子节点
即令i = j,并循环③;
如果当前儿子节点的优先级低于parent,处理结束。
6) priority_queue_size用于取得队列中元素个数，priority_queue_empty用于判断队列是否为空。
7）priority_queue_print用于输出队列中的内容。
文件pq.h给出了数据结构和函数的声明，文件pq.c给出了具体实现，main.c文件用于测试。虽然是使用过程化编程的C语言，可以看到具体的编码中应用了基于对象的思想，我们对数据结构和相关函数做了一定程度的聚集和封装。
/*
*File: pq.h
*purpose: declaration of priority queue in C
*Author:puresky
*Date:2011/04/27
*/
#ifndef _PRIORITY_QUEUE_H
#define _PRIORITY_QUEUE_H
// =============KeyValue Struct==================================
typedef struct key_value_struct KeyValue;
struct key_value_struct
{
int _key;
void *_value;
};
KeyValue *key_value_new(int key, void *value);
void key_value_free(KeyValue *kv, void (*freevalue)(void *));
// =============PriorityQueue Struct==============================
#define PRIORITY_MAX 1
#define PRIORITY_MIN 2
typedef struct priority_queue_struct PriorityQueue;
struct priority_queue_struct
{
KeyValue **_nodes;
int _size;
int _capacity;
int _priority;
};
PriorityQueue *priority_queue_new(int priority);
void priority_queue_free(PriorityQueue *pq, void (*freevalue)(void *));
const KeyValue *priority_queue_top(PriorityQueue *pq);
KeyValue *priority_queue_dequeue(PriorityQueue *pq);
void priority_queue_enqueue(PriorityQueue *pq, KeyValue *kv);
int priority_queue_size(PriorityQueue *pq);
int priority_queue_empty(PriorityQueue *pq);
void priority_queue_print(PriorityQueue *pq);
#endif
/*
*File:pq.c
*purpose: definition of priority queue in C
*Author:puresky
*Date:2011/04/27
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "pq.h"
//Private Functions
static void priority_queue_realloc(PriorityQueue *pq);
static void priority_queue_adjust_head(PriorityQueue *pq);
static void priority_queue_adjust_tail(PriorityQueue *pq);
static int priority_queue_compare(PriorityQueue *pq,
int pos1,
int pos2);
static void priority_queue_swap(KeyValue **nodes,
int pos1,
int pos2);
//Functions of KeyValue Struct
KeyValue *key_value_new(int key,
void *value)
{
KeyValue *pkv = (KeyValue *)malloc(sizeof(KeyValue));
pkv->_key = key;
pkv->_value = value;
return pkv;
}
void key_value_free(KeyValue *kv,
void (*freevalue)(void *))
{
if(kv)
{
if(freevalue)
{
freevalue(kv->_value);
}
free(kv);
}
}
//Functions of PriorityQueue Struct
PriorityQueue *priority_queue_new(int priority)
{
PriorityQueue *pq = (PriorityQueue *)malloc(sizeof(PriorityQueue));
pq->_capacity = 11; //default initial value
pq->_size = 0;
pq->_priority = priority;
pq->_nodes = (KeyValue **)malloc(sizeof(KeyValue *) * pq->_capacity);
return pq;
}
void priority_queue_free(PriorityQueue *pq,
void (*freevalue)(void *))
{
int i;
if(pq)
{
for(i = 0; i < pq->_size; ++i)
key_value_free(pq->_nodes[i], freevalue);
free(pq->_nodes);
free(pq);
}
}
const KeyValue *priority_queue_top(PriorityQueue *pq)
{
if(pq->_size > 0)
return pq->_nodes[0];
return NULL;
}
KeyValue *priority_queue_dequeue(PriorityQueue *pq)
{
KeyValue *pkv = NULL;
if(pq->_size > 0)
{
pkv = pq->_nodes[0];
priority_queue_adjust_head(pq);
}
return pkv;
}
void priority_queue_enqueue(PriorityQueue *pq,
KeyValue *kv)
{
printf("add key:%d\n", kv->_key);
pq->_nodes[pq->_size] = kv;
priority_queue_adjust_tail(pq);
if(pq->_size >= pq->_capacity)
priority_queue_realloc(pq);
}
int priority_queue_size(PriorityQueue *pq)
{
return pq->_size;
}
int priority_queue_empty(PriorityQueue *pq)
{
return pq->_size <= 0;
}
void priority_queue_print(PriorityQueue *pq)
{
int i;
KeyValue *kv;
printf("data in the pq->_nodes\n");
for(i = 0; i < pq->_size; ++i)
printf("%d ", pq->_nodes[i]->_key);
printf("\n");
printf("dequeue all data\n");
while(!priority_queue_empty(pq))
{
kv = priority_queue_dequeue(pq);
printf("%d ", kv->_key);
}
printf("\n");
}
static void priority_queue_realloc(PriorityQueue *pq)
{
pq->_capacity = pq->_capacity * 2;
pq->_nodes = realloc(pq->_nodes, sizeof(KeyValue *) * pq->_capacity);
}
static void priority_queue_adjust_head(PriorityQueue *pq)
{
int i, j, parent, left, right;
i = 0, j = 0;
parent = left = right = 0;
priority_queue_swap(pq->_nodes, 0, pq->_size - 1);
pq->_size--;
while(i < (pq->_size - 1) / 2)
{
parent = i;
left = i * 2 + 1;
right = left + 1;
j = left;
if(priority_queue_compare(pq, left, right) > 0)
j++;
if(priority_queue_compare(pq, parent, j) > 0)
{
priority_queue_swap(pq->_nodes, i, j);
i = j;
}
else
break;
}
}
static void priority_queue_adjust_tail(PriorityQueue *pq)
{
int i, parent, child;
i = pq->_size - 1;
pq->_size++;
while(i > 0)
{
child = i;
parent = (child - 1) / 2;
if(priority_queue_compare(pq, parent, child) > 0)
{
priority_queue_swap(pq->_nodes, child, parent);
i = parent;
}
else
break;
}
}
static int priority_queue_compare(PriorityQueue *pq,
int pos1,
int pos2)
{
int adjust = -1;
int r = pq->_nodes[pos1]->_key - pq->_nodes[pos2]->_key;
if(pq->_priority == PRIORITY_MAX)
r *= adjust;
return r;
}
static void priority_queue_swap(KeyValue **nodes,
int pos1,
int pos2)
{
KeyValue *temp = nodes[pos1];
nodes[pos1] = nodes[pos2];
nodes[pos2] = temp;
}
/*
*File: main.c
*purpose: tesing priority queue in C
*Author:puresky
*Date:2011/04/27
*/
#include <stdio.h>
#include <stdlib.h>
#include "pq.h"
int main(int argc, char **argv)
{
int i;
PriorityQueue *pq = priority_queue_new(PRIORITY_MAX);
int a[]={1, 9, 7, 8, 5, 4, 3, 2, 1, 100, 50, 17};
for(i = 0; i < sizeof(a)/ sizeof(int); ++i)
{
KeyValue *kv = key_value_new(a[i], NULL);
priority_queue_enqueue(pq, kv);
}
priority_queue_print(pq);
priority_queue_free(pq, NULL);
system("pause");
return 0;
}
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