循环队列FIFO原理及C实现

循环队列是把顺序队列首尾相连，把存储队列元素的表从逻辑上看成一个环，成为循环队列。

 

入队时尾指针向前追赶头指针；出队时头指针向前追赶尾指针。

定义一个循环队列结构：

#define FIFO_HEAD(name, type)                                                
        struct name {                                                        
                struct type *fifo;                                        
                int front, tail, capacity;                                
        }

front表示首元素索引

struct type *fifo表示该队列中的元素指针，可以指向任意结构体指针

tail表示最后一个元素索引

capacity表示队列的长度

循环队列初始化：

分配一个连续的空间存储队列元素。

 

#define FIFO_INIT(head, _capacity) do {                                        
        (head)->fifo = malloc(sizeof(*(head)->fifo) * _capacity);        
        (head)->front = (head)->tail = -1;                                
        (head)->capacity = _capacity;                                        
} while (0)

循环队列销毁：

#define FIFO_EXIT(head) do {                                                
        (head)->front = (head)->tail = -1;                                
        (head)->capacity = 0;                                                
        if ((head)->fifo)                                                
                free((head)->fifo);                                        
} while (0)

队列空判断： 

#define FIFO_EMPTY(head)    ((head)->front == -1)

①队列初始化时，队列是空的，会让front为-1

②出队列时，font++, 当font追上tail表示空了，则可以重新设置起始点，令front = tail = -1

综合①②所以可以用-1判断

队列满判断：

#define FIFO_FULL(head)     (((head)->front == ((head)->tail + 1)%(head)->capacity))

①当front=0时，那么tail到达capacity-1表示FIFO full.

 

②否则，tail追上front后(front = tail + 1)表示FIFO full.

 

入队列：

入队列就是尾元素的索引++，也就是tail++，让新元素放进队列的尾部。

#define FIFO_PUSH(head, elm) do {                                        
        if (FIFO_EMPTY(head))                                                
                (head)->front = (head)->tail = 0;                        
        else                                                                
                (head)->tail = ((head)->tail == (head)->capacity - 1)        
                        ? 0 : (head)->tail + 1;                        
        (head)->fifo[(head)->tail] = elm;                                
} while (0)

如果队列是空的，则第一个元素入队列，front和tail索引都指向第一个元素，front = tail = 0;

 

其他情况入队，让tail++

 

 

 

出队列：

出队列就是让font对应的元素丢出去，font++。

#define FIFO_POP(head, pelm) do {                                        
        *(pelm) = (head)->fifo[(head)->front];                                
        if ((head)->front == (head)->tail)                                
                (head)->front = (head)->tail = -1;                        
        else                                                                
                (head)->front = ((head)->front == (head)->capacity - 1)        
                        ? 0 : (head)->front + 1;                
} while (0)

当front追上tail后，表示队列空了，重新设置起始点，需要将front = tail = -1 

 

其他情况出队，丢出front元素，让front++

 

队列总元素个数：

#define FIFO_CAPACITY(head) ((head)->capacity)

队列有效元素个数：

#define FIFO_SIZE(head)     (FIFO_EMPTY(head) ? 
        0 : ((((head)->tail + (head)->capacity - (head)->front) % (head)->capacity) + 1))

 用tail - front就表示有效元素个数，不过由于循环FIFO,可能tail<front,这个时候就需要取余运算，如下图。

 

循环队列遍历：

#define FIFO_FOREACH(var, head, idx) 
　　for (idx = (head)->front, var = (head)->fifo[idx]; 
　　　　idx < (head)->front + FIFO_SIZE(head); 
　　　　var = (head)->fifo[++idx % (head)->capacity])

获取队列尾元素:

#define FIFO_GET_TAIL(head, pelm) (*(pelm) = (head)->fifo[(head)->tail])

获取队列头元素：

#define FIFO_GET_FRONT(head, pelm) (*(pelm) = (head)->fifo[(head)->front])

测试：

#include "fifo.h"
#include <stdio.h>

struct person{
        int age;
        int id;
        char name[20];
};
 
FIFO_HEAD(person_q, person*);
/*
        struct person_q {                                                        
                struct person* *fifo;                                        
                int front, tail, capacity;                                
        }
*/
struct person_q person1_queue;
struct person_q person2_queue;

int main(void)
{
        FIFO_INIT(&person1_queue, 1);
        FIFO_INIT(&person2_queue, 5);

        if (FIFO_CAPACITY(&person1_queue) != 1) {
                printf( "FIFO_CAPACITY 1 NG.
");
                return -1;
        }

        if (FIFO_CAPACITY(&person2_queue) != 5) {
                printf( "FIFO_CAPACITY 2 NG.
");
                return -1;
        }
 
        if (FIFO_SIZE(&person1_queue) != 0) {
                printf( "FIFO_SIZE 1 NG.
");
                return -1;
        }
 
        if (FIFO_SIZE(&person2_queue) != 0) {
                printf( "FIFO_SIZE 2 NG.
");
                return -1;
        }

        if (!FIFO_EMPTY(&person1_queue)) {
                printf( "FIFO_EMPTY 1 NG.
");
                return -1;
        }
 
        if (!FIFO_EMPTY(&person2_queue)) {
                printf( "FIFO_EMPTY 2 NG.
");
                return -1;
        }
 
        struct person *person_a = malloc(sizeof(*person_a));
        person_a->age = 20;
        person_a->id = 1001;
        FIFO_PUSH(&person1_queue, person_a);//把person_a这个结构体指针元素丢进FIFO,
        //后面对它pop出来又能拿到它，所以不用担心地址弄丢导致无法释放.
 
        if (!FIFO_FULL(&person1_queue)) {
                printf( "FIFO_FULL 1 NG.
");
                return -1;
        }
 
        person_a = malloc(sizeof(*person_a));
        person_a->age = 30;
        person_a->id = 1002;
        FIFO_PUSH(&person2_queue, person_a);

        if (FIFO_FULL(&person2_queue)) {
                printf( "FIFO_FULL 2 NG.
");
                return -1;
        }

        if (FIFO_SIZE(&person1_queue) != 1) {
                printf( "FIFO_SIZE 3 NG.
");
                return -1;
        }

        if (FIFO_SIZE(&person2_queue) != 1) {
                printf( "FIFO_SIZE 4 NG.
");
                return -1;
        }

        FIFO_POP(&person1_queue, &person_a);
        if (person_a->age != 20) {
                printf( "FIFO_POP content NG.
");
                return -1;
        }
        free(person_a);

        if (FIFO_SIZE(&person1_queue) != 0) {
                printf( "FIFO_SIZE 5 NG.
");
                return -1;
        }
 
        person_a = malloc(sizeof(*person_a));
        person_a->age = 40;
        person_a->id = 1003;
        FIFO_PUSH(&person2_queue, person_a);

        FIFO_GET_FRONT(&person2_queue, &person_a);
        if (person_a->age != 30) {
                printf( "FIFO_GET_FRONT NG.
");
                return -1;
        }
 
        FIFO_GET_TAIL(&person2_queue, &person_a);
        if (person_a->age != 40) {
                printf( "FIFO_GET_TAIL NG.
");
                return -1;
        }
 
        FIFO_POP(&person2_queue, &person_a);
        if (person_a->age != 30) {
                printf( "FIFO_POP content NG.
");
                return -1;
        }
        free(person_a);

        if (FIFO_SIZE(&person2_queue) != 1) {
                printf( "FIFO_SIZE 6 NG.
");
                return -1;
        }

        FIFO_POP(&person2_queue, &person_a);
        if (person_a->age != 40) {
                printf( "FIFO_POP content NG.
");
                return -1;
        }
        free(person_a);
 
        if (FIFO_SIZE(&person2_queue) != 0) {
                printf( "FIFO_SIZE 7 NG.
");
                return -1;
        }
 
        struct person *person_arr[5];
        int i=0;

        while (!FIFO_FULL(&person2_queue))
        {
                person_arr[i] = malloc(sizeof(*person_arr[0]));
                person_arr[i]->age = i;
                person_arr[i]->id = 1000 + i;
                FIFO_PUSH(&person2_queue, person_arr[i]);
                i++;
        }

        while (!FIFO_EMPTY(&person2_queue))
        {
                FIFO_POP(&person2_queue, &person_a);
                printf( "age:%d, id:%d.
", person_a->age, person_a->id);
                free(person_a);
        }

        FIFO_EXIT(&person1_queue);
        FIFO_EXIT(&person2_queue);
        return 0;
}