queue.h

#ifndef LAB0_QUEUE_H
#define LAB0_QUEUE_H

/* This program implements a queue supporting both FIFO and LIFO
 * operations.
 *
 * It uses a circular doubly-linked list to represent the set of queue elements
 */

#include <stdbool.h>
#include <stddef.h>
#include "list.h"

/* Linked list element */
typedef struct {
    /* Pointer to array holding string.
     * This array needs to be explicitly allocated and freed
     */
    char *value;
    struct list_head list;
} element_t;

/* Operations on queue */

/* Create empty queue.
 * Return NULL if could not allocate space.
 */
struct list_head *q_new();

/* Free ALL storage used by queue.
 * No effect if q is NULL
 */
void q_free(struct list_head *head);

/* Attempt to insert element at head of queue.
 * Return true if successful.
 * Return false if q is NULL or could not allocate space.
 * Argument s points to the string to be stored.
 * The function must explicitly allocate space and copy the string into it.
 */
bool q_insert_head(struct list_head *head, char *s);

/* Attempt to insert element at tail of queue.
 * Return true if successful.
 * Return false if q is NULL or could not allocate space.
 * Argument s points to the string to be stored.
 * The function must explicitly allocate space and copy the string into it.
 */
bool q_insert_tail(struct list_head *head, char *s);

/* Attempt to remove element from head of queue.
 * Return target element.
 * Return NULL if queue is NULL or empty.
 * If sp is non-NULL and an element is removed, copy the removed string to *sp
 * (up to a maximum of bufsize-1 characters, plus a null terminator.)
 *
 * NOTE: "remove" is different from "delete"
 * The space used by the list element and the string should not be freed.
 * The only thing "remove" need to do is unlink it.
 *
 * Referenceerence:
 * https://english.stackexchange.com/questions/52508/difference-between-delete-and-remove
 */
element_t *q_remove_head(struct list_head *head, char *sp, size_t bufsize);

/* Attempt to remove element from tail of queue.
 * Other attribute is as same as q_remove_head.
 */
element_t *q_remove_tail(struct list_head *head, char *sp, size_t bufsize);

/* Attempt to release element */
void q_release_element(element_t *e);

/* Return number of elements in queue.
 * Return 0 if q is NULL or empty
 */
int q_size(struct list_head *head);

/* Delete the middle node in list.
 * The middle node of a linked list of size n is the
 * ⌊n / 2⌋th node from the start using 0-based indexing.
 * If there're six element, the third member should be return.
 * Return true if successful.
 * Return false if list is NULL or empty.
 *
 * Reference:
 * https://leetcode.com/problems/delete-the-middle-node-of-a-linked-list/
 */
bool q_delete_mid(struct list_head *head);

/* Delete all nodes that have duplicate string,
 * leaving only distinct strings from the original list.
 * Return true if successful.
 * Return false if list is NULL.
 *
 * Reference:
 * https://leetcode.com/problems/remove-duplicates-from-sorted-list-ii/
 */
bool q_delete_dup(struct list_head *head);

/* Attempt to swap every two adjacent nodes.
 *
 * Reference:
 * https://leetcode.com/problems/swap-nodes-in-pairs/
 */
void q_swap(struct list_head *head);

/* Reverse elements in queue
 * No effect if q is NULL or empty
 * This function should not allocate or free any list elements
 * (e.g., by calling q_insert_head, q_insert_tail, or q_remove_head).
 * It should rearrange the existing ones.
 */
void q_reverse(struct list_head *head);

/* Sort elements of queue in ascending order
 * No effect if q is NULL or empty. In addition, if q has only one
 * element, do nothing.
 */
void q_sort(struct list_head *head);

#endif /* LAB0_QUEUE_H */