QUEUE - Implement a first in, first out linked list

/*
 * File:    queue.h
 * Purpose: Implement a first in, first out linked list
 *
 * Notes:
 */

#ifndef _QUEUE_H_
#define _QUEUE_H_

/********************************************************************/

/* 
 * Individual queue node
 */
typedef struct NODE
{
  struct NODE *next;
} QNODE;

/* 
 * Queue Struture - linked list of qentry items 
 */
typedef struct
{
  QNODE *head;
  QNODE *tail;
} QUEUE;

/*
 * Functions provided by queue.c
 */
void queue_init( QUEUE * );

int queue_isempty( QUEUE * );

void queue_add( QUEUE *, QNODE * );

QNODE* queue_remove( QUEUE * );

QNODE* queue_peek( QUEUE * );

void queue_move( QUEUE *, QUEUE * );

/********************************************************************/

#endif /* _QUEUE_H_ */

/*
 * File:    queue.c
 * Purpose: Implement a first in, first out linked list
 *
 * Notes:   
 */

#include "queue.h"

/********************************************************************/
/* 
 * Initialize the specified queue to an empty state
 * 
 * Parameters:
 *  q   Pointer to queue structure
 */
void queue_init( QUEUE *q )
{
  q->head = NULL;
}
/********************************************************************/
/* 
 * Check for an empty queue
 *
 * Parameters:
 *  q       Pointer to queue structure
 * 
 * Return Value:
 *  1 if Queue is empty
 *  0 otherwise
 */
int queue_isempty( QUEUE *q )
{
  return ( q->head == NULL );
}
/********************************************************************/
/* 
 * Add an item to the end of the queue 
 *
 * Parameters:
 *  q       Pointer to queue structure
 *  node    New node to add to the queue
 */
void queue_add( QUEUE *q, QNODE *node )
{
  if ( queue_isempty( q ) )
  {
    q->head = q->tail = node;
  }
  else
  {
    q->tail->next = node;
    q->tail = node;
  }

  node->next = NULL;
}

/********************************************************************/
/* 
 * Remove and return first (oldest) entry from the specified queue 
 *
 * Parameters:
 *  q       Pointer to queue structure
 *
 * Return Value:
 *  Node at head of queue - NULL if queue is empty
 */
QNODE*
queue_remove( QUEUE *q )
{
  QNODE *oldest;

  if ( queue_isempty( q ) )
    return NULL;

  oldest = q->head;
  q->head = oldest->next;
  return oldest;
}
/********************************************************************/
/* 
 * Peek into the queue and return pointer to first (oldest) entry.
 * The queue is not modified
 *
 * Parameters:
 *  q       Pointer to queue structure
 *
 * Return Value:
 *  Node at head of queue - NULL if queue is empty
 */
QNODE*
queue_peek( QUEUE *q )
{
  return q->head;
}
/********************************************************************/
/* 
 * Move entire contents of one queue to the other
 *
 * Parameters:
 *  src     Pointer to source queue
 *  dst     Pointer to destination queue
 */
void queue_move( QUEUE *dst, QUEUE *src )
{
  if ( queue_isempty( src ) )
    return;

  if ( queue_isempty( dst ) )
    dst->head = src->head;
  else
    dst->tail->next = src->head;

  dst->tail = src->tail;
  src->head = NULL;
  return;
}
/********************************************************************/