LISTING 15.5 L15-5.C

/* Finds the first node in a value-sorted linked list that
   has a Value field greater than or equal to a key value, and
   returns a pointer to the node preceding that node (to facilitate
   insertion and deletion), or a NULL pointer if no such value was
   found. Assumes the list is terminated with a sentinel tail node
   containing the largest possible Value field setting and pointing
   to itself as the next node. */
#include <stdio.h>
#include “llist.h”
struct LinkNode *FindNodeBeforeValueNotLess(
   struct LinkNode *HeadOfListNode, int SearchValue)
{
   struct LinkNode *NodePtr = HeadOfListNode;
   while (NodePtr->NextNode->Value < SearchValue)
      NodePtr = NodePtr->NextNode;
   if (NodePtr->NextNode->NextNode == NodePtr->NextNode)
      return(NULL);     /* we found the sentinel; failed search */
   else
      return(NodePtr);  /* success; return pointer to node preceding
                           node that was >= */
}

Figure 15.4  List terminated by a sentinel.

Circular Lists

One minor but elegant refinement yet remains: Use a single node as both the head and the tail of the list. We can do this by connecting the last node back to the first through the head/tail node in a circular fashion, as shown in Figure 15.5. This head/tail node can also, of course, be a sentinel; when it’s necessary to check for the end of the list explicitly, that can be done by comparing the current node pointer to the head pointer. If they’re equal, you’re at the head/tail node.

Why am I so fond of this circular list architecture? For one thing, it saves a node, and most of my linked list programming has been done in severely memory-constrained environments. Mostly, though, it’s just so neat; with this setup, there’s not a single node or inner-loop instruction wasted. Perfect economy of programming, if you ask me.

I must admit that I racked my brains for quite a while to come up with the circular list, simple as it may seem. Shortly after coming up with it, I happened to look in Sedgewick’s book, only to find my nifty optimization described plain as day; and a little while after that, I came across a thread in the algorithms/computer.sci topic on BIX that described it in considerable detail. Folks, the information is out there. Look it up before turning on your optimizer afterburners!

Listings 15.1 and 15.6 together form a suite of C functions for maintaining a circular linked list sorted by ascending value. (Listing 15.5 requires modification before it will work with circular lists.) Listing 15.7 is an assembly language version of InsertNodeSorted(); note the tremendous efficiency of the scanning loop in InsertNodeSorted()—four instructions per node!—thanks to the dummy head/tail/sentinel node. Listing 15.8 is a simple application that illustrates the use of the linked-list functions in Listings 15.1 and 15.6.

Contrast Figure 15.5 with Figure 15.1, and Listings 15.1, 15.5, 15.6, and 15.7 with Listings 15.3 and 15.4. Yes, linked lists are simple, but not so simple that a little knowledge doesn’t make a substantial difference. Make it a habit to read Knuth or Sedgewick or the like before you write a single line of code.

Figure 15.5  Representing a circular list.

LISTING 15.6 L15-6.C

/*  Suite of functions for maintaining a linked list sorted by
    ascending order of the Value field. The list is circular; that
    is,it has a dummy node as both the head and the tail of the list.
    The dummy node is a sentinel, containing the largest possible
    Value field setting. Tested with Borland C++ in C mode. */
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include “llist.h”
/* Initializes an empty linked list of LinkNode structures,
   consisting of a single head/tail/sentinel node, and returns a
   pointer to the list. Returns NULL for failure. */
struct LinkNode *InitLinkedList()
{
   struct LinkNode *Sentinel;

   if ((Sentinel = malloc(sizeof(struct LinkNode))) == NULL)
      return(NULL);
   Sentinel->NextNode = Sentinel;
   Sentinel->Value = SENTINEL;
   strcpy(Sentinel->Text, “*** sentinel ***”);
   return(Sentinel);
}

/* Finds the first node in a value-sorted linked list with a value
   field equal to a key value, and returns a pointer to the node
   preceding that node (to facilitate insertion and deletion), or a
   NULL pointer if no value was found. Assumes list is terminated
   with a sentinel node containing the largest possible value. */

struct LinkNode *FindNodeBeforeValue(struct LinkNode *HeadOfListNode,
int SearchValue)
{
   struct LinkNode *NodePtr = HeadOfListNode;

   while (NodePtr->NextNode->Value < SearchValue)
      NodePtr = NodePtr->NextNode;
   if (NodePtr->NextNode->Value == SearchValue) {
      /* Found the search value; success unless we found the
         sentinel (can happen only if SearchValue == SENTINEL) */
      if (NodePtr->NextNode == HeadOfListNode) {
         return(NULL);     /* failure; we found the sentinel */
      } else {
         return(NodePtr);  /* success; return pointer to node
                              preceding the node that was equal */
      }
   } else {
       return(NULL);       /* No match; return failure status */
   }
}

/* Inserts the specified node into a value-sorted linked list, such
   that value-sorting is maintained. Returns a pointer to the node
   after which the new node is inserted. */
struct LinkNode *InsertNodeSorted(struct LinkNode *HeadOfListNode,
   struct LinkNode *NodeToInsert)
{
   struct LinkNode *NodePtr = HeadOfListNode;
   int SearchValue = NodeToInsert->Value;
   while (NodePtr->NextNode->Value < SearchValue)
      NodePtr = NodePtr->NextNode;
   NodeToInsert->NextNode = NodePtr->NextNode;
   NodePtr->NextNode = NodeToInsert;
   return(NodePtr);
}