list_pop

src/passbook.c

@@ -192,12 +192,14 @@ char inst[INSTRUCTION_LENGTH];
 /* password mapping for each url: initially empty */
 node_t * map = NULL;
 
-/* a list of node pointers -- the idea is to use these to implement a node
-   stack that we can then use to traverse the tree iteratively, avoiding
-   recursion. We need tree traversal e.g. when saving the passbook to a file */
+/* a doubly-linked list of node pointers
+   is used to implement stacks/queues of nodes so we can implement various
+   tree traversal algorithms without using recursion (to avoid stack overflow
+   for very large trees). */
 typedef struct nodeptr_list {
   const node_t *p;
   struct nodeptr_list *next;
+  struct nodeptr_list *prev;
 } nodeptr_list_t;
 
 
@@ -206,26 +208,47 @@ nodeptr_list_t * list_push(nodeptr_list_t *lst, const node_t *p){
   assert(n != NULL && "push: malloc failed");
   n->p = p;
   n->next = lst;
+  if (lst != NULL){
+    lst->prev = n;
+  }
+  n->prev = NULL;
   return n;
 }
 
+/* when out is non-NULL we place a pointer to the first node into it */
+nodeptr_list_t * list_pop(nodeptr_list_t *lst, const node_t **out){
+  if (lst != NULL){
+    if (out != NULL){
+      *out = lst->p;
+    }
+    nodeptr_list_t *ret = lst->next;
+    free(lst);
+    return ret;
+  }else{
+    if (out != NULL){
+      *out = (const node_t *)NULL;
+    }
+    return lst;
+  }
+}
+
 void list_destroy(nodeptr_list_t *lst){
   while (lst != NULL){
-    nodeptr_list_t *next = lst->next;
-    free(lst);
-    lst = next;
+    lst = list_pop(lst,NULL);
   }
 }
 
 void list_print(nodeptr_list_t *lst){
   printf("\nBegin list print:\n");
   while (lst != NULL){
-    node_print(lst->p);
-    lst = lst->next;
+    const node_t *p;
+    lst = list_pop(lst,&p);
+    node_print(p);
   }
   printf("End list print.\n\n");
 }
 
+
 void print_all(const node_t *p){
   nodeptr_list_t *lst = NULL;
   if (p != NULL){
@@ -239,10 +262,8 @@ void print_all(const node_t *p){
       }
       
       // pop from the stack to simulate the return
-      const node_t *q = lst->p;
-      nodeptr_list_t *temp = lst;
-      lst = lst->next;
-      free(temp);
+      const node_t *q;
+      lst = list_pop(lst,&q);
 
       // print the node following the return
       node_print(q);