improve doubly linked list in the same way as the last commit

lab01/zad3/include/doubly_linked_list.h

@@ -14,6 +14,7 @@ typedef struct {
     size_t length;
     size_t element_size;
     Node *head;
+    Node *tail;
 } DoublyLinkedList;
 
 typedef struct {

lab01/zad3/src/doubly_linked_list.c

@@ -17,6 +17,7 @@ DoublyLinkedList doubly_linked_list_new(size_t element_size) {
     ret.length = 0;
     ret.element_size = element_size;
     ret.head = NULL;
+    ret.tail = NULL;
 
     return ret;
 }
@@ -26,20 +27,18 @@ void doubly_linked_list_insert(DoublyLinkedList *l, void *value) {
         l->head = node_new(value, l->element_size);
         l->head->next = l->head;
         l->head->prev = l->head;
+        l->tail = l->head;
         l->length += 1;
         return;
     }
 
-    Node *tail = l->head;
-    while (tail->next != l->head) {
-        tail = tail->next;
-    }
+    l->tail->next = node_new(value, l->element_size);
+    l->tail->next->next = l->head;
+    l->tail->next->prev = l->tail;
 
-    tail->next = node_new(value, l->element_size);
-    tail->next->next = l->head;
-    tail->next->prev = tail;
+    l->tail = l->tail->next;
 
-    l->head->prev = tail->next;
+    l->head->prev = l->tail;
     l->length += 1;
 }
 
@@ -47,21 +46,11 @@ DoublyLinkedList doubly_linked_list_merge(DoublyLinkedList l1, DoublyLinkedList
     if (l1.head == NULL) return l2;
     if (l2.head == NULL) return l1;
 
-    Node *l1_tail = l1.head;
-    while (l1_tail->next != l1.head) {
-        l1_tail = l1_tail->next;
-    }
+    l2.head->prev = l1.tail;
+    l1.tail->next = l2.head;
 
-    l2.head->prev = l1_tail;
-    l1_tail->next = l2.head;
-
-    Node *l2_tail = l2.head;
-    while (l2_tail->next != l2.head) {
-        l2_tail = l2_tail->next;
-    }
-
-    l1.head->prev = l2_tail;
-    l2_tail->next = l1.head;
+    l1.head->prev = l2.tail;
+    l2.tail->next = l1.head;
 
     l1.length += l2.length;