fix errors and docs in cantor set

misc/cantor_set.c

@@ -1,54 +1,61 @@
+/**
+ * @file
+ * @brief function to generate [Cantor ternary
+ * set](https://en.wikipedia.org/wiki/Cantor_set)
+ */
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 
-struct contour
+/** structure to define Cantor set */
+typedef struct contour
 {
-    double start;
-    double end;
-    struct contour *next;
-};
-typedef struct contour Contour;
-Contour *head;
-
-void startList(double start_num, double end_num)
-{
-    if (head == NULL)
-    {
-        head = (Contour *)malloc(sizeof(Contour));
-        head->start = start_num;
-        head->end = end_num;
-        head->next = NULL;
-    }
-}
+    double start;         /**< start of interval */
+    double end;           /**< end of interval */
+    struct contour *next; /**< pointer to next set */
+} Contour;
 
+/** Iterative constructor of all sets in the current level. This function
+ * dynamically allocates memory when creating new sets. These are freed by the
+ * function ::free_memory.
+ * @param head pointer to interval set instance to update
+ */
 void propagate(Contour *head)
 {
-    Contour *temp = head;
-
-    if (temp != NULL)
-    {
-        Contour *newNode = (Contour *)malloc(sizeof(Contour));
-        double diff = (((temp->end) - (temp->start)) / 3);
-
-        newNode->end = temp->end;
-        temp->end = ((temp->start) + diff);
-        newNode->start = (newNode->end) - diff;
-
-        newNode->next = temp->next;
-
-        temp->next = newNode;
-
-        propagate(temp->next->next);
-    }
-    else
+    // if input is NULL, ignore the process
+    if (head == NULL)
         return;
+
+    Contour *temp = head;  // local pointer to track propagation
+
+    // create new node for the new set
+    Contour *newNode = (Contour *)malloc(sizeof(Contour));
+
+    // get 1/3rd of interval
+    double diff = (((temp->end) - (temp->start)) / 3);
+
+    // update interval ranges
+    newNode->end = temp->end;
+    temp->end = ((temp->start) + diff);
+    newNode->start = (newNode->end) - diff;
+
+    // update pointer to next set in this level
+    newNode->next = temp->next;
+
+    // point to next set
+    temp->next = newNode;
+
+    // create next set
+    propagate(temp->next->next);
 }
 
+/** Print sets in the current range to `stdout`
+ * @param head pointer to first set in the current level
+ */
 void print(Contour *head)
 {
     Contour *temp = head;
-    while (temp != NULL)
+    while (temp != NULL)  // print while a valid set is found
     {
         printf("\t");
         printf("[%lf] -- ", temp->start);
@@ -59,10 +66,23 @@ void print(Contour *head)
     printf("\n");
 }
 
+/** Clear memory allocated by ::propagate function.
+ * @param head pointer to first allocated instance.
+ */
+void free_memory(Contour *head)
+{
+    if (!head)
+        return;
+
+    if (head->next)
+        free_memory(head->next);
+
+    free(head);
+}
+
+/** Main function */
 int main(int argc, char const *argv[])
 {
-    head = NULL;
-
     int start_num, end_num, levels;
 
     if (argc < 2)
@@ -77,17 +97,27 @@ int main(int argc, char const *argv[])
         levels = atoi(argv[3]);
     }
 
-    startList(start_num, end_num);
+    if (start_num < 0 || end_num < 0 || levels < 0)
+    {
+        fprintf(stderr, "All numbers must be positive\n");
+        return -1;
+    }
 
+    Contour head = {.start = start_num, .end = end_num, .next = NULL};
+
+    // loop to propagate each level from top to bottom
     for (int i = 0; i < levels; i++)
     {
         printf("Level %d\t", i);
-        print(head);
-        propagate(head);
+        print(&head);
+        propagate(&head);
         printf("\n");
     }
     printf("Level %d\t", levels);
-    print(head);
+    print(&head);
+
+    // delete all memory allocated
+    free_memory(head.next);
 
     return 0;
 }