fix: getDuplicate() must not modify const numbers array

numbers.c

@@ -74,13 +74,27 @@ unsigned int *createNumbers(unsigned int len)
 // Returns only the only number in numbers which is present twice. Returns zero on errors.
 unsigned int getDuplicate(const unsigned int numbers[], unsigned int len)
 {
-  qsort((void *)numbers, len, sizeof(int), compareInt); // sort the array
+  unsigned int *numbersCpy = malloc(sizeof(unsigned int) * len);
+  if (!numbersCpy)
+  {
+    return 0;
+  }
+  memcpy(numbersCpy, numbers, len * sizeof(unsigned int));
+  numbersCpy = numbersCpy; // shadow the numbers array with copy
+
+  qsort((void *)numbersCpy, len, sizeof(int), compareInt); // sort the array
+
+  unsigned int duplicateFound = 0; // zero on errors
   for (int i = 0; i < len - 1; i++)
   {
-    if (numbers[i] == numbers[i + 1])
-      return numbers[i];
+    if (numbersCpy[i] == numbersCpy[i + 1])
+    {
+      duplicateFound = numbersCpy[i];
+      break;
+    }
   }
-  return 0; // zero on errors
+  free(numbersCpy);
+  return duplicateFound;
 }
 
 static int compareInt(const void *ptr1, const void *ptr2)

test_numbers.c

@@ -61,12 +61,12 @@ void test_get_duplicate_does_modify()
     unsigned int arr1[] = {1, 2, 3, 4, 5, 4, 3, 2, 1}; // sorting would change this
     size_t len1 = sizeof(arr1) / sizeof(arr1[0]);
     unsigned int arr1Copy[len1];
-    memcpy(arr1Copy, arr1, len1 * sizeof(arr1[0]));
+    memcpy(arr1Copy, arr1, len1 * sizeof(unsigned int));
 
     getDuplicate(arr1, len1); // return value does not matter
 
     // check if the arrays are still the same
-    if (memcmp(arr1, arr1Copy, len1 * sizeof(arr1[0])))
+    if (memcmp(arr1, arr1Copy, len1 * sizeof(unsigned int)))
     {
         TEST_FAIL_MESSAGE("Arrays have diverged");
     }