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

@@ -1,6 +1,7 @@
 #include "unity.h"
 #include "numbers.h"
 #include "stdlib.h"
+#include "string.h"
 
 static int compareInt(const void *ptr1, const void *ptr2);
 
@@ -27,7 +28,7 @@ void test_get_duplicate_without_duplicates(void)
 // getDuplicate() on some arrays with 1 duplicate
 void test_get_duplicate(void)
 {
-    unsigned int arr1[] = {4, 8, 32, 5, 3, 8, 8};
+    unsigned int arr1[] = {4, 15, 32, 5, 3, 8, 8};
     unsigned int len1 = sizeof(arr1) / sizeof(arr1[0]);
     unsigned int arr2[] = {1, 3, 3, 7};
     unsigned int len2 = sizeof(arr2) / sizeof(arr2[0]);
@@ -48,13 +49,29 @@ void test_for_triple(void)
         unsigned int *numbers = createNumbers(3);
         if (numbers[0] == numbers[1] && numbers[1] == numbers[2])
         {
-            // fail the test
-            TEST_ASSERT(0);
+            TEST_FAIL_MESSAGE("triple generated");
         }
         free(numbers);
     }
 }
 
+// check if getDuplicate() modifies the original array (it should not)
+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(unsigned int));
+
+    getDuplicate(arr1, len1); // return value does not matter
+
+    // check if the arrays are still the same
+    if (memcmp(arr1, arr1Copy, len1 * sizeof(unsigned int)))
+    {
+        TEST_FAIL_MESSAGE("Arrays have diverged");
+    }
+}
+
 // checks if there is exactly 1 duplicate number at varying array sizes
 void test_exactly_one_duplicate()
 {
@@ -105,5 +122,6 @@ int main(void)
     RUN_TEST(test_for_triple);
     RUN_TEST(test_exactly_one_duplicate);
     RUN_TEST(test_get_duplicate);
+    RUN_TEST(test_get_duplicate_does_modify);
     return UNITY_END();
 }