info2aufgabe3/numbers.c

#include <stdlib.h>
#include <stdio.h>
#include <time.h>
#include <string.h>
#include "numbers.h"
#include "bintree.h"

//TODO: getDuplicate und createNumbers implementieren
/*  *   * Erzeugen eines Arrays mit der vom Nutzer eingegebenen Anzahl an Zufallszahlen.
  * Sicherstellen, dass beim Befüllen keine Duplikate entstehen.
  * Duplizieren eines zufälligen Eintrags im Array.
  * in `getDuplicate()`: Sortieren des Arrays und Erkennen der doppelten Zahl durch Vergleich benachbarter Elemente. */

// Comparison function for unsigned integers used in BST and sorting operations.
// Returns: <0 if a < b, 0 if a == b, >0 if a > b
// This is an internal function (static) since it's not part of the public API.
static int compareUnsignedInts(const void *a, const void *b)
{
    unsigned int valA = *(const unsigned int *)a;
    unsigned int valB = *(const unsigned int *)b;
    
    // Safe comparison without overflow concerns for our range
    if (valA < valB) return -1;
    if (valA > valB) return 1;
    return 0;
}

// Returns len random numbers between 1 and 2x len in random order which are all different, except for two entries.
// Returns NULL on errors. Use the implementation of the binary search tree to check for possible duplicates while
// creating random numbers.
//
// Algorithm:
// 1. Generate len-1 unique random numbers using the BST for duplicate detection
// 2. Randomly duplicate one of those numbers by inserting it at a random position
// 3. Final array has len entries with exactly one value appearing twice
unsigned int *createNumbers(unsigned int len)
{
    // Step 1: Validate input - we need at least 2 elements to create a duplicate
    if (len < 2)
    {
        return NULL;
    }
    
    // Step 2: Allocate memory for the final array (len elements total)
    unsigned int *numbers = (unsigned int *)malloc(len * sizeof(unsigned int));
    if (numbers == NULL)
    {
        return NULL;
    }
    
    // Step 3: Create binary search tree to track generated numbers and prevent accidental duplicates
    // Note: Random seed should be initialized once in main() with srand(time(NULL))
    // Calling srand() here would reset the seed each time, reducing randomness
    TreeNode *usedNumbers = NULL;
    
    // Step 4: Generate len-1 unique random numbers (we'll add one duplicate later)
    unsigned int arrayIndex = 0;
    
    while (arrayIndex < len - 1)
    {
        // Generate random number in range [1, 2×len]
        unsigned int randomNumber = (rand() % (2 * len)) + 1;
        
        // Check if this number was already generated
        int isDuplicate = 0;
        usedNumbers = addToTree(usedNumbers, &randomNumber, sizeof(unsigned int), compareUnsignedInts, &isDuplicate);
        
        if (usedNumbers == NULL) // checks if addTOTree failed 
        {
            free(numbers);
            return NULL;
        }
        
        // If new number, add to result array; if duplicate, retry
        if (isDuplicate == 0)
        {
            numbers[arrayIndex] = randomNumber;
            arrayIndex++;
        }
    }
    
    // Step 5: Pick a random value from our generated numbers to duplicate
    unsigned int indexToDuplicate = rand() % (len - 1);
    unsigned int valueToDuplicate = numbers[indexToDuplicate];
    
    // Step 6: Pick a random position to insert the duplicate (anywhere in the array)
    unsigned int insertPos = rand() % len;
    
    // Step 7: Shift elements right to make room for the duplicate
    for (unsigned int i = len - 1; i > insertPos; i--)
    {
        numbers[i] = numbers[i - 1];
    }
    
    // Step 8: Insert the duplicate at the chosen position
    numbers[insertPos] = valueToDuplicate;
    
    // Step 9: Clean up the binary search tree (we don't need it anymore)
    clearTree(usedNumbers);
    
    // Return the array with len elements (len-1 unique + 1 duplicate)
    return numbers;
}



// Returns only the only number in numbers which is present twice. Returns zero on errors.
//
// Algorithm:
// 1. Create a copy of the array to avoid modifying the input
// 2. Sort the copy using qsort()
// 3. Compare adjacent elements to find the duplicate
// 4. Return the duplicate value, or 0 on error
unsigned int getDuplicate(const unsigned int *numbers, unsigned int len)
{
    // Step 1: Input validation - need at least 2 elements to have a duplicate
    if (numbers == NULL || len < 2)
    {
        return 0;
    }
    
    // Step 2: Create a copy of the array to sort (we don't modify the input)
    unsigned int *sorted = (unsigned int *)malloc(len * sizeof(unsigned int));
    if (sorted == NULL)
    {
        return 0;
    }
    
    // Step 3: Copy the array
    memcpy(sorted, numbers, len * sizeof(unsigned int));
    
    // Step 4: Sort the copy using the shared comparison function
    qsort(sorted, len, sizeof(unsigned int), compareUnsignedInts);
    
    // Step 5: Compare adjacent elements to find the duplicate
    for (unsigned int i = 0; i < len - 1; i++)
    {
        if (sorted[i] == sorted[i + 1])
        {
            // Found the duplicate!
            unsigned int result = sorted[i];
            free(sorted);
            return result;
        }
    }
    
    // Step 6: No duplicate found (should not happen if input is valid)
    free(sorted);
    return 0;
}