## C, search, floyd_cycle_detection_algorithm.c

``````/**
* @file
* @brief Implementation of [Floyd's Cycle
* Detection](https://en.wikipedia.org/wiki/Cycle_detection) algorithm
* @details
* Given an array of integers containing `n + 1` integers, where each
* integer is in the range [1, n] inclusive. If there is only one duplicate
* number in the input array, this algorithm returns the duplicate number in
* O(1) space and the time complexity is less than O(n^2) without modifying the
* original array, otherwise, it returns -1.
* @author [Swastika Gupta](https://github.com/Swastyy)
*/

#include <assert.h>   /// for assert
#include <inttypes.h> /// for uint32_t
#include <stdio.h>    /// for IO operations

/**
* @brief The main function implements the search algorithm
* @tparam T type of array
* @param in_arr the input array
* @param n size of the array
* @returns the duplicate number
*/
uint32_t duplicateNumber(const uint32_t *in_arr, size_t n)
{
if (n <= 1) {  // to find duplicate in an array its size should be atleast 2
return -1;
}
uint32_t tortoise = in_arr[0];  ///< variable tortoise is used for the longer
///< jumps in the array
uint32_t hare = in_arr[0];  ///< variable hare is used for shorter jumps in the array
do {                                   // loop to enter the cycle
tortoise = in_arr[tortoise];       // tortoise is moving by one step
hare = in_arr[in_arr[hare]];       // hare is moving by two steps
} while (tortoise != hare);
tortoise = in_arr[0];
while (tortoise != hare) {             // loop to find the entry point of cycle
tortoise = in_arr[tortoise];
hare = in_arr[hare];
}
}

/**
* @brief Self-test implementations
* @returns void
*/
static void test()
{
uint32_t arr[] = {1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610}; // input array
size_t n = sizeof(arr) / sizeof(int);

printf("1st test... ");
uint32_t index = duplicateNumber(arr, n); // calling the duplicateNumber function to check which number occurs twice in the array
assert(index == 1); // the number which occurs twice is 1 or not
printf("passed\n");
}

/**
* @brief Main function
* @returns 0 on exit
*/
int main()
{
test();  // run self-test implementations
return 0;
}
``````