mirror of
https://github.com/TheAlgorithms/C
synced 2024-11-25 14:59:36 +03:00
80 lines
2.1 KiB
C
80 lines
2.1 KiB
C
/**
|
|
* \file
|
|
* \brief [Problem 14](https://projecteuler.net/problem=14) solution
|
|
* \author [Krishna Vedala](https://github.com/kvedala)
|
|
*
|
|
* Since the computational values for each iteration step are independent,
|
|
* we can compute them in parallel. However, the maximum values should be
|
|
* updated in synchrony so that we do not get into a "race condition".
|
|
*
|
|
* To compile with supporintg gcc or clang, the flag "-fopenmp" should be
|
|
* passes while with Microsoft C compiler, the flag "/fopenmp" should be
|
|
* used. If you are using the provided CMAKE compilation, it will automatically
|
|
* detect OPENMP and compile with it for you.
|
|
*
|
|
* Automatically detects for OPENMP using the _OPENMP macro.
|
|
*/
|
|
#include <stdio.h>
|
|
#include <stdlib.h>
|
|
#ifdef _OPENMP
|
|
#include <omp.h>
|
|
#endif
|
|
|
|
/**
|
|
* Computes the length of collatz sequence for a given
|
|
* starting number
|
|
*/
|
|
long long collatz(long long start_num)
|
|
{
|
|
long long length = 1;
|
|
|
|
while (start_num != 1) /* loop till we reach 1 */
|
|
{
|
|
if (start_num & 0x01) /* check for odd */
|
|
start_num = 3 * start_num + 1;
|
|
else
|
|
start_num >>= 1; /* simpler divide by 2 */
|
|
length++;
|
|
}
|
|
|
|
return length;
|
|
}
|
|
|
|
/** Main function */
|
|
int main(int argc, char **argv)
|
|
{
|
|
long long max_len = 0, max_len_num = 0;
|
|
long long MAX_NUM = 1000000;
|
|
|
|
if (argc ==
|
|
2) /* set commandline argumnet as the maximum iteration number */
|
|
{
|
|
MAX_NUM = atoll(argv[1]);
|
|
printf("Maximum number: %lld\n", MAX_NUM);
|
|
}
|
|
|
|
long long i;
|
|
#ifdef _OPENMP
|
|
#pragma omp parallel for shared(max_len, max_len_num) schedule(guided)
|
|
#endif
|
|
for (i = 1; i < MAX_NUM; i++)
|
|
{
|
|
long long L = collatz(i);
|
|
if (L > max_len)
|
|
{
|
|
max_len = L; /* length of sequence */
|
|
max_len_num = i; /* starting number */
|
|
}
|
|
|
|
#if defined(_OPENMP) && defined(DEBUG)
|
|
printf("Thread: %2d\t %3lld: \t%5lld\n", omp_get_thread_num(), i, L);
|
|
#elif defined(DEBUG)
|
|
printf("%3lld: \t%5lld\n", i, L);
|
|
#endif
|
|
}
|
|
|
|
printf("Start: %3lld: \tLength: %5lld\n", max_len_num, max_len);
|
|
|
|
return 0;
|
|
}
|