TheAlgorithms-C/project_euler/problem_14/sol1.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;
}