optimized solution with option to compile using platform independent OpenMP parallelization.

project_euler/Problem 14/sol1.c

@@ -0,0 +1,72 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+#ifdef _OPENMP
+    #include <omp.h>
+    #pragma message ("Using OpenMP parallelization")
+#else
+    #pragma message ("Not using OpenMP parallelization")
+#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;
+}
+
+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]);
+
+    /** 
+     * 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.
+     * 
+     * Automatically detects for OPENMP using the _OPENMP macro.
+     **/
+    #ifdef _OPENMP
+    printf("\nUsing %d threads for parallelization.\n", omp_get_num_threads());
+    #pragma omp parallel for shared(max_len, max_len_num) schedule(guided)
+    #endif
+    for (long long i = 1; i < MAX_NUM; i++)
+    {
+        long long L = collatz(i);
+        if (L > max_len)
+        {
+            max_len = L;
+            max_len_num = i;
+        }
+
+        #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("Max: %3lld: \t%5lld\n", max_len_num, max_len);
+
+    return 0;
+}