coolatz-fast.c

#include <stdio.h>
#include <stdlib.h>


int main(){
	unsigned int maxRecord = 0;
	unsigned int upperBound = 1000000;
	unsigned int chainLen[upperBound + 1];

	size_t x;
	int max_x;
	for(x = 1; x < upperBound; ++x) {

	   unsigned int n = x;
	   unsigned int max = 0;

	   while(n != 1) {

		  if(n < upperBound && chainLen[n] != 0 && chainLen[n] + max < maxRecord) {break;}
		  if(n % 2 == 0) {n /= 2;}
		  else {n = (n * 3 + 1);}
		  max++;

	   }

	   chainLen[x] = max;

	   if(max >= maxRecord) {
		  maxRecord = max;
		  max_x = x;
	   }
	}
	fprintf(stdout, "%u\n", max_x);
	return 0;
}