persistence.c

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

size_t	digit_mul(long long n)
{
	size_t i;

	i = n % 10;
	n /= 10;
	while (n > 0)
	{
		if (n % 10 == 0)
			return (0);
		i *= (n % 10);
		n /= 10;
	}
	return (i);
}

size_t	persistence(long long n)
{
	size_t step_count;

	step_count = 0;
	while (n > 10)
	{
		n = digit_mul(n);
		step_count++;
	}
	return (step_count);
}

int		main(int argc, char **argv)
{
	long long	max;
	long long	num;
	long long	best;
	size_t		per;

	num = 0;
	best = 1;
	if (argc == 3)
	{
		num = atoll(argv[1]);
		max = atoll(argv[2]);
		while (num < max)
		{
			per = persistence(num);
			if (per > best)
			{
				printf("%lu: %llu\n", per, num);
				best = per;
			}
			num++;
		}
	}
	else
	{
		ft_putendl("Enter two numbers (min, max).");
	}
}