main.c

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

enum Result
{
	CantReadFile,
	BadInput,
	Success
};

enum Result calculate_fuel_requirements(int *req)
{
	int read_ret = 0, mass = 0, total_fuel = 0;
	FILE *f = NULL;
	errno_t ferr;
	
	ferr = fopen_s(&f, "input", "r");
	if (ferr != 0 || !f)
	{
		return CantReadFile;
	}

	while (read_ret != EOF)
	{
		mass = 0;
		read_ret = fscanf_s(f, "%d\n", &mass, sizeof(mass));
		if (read_ret == EOF)
		{
			break;
		}
		else if (read_ret == 0)
		{
			goto parse_error;
		}
		
		total_fuel += mass / 3 - 2;
	}

	*req = total_fuel;

	fclose(f);
	return Success;

parse_error:

	fclose(f);
	return BadInput;
}

int main(int argc, char **argv)
{
	int req = 0;

	switch (calculate_fuel_requirements(&req))
	{
	case Success:
		printf("Requires %d fuel\n", req);
		return EXIT_SUCCESS;
	case CantReadFile:
		printf("Can't read input file\n");
		return EXIT_FAILURE;
	case BadInput:
		printf("Bad input\n");
	default:
		return EXIT_FAILURE;
	}
}