public string GetAnswer()
{
int[] fn_2 = new int[] { 1 };
int[] fn_1 = new int[] { 1 };
BigInteger f1 = 1;
BigInteger f2 = 1;
int i = 3;
while (true)
{
BigInteger fnBig = f1 + f2;
int[] fn = Add(fn_1, fn_2);
if (IsPandigitalFibonacci(fn))
{
int digits = (int)Math.Floor(BigInteger.Log10(fnBig) + 1);
var dividend = BigInteger.Pow(10, digits - 9);
var firstNineDigits = fnBig / dividend;
if (Utils.IsPandigital(firstNineDigits.ToString(), false))
{
return(i.ToString());
}
}
fn_2 = fn_1;
fn_1 = fn;
f2 = f1;
f1 = fnBig;
i++;
}
}
public static int IntLength(BigInteger i)
{
if (i < 0)
{
throw new ArgumentOutOfRangeException();
}
if (i == 0)
{
return(1);
}
{
return((int)Math.Floor(BigInteger.Log10(i)) + 1);
}
}
public static int Problem104Old()
{
var i = 1;
BigInteger prev = 0;
BigInteger cur = 1;
while (cur.ToString().Length < 18)
{
var temp = cur;
cur = cur + prev;
prev = temp;
i++;
}
while (true)
{
var tail = (long)(cur % 1000000000);
if (tail.IsPandigital())
{
var l = (int)(BigInteger.Log10(cur) + 1);
var head = (long)(cur / BigInteger.Pow(10, l - 9));
if (head.IsPandigital())
{
return(i);
}
}
var temp = cur;
cur = cur + prev;
prev = temp;
i++;
}
}
