Larger data decimals than BigDecimal - bigdecimal

Question

I recently made a basic Pi calculator that works with Math.BigDecimal. It outputs to a text file, and calculates extremely fast, but even BigDecimal has it's limits. I was wondering what the basic code would be for numbers even more precise (Not that any code would actually need that). Here is the Pi code:
package mainPack;
import java.io.IOException;
import java.math.BigDecimal;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.Scanner;
public class Main {
public static BigDecimal piDigCalc(int i){
double o = (double)i;
BigDecimal ret = new BigDecimal((1/(Math.pow(16.0D, o))*((4/((8*o) + 1))-(2/((8*o) + 4))-(1/((8*o) + 5))-(1/((8*o)+6))))); //Just the code for a hexdigit of Pi.
return ret;
}
public static void main(String[] args) throws IOException {
System.out.println("Enter accuracy:");
Scanner s = new Scanner(System.in);
int acc = s.nextInt();
s.close();
BigDecimal Pi = new BigDecimal(0);
for(int i = 0; i < acc; i++){
Pi = Pi.add(piDigCalc(i));
}
Path file = Paths.get("C:\\tmp\\PI.txt");
String pi = "" + Pi;
Files.write(file, pi.getBytes());
System.out.println("File has been saved at "+ file.toString());
}
}

Answer 1

I rewrote your piDigCalc function to use entirely arbitrary precision values. You can see it running on ideone (1300 was about the largest precision I could get to finish under ideone's 15 second time limit).
It is in no way optimized, it's just a verbatim translation of the math in your code to bigdecimals.
You might notice that there is actually a precision limit to the values, but this is partly because setting huge limits increases runtime by a lot. The limit I set scales with the provided accuracy.
In case ideone ever explodes, here's the code for piDigCalc:
public static BigDecimal piDigCalc(int _i, int x){
BigDecimal
i = BigDecimal.valueOf(_i),
a = SIXTEEN.pow(_i),
b = EIGHT.multiply(i),
c = ONE.divide(a, x, BigDecimal.ROUND_HALF_EVEN),
d = b.add(ONE), // b + 1
e = b.add(FOUR), // b + 4
f = e.add(ONE), // b + 5
g = f.add(ONE), // b + 6
h = FOUR.divide(d, x, BigDecimal.ROUND_HALF_EVEN),
j = TWO.divide(e, x, BigDecimal.ROUND_HALF_EVEN),
k = ONE.divide(f, x, BigDecimal.ROUND_HALF_EVEN),
l = ONE.divide(g, x, BigDecimal.ROUND_HALF_EVEN),
m = h.subtract(j).subtract(k).subtract(l),
n = c.multiply(m);
return n;
}
The constants ONE, TWO, FOUR, EIGHT, SIXTEEN are defined static to the class and have exact integer values (ONE also exists as a constant in BigDecimal).