How can I find the Square Root of a Java BigInteger?

Question

Is there a library that will find the square root of a BigInteger? I want it computed offline - only once, and not inside any loop. So even computationally expensive solutio

Answer 1

The C# language has similar syntax to Java. I wrote this recursive solution.

    static BigInteger fsqrt(BigInteger n)
    {
        string sn = n.ToString();
        return guess(n, BigInteger.Parse(sn.Substring(0, sn.Length >> 1)), 0);          
    }
    static BigInteger guess(BigInteger n, BigInteger g, BigInteger last)
    {
        if (last >= g - 1 && last <= g + 1) return g;
        else return guess(n, (g + (n / g)) >> 1, g);
    }

Call this code like this (in Java I guess it would be "System.out.print").

Console.WriteLine(fsqrt(BigInteger.Parse("783648276815623658365871365876257862874628734627835648726")));

And the answer is: 27993718524262253829858552106

Disclaimer: I understand this method doesn't work for numbers less than 10; this is a BigInteger square root method.

This is easily remedied. Change the first method to the following to give the recursive portion some room to breathe.

    static BigInteger fsqrt(BigInteger n)
    {
        if (n > 999)
        {
           string sn = n.ToString();
           return guess(n, BigInteger.Parse(sn.Substring(0, sn.Length >> 1)), 0);
        }
        else return guess(n, n >> 1, 0);            
    }