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

This is the best (and shortest) working solution I've found

http://faruk.akgul.org/blog/javas-missing-algorithm-biginteger-sqrt/

Here is the code:

  public static BigInteger sqrt(BigInteger n) {
    BigInteger a = BigInteger.ONE;
    BigInteger b = new BigInteger(n.shiftRight(5).add(new BigInteger("8")).toString());
    while(b.compareTo(a) >= 0) {
      BigInteger mid = new BigInteger(a.add(b).shiftRight(1).toString());
      if(mid.multiply(mid).compareTo(n) > 0) b = mid.subtract(BigInteger.ONE);
      else a = mid.add(BigInteger.ONE);
    }
    return a.subtract(BigInteger.ONE);
  }

I've tested it and it's working correctly (and seems fast)