BigInteger.FermatLittleTest C# (CSharp) Method

        public bool FermatLittleTest(int confidence)
        {
                BigInteger thisVal;
                if((this.data[maxLength-1] & 0x80000000) != 0)        // negative
                        thisVal = -this;
                else
                        thisVal = this;

                if(thisVal.dataLength == 1)
                {
                        // test small numbers
                        if(thisVal.data[0] == 0 || thisVal.data[0] == 1)
                                return false;
                        else if(thisVal.data[0] == 2 || thisVal.data[0] == 3)
                                return true;
                }

                if((thisVal.data[0] & 0x1) == 0)     // even numbers
                        return false;

	        int bits = thisVal.bitCount();
	        BigInteger a = new BigInteger();
	        BigInteger p_sub1 = thisVal - (new BigInteger(1));
	        Random rand = new Random();

	        for(int round = 0; round < confidence; round++)
	        {
		        bool done = false;

		        while(!done)		// generate a < n
		        {
			        int testBits = 0;

			        // make sure "a" has at least 2 bits
			        while(testBits < 2)
				        testBits = (int)(rand.NextDouble() * bits);

			        a.genRandomBits(testBits, rand);

			        int byteLen = a.dataLength;

                                // make sure "a" is not 0
			        if(byteLen > 1 || (byteLen == 1 && a.data[0] != 1))
                                        done = true;
		        }

                        // check whether a factor exists (fix for version 1.03)
		        BigInteger gcdTest = a.gcd(thisVal);
                        if(gcdTest.dataLength == 1 && gcdTest.data[0] != 1)
                                return false;

		        // calculate a^(p-1) mod p
		        BigInteger expResult = a.modPow(p_sub1, thisVal);

		        int resultLen = expResult.dataLength;

                        // is NOT prime is a^(p-1) mod p != 1

		        if(resultLen > 1 || (resultLen == 1 && expResult.data[0] != 1))
		        {
		                //Console.WriteLine("a = " + a.ToString());
			        return false;
                        }
	        }

	        return true;
        }

    public ShamirSS(uint _numAllParts, uint _numNecessaryParts, long _modP)
    {
        numAllParts       = _numAllParts;
        numNecessaryParts = _numNecessaryParts;
        modP = _modP;

        //find the next prime larger than secret
        bool found = false;

        while (!found)
        {
            if (modP.FermatLittleTest(5))
            {
                found = true;
            }
            else
            {
                modP++;
            }
        }
    }