C# BigInteger.QuickPow2Check方法代码示例

        public BigInteger Remainder(
            BigInteger n)
        {
            if (n.sign == 0)
                throw new ArithmeticException("Division by zero error");

            if (this.sign == 0)
                return Zero;

            // For small values, use fast remainder method
            if (n.magnitude.Length == 1)
            {
                int val = n.magnitude[0];

                if (val > 0)
                {
                    if (val == 1)
                        return Zero;

                    // TODO Make this func work on uint, and handle val == 1?
                    int rem = Remainder(val);

                    return rem == 0
                        ?	Zero
                        :	new BigInteger(sign, new int[]{ rem }, false);
                }
            }

            if (CompareNoLeadingZeroes(0, magnitude, 0, n.magnitude) < 0)
                return this;

            int[] result;
            if (n.QuickPow2Check())  // n is power of two
            {
                // TODO Move before small values branch above?
                result = LastNBits(n.Abs().BitLength - 1);
            }
            else
            {
                result = (int[]) this.magnitude.Clone();
                result = Remainder(result, n.magnitude);
            }

            return new BigInteger(sign, result, true);
        }

        public BigInteger Multiply(
            BigInteger val)
        {
            if (val == this)
                return Square();

            if ((sign & val.sign) == 0)
                return Zero;

            if (val.QuickPow2Check()) // val is power of two
            {
                BigInteger result = this.ShiftLeft(val.Abs().BitLength - 1);
                return val.sign > 0 ? result : result.Negate();
            }

            if (this.QuickPow2Check()) // this is power of two
            {
                BigInteger result = val.ShiftLeft(this.Abs().BitLength - 1);
                return this.sign > 0 ? result : result.Negate();
            }

            int resLength = magnitude.Length + val.magnitude.Length;
            int[] res = new int[resLength];

            Multiply(res, this.magnitude, val.magnitude);

            int resSign = sign ^ val.sign ^ 1;
            return new BigInteger(resSign, res, true);
        }

        public BigInteger[] DivideAndRemainder(
            BigInteger val)
        {
            if (val.sign == 0)
                throw new ArithmeticException("Division by zero error");

            BigInteger[] biggies = new BigInteger[2];

            if (sign == 0)
            {
                biggies[0] = Zero;
                biggies[1] = Zero;
            }
            else if (val.QuickPow2Check()) // val is power of two
            {
                int e = val.Abs().BitLength - 1;
                BigInteger quotient = this.Abs().ShiftRight(e);
                int[] remainder = this.LastNBits(e);

                biggies[0] = val.sign == this.sign ? quotient : quotient.Negate();
                biggies[1] = new BigInteger(this.sign, remainder, true);
            }
            else
            {
                int[] remainder = (int[]) this.magnitude.Clone();
                int[] quotient = Divide(remainder, val.magnitude);

                biggies[0] = new BigInteger(this.sign * val.sign, quotient, true);
                biggies[1] = new BigInteger(this.sign, remainder, true);
            }

            return biggies;
        }

        public BigInteger ModInverse(
            BigInteger m)
        {
            if (m.sign < 1)
                throw new ArithmeticException("Modulus must be positive");

            // TODO Too slow at the moment
//			// "Fast Key Exchange with Elliptic Curve Systems" R.Schoeppel
//			if (m.TestBit(0))
//			{
//				//The Almost Inverse Algorithm
//				int k = 0;
//				BigInteger B = One, C = Zero, F = this, G = m, tmp;
//
//				for (;;)
//				{
//					// While F is even, do F=F/u, C=C*u, k=k+1.
//					int zeroes = F.GetLowestSetBit();
//					if (zeroes > 0)
//					{
//						F = F.ShiftRight(zeroes);
//						C = C.ShiftLeft(zeroes);
//						k += zeroes;
//					}
//
//					// If F = 1, then return B,k.
//					if (F.Equals(One))
//					{
//						BigInteger half = m.Add(One).ShiftRight(1);
//						BigInteger halfK = half.ModPow(BigInteger.ValueOf(k), m);
//						return B.Multiply(halfK).Mod(m);
//					}
//
//					if (F.CompareTo(G) < 0)
//					{
//						tmp = G; G = F; F = tmp;
//						tmp = B; B = C; C = tmp;
//					}
//
//					F = F.Add(G);
//					B = B.Add(C);
//				}
//			}

            if (m.QuickPow2Check())
            {
                return ModInversePow2(m);
            }

            BigInteger d = this.Remainder(m);
            BigInteger x;
            BigInteger gcd = ExtEuclid(d, m, out x);

            if (!gcd.Equals(One))
                throw new ArithmeticException("Numbers not relatively prime.");

            if (x.sign < 0)
            {
                x = x.Add(m);
            }

            return x;
        }

        public BigInteger Divide(
            BigInteger val)
        {
            if (val.sign == 0)
                throw new ArithmeticException("Division by zero error");

            if (sign == 0)
                return Zero;

            if (val.QuickPow2Check()) // val is power of two
            {
                BigInteger result = this.Abs().ShiftRight(val.Abs().BitLength - 1);
                return val.sign == this.sign ? result : result.Negate();
            }

            int[] mag = (int[]) this.magnitude.Clone();

            return new BigInteger(this.sign * val.sign, Divide(mag, val.magnitude), true);
        }