public static BigInteger operator +(BigInteger x, BigInteger y)
{
if (x == null || y == null)
{
return(null);
}
if (x.AbsCompareTo(y) < 0)
{
Utility.Swap(ref x, ref y);
}
BigInteger z = new BigInteger();
z.sign = x.sign;
byte[] bs = Utility.Expand(y.data, x.data.Length);
bool isAdd = x.sign * y.sign == 1;
z.data = new byte[x.data.Length + (isAdd ? 1 : 0)];
if (isAdd)
{
BigArithmetic.Add(z.data, x.data, bs, bs.Length);
}
else
{
BigArithmetic.Subtract(z.data, x.data, bs, bs.Length);
}
z.Shrink();
return(z);
}
//= pp.780-781, mppi, 20.6 任意精度的运算
/// <summary>
/// 计算圆周率到小数点后 digits 位数字。
/// 计算结果保存在返回的字节数组中，每个字节存放两个十进制数字，字节数组的第一个元素是“03”。
/// 字节数组的长度可能大于 (digits + 1) / 2 + 1，只保证小数点后前 digits 个十进制数字是准确的。
/// </summary>
/// <param name="digits">小数点后的十进制数字个数</param>
/// <returns>存放圆周率的字节数组</returns>
public static byte[] Compute(int digits)
{
if (digits < 0)
{
throw new ArgumentOutOfRangeException("digits", "can't less than zero");
}
int n = Math.Max(5, (digits + 1) / 2 + 2);
byte[] pi = new byte[n + 1];
byte[] x = new byte[n + 1], y = new byte[n << 1];
byte[] sx = new byte[n], sxi = new byte[n];
byte[] t = new byte[n << 1], s = new byte[3 * n];
t[0] = 2; // t = 2
BigArithmetic.Sqrt(x, x, n, t, n); // x = sqrt(2)
BigArithmetic.Add(pi, t, x, n); // pi = 2 + sqrt(2)
Array.Copy(pi, 1, pi, 0, n);
BigArithmetic.Sqrt(sx, sxi, n, x, n); // sx = sqrt(x)
Array.Copy(sx, y, n); // y = sqrt(x)
for (; ;)
{
BigArithmetic.Add(x, sx, sxi, n); // x = sqrt(x) + 1/sqrt(x)
Array.Copy(x, 1, x, 0, n);
BigArithmetic.Divide(x, x, n, 2); // x = x / 2
BigArithmetic.Sqrt(sx, sxi, n, x, n); // sx = sqrt(x), sxi = 1/sqrt(x)
BigArithmetic.Multiply(t, y, n, sx, n); // t = y * sx
Array.Copy(t, 1, t, 0, n);
BigArithmetic.Add(t, t, sxi, n); // t = y * sx + sxi
x[0]++;
y[0]++;
BigArithmetic.Inverse(s, n, y, n); // s = 1 / (y + 1)
Array.Copy(t, 1, t, 0, n);
BigArithmetic.Multiply(y, t, n, s, n); // y = t / (y + 1)
Array.Copy(y, 1, y, 0, n);
BigArithmetic.Multiply(t, x, n, s, n); // t = (x + 1) / (y + 1)
int mm = t[1] - 1; // 若 t == 1 则收敛
int j = t[n] - mm;
if (j > 1 || j < -1)
{
for (j = 2; j < n; j++)
{
if (t[j] != mm)
{
Array.Copy(t, 1, t, 0, n);
BigArithmetic.Multiply(s, pi, n, t, n); // s = t * pi
Array.Copy(s, 1, pi, 0, n); // pi = t * pi
break;
}
}
if (j < n)
{
continue;
}
}
break;
}
return(pi);
}