/// <summary>
/// Tried the newton method for logs, but the exponential function is too slow to do it.
/// </summary>
private void LogNewton()
{
if (mantissa.IsZero() || mantissa.Sign)
{
return;
}
//Compute ln2.
if (ln2cache == null || mantissa.Precision.NumBits > ln2cache.mantissa.Precision.NumBits)
{
CalculateLog2(mantissa.Precision.NumBits);
}
int numBits = mantissa.Precision.NumBits;
//Use inverse exp function with Newton's method.
BigFloat xn = new BigFloat(this);
BigFloat oldExponent = new BigFloat(xn.exponent, mantissa.Precision);
xn.exponent = 0;
this.exponent = 0;
//Hack to subtract 1
xn.mantissa.ClearBit(numBits - 1);
//x0 = (x - 1) * log2 - this is a straight line fit between log(1) = 0 and log(2) = ln2
xn.Mul(ln2cache);
//x0 = (x - 1) * log2 + C - this corrects for minimum error over the range.
xn.Add(logNewtonConstant);
BigFloat term = new BigFloat(mantissa.Precision);
BigFloat one = new BigFloat(1, mantissa.Precision);
int precision = 32;
int normalPrecision = mantissa.Precision.NumBits;
int iterations = 0;
while (true)
{
term.Assign(xn);
term.mantissa.Sign = true;
term.Exp(precision);
term.Mul(this);
term.Sub(one);
iterations++;
if (term.exponent < -((precision >> 1) - 4))
{
if (precision == normalPrecision)
{
if (term.exponent < -(precision - 4)) break;
}
else
{
precision = precision << 1;
if (precision > normalPrecision) precision = normalPrecision;
}
}
xn.Add(term);
}
//log(2^n*s) = log(2^n) + log(s) = nlog(2) + log(s)
term.Assign(ln2cache);
term.Mul(oldExponent);
this.Assign(xn);
this.Add(term);
}
/// <summary>
/// The exponential function. Less accurate for high exponents, scales poorly with the number
/// of bits. This is quite fast for low-precision arguments.
/// </summary>
public static BigFloat Exp(BigFloat n1)
{
BigFloat res = new BigFloat(n1);
res.Exp();
return res;
}