Math libarary

I wrote a routine to replace Math's Exp method but it turns out to be almost
2x slower ;/ (well, actually its about 1.5x in release)

I'm essentially using a lookup table and interpolate between integer
values(but even just looking up its still as slow).

public static double Exp(double x)
{
int n = (int)Math.Floor(x);
//if ((n > 2351) || (n < -2351)) return Math.Sign(x) *
double.PositiveInfinity;

double f = x - n;
double ff = 1;
double e = 1;

if (f != 0)
for (int i = 1; i < 10; i++)
{
ff = ff * f;
e += invnfac[i] * ff;
}

return ExpIp[n + 2351]*e;
}

What I did was precompute n! and ExpIp is exp(k) for integer k. exp(+-2351)
is the maximum that a double can hold so no reason to go beyond that.

If I comment out the loop I get almost there speed but still about 1%
slower.

How the hell are they computing so fast? Normally these Math library
routines are very slow but either my code/method sucks or Math .NET is
fairly optimized?

Anyone know whats going on here?

Thanks,
Jon


.

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