static double ErfImp(double z, bool invert)
{
if (z < 0)
{
if (!invert)
{
return(-ErfImp(-z, false));
}
if (z < -0.5)
{
return(2 - ErfImp(-z, true));
}
return(1 + ErfImp(-z, false));
}
double result;
// Big bunch of selection statements now to pick which
// implementation to use, try to put most likely options
// first:
if (z < 0.5)
{
// We're going to calculate erf:
if (z < 1e-10)
{
result = (z * 1.125) + (z * 0.003379167095512573896158903121545171688);
}
else
{
// Worst case absolute error found: 6.688618532e-21
result = (z * 1.125) + (z * Evaluate.Polynomial(z, ErfImpAn) / Evaluate.Polynomial(z, ErfImpAd));
}
}
else if ((z < 110) || ((z < 110) && invert))
{
// We'll be calculating erfc:
invert = !invert;
double r, b;
if (z < 0.75)
{
// Worst case absolute error found: 5.582813374e-21
r = Evaluate.Polynomial(z - 0.5, ErfImpBn) / Evaluate.Polynomial(z - 0.5, ErfImpBd);
b = 0.3440242112F;
}
else if (z < 1.25)
{
// Worst case absolute error found: 4.01854729e-21
r = Evaluate.Polynomial(z - 0.75, ErfImpCn) / Evaluate.Polynomial(z - 0.75, ErfImpCd);
b = 0.419990927F;
}
else if (z < 2.25)
{
// Worst case absolute error found: 2.866005373e-21
r = Evaluate.Polynomial(z - 1.25, ErfImpDn) / Evaluate.Polynomial(z - 1.25, ErfImpDd);
b = 0.4898625016F;
}
else if (z < 3.5)
{
// Worst case absolute error found: 1.045355789e-21
r = Evaluate.Polynomial(z - 2.25, ErfImpEn) / Evaluate.Polynomial(z - 2.25, ErfImpEd);
b = 0.5317370892F;
}
else if (z < 5.25)
{
// Worst case absolute error found: 8.300028706e-22
r = Evaluate.Polynomial(z - 3.5, ErfImpFn) / Evaluate.Polynomial(z - 3.5, ErfImpFd);
b = 0.5489973426F;
}
else if (z < 8)
{
// Worst case absolute error found: 1.700157534e-21
r = Evaluate.Polynomial(z - 5.25, ErfImpGn) / Evaluate.Polynomial(z - 5.25, ErfImpGd);
b = 0.5571740866F;
}
else if (z < 11.5)
{
// Worst case absolute error found: 3.002278011e-22
r = Evaluate.Polynomial(z - 8, ErfImpHn) / Evaluate.Polynomial(z - 8, ErfImpHd);
b = 0.5609807968F;
}
else if (z < 17)
{
// Worst case absolute error found: 6.741114695e-21
r = Evaluate.Polynomial(z - 11.5, ErfImpIn) / Evaluate.Polynomial(z - 11.5, ErfImpId);
b = 0.5626493692F;
}
else if (z < 24)
{
// Worst case absolute error found: 7.802346984e-22
r = Evaluate.Polynomial(z - 17, ErfImpJn) / Evaluate.Polynomial(z - 17, ErfImpJd);
b = 0.5634598136F;
}
else if (z < 38)
{
// Worst case absolute error found: 2.414228989e-22
r = Evaluate.Polynomial(z - 24, ErfImpKn) / Evaluate.Polynomial(z - 24, ErfImpKd);
b = 0.5638477802F;
}
else if (z < 60)
{
// Worst case absolute error found: 5.896543869e-24
r = Evaluate.Polynomial(z - 38, ErfImpLn) / Evaluate.Polynomial(z - 38, ErfImpLd);
b = 0.5640528202F;
}
else if (z < 85)
{
// Worst case absolute error found: 3.080612264e-21
r = Evaluate.Polynomial(z - 60, ErfImpMn) / Evaluate.Polynomial(z - 60, ErfImpMd);
b = 0.5641309023F;
}
else
{
// Worst case absolute error found: 8.094633491e-22
r = Evaluate.Polynomial(z - 85, ErfImpNn) / Evaluate.Polynomial(z - 85, ErfImpNd);
b = 0.5641584396F;
}
double g = Math.Exp(-z * z) / z;
result = (g * b) + (g * r);
}
else
{
// Any value of z larger than 28 will underflow to zero:
result = 0;
invert = !invert;
}
if (invert)
{
result = 1 - result;
}
return(result);
}
static double ErfInvImpl(double p, double q, double s)
{
double result;
if (p <= 0.5)
{
// Evaluate inverse erf using the rational approximation:
//
// x = p(p+10)(Y+R(p))
//
// Where Y is a constant, and R(p) is optimized for a low
// absolute error compared to |Y|.
//
// double: Max error found: 2.001849e-18
// long double: Max error found: 1.017064e-20
// Maximum Deviation Found (actual error term at infinite precision) 8.030e-21
const float y = 0.0891314744949340820313f;
double g = p * (p + 10);
double r = Evaluate.Polynomial(p, ErvInvImpAn) / Evaluate.Polynomial(p, ErvInvImpAd);
result = (g * y) + (g * r);
}
else if (q >= 0.25)
{
// Rational approximation for 0.5 > q >= 0.25
//
// x = sqrt(-2*log(q)) / (Y + R(q))
//
// Where Y is a constant, and R(q) is optimized for a low
// absolute error compared to Y.
//
// double : Max error found: 7.403372e-17
// long double : Max error found: 6.084616e-20
// Maximum Deviation Found (error term) 4.811e-20
const float y = 2.249481201171875f;
double g = Math.Sqrt(-2 * Math.Log(q));
double xs = q - 0.25;
double r = Evaluate.Polynomial(xs, ErvInvImpBn) / Evaluate.Polynomial(xs, ErvInvImpBd);
result = g / (y + r);
}
else
{
// For q < 0.25 we have a series of rational approximations all
// of the general form:
//
// let: x = sqrt(-log(q))
//
// Then the result is given by:
//
// x(Y+R(x-B))
//
// where Y is a constant, B is the lowest value of x for which
// the approximation is valid, and R(x-B) is optimized for a low
// absolute error compared to Y.
//
// Note that almost all code will really go through the first
// or maybe second approximation. After than we're dealing with very
// small input values indeed: 80 and 128 bit long double's go all the
// way down to ~ 1e-5000 so the "tail" is rather long...
double x = Math.Sqrt(-Math.Log(q));
if (x < 3)
{
// Max error found: 1.089051e-20
const float y = 0.807220458984375f;
double xs = x - 1.125;
double r = Evaluate.Polynomial(xs, ErvInvImpCn) / Evaluate.Polynomial(xs, ErvInvImpCd);
result = (y * x) + (r * x);
}
else if (x < 6)
{
// Max error found: 8.389174e-21
const float y = 0.93995571136474609375f;
double xs = x - 3;
double r = Evaluate.Polynomial(xs, ErvInvImpDn) / Evaluate.Polynomial(xs, ErvInvImpDd);
result = (y * x) + (r * x);
}
else if (x < 18)
{
// Max error found: 1.481312e-19
const float y = 0.98362827301025390625f;
double xs = x - 6;
double r = Evaluate.Polynomial(xs, ErvInvImpEn) / Evaluate.Polynomial(xs, ErvInvImpEd);
result = (y * x) + (r * x);
}
else if (x < 44)
{
// Max error found: 5.697761e-20
const float y = 0.99714565277099609375f;
double xs = x - 18;
double r = Evaluate.Polynomial(xs, ErvInvImpFn) / Evaluate.Polynomial(xs, ErvInvImpFd);
result = (y * x) + (r * x);
}
else
{
// Max error found: 1.279746e-20
const float y = 0.99941349029541015625f;
double xs = x - 44;
double r = Evaluate.Polynomial(xs, ErvInvImpGn) / Evaluate.Polynomial(xs, ErvInvImpGd);
result = (y * x) + (r * x);
}
}
return(s * result);
}
