/// <summary>
/// Computes the factorial of a number (n!)
/// </summary>
public static double Factorial(int n)
{
if (fcache == null)
{
// Initialize factorial cache
fcache = new double[33];
fcache[0] = 1; fcache[1] = 1;
fcache[2] = 2; fcache[3] = 6;
fcache[4] = 24; ftop = 4;
}
if (n < 0)
{
// Factorial is not defined for negative numbers
throw new ArgumentException("Argument can not be negative", "n");
}
if (n > 32)
{
// Return Gamma approximation using exp(gammaln(n+1)),
// which for some reason is faster than gamma(n+1).
return(Math.Exp(Gamma.Log(n + 1.0)));
}
else
{
// Compute in the standard way, but use the
// factorial cache to speed up computations.
while (ftop < n)
{
int j = ftop++;
fcache[ftop] = fcache[j] * ftop;
}
return(fcache[n]);
}
}
/// <summary>
/// Returns the log factorial of a number (ln(n!))
/// </summary>
///
public static double LogFactorial(int n)
{
if (lnfcache == null)
{
lnfcache = new double[101];
}
if (n < 0)
{
// Factorial is not defined for negative numbers.
throw new ArgumentException("Argument cannot be negative.", "n");
}
if (n <= 1)
{
// Factorial for n between 0 and 1 is 1, so log(factorial(n)) is 0.
return(0.0);
}
if (n <= 100)
{
// Compute the factorial using ln(gamma(n)) approximation, using the cache
// if the value has been previously computed.
return((lnfcache[n] > 0) ? lnfcache[n] : (lnfcache[n] = Gamma.Log(n + 1.0)));
}
else
{
// Just compute the factorial using ln(gamma(n)) approximation.
return(Gamma.Log(n + 1.0));
}
}
/// <summary>
/// Power series for incomplete beta integral. Use when b*x
/// is small and x not too close to 1.
/// </summary>
///
/// <example>
/// Please see <see cref="Beta"/>
/// </example>
///
public static double PowerSeries(double a, double b, double x)
{
double s, t, u, v, n, t1, z, ai;
ai = 1.0 / a;
u = (1.0 - b) * x;
v = u / (a + 1.0);
t1 = v;
t = u;
n = 2.0;
s = 0.0;
z = Constants.DoubleEpsilon * ai;
while (System.Math.Abs(v) > z)
{
u = (n - b) * x / n;
t *= u;
v = t / (a + n);
s += v;
n += 1.0;
}
s += t1;
s += ai;
u = a * System.Math.Log(x);
if ((a + b) < Gamma.GammaMax && System.Math.Abs(u) < Constants.LogMax)
{
t = Gamma.Function(a + b) / (Gamma.Function(a) * Gamma.Function(b));
s = s * t * System.Math.Pow(x, a);
}
else
{
t = Gamma.Log(a + b) - Gamma.Log(a) - Gamma.Log(b) + u + System.Math.Log(s);
if (t < Constants.LogMin)
{
s = 0.0;
}
else
{
s = System.Math.Exp(t);
}
}
return(s);
}
private static double inverse(double a, double y)
{
// bound the solution
double x0 = Double.MaxValue;
double yl = 0;
double x1 = 0;
double yh = 1.0;
double dithresh = 5.0 * Constants.DoubleEpsilon;
// approximation to inverse function
double d = 1.0 / (9.0 * a);
double yy = (1.0 - d - Normal.Inverse(y) * Math.Sqrt(d));
double x = a * yy * yy * yy;
double lgm = Gamma.Log(a);
for (int i = 0; i < 10; i++)
{
if (x > x0 || x < x1)
{
goto ihalve;
}
yy = Gamma.UpperIncomplete(a, x);
if (yy < yl || yy > yh)
{
goto ihalve;
}
if (yy < y)
{
x0 = x;
yl = yy;
}
else
{
x1 = x;
yh = yy;
}
// compute the derivative of the function at this point
d = (a - 1.0) * Math.Log(x) - x - lgm;
if (d < -Constants.LogMax)
{
goto ihalve;
}
d = -Math.Exp(d);
// compute the step to the next approximation of x
d = (yy - y) / d;
if (Math.Abs(d / x) < Constants.DoubleEpsilon)
{
return(x);
}
x = x - d;
}
// Resort to interval halving if Newton iteration did not converge.
ihalve:
d = 0.0625;
if (x0 == Double.MaxValue)
{
if (x <= 0.0)
{
x = 1.0;
}
while (x0 == Double.MaxValue && !Double.IsNaN(x))
{
x = (1.0 + d) * x;
yy = Gamma.UpperIncomplete(a, x);
if (yy < y)
{
x0 = x;
yl = yy;
break;
}
d = d + d;
}
}
d = 0.5;
double dir = 0;
for (int i = 0; i < 400; i++)
{
double t = x1 + d * (x0 - x1);
if (Double.IsNaN(t))
{
break;
}
x = t;
yy = Gamma.UpperIncomplete(a, x);
lgm = (x0 - x1) / (x1 + x0);
if (Math.Abs(lgm) < dithresh)
{
break;
}
lgm = (yy - y) / y;
if (Math.Abs(lgm) < dithresh)
{
break;
}
if (x <= 0.0)
{
break;
}
if (yy >= y)
{
x1 = x;
yh = yy;
if (dir < 0)
{
dir = 0;
d = 0.5;
}
else if (dir > 1)
{
d = 0.5 * d + 0.5;
}
else
{
d = (y - yl) / (yh - yl);
}
dir += 1;
}
else
{
x0 = x;
yl = yy;
if (dir > 0)
{
dir = 0;
d = 0.5;
}
else if (dir < -1)
{
d = 0.5 * d;
}
else
{
d = (y - yl) / (yh - yl);
}
dir -= 1;
}
}
if (x == 0.0 || Double.IsNaN(x))
{
throw new ArithmeticException();
}
return(x);
}
/// <summary>
/// Inverse of incomplete beta integral.
/// </summary>
///
/// <example>
/// Please see <see cref="Beta"/>
/// </example>
///
public static double IncompleteInverse(double aa, double bb, double yy0)
{
double a, b, y0, d, y, x, x0, x1, lgm, yp, di, dithresh, yl, yh;
int i, dir;
bool nflg;
bool rflg;
if (yy0 <= 0)
{
return(0.0);
}
if (yy0 >= 1.0)
{
return(1.0);
}
if (aa <= 1.0 || bb <= 1.0)
{
nflg = true;
dithresh = 4.0 * Constants.DoubleEpsilon;
rflg = false;
a = aa;
b = bb;
y0 = yy0;
x = a / (a + b);
y = Incomplete(a, b, x);
goto ihalve;
}
else
{
nflg = false;
dithresh = 1.0e-4;
}
/* approximation to inverse function */
yp = -Normal.Inverse(yy0);
if (yy0 > 0.5)
{
rflg = true;
a = bb;
b = aa;
y0 = 1.0 - yy0;
yp = -yp;
}
else
{
rflg = false;
a = aa;
b = bb;
y0 = yy0;
}
lgm = (yp * yp - 3.0) / 6.0;
x0 = 2.0 / (1.0 / (2.0 * a - 1.0) + 1.0 / (2.0 * b - 1.0));
y = yp * Math.Sqrt(x0 + lgm) / x0
- (1.0 / (2.0 * b - 1.0) - 1.0 / (2.0 * a - 1.0))
* (lgm + 5.0 / 6.0 - 2.0 / (3.0 * x0));
y = 2.0 * y;
if (y < Constants.LogMin)
{
x0 = 1.0;
throw new ArithmeticException("underflow");
}
x = a / (a + b * Math.Exp(y));
y = Incomplete(a, b, x);
yp = (y - y0) / y0;
if (Math.Abs(yp) < 1.0e-2)
{
goto newt;
}
ihalve:
/* Resort to interval halving if not close enough */
x0 = 0.0;
yl = 0.0;
x1 = 1.0;
yh = 1.0;
di = 0.5;
dir = 0;
for (i = 0; i < 400; i++)
{
if (i != 0)
{
x = x0 + di * (x1 - x0);
if (x == 1.0)
{
x = 1.0 - Constants.DoubleEpsilon;
}
y = Incomplete(a, b, x);
yp = (x1 - x0) / (x1 + x0);
if (Math.Abs(yp) < dithresh)
{
x0 = x;
goto newt;
}
}
if (y < y0)
{
x0 = x;
yl = y;
if (dir < 0)
{
dir = 0;
di = 0.5;
}
else if (dir > 1)
{
di = 0.5 * di + 0.5;
}
else
{
di = (y0 - y) / (yh - yl);
}
dir += 1;
if (x0 > 0.75)
{
if (rflg)
{
rflg = false;
a = aa;
b = bb;
y0 = yy0;
}
else
{
rflg = true;
a = bb;
b = aa;
y0 = 1.0 - yy0;
}
x = 1.0 - x;
y = Incomplete(a, b, x);
goto ihalve;
}
}
else
{
x1 = x;
if (rflg && x1 < Constants.DoubleEpsilon)
{
x0 = 0.0;
goto done;
}
yh = y;
if (dir > 0)
{
dir = 0;
di = 0.5;
}
else if (dir < -1)
{
di = 0.5 * di;
}
else
{
di = (y - y0) / (yh - yl);
}
dir -= 1;
}
}
if (x0 >= 1.0)
{
x0 = 1.0 - Constants.DoubleEpsilon;
goto done;
}
if (x == 0.0)
{
throw new ArithmeticException("underflow");
}
newt:
if (nflg)
{
goto done;
}
x0 = x;
lgm = Gamma.Log(a + b) - Gamma.Log(a) - Gamma.Log(b);
for (i = 0; i < 10; i++)
{
/* Compute the function at this point. */
if (i != 0)
{
y = Incomplete(a, b, x0);
}
/* Compute the derivative of the function at this point. */
d = (a - 1.0) * Math.Log(x0) + (b - 1.0) * Math.Log(1.0 - x0) + lgm;
if (d < Constants.LogMin)
{
throw new ArithmeticException("underflow");
}
d = Math.Exp(d);
/* compute the step to the next approximation of x */
d = (y - y0) / d;
x = x0;
x0 = x0 - d;
if (x0 <= 0.0)
{
throw new ArithmeticException("underflow");
}
if (x0 >= 1.0)
{
x0 = 1.0 - Constants.DoubleEpsilon;
goto done;
}
if (Math.Abs(d / x0) < 64.0 * Constants.DoubleEpsilon)
{
goto done;
}
}
done:
if (rflg)
{
if (x0 <= Double.Epsilon)
{
x0 = 1.0 - Double.Epsilon;
}
else
{
x0 = 1.0 - x0;
}
}
return(x0);
}
/// <summary>
/// Incomplete (regularized) Beta function Ix(a, b).
/// </summary>
///
/// <example>
/// Please see <see cref="Beta"/>
/// </example>
///
public static double Incomplete(double a, double b, double x)
{
double aa, bb, t, xx, xc, w, y;
bool flag;
if (a <= 0.0)
{
throw new ArgumentOutOfRangeException("a", "Lower limit must be greater than zero.");
}
if (b <= 0.0)
{
throw new ArgumentOutOfRangeException("b", "Upper limit must be greater than zero.");
}
if ((x <= 0.0) || (x >= 1.0))
{
if (x == 0.0)
{
return(0.0);
}
if (x == 1.0)
{
return(1.0);
}
throw new ArgumentOutOfRangeException("x", "Value must be between 0 and 1.");
}
flag = false;
if ((b * x) <= 1.0 && x <= 0.95)
{
t = PowerSeries(a, b, x);
return(t);
}
w = 1.0 - x;
if (x > (a / (a + b)))
{
flag = true;
aa = b;
bb = a;
xc = x;
xx = w;
}
else
{
aa = a;
bb = b;
xc = w;
xx = x;
}
if (flag && (bb * xx) <= 1.0 && xx <= 0.95)
{
t = PowerSeries(aa, bb, xx);
if (t <= Constants.DoubleEpsilon)
{
t = 1.0 - Constants.DoubleEpsilon;
}
else
{
t = 1.0 - t;
}
return(t);
}
y = xx * (aa + bb - 2.0) - (aa - 1.0);
if (y < 0.0)
{
w = Incbcf(aa, bb, xx);
}
else
{
w = Incbd(aa, bb, xx) / xc;
}
y = aa * System.Math.Log(xx);
t = bb * System.Math.Log(xc);
if ((aa + bb) < Gamma.GammaMax && System.Math.Abs(y) < Constants.LogMax && System.Math.Abs(t) < Constants.LogMax)
{
t = System.Math.Pow(xc, bb);
t *= System.Math.Pow(xx, aa);
t /= aa;
t *= w;
t *= Gamma.Function(aa + bb) / (Gamma.Function(aa) * Gamma.Function(bb));
if (flag)
{
if (t <= Constants.DoubleEpsilon)
{
t = 1.0 - Constants.DoubleEpsilon;
}
else
{
t = 1.0 - t;
}
}
return(t);
}
y += t + Gamma.Log(aa + bb) - Gamma.Log(aa) - Gamma.Log(bb);
y += System.Math.Log(w / aa);
if (y < Constants.LogMin)
{
t = 0.0;
}
else
{
t = System.Math.Exp(y);
}
if (flag)
{
if (t <= Constants.DoubleEpsilon)
{
t = 1.0 - Constants.DoubleEpsilon;
}
else
{
t = 1.0 - t;
}
}
return(t);
}
/// <summary>
/// Natural logarithm of the Beta function.
/// </summary>
///
/// <example>
/// Please see <see cref="Beta"/>
/// </example>
///
public static double Log(double a, double b)
{
return(Gamma.Log(a) + Gamma.Log(b) - Gamma.Log(a + b));
}
/// <summary>
/// Returns the log factorial of a number (ln(n!))
/// </summary>
///
public static double LogFactorial(double n)
{
return(Gamma.Log(n + 1.0));
}