/// <summary>
/// Gets the expected logarithm of that distribution under this distribution.
/// </summary>
/// <param name="that">The distribution to take the logarithm of.</param>
/// <returns><c>sum_x this.Evaluate(x)*Math.Log(that.Evaluate(x))</c></returns>
/// <remarks>This is also known as the cross entropy.</remarks>
public double GetAverageLog(Gaussian that)
{
if (that.IsPointMass)
{
if (this.IsPointMass && this.Point == that.Point)
{
return(0.0);
}
else
{
return(Double.NegativeInfinity);
}
}
else if (!IsProper())
{
throw new ImproperDistributionException(this);
}
else
{
// that.Precision != inf
double mean, variance;
GetMeanAndVariance(out mean, out variance);
return(that.GetLogProb(mean) - 0.5 * that.Precision * variance);
}
}
/// <summary>
/// Get the log probability density at value.
/// </summary>
/// <param name="value"></param>
/// <returns></returns>
public double GetLogProb(double value)
{
// p(x) = sum_k N(x + L*k; m, v)
// = a/(2*pi) + a/pi sum_{k=1}^inf cos(a*k*(x-m)) exp(-(a*k)^2 v/2) where a = 2*pi/L
double m, v;
Gaussian.GetMeanAndVariance(out m, out v);
if (v < 0.15)
{
// use Gaussian summation formula
double result = double.NegativeInfinity;
for (int k = -1; k <= 1; k++)
{
double logp = Gaussian.GetLogProb(value + k * Period);
result = MMath.LogSumExp(result, logp);
}
return(result);
}
else
{
// v >= 0.15
// use the cosine formula
double result = 0.5;
double aOverPi = 2 / Period;
double a = aOverPi * Math.PI;
double diff = value - m;
double vHalf = v * 0.5;
for (int k = 1; k <= 8; k++)
{
double ak = a * k;
result += Math.Cos(ak * diff) * Math.Exp(-ak * ak * vHalf);
}
return(Math.Log(result * aOverPi));
}
}
/// <summary>
/// Get the integral of this distribution times another distribution raised to a power.
/// </summary>
/// <param name="that"></param>
/// <param name="power"></param>
/// <returns></returns>
public double GetLogAverageOfPower(Gaussian that, double power)
{
if (IsPointMass)
{
return(power * that.GetLogProb(Point));
}
else if (that.IsPointMass)
{
if (power < 0)
{
throw new DivideByZeroException("The exponent is negative and the distribution is a point mass");
}
return(this.GetLogProb(that.Point));
}
else
{
var product = this * (that ^ power);
return(product.GetLogNormalizer() - this.GetLogNormalizer() - power * that.GetLogNormalizer());
}
}
/// <summary>
/// Gets the integral of the product of two Gaussians.
/// </summary>
/// <param name="that"></param>
/// <remarks>
/// <c>this = N(x;m1,v1)</c>.
/// <c>that = N(x;m2,v2)</c>.
/// <c>int_(-infinity)^(infinity) N(x;m1,v1) N(x;m2,v2) dx = N(m1; m2, v1+v2)</c>.
/// When improper, the density is redefined to be <c>exp(-0.5*x^2*(1/v) + x*(m/v))</c>,
/// i.e. we drop the terms <c>exp(-m^2/(2v))/sqrt(2*pi*v)</c>.
/// </remarks>
/// <returns>log(N(m1;m2,v1+v2)).</returns>
public double GetLogAverageOf(Gaussian that)
{
if (IsPointMass)
{
return(that.GetLogProb(Point));
}
else if (that.IsPointMass)
{
return(GetLogProb(that.Point));
}
else
{
// neither this nor that is a point mass.
// int_x N(x;m1,v1) N(x;m2,v2) dx = N(m1;m2,v1+v2)
// (m1-m2)^2/(v1+v2) = (m1-m2)^2/(v1*v2*product.Prec)
// (m1-m2)^2/(v1*v2) = (m1/(v1*v2) - m2/(v1*v2))^2 *v1*v2
Gaussian product = this * that;
//if (!product.IsProper()) throw new ArgumentException("The product is improper.");
return(product.GetLogNormalizer() - this.GetLogNormalizer() - that.GetLogNormalizer());
}
}
