/// <summary>
/// Evidence message for EP.
/// </summary>
/// <param name="cases">Incoming message from 'cases'.</param>
/// <param name="b">Incoming message from 'b'.</param>
public static double LogEvidenceRatio(IList <Bernoulli> cases, Bernoulli b)
{
// result = log (p(data|b=true) p(b=true) + p(data|b=false) p(b=false))
// log (p(data|b=true) p(b=true) + p(data|b=false) (1-p(b=true))
// log ((p(data|b=true) - p(data|b=false)) p(b=true) + p(data|b=false))
// log ((p(data|b=true)/p(data|b=false) - 1) p(b=true) + 1) + log p(data|b=false)
// where cases[0].LogOdds = log p(data|b=true)
// cases[1].LogOdds = log p(data|b=false)
if (b.IsPointMass)
{
return(b.Point ? cases[0].LogOdds : cases[1].LogOdds);
}
//else return MMath.LogSumExp(cases[0].LogOdds + b.GetLogProbTrue(), cases[1].LogOdds + b.GetLogProbFalse());
else
{
// the common case is when cases[0].LogOdds == cases[1].LogOdds. we must not introduce rounding error in that case.
if (cases[0].LogOdds >= cases[1].LogOdds)
{
if (Double.IsNegativeInfinity(cases[1].LogOdds))
{
return(cases[0].LogOdds + b.GetLogProbTrue());
}
else
{
return(cases[1].LogOdds + MMath.Log1Plus(b.GetProbTrue() * MMath.ExpMinus1(cases[0].LogOdds - cases[1].LogOdds)));
}
}
else
{
if (Double.IsNegativeInfinity(cases[0].LogOdds))
{
return(cases[1].LogOdds + b.GetLogProbFalse());
}
else
{
return(cases[0].LogOdds + MMath.Log1Plus(b.GetProbFalse() * MMath.ExpMinus1(cases[1].LogOdds - cases[0].LogOdds)));
}
}
}
}
/// <summary>
/// Sample from a Gaussian(0,1) truncated at the given upper and lower bounds
/// </summary>
/// <param name="lowerBound">Can be -Infinity.</param>
/// <param name="upperBound">Must be >= <paramref name="lowerBound"/>. Can be Infinity.</param>
/// <returns>A real number >= <paramref name="lowerBound"/> and < <paramref name="upperBound"/></returns>
public static double NormalBetween(double lowerBound, double upperBound)
{
if (double.IsNaN(lowerBound))
{
throw new ArgumentException("lowerBound is NaN");
}
if (double.IsNaN(upperBound))
{
throw new ArgumentException("upperBound is NaN");
}
double delta = upperBound - lowerBound;
if (delta == 0)
{
return(lowerBound);
}
if (delta < 0)
{
throw new ArgumentException("upperBound (" + upperBound + ") < lowerBound (" + lowerBound + ")");
}
// Switch between the following 3 options:
// 1. Gaussian rejection, with acceptance rate Z = NormalCdf(upperBound) - NormalCdf(lowerBound)
// 2. Uniform rejection, with acceptance rate sqrt(2*pi)*Z/delta if the interval contains 0
// 3. Truncated exponential rejection, with acceptance rate
// = sqrt(2*pi)*Z*lambda*exp(-lambda^2/2)/(exp(-lambda*lowerBound)-exp(-lambda*upperBound))
// = sqrt(2*pi)*Z*lowerBound*exp(lowerBound^2/2)/(1-exp(-lowerBound*(upperBound-lowerBound)))
// (3) has the highest acceptance rate under the following conditions:
// lowerBound > 0.5 or (lowerBound > 0 and delta < 2.5)
// (2) has the highest acceptance rate if the interval contains 0 and delta < sqrt(2*pi)
// (1) has the highest acceptance rate otherwise
if (lowerBound > 0.5 || (lowerBound > 0 && delta < 2.5))
{
// Rejection sampler using truncated exponential proposal
double lambda = lowerBound;
double s = MMath.ExpMinus1(-lambda * delta);
double c = 2 * lambda * lambda;
while (true)
{
double x = -MMath.Log1Plus(s * Rand.Double());
double u = -System.Math.Log(Rand.Double());
if (c * u > x * x)
{
return(x / lambda + lowerBound);
}
}
throw new InferRuntimeException("failed to sample");
}
else if (upperBound < -0.5 || (upperBound < 0 && delta < 2.5))
{
return(-NormalBetween(-upperBound, -lowerBound));
}
else if (lowerBound <= 0 && upperBound >= 0 && delta < MMath.Sqrt2PI)
{
// Uniform rejection
while (true)
{
double x = Rand.Double() * delta + lowerBound;
double u = -System.Math.Log(Rand.Double());
if (2 * u > x * x)
{
return(x);
}
}
}
else
{
// Gaussian rejection
while (true)
{
double x = Rand.Normal();
if (x >= lowerBound && x < upperBound)
{
return(x);
}
}
}
}
