logsigmoid.h

#ifndef LOG_SIGMOID_DEC_2007
#define LOG_SIGMOID_DEC_2007


//currently LogSigMoid(t, a) is taking the form -log(1+exp(a*(t-x)))
class LogSigMoid
{
        public:
                LogSigMoid()
                {
                        strictness_ = 0;
                        threshold_ = 0;
                }

                LogSigMoid(double strictness, double threshold): strictness_(strictness), threshold_(threshold)
                {}

                double funcValue(double x) //compute function value
                {
                        return -log(1+exp(strictness_*(threshold_-x)));
                }

                double firstDerivative(double x) //compute first derivative
                {
                        // first derivative is of the form 
                        // a*exp(a*(t-x))/(1+exp(a*(t-x)))
                        return strictness_*exp(strictness_*(threshold_-x))/(1+exp(strictness_*(threshold_-x)));
                }

                double solveConstraint(double th) // solve constraint of the form logSigmoid(a, t, x) > th
                {
                        if (th >=0 ) return NOSOL;

                        double root = threshold_ - log(exp(-1*th) - 1)/strictness_;

                        return root;
                }
        public:
                double strictness_; // x > t if strictness_ > 0, x < t if strictness < 0
                double threshold_; //
};
#endif

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