plda源碼(十一)

plda源碼(十一)

Alias Table

sample alias

class VoseAlias {

    unsigned short n;				//Dimension
    double wsum;		//Sum of proportions
    std::vector<std::pair<double, unsigned short>> table; //Alias probabilities and indices

    std::vector<double> w;			//Contains proportions and alias probabilities

    unsigned noSamples;		// number of samples between replenishment

    inline unsigned short sample(unsigned short fair_die , double u) const
    {
        bool res = u < table[fair_die].first; 
        return res ? fair_die : table[fair_die].second; 
    }
};

#endif

	double *p = new double[n];
	std::queue<unsigned short> Small, Large;

	for (unsigned short i = 0; i < n; ++i)
	{
		p[i] = (w[i] * n) / wsum;// lp
	}

	for (unsigned short i = 0; i < n; ++i)
	{
		if (p[i] < 1)// $p_i < l^{-1}$
			Small.push(i);//(i, p_i)，因爲p_i可以通過i取到，所以只push i就可以了
		else
			Large.push(i);
	}

	while (!(Small.empty() || Large.empty()))
	{
		unsigned short l = Small.front(); Small.pop();
		unsigned short g = Large.front(); Large.pop();
		table[l].first = p[l]; 
		table[l].second = g; //（i, h, p_i), first是p_i, second是h， l就是i
		p[g] = (p[g] + p[l]) - 1;//對比p_h - p_i, This is a more numerically stable option
		if (p[g] < 1)
			Small.push(g);
		else
			Large.push(g);
	}

	while (!Large.empty())
	{
		unsigned short g = Large.front(); Large.pop();
		table[g].first = 1;	
	}

	while (!Small.empty())
	{
		unsigned short l = Small.front(); Small.pop();
		table[l].first = 1;	
	}

	noSamples = 0;
	delete[] p;