70 lines
1.8 KiB
C++
70 lines
1.8 KiB
C++
/*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 3 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program. If not, see <http://www.gnu.org/licenses/>.
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*/
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#include <numeric>
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#include <cstdlib>
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#include "Probability.h"
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namespace Probability
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{
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float binomial_gte1(int n, float p)
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{
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return 1.0f - std::pow(1.0f-p, n);
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}
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float randFloat()
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{
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return static_cast<float>(rand())/RAND_MAX;
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}
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SmallKBinomialGenerator::SmallKBinomialGenerator(unsigned int n, float p, unsigned int maxK_)
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{
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maxK = maxK_;
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cdf = new float[maxK];
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float *pdf = new float[maxK];
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// initial values, k=0:
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float pTerm = std::pow(1.0f-p, static_cast<float>(n)); // the p^k * (1-p)^(n-k) term
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unsigned int coeffN = 1, coeffD = 1; // (N / D) evaluates to the same result as the n!/(k!(n-k)!) term
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for (unsigned int k=0; k<maxK; k++)
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{
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pdf[k] = pTerm * (float(coeffN) / coeffD);
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pTerm *= p/(1.0f-p);
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coeffN *= n-k; // Part of the n! will no longer cancelled out by (n-k)!
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coeffD *= k+1; // k! (k+1 because this is for next k)
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}
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std::partial_sum(pdf, pdf+maxK, cdf);
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delete[] pdf;
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}
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SmallKBinomialGenerator::~SmallKBinomialGenerator()
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{
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delete[] cdf;
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}
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unsigned int SmallKBinomialGenerator::calc(float randFloat)
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{
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for (unsigned int k=0; k<maxK; k++)
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{
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if (randFloat<cdf[k])
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return k;
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}
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return maxK;
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}
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}
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