pascaltriangle.cpp

/* -*- mode: c++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
 
/*
 Copyright (C) 2015 Ferdinando Ametrano
 Copyright (C) 2015 Paolo Mazzocchi
 
 This file is part of QuantLib, a free-software/open-source library
 for financial quantitative analysts and developers - http://quantlib.org/
 
 QuantLib is free software: you can redistribute it and/or modify it
 under the terms of the QuantLib license.  You should have received a
 copy of the license along with this program; if not, please email
 <quantlib-dev@lists.sf.net>. The license is also available online at
 <http://quantlib.org/license.shtml>.
 
 This program is distributed in the hope that it will be useful, but WITHOUT
 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
 FOR A PARTICULAR PURPOSE.  See the license for more details.
*/
 
#include <ql/math/pascaltriangle.hpp>
#include <iterator>
 
namespace QuantLib {
 
    std::vector<std::vector<BigNatural> > PascalTriangle::coefficients_;
 
    const std::vector<BigNatural>& PascalTriangle::get(Size order) {
        if (coefficients_.empty()) {
            // order zero mandatory for bootstrap
            coefficients_.emplace_back(1, 1);
 
            coefficients_.emplace_back(2, 1);
            coefficients_.emplace_back(3, 1);
            coefficients_[2][1] = 2;
            coefficients_.emplace_back(4, 1);
            coefficients_[3][1] = coefficients_[3][2] = 3;
        }
        while (coefficients_.size()<=order)
            nextOrder();
        return coefficients_[order];
    }
 
    void PascalTriangle::nextOrder() {
        Size order = coefficients_.size();
        coefficients_.emplace_back(order + 1);
        coefficients_[order][0] = coefficients_[order][order] = 1;
        for (Size i=1; i<order/2+1; ++i) {
            coefficients_[order][i] = coefficients_[order][order-i] =
                coefficients_[order-1][i-1] + coefficients_[order-1][i];
        }
    }
 
}