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Diffstat (limited to '')
| -rw-r--r-- | src/2geom/choose.h | 140 |
1 files changed, 140 insertions, 0 deletions
diff --git a/src/2geom/choose.h b/src/2geom/choose.h new file mode 100644 index 0000000..64ce76f --- /dev/null +++ b/src/2geom/choose.h @@ -0,0 +1,140 @@ +/** + * \file + * \brief Calculation of binomial cefficients + *//* + * Copyright 2006 Nathan Hurst <njh@mail.csse.monash.edu.au> + * + * This library is free software; you can redistribute it and/or + * modify it either under the terms of the GNU Lesser General Public + * License version 2.1 as published by the Free Software Foundation + * (the "LGPL") or, at your option, under the terms of the Mozilla + * Public License Version 1.1 (the "MPL"). If you do not alter this + * notice, a recipient may use your version of this file under either + * the MPL or the LGPL. + * + * You should have received a copy of the LGPL along with this library + * in the file COPYING-LGPL-2.1; if not, write to the Free Software + * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA + * You should have received a copy of the MPL along with this library + * in the file COPYING-MPL-1.1 + * + * The contents of this file are subject to the Mozilla Public License + * Version 1.1 (the "License"); you may not use this file except in + * compliance with the License. You may obtain a copy of the License at + * http://www.mozilla.org/MPL/ + * + * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY + * OF ANY KIND, either express or implied. See the LGPL or the MPL for + * the specific language governing rights and limitations. + * + */ + +#ifndef LIB2GEOM_SEEN_CHOOSE_H +#define LIB2GEOM_SEEN_CHOOSE_H + +#include <vector> + +namespace Geom { + +// XXX: Can we keep only the left terms easily? +// this would more than halve the array +// row index becomes n2 = n/2, row2 = n2*(n2+1)/2, row = row2*2+(n&1)?n2:0 +// we could also leave off the ones + +template <typename T> +T choose(unsigned n, unsigned k) { + static std::vector<T> pascals_triangle; + static unsigned rows_done = 0; + // indexing is (0,0,), (1,0), (1,1), (2, 0)... + // to get (i, j) i*(i+1)/2 + j + if(/*k < 0 ||*/ k > n) return 0; + if(rows_done <= n) {// we haven't got there yet + if(rows_done == 0) { + pascals_triangle.push_back(1); + rows_done = 1; + } + while(rows_done <= n) { + unsigned p = pascals_triangle.size() - rows_done; + pascals_triangle.push_back(1); + for(unsigned i = 0; i < rows_done-1; i++) { + pascals_triangle.push_back(pascals_triangle[p] + + pascals_triangle[p+1]); + p++; + } + pascals_triangle.push_back(1); + rows_done ++; + } + } + unsigned row = (n*(n+1))/2; + return pascals_triangle[row+k]; +} + +// Is it faster to store them or compute them on demand? +/*template <typename T> +T choose(unsigned n, unsigned k) { + T r = 1; + for(unsigned i = 1; i <= k; i++) + r = (r*(n-k+i))/i; + return r; + }*/ + + + +template <typename ValueType> +class BinomialCoefficient +{ + public: + typedef ValueType value_type; + typedef std::vector<value_type> container_type; + + BinomialCoefficient(unsigned int _n) + : n(_n), m(n >> 1) + { + coefficients.reserve(m+1); + coefficients.push_back(1); + int h = m + 1; + value_type bct = 1; + for (int i = 1; i < h; ++i) + { + bct *= (n-i+1); + bct /= i; + coefficients.push_back(bct); + } + } + + unsigned int size() const + { + return degree() +1; + } + + unsigned int degree() const + { + return n; + } + + value_type operator[] (unsigned int k) const + { + if (k > m) k = n - k; + return coefficients[k]; + } + + private: + const int n; + const unsigned int m; + container_type coefficients; +}; + +} // end namespace Geom + +#endif // LIB2GEOM_SEEN_CHOOSE_H + +/* + Local Variables: + mode:c++ + c-file-style:"stroustrup" + c-file-offsets:((innamespace . 0)(inline-open . 0)(case-label . +)) + indent-tabs-mode:nil + fill-column:99 + End: +*/ +// vim: filetype=cpp:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:fileencoding=utf-8:textwidth=99 : |