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15347 | fschmid | 1 | `/*` |

2 | ` * choose.h` |
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12065 | fschmid | 3 | ` *` |

4 | ` * Copyright 2006 Nathan Hurst <njh@mail.csse.monash.edu.au>` |
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5 | ` *` |
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6 | ` * This library is free software; you can redistribute it and/or` |
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7 | ` * modify it either under the terms of the GNU Lesser General Public` |
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8 | ` * License version 2.1 as published by the Free Software Foundation` |
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9 | ` * (the "LGPL") or, at your option, under the terms of the Mozilla` |
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10 | ` * Public License Version 1.1 (the "MPL"). If you do not alter this` |
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11 | ` * notice, a recipient may use your version of this file under either` |
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12 | ` * the MPL or the LGPL.` |
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13 | ` *` |
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14 | ` * You should have received a copy of the LGPL along with this library` |
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15 | ` * in the file COPYING-LGPL-2.1; if not, write to the Free Software` |
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18122 | mrdocs | 16 | ` * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA` |

12065 | fschmid | 17 | ` * You should have received a copy of the MPL along with this library` |

18 | ` * in the file COPYING-MPL-1.1` |
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19 | ` *` |
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20 | ` * The contents of this file are subject to the Mozilla Public License` |
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21 | ` * Version 1.1 (the "License"); you may not use this file except in` |
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22 | ` * compliance with the License. You may obtain a copy of the License at` |
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23 | ` * http://www.mozilla.org/MPL/` |
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24 | ` *` |
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25 | ` * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY` |
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26 | ` * OF ANY KIND, either express or implied. See the LGPL or the MPL for` |
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27 | ` * the specific language governing rights and limitations.` |
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28 | ` *` |
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29 | ` */` |
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30 | |||

31 | `#ifndef _CHOOSE_H` |
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32 | `#define _CHOOSE_H` |
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33 | |||

15347 | fschmid | 34 | `// XXX: Can we keep only the left terms easily? ` |

12065 | fschmid | 35 | `// this would more than halve the array` |

36 | `// row index becomes n2 = n/2, row2 = n2*(n2+1)/2, row = row2*2+(n&1)?n2:0` |
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37 | `// we could also leave off the ones` |
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38 | |||

39 | template <typename T> |
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40 | T choose(unsigned n, unsigned k) { |
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41 | static std::vector<T> pascals_triangle; |
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42 | static unsigned rows_done = 0; |
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43 | ` // indexing is (0,0,), (1,0), (1,1), (2, 0)...` |
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44 | ` // to get (i, j) i*(i+1)/2 + j` |
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15347 | fschmid | 45 | if(k < 0 || k > n) return 0; |

12065 | fschmid | 46 | if(rows_done <= n) {// we haven't got there yet |

47 | if(rows_done == 0) { |
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48 | pascals_triangle.push_back(1); |
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49 | rows_done = 1; |
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50 | ` }` |
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51 | while(rows_done <= n) { |
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52 | unsigned p = pascals_triangle.size() - rows_done; |
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53 | pascals_triangle.push_back(1); |
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54 | for(unsigned i = 0; i < rows_done-1; i++) { |
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15347 | fschmid | 55 | pascals_triangle.push_back(pascals_triangle[p] |

12065 | fschmid | 56 | + pascals_triangle[p+1]); |

57 | p++; |
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58 | ` }` |
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59 | pascals_triangle.push_back(1); |
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60 | rows_done ++; |
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61 | ` }` |
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62 | ` }` |
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63 | unsigned row = (n*(n+1))/2; |
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64 | return pascals_triangle[row+k]; |
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65 | `}` |
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66 | |||

67 | `#endif` |