1 /**
2 * \file choose.h
3 * \brief \todo brief description
4 *
5 * Copyright 2006 Nathan Hurst <njh@mail.csse.monash.edu.au>
6 *
7 * This library is free software; you can redistribute it and/or
8 * modify it either under the terms of the GNU Lesser General Public
9 * License version 2.1 as published by the Free Software Foundation
10 * (the "LGPL") or, at your option, under the terms of the Mozilla
11 * Public License Version 1.1 (the "MPL"). If you do not alter this
12 * notice, a recipient may use your version of this file under either
13 * the MPL or the LGPL.
14 *
15 * You should have received a copy of the LGPL along with this library
16 * in the file COPYING-LGPL-2.1; if not, write to the Free Software
17 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
18 * You should have received a copy of the MPL along with this library
19 * in the file COPYING-MPL-1.1
20 *
21 * The contents of this file are subject to the Mozilla Public License
22 * Version 1.1 (the "License"); you may not use this file except in
23 * compliance with the License. You may obtain a copy of the License at
24 * http://www.mozilla.org/MPL/
25 *
26 * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY
27 * OF ANY KIND, either express or implied. See the LGPL or the MPL for
28 * the specific language governing rights and limitations.
29 *
30 */
32 #ifndef _CHOOSE_H
33 #define _CHOOSE_H
34 #include <vector>
36 // XXX: Can we keep only the left terms easily?
37 // this would more than halve the array
38 // row index becomes n2 = n/2, row2 = n2*(n2+1)/2, row = row2*2+(n&1)?n2:0
39 // we could also leave off the ones
41 template <typename T>
42 T choose(unsigned n, unsigned k) {
43 static std::vector<T> pascals_triangle;
44 static unsigned rows_done = 0;
45 // indexing is (0,0,), (1,0), (1,1), (2, 0)...
46 // to get (i, j) i*(i+1)/2 + j
47 if(/*k < 0 ||*/ k > n) return 0;
48 if(rows_done <= n) {// we haven't got there yet
49 if(rows_done == 0) {
50 pascals_triangle.push_back(1);
51 rows_done = 1;
52 }
53 while(rows_done <= n) {
54 unsigned p = pascals_triangle.size() - rows_done;
55 pascals_triangle.push_back(1);
56 for(unsigned i = 0; i < rows_done-1; i++) {
57 pascals_triangle.push_back(pascals_triangle[p]
58 + pascals_triangle[p+1]);
59 p++;
60 }
61 pascals_triangle.push_back(1);
62 rows_done ++;
63 }
64 }
65 unsigned row = (n*(n+1))/2;
66 return pascals_triangle[row+k];
67 }
69 // Is it faster to store them or compute them on demand?
70 /*template <typename T>
71 T choose(unsigned n, unsigned k) {
72 T r = 1;
73 for(unsigned i = 1; i <= k; i++)
74 r = (r*(n-k+i))/i;
75 return r;
76 }*/
78 #endif
80 /*
81 Local Variables:
82 mode:c++
83 c-file-style:"stroustrup"
84 c-file-offsets:((innamespace . 0)(inline-open . 0)(case-label . +))
85 indent-tabs-mode:nil
86 fill-column:99
87 End:
88 */
89 // vim: filetype=cpp:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:encoding=utf-8:textwidth=99 :