| /* rng/schrage.c |
| * Copyright (C) 2003 Carlo Perassi and Heiko Bauke. |
| * |
| * This program is free software; you can redistribute it and/or modify |
| * it under the terms of the GNU General Public License as published by |
| * the Free Software Foundation; either version 2 of the License, or (at |
| * your option) any later version. |
| * |
| * 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 GNU |
| * General Public License for more details. |
| * |
| * You should have received a copy of the GNU General Public License |
| * along with this program; if not, write to the Free Software |
| * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. |
| */ |
| |
| static inline unsigned long int |
| schrage (unsigned long int a, unsigned long int b, unsigned long int m) |
| { |
| /* This is a modified version of Schrage's method. It ensures that no |
| * overflow or underflow occurs even if a=ceil(sqrt(m)). Usual |
| * Schrage's method works only until a=floor(sqrt(m)). |
| */ |
| unsigned long int q, t; |
| if (a == 0UL) |
| return 0UL; |
| q = m / a; |
| t = 2 * m - (m % a) * (b / q); |
| if (t >= m) |
| t -= m; |
| t += a * (b % q); |
| return (t >= m) ? (t - m) : t; |
| } |
| |
| static inline unsigned long int |
| schrage_mult (unsigned long int a, unsigned long int b, |
| unsigned long int m, |
| unsigned long int sqrtm) |
| { |
| /* To multiply a and b use Schrage's method 3 times. |
| * represent a in base ceil(sqrt(m)) a = a1*ceil(sqrt(m)) + a0 |
| * a*b = (a1*ceil(sqrt(m)) + a0)*b = a1*ceil(sqrt(m))*b + a0*b |
| */ |
| unsigned long int t0 = schrage (sqrtm, b, m); |
| unsigned long int t1 = schrage (a / sqrtm, t0, m); |
| unsigned long int t2 = schrage (a % sqrtm, b, m); |
| unsigned long int t = t1 + t2; |
| return (t >= m) ? (t - m) : t; |
| } |