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- /* 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 3 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;
- }
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