1 /* gmp_urandomm_ui -- uniform random number 0 to N-1 for ulong N. 2 3 Copyright 2003, 2004 Free Software Foundation, Inc. 4 5 This file is part of the GNU MP Library. 6 7 The GNU MP Library is free software; you can redistribute it and/or modify 8 it under the terms of the GNU Lesser General Public License as published by 9 the Free Software Foundation; either version 3 of the License, or (at your 10 option) any later version. 11 12 The GNU MP Library is distributed in the hope that it will be useful, but 13 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY 14 or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public 15 License for more details. 16 17 You should have received a copy of the GNU Lesser General Public License 18 along with the GNU MP Library. If not, see http://www.gnu.org/licenses/. */ 19 20 #include "gmp.h" 21 #include "gmp-impl.h" 22 #include "longlong.h" 23 24 25 /* If n is a power of 2 then the test ret<n is always true and the loop is 26 unnecessary, but there's no need to add special code for this. Just get 27 the "bits" calculation correct and let it go through normally. 28 29 If n is 1 then will have bits==0 and _gmp_rand will produce no output and 30 we always return 0. Again there seems no need for a special case, just 31 initialize a[0]=0 and let it go through normally. */ 32 33 #define MAX_URANDOMM_ITER 80 34 35 unsigned long 36 gmp_urandomm_ui (gmp_randstate_ptr rstate, unsigned long n) 37 { 38 mp_limb_t a[LIMBS_PER_ULONG]; 39 unsigned long ret, bits, leading; 40 int i; 41 42 if (UNLIKELY (n == 0)) 43 DIVIDE_BY_ZERO; 44 45 /* start with zeros, since if bits==0 then _gmp_rand will store nothing at 46 all (bits==0 arises when n==1), or if bits <= GMP_NUMB_BITS then it 47 will store only a[0]. */ 48 a[0] = 0; 49 #if LIMBS_PER_ULONG > 1 50 a[1] = 0; 51 #endif 52 53 count_leading_zeros (leading, (mp_limb_t) n); 54 bits = GMP_LIMB_BITS - leading - (POW2_P(n) != 0); 55 56 for (i = 0; i < MAX_URANDOMM_ITER; i++) 57 { 58 _gmp_rand (a, rstate, bits); 59 #if LIMBS_PER_ULONG == 1 60 ret = a[0]; 61 #else 62 ret = a[0] | (a[1] << GMP_NUMB_BITS); 63 #endif 64 if (LIKELY (ret < n)) /* usually one iteration suffices */ 65 goto done; 66 } 67 68 /* Too many iterations, there must be something degenerate about the 69 rstate algorithm. Return r%n. */ 70 ret -= n; 71 ASSERT (ret < n); 72 73 done: 74 return ret; 75 } 76