| Austin Schuh | bb1338c | 2024-06-15 19:31:16 -0700 | [diff] [blame] | 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 either: |
| 9 | |
| 10 | * the GNU Lesser General Public License as published by the Free |
| 11 | Software Foundation; either version 3 of the License, or (at your |
| 12 | option) any later version. |
| 13 | |
| 14 | or |
| 15 | |
| 16 | * the GNU General Public License as published by the Free Software |
| 17 | Foundation; either version 2 of the License, or (at your option) any |
| 18 | later version. |
| 19 | |
| 20 | or both in parallel, as here. |
| 21 | |
| 22 | The GNU MP Library is distributed in the hope that it will be useful, but |
| 23 | WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY |
| 24 | or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
| 25 | for more details. |
| 26 | |
| 27 | You should have received copies of the GNU General Public License and the |
| 28 | GNU Lesser General Public License along with the GNU MP Library. If not, |
| 29 | see https://www.gnu.org/licenses/. */ |
| 30 | |
| 31 | #include "gmp-impl.h" |
| 32 | #include "longlong.h" |
| 33 | |
| 34 | |
| 35 | /* If n is a power of 2 then the test ret<n is always true and the loop is |
| 36 | unnecessary, but there's no need to add special code for this. Just get |
| 37 | the "bits" calculation correct and let it go through normally. |
| 38 | |
| 39 | If n is 1 then will have bits==0 and _gmp_rand will produce no output and |
| 40 | we always return 0. Again there seems no need for a special case, just |
| 41 | initialize a[0]=0 and let it go through normally. */ |
| 42 | |
| 43 | #define MAX_URANDOMM_ITER 80 |
| 44 | |
| 45 | unsigned long |
| 46 | gmp_urandomm_ui (gmp_randstate_ptr rstate, unsigned long n) |
| 47 | { |
| 48 | mp_limb_t a[LIMBS_PER_ULONG]; |
| 49 | unsigned long ret, bits, leading; |
| 50 | int i; |
| 51 | |
| 52 | if (UNLIKELY (n == 0)) |
| 53 | DIVIDE_BY_ZERO; |
| 54 | |
| 55 | /* start with zeros, since if bits==0 then _gmp_rand will store nothing at |
| 56 | all (bits==0 arises when n==1), or if bits <= GMP_NUMB_BITS then it |
| 57 | will store only a[0]. */ |
| 58 | a[0] = 0; |
| 59 | #if LIMBS_PER_ULONG > 1 |
| 60 | a[1] = 0; |
| 61 | #endif |
| 62 | |
| 63 | count_leading_zeros (leading, (mp_limb_t) n); |
| 64 | bits = GMP_LIMB_BITS - leading - (POW2_P(n) != 0); |
| 65 | |
| 66 | for (i = 0; i < MAX_URANDOMM_ITER; i++) |
| 67 | { |
| 68 | _gmp_rand (a, rstate, bits); |
| 69 | #if LIMBS_PER_ULONG == 1 |
| 70 | ret = a[0]; |
| 71 | #else |
| 72 | ret = a[0] | (a[1] << GMP_NUMB_BITS); |
| 73 | #endif |
| 74 | if (LIKELY (ret < n)) /* usually one iteration suffices */ |
| 75 | goto done; |
| 76 | } |
| 77 | |
| 78 | /* Too many iterations, there must be something degenerate about the |
| 79 | rstate algorithm. Return r%n. */ |
| 80 | ret -= n; |
| 81 | ASSERT (ret < n); |
| 82 | |
| 83 | done: |
| 84 | return ret; |
| 85 | } |