| 1 | /* mpn_rshift -- Shift right a low-level natural-number integer. |
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| 2 | |
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| 3 | Copyright (C) 1991-2022 Free Software Foundation, Inc. |
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| 4 | |
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| 5 | This file is part of the GNU MP Library. |
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| 6 | |
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| 7 | The GNU MP Library is free software; you can redistribute it and/or modify |
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| 8 | it under the terms of the GNU Lesser General Public License as published by |
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| 9 | the Free Software Foundation; either version 2.1 of the License, or (at your |
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| 10 | option) any later version. |
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| 11 | |
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| 12 | The GNU MP Library is distributed in the hope that it will be useful, but |
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| 13 | WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY |
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| 14 | or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public |
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| 15 | License for more details. |
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| 16 | |
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| 17 | You should have received a copy of the GNU Lesser General Public License |
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| 18 | along with the GNU MP Library; see the file COPYING.LIB. If not, see |
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| 19 | <https://www.gnu.org/licenses/>. */ |
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| 20 | |
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| 21 | #include <gmp.h> |
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| 22 | #include "gmp-impl.h" |
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| 23 | |
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| 24 | #include <assert.h> |
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| 25 | |
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| 26 | /* Shift U (pointed to by UP and USIZE limbs long) CNT bits to the right |
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| 27 | and store the USIZE least significant limbs of the result at WP. |
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| 28 | The bits shifted out to the right are returned. |
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| 29 | |
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| 30 | Argument constraints: |
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| 31 | 1. 0 < CNT < BITS_PER_MP_LIMB |
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| 32 | 2. If the result is to be written over the input, WP must be <= UP. |
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| 33 | */ |
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| 34 | |
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| 35 | mp_limb_t |
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| 36 | mpn_rshift (register mp_ptr wp, |
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| 37 | register mp_srcptr up, mp_size_t usize, |
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| 38 | register unsigned int cnt) |
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| 39 | { |
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| 40 | register mp_limb_t high_limb, low_limb; |
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| 41 | register unsigned sh_1, sh_2; |
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| 42 | register mp_size_t i; |
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| 43 | mp_limb_t retval; |
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| 44 | |
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| 45 | assert (usize != 0 && cnt != 0); |
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| 46 | |
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| 47 | sh_1 = cnt; |
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| 48 | |
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| 49 | #if 0 |
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| 50 | if (sh_1 == 0) |
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| 51 | { |
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| 52 | if (wp != up) |
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| 53 | { |
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| 54 | /* Copy from low end to high end, to allow specified input/output |
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| 55 | overlapping. */ |
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| 56 | for (i = 0; i < usize; i++) |
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| 57 | wp[i] = up[i]; |
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| 58 | } |
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| 59 | return usize; |
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| 60 | } |
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| 61 | #endif |
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| 62 | |
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| 63 | wp -= 1; |
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| 64 | sh_2 = BITS_PER_MP_LIMB - sh_1; |
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| 65 | high_limb = up[0]; |
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| 66 | retval = high_limb << sh_2; |
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| 67 | low_limb = high_limb; |
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| 68 | |
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| 69 | for (i = 1; i < usize; i++) |
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| 70 | { |
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| 71 | high_limb = up[i]; |
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| 72 | wp[i] = (low_limb >> sh_1) | (high_limb << sh_2); |
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| 73 | low_limb = high_limb; |
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| 74 | } |
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| 75 | wp[i] = low_limb >> sh_1; |
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| 76 | |
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| 77 | return retval; |
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| 78 | } |
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| 79 | |
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