Mercurial > dropbear
annotate libtommath/bn_mp_div_3.c @ 1723:5386011b740d
Disable toom and karatsuba for new libtommath
author | Matt Johnston <matt@ucc.asn.au> |
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date | Thu, 18 Jun 2020 19:12:07 +0800 |
parents | 1051e4eea25a |
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1 #include "tommath_private.h" |
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2 #ifdef BN_MP_DIV_3_C |
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3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */ |
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4 /* SPDX-License-Identifier: Unlicense */ |
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5 |
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6 /* divide by three (based on routine from MPI and the GMP manual) */ |
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7 mp_err mp_div_3(const mp_int *a, mp_int *c, mp_digit *d) |
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8 { |
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9 mp_int q; |
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10 mp_word w, t; |
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11 mp_digit b; |
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12 mp_err err; |
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13 int ix; |
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14 |
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15 /* b = 2**MP_DIGIT_BIT / 3 */ |
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16 b = ((mp_word)1 << (mp_word)MP_DIGIT_BIT) / (mp_word)3; |
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17 |
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18 if ((err = mp_init_size(&q, a->used)) != MP_OKAY) { |
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19 return err; |
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20 } |
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21 |
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22 q.used = a->used; |
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23 q.sign = a->sign; |
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24 w = 0; |
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25 for (ix = a->used - 1; ix >= 0; ix--) { |
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26 w = (w << (mp_word)MP_DIGIT_BIT) | (mp_word)a->dp[ix]; |
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27 |
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28 if (w >= 3u) { |
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29 /* multiply w by [1/3] */ |
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30 t = (w * (mp_word)b) >> (mp_word)MP_DIGIT_BIT; |
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31 |
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32 /* now subtract 3 * [w/3] from w, to get the remainder */ |
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33 w -= t+t+t; |
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34 |
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35 /* fixup the remainder as required since |
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36 * the optimization is not exact. |
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37 */ |
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38 while (w >= 3u) { |
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39 t += 1u; |
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40 w -= 3u; |
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41 } |
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42 } else { |
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43 t = 0; |
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44 } |
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45 q.dp[ix] = (mp_digit)t; |
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46 } |
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47 |
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48 /* [optional] store the remainder */ |
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49 if (d != NULL) { |
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50 *d = (mp_digit)w; |
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51 } |
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52 |
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53 /* [optional] store the quotient */ |
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54 if (c != NULL) { |
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55 mp_clamp(&q); |
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56 mp_exch(&q, c); |
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57 } |
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58 mp_clear(&q); |
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59 |
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60 return err; |
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61 } |
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62 |
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63 #endif |