[25] | 1 | #include <tommath.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 | * |
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| 5 | * LibTomMath is a library that provides multiple-precision |
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| 6 | * integer arithmetic as well as number theoretic functionality. |
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| 7 | * |
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| 8 | * The library was designed directly after the MPI library by |
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| 9 | * Michael Fromberger but has been written from scratch with |
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| 10 | * additional optimizations in place. |
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| 11 | * |
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| 12 | * The library is free for all purposes without any express |
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| 13 | * guarantee it works. |
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| 14 | * |
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| 15 | * Tom St Denis, tomstdenis@gmail.com, http://math.libtomcrypt.com |
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| 16 | */ |
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| 17 | |
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| 18 | /* divide by three (based on routine from MPI and the GMP manual) */ |
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| 19 | int |
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| 20 | mp_div_3 (mp_int * a, mp_int *c, mp_digit * d) |
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| 21 | { |
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| 22 | mp_int q; |
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| 23 | mp_word w, t; |
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| 24 | mp_digit b; |
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| 25 | int res, ix; |
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| 26 | |
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| 27 | /* b = 2**DIGIT_BIT / 3 */ |
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| 28 | b = (((mp_word)1) << ((mp_word)DIGIT_BIT)) / ((mp_word)3); |
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| 29 | |
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| 30 | if ((res = mp_init_size(&q, a->used)) != MP_OKAY) { |
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| 31 | return res; |
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| 32 | } |
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| 33 | |
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| 34 | q.used = a->used; |
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| 35 | q.sign = a->sign; |
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| 36 | w = 0; |
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| 37 | for (ix = a->used - 1; ix >= 0; ix--) { |
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| 38 | w = (w << ((mp_word)DIGIT_BIT)) | ((mp_word)a->dp[ix]); |
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| 39 | |
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| 40 | if (w >= 3) { |
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| 41 | /* multiply w by [1/3] */ |
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| 42 | t = (w * ((mp_word)b)) >> ((mp_word)DIGIT_BIT); |
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| 43 | |
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| 44 | /* now subtract 3 * [w/3] from w, to get the remainder */ |
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| 45 | w -= t+t+t; |
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| 46 | |
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| 47 | /* fixup the remainder as required since |
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| 48 | * the optimization is not exact. |
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| 49 | */ |
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| 50 | while (w >= 3) { |
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| 51 | t += 1; |
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| 52 | w -= 3; |
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| 53 | } |
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| 54 | } else { |
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| 55 | t = 0; |
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| 56 | } |
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| 57 | q.dp[ix] = (mp_digit)t; |
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| 58 | } |
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| 59 | |
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| 60 | /* [optional] store the remainder */ |
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| 61 | if (d != NULL) { |
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| 62 | *d = (mp_digit)w; |
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| 63 | } |
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| 64 | |
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| 65 | /* [optional] store the quotient */ |
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| 66 | if (c != NULL) { |
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| 67 | mp_clamp(&q); |
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| 68 | mp_exch(&q, c); |
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| 69 | } |
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| 70 | mp_clear(&q); |
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| 71 | |
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| 72 | return res; |
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| 73 | } |
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| 74 | |
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| 75 | #endif |
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| 76 | |
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| 77 | /* $Source: /cvsroot/tcl/libtommath/bn_mp_div_3.c,v $ */ |
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| 78 | /* $Revision: 1.1.1.3 $ */ |
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| 79 | /* $Date: 2006/12/01 00:08:11 $ */ |
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