mp_div_2d.c

   1 #include "tommath_private.h"
   2 #ifdef MP_DIV_2D_C
   3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
   4 /* SPDX-License-Identifier: Unlicense */
   5
   6 /* shift right by a certain bit count (store quotient in c, optional remainder in d) */
   7 mp_err mp_div_2d(const mp_int *a, int b, mp_int *c, mp_int *d)
   8 {
   9    mp_err err;
  10
  11    if (b < 0) {
  12       return MP_VAL;
  13    }
  14
  15    if ((err = mp_copy(a, c)) != MP_OKAY) {
  16       return err;
  17    }
  18
  19    /* 'a' should not be used after here - it might be the same as d */
  20
  21    /* get the remainder */
  22    if (d != NULL) {
  23       if ((err = mp_mod_2d(a, b, d)) != MP_OKAY) {
  24          return err;
  25       }
  26    }
  27
  28    /* shift by as many digits in the bit count */
  29    if (b >= MP_DIGIT_BIT) {
  30       mp_rshd(c, b / MP_DIGIT_BIT);
  31    }
  32
  33    /* shift any bit count < MP_DIGIT_BIT */
  34    b %= MP_DIGIT_BIT;
  35    if (b != 0u) {
  36       int x;
  37       mp_digit r, mask, shift;
  38
  39       /* mask */
  40       mask = ((mp_digit)1 << b) - 1uL;
  41
  42       /* shift for lsb */
  43       shift = (mp_digit)(MP_DIGIT_BIT - b);
  44
  45       /* carry */
  46       r = 0;
  47       for (x = c->used; x --> 0;) {
  48          /* get the lower  bits of this word in a temp */
  49          mp_digit rr = c->dp[x] & mask;
  50
  51          /* shift the current word and mix in the carry bits from the previous word */
  52          c->dp[x] = (c->dp[x] >> b) | (r << shift);
  53
  54          /* set the carry to the carry bits of the current word found above */
  55          r = rr;
  56       }
  57    }
  58    mp_clamp(c);
  59    return MP_OKAY;
  60 }
  61 #endif