diff options
Diffstat (limited to 'libtommath/bn_mp_root_u32.c')
| -rw-r--r-- | libtommath/bn_mp_root_u32.c | 139 |
1 files changed, 139 insertions, 0 deletions
diff --git a/libtommath/bn_mp_root_u32.c b/libtommath/bn_mp_root_u32.c new file mode 100644 index 0000000..ba65549 --- /dev/null +++ b/libtommath/bn_mp_root_u32.c @@ -0,0 +1,139 @@ +#include "tommath_private.h" +#ifdef BN_MP_ROOT_U32_C +/* LibTomMath, multiple-precision integer library -- Tom St Denis */ +/* SPDX-License-Identifier: Unlicense */ + +/* find the n'th root of an integer + * + * Result found such that (c)**b <= a and (c+1)**b > a + * + * This algorithm uses Newton's approximation + * x[i+1] = x[i] - f(x[i])/f'(x[i]) + * which will find the root in log(N) time where + * each step involves a fair bit. + */ +mp_err mp_root_u32(const mp_int *a, uint32_t b, mp_int *c) +{ + mp_int t1, t2, t3, a_; + mp_ord cmp; + int ilog2; + mp_err err; + + /* input must be positive if b is even */ + if (((b & 1u) == 0u) && (a->sign == MP_NEG)) { + return MP_VAL; + } + + if ((err = mp_init_multi(&t1, &t2, &t3, NULL)) != MP_OKAY) { + return err; + } + + /* if a is negative fudge the sign but keep track */ + a_ = *a; + a_.sign = MP_ZPOS; + + /* Compute seed: 2^(log_2(n)/b + 2)*/ + ilog2 = mp_count_bits(a); + + /* + If "b" is larger than INT_MAX it is also larger than + log_2(n) because the bit-length of the "n" is measured + with an int and hence the root is always < 2 (two). + */ + if (b > (uint32_t)(INT_MAX/2)) { + mp_set(c, 1uL); + c->sign = a->sign; + err = MP_OKAY; + goto LBL_ERR; + } + + /* "b" is smaller than INT_MAX, we can cast safely */ + if (ilog2 < (int)b) { + mp_set(c, 1uL); + c->sign = a->sign; + err = MP_OKAY; + goto LBL_ERR; + } + ilog2 = ilog2 / ((int)b); + if (ilog2 == 0) { + mp_set(c, 1uL); + c->sign = a->sign; + err = MP_OKAY; + goto LBL_ERR; + } + /* Start value must be larger than root */ + ilog2 += 2; + if ((err = mp_2expt(&t2,ilog2)) != MP_OKAY) goto LBL_ERR; + do { + /* t1 = t2 */ + if ((err = mp_copy(&t2, &t1)) != MP_OKAY) goto LBL_ERR; + + /* t2 = t1 - ((t1**b - a) / (b * t1**(b-1))) */ + + /* t3 = t1**(b-1) */ + if ((err = mp_expt_u32(&t1, b - 1u, &t3)) != MP_OKAY) goto LBL_ERR; + + /* numerator */ + /* t2 = t1**b */ + if ((err = mp_mul(&t3, &t1, &t2)) != MP_OKAY) goto LBL_ERR; + + /* t2 = t1**b - a */ + if ((err = mp_sub(&t2, &a_, &t2)) != MP_OKAY) goto LBL_ERR; + + /* denominator */ + /* t3 = t1**(b-1) * b */ + if ((err = mp_mul_d(&t3, b, &t3)) != MP_OKAY) goto LBL_ERR; + + /* t3 = (t1**b - a)/(b * t1**(b-1)) */ + if ((err = mp_div(&t2, &t3, &t3, NULL)) != MP_OKAY) goto LBL_ERR; + + if ((err = mp_sub(&t1, &t3, &t2)) != MP_OKAY) goto LBL_ERR; + + /* + Number of rounds is at most log_2(root). If it is more it + got stuck, so break out of the loop and do the rest manually. + */ + if (ilog2-- == 0) { + break; + } + } while (mp_cmp(&t1, &t2) != MP_EQ); + + /* result can be off by a few so check */ + /* Loop beneath can overshoot by one if found root is smaller than actual root */ + for (;;) { + if ((err = mp_expt_u32(&t1, b, &t2)) != MP_OKAY) goto LBL_ERR; + cmp = mp_cmp(&t2, &a_); + if (cmp == MP_EQ) { + err = MP_OKAY; + goto LBL_ERR; + } + if (cmp == MP_LT) { + if ((err = mp_add_d(&t1, 1uL, &t1)) != MP_OKAY) goto LBL_ERR; + } else { + break; + } + } + /* correct overshoot from above or from recurrence */ + for (;;) { + if ((err = mp_expt_u32(&t1, b, &t2)) != MP_OKAY) goto LBL_ERR; + if (mp_cmp(&t2, &a_) == MP_GT) { + if ((err = mp_sub_d(&t1, 1uL, &t1)) != MP_OKAY) goto LBL_ERR; + } else { + break; + } + } + + /* set the result */ + mp_exch(&t1, c); + + /* set the sign of the result */ + c->sign = a->sign; + + err = MP_OKAY; + +LBL_ERR: + mp_clear_multi(&t1, &t2, &t3, NULL); + return err; +} + +#endif |