83 lines
3.0 KiB
C
83 lines
3.0 KiB
C
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#include "tommath_private.h"
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#ifdef S_MP_RADIX_SIZE_OVERESTIMATE_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis */
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/* SPDX-License-Identifier: Unlicense */
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/*
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Overestimate the size needed for the bigint to string conversion by a very small amount.
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The error is about 10^-8; it will overestimate the result by at most 11 elements for
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a number of the size 2^(2^31)-1 which is currently the largest possible in this library.
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Some short tests gave no results larger than 5 (plus 2 for sign and EOS).
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*/
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/*
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Table of {0, INT(log_2([1..64])*2^p)+1 } where p is the scale
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factor defined in MP_RADIX_SIZE_SCALE and INT() extracts the integer part (truncating).
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Good for 32 bit "int". Set MP_RADIX_SIZE_SCALE = 61 and recompute values
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for 64 bit "int".
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*/
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/* *INDENT-OFF* */
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#define MP_RADIX_SIZE_SCALE 29
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static const uint32_t s_log_bases[65] = {
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0u, 0u, 0x20000001u, 0x14309399u, 0x10000001u,
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0xdc81a35u, 0xc611924u, 0xb660c9eu, 0xaaaaaabu, 0xa1849cdu,
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0x9a209a9u, 0x94004e1u, 0x8ed19c2u, 0x8a5ca7du, 0x867a000u,
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0x830cee3u, 0x8000001u, 0x7d42d60u, 0x7ac8b32u, 0x7887847u,
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0x7677349u, 0x749131fu, 0x72d0163u, 0x712f657u, 0x6fab5dbu,
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0x6e40d1bu, 0x6ced0d0u, 0x6badbdeu, 0x6a80e3bu, 0x6964c19u,
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0x6857d31u, 0x6758c38u, 0x6666667u, 0x657fb21u, 0x64a3b9fu,
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0x63d1ab4u, 0x6308c92u, 0x624869eu, 0x618ff47u, 0x60dedeau,
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0x6034ab0u, 0x5f90e7bu, 0x5ef32cbu, 0x5e5b1b2u, 0x5dc85c3u,
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0x5d3aa02u, 0x5cb19d9u, 0x5c2d10fu, 0x5bacbbfu, 0x5b3064fu,
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0x5ab7d68u, 0x5a42df0u, 0x59d1506u, 0x5962ffeu, 0x58f7c57u,
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0x588f7bcu, 0x582a000u, 0x57c7319u, 0x5766f1du, 0x5709243u,
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0x56adad9u, 0x565474du, 0x55fd61fu, 0x55a85e8u, 0x5555556u
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};
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/* *INDENT-ON* */
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mp_err s_mp_radix_size_overestimate(const mp_int *a, const int radix, size_t *size)
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{
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int bit_count;
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mp_int bi_bit_count, bi_k;
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mp_err err = MP_OKAY;
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if ((radix < 2) || (radix > 64)) {
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return MP_VAL;
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}
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if (mp_iszero(a)) {
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*size = 2U;
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return MP_OKAY;
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}
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if (MP_HAS(S_MP_LOG_2EXPT) && MP_IS_2EXPT((mp_digit)radix)) {
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/* floor(log_{2^n}(a)) + 1 + EOS + sign */
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*size = (size_t)(s_mp_log_2expt(a, (mp_digit)radix) + 3);
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return MP_OKAY;
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}
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if ((err = mp_init_multi(&bi_bit_count, &bi_k, NULL)) != MP_OKAY) {
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return err;
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}
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/* la = floor(log_2(a)) + 1 */
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bit_count = mp_count_bits(a);
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mp_set_u32(&bi_bit_count, (uint32_t)bit_count);
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/* k = floor(2^29/log_2(radix)) + 1 */
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mp_set_u32(&bi_k, s_log_bases[radix]);
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/* n = floor((la * k) / 2^29) + 1 */
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if ((err = mp_mul(&bi_bit_count, &bi_k, &bi_bit_count)) != MP_OKAY) goto LBL_ERR;
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if ((err = mp_div_2d(&bi_bit_count, MP_RADIX_SIZE_SCALE, &bi_bit_count, NULL)) != MP_OKAY) goto LBL_ERR;
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/* The "+1" here is the "+1" in "floor((la * k) / 2^29) + 1" */
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/* n = n + 1 + EOS + sign */
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*size = (size_t)(mp_get_u64(&bi_bit_count) + 3U);
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LBL_ERR:
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mp_clear_multi(&bi_bit_count, &bi_k, NULL);
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return err;
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}
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#endif
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