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/*
* Copyright (c) 2016 Thomas Pornin <pornin@bolet.org>
*
* Permission is hereby granted, free of charge, to any person obtaining
* a copy of this software and associated documentation files (the
* "Software"), to deal in the Software without restriction, including
* without limitation the rights to use, copy, modify, merge, publish,
* distribute, sublicense, and/or sell copies of the Software, and to
* permit persons to whom the Software is furnished to do so, subject to
* the following conditions:
*
* The above copyright notice and this permission notice shall be
* included in all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
* NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
* BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
* ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
* CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
* SOFTWARE.
*/
#include "inner.h"
/* see inner.h */
void
br_i31_decode_reduce(uint32_t *x,
const void *src, size_t len, const uint32_t *m)
{
uint32_t m_ebitlen, m_rbitlen;
size_t mblen, k;
const unsigned char *buf;
uint32_t acc;
int acc_len;
/*
* Get the encoded bit length.
*/
m_ebitlen = m[0];
/*
* Special case for an invalid (null) modulus.
*/
if (m_ebitlen == 0) {
x[0] = 0;
return;
}
/*
* Clear the destination.
*/
br_i31_zero(x, m_ebitlen);
/*
* First decode directly as many bytes as possible. This requires
* computing the actual bit length.
*/
m_rbitlen = m_ebitlen >> 5;
m_rbitlen = (m_ebitlen & 31) + (m_rbitlen << 5) - m_rbitlen;
mblen = (m_rbitlen + 7) >> 3;
k = mblen - 1;
if (k >= len) {
br_i31_decode(x, src, len);
x[0] = m_ebitlen;
return;
}
buf = src;
br_i31_decode(x, buf, k);
x[0] = m_ebitlen;
/*
* Input remaining bytes, using 31-bit words.
*/
acc = 0;
acc_len = 0;
while (k < len) {
uint32_t v;
v = buf[k ++];
if (acc_len >= 23) {
acc_len -= 23;
acc <<= (8 - acc_len);
acc |= v >> acc_len;
br_i31_muladd_small(x, acc, m);
acc = v & (0xFF >> (8 - acc_len));
} else {
acc = (acc << 8) | v;
acc_len += 8;
}
}
/*
* We may have some bits accumulated. We then perform a shift to
* be able to inject these bits as a full 31-bit word.
*/
if (acc_len != 0) {
acc = (acc | (x[1] << acc_len)) & 0x7FFFFFFF;
br_i31_rshift(x, 31 - acc_len);
br_i31_muladd_small(x, acc, m);
}
}
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