/*
* Copyright (c) 2016 Thomas Pornin
*
* 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 */
uint32_t
br_i31_decode_mod(uint32_t *x, const void *src, size_t len, const uint32_t *m)
{
/*
* Two-pass algorithm: in the first pass, we determine whether the
* value fits; in the second pass, we do the actual write.
*
* During the first pass, 'r' contains the comparison result so
* far:
* 0x00000000 value is equal to the modulus
* 0x00000001 value is greater than the modulus
* 0xFFFFFFFF value is lower than the modulus
*
* Since we iterate starting with the least significant bytes (at
* the end of src[]), each new comparison overrides the previous
* except when the comparison yields 0 (equal).
*
* During the second pass, 'r' is either 0xFFFFFFFF (value fits)
* or 0x00000000 (value does not fit).
*
* We must iterate over all bytes of the source, _and_ possibly
* some extra virutal bytes (with value 0) so as to cover the
* complete modulus as well. We also add 4 such extra bytes beyond
* the modulus length because it then guarantees that no accumulated
* partial word remains to be processed.
*/
const unsigned char *buf;
size_t mlen, tlen;
int pass;
uint32_t r;
buf = src;
mlen = (m[0] + 31) >> 5;
tlen = (mlen << 2);
if (tlen < len) {
tlen = len;
}
tlen += 4;
r = 0;
for (pass = 0; pass < 2; pass ++) {
size_t u, v;
uint32_t acc;
int acc_len;
v = 1;
acc = 0;
acc_len = 0;
for (u = 0; u < tlen; u ++) {
uint32_t b;
if (u < len) {
b = buf[len - 1 - u];
} else {
b = 0;
}
acc |= (b << acc_len);
acc_len += 8;
if (acc_len >= 31) {
uint32_t xw;
xw = acc & (uint32_t)0x7FFFFFFF;
acc_len -= 31;
acc = b >> (8 - acc_len);
if (v <= mlen) {
if (pass) {
x[v] = r & xw;
} else {
uint32_t cc;
cc = (uint32_t)CMP(xw, m[v]);
r = MUX(EQ(cc, 0), r, cc);
}
} else {
if (!pass) {
r = MUX(EQ(xw, 0), r, 1);
}
}
v ++;
}
}
/*
* When we reach this point at the end of the first pass:
* r is either 0, 1 or -1; we want to set r to 0 if it
* is equal to 0 or 1, and leave it to -1 otherwise.
*
* When we reach this point at the end of the second pass:
* r is either 0 or -1; we want to leave that value
* untouched. This is a subcase of the previous.
*/
r >>= 1;
r |= (r << 1);
}
x[0] = m[0];
return r & (uint32_t)1;
}