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