91 lines
2.9 KiB
C
91 lines
2.9 KiB
C
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/* Copyright (c) 2001-2011 Timothy B. Terriberry
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*/
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/*
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Redistribution and use in source and binary forms, with or without
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modification, are permitted provided that the following conditions
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are met:
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- Redistributions of source code must retain the above copyright
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notice, this list of conditions and the following disclaimer.
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- Redistributions in binary form must reproduce the above copyright
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notice, this list of conditions and the following disclaimer in the
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documentation and/or other materials provided with the distribution.
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THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR
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CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
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EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
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PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
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PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
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LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
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NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
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SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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*/
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#ifdef HAVE_CONFIG_H
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#include "config.h"
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#endif
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#include "entcode.h"
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#if !defined(EC_CLZ)
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int ec_ilog(ec_uint32 _v){
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/*On a Pentium M, this branchless version tested as the fastest on
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1,000,000,000 random 32-bit integers, edging out a similar version with
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branches, and a 256-entry LUT version.*/
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int ret;
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int m;
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ret=!!_v;
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m=!!(_v&0xFFFF0000)<<4;
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_v>>=m;
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ret|=m;
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m=!!(_v&0xFF00)<<3;
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_v>>=m;
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ret|=m;
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m=!!(_v&0xF0)<<2;
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_v>>=m;
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ret|=m;
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m=!!(_v&0xC)<<1;
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_v>>=m;
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ret|=m;
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ret+=!!(_v&0x2);
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return ret;
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}
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#endif
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ec_uint32 ec_tell_frac(ec_ctx *_this){
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ec_uint32 nbits;
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ec_uint32 r;
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int l;
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int i;
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/*To handle the non-integral number of bits still left in the encoder/decoder
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state, we compute the worst-case number of bits of val that must be
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encoded to ensure that the value is inside the range for any possible
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subsequent bits.
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The computation here is independent of val itself (the decoder does not
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even track that value), even though the real number of bits used after
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ec_enc_done() may be 1 smaller if rng is a power of two and the
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corresponding trailing bits of val are all zeros.
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If we did try to track that special case, then coding a value with a
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probability of 1/(1<<n) might sometimes appear to use more than n bits.
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This may help explain the surprising result that a newly initialized
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encoder or decoder claims to have used 1 bit.*/
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nbits=_this->nbits_total<<BITRES;
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l=EC_ILOG(_this->rng);
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r=_this->rng>>l-16;
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for(i=BITRES;i-->0;){
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int b;
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r=r*r>>15;
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b=(int)(r>>16);
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l=l<<1|b;
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r>>=b;
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}
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return nbits-l;
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}
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