54 строки
2.3 KiB
C
54 строки
2.3 KiB
C
/*
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* Copyright (c) 2001-2016, Alliance for Open Media. All rights reserved
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*
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* This source code is subject to the terms of the BSD 2 Clause License and
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* the Alliance for Open Media Patent License 1.0. If the BSD 2 Clause License
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* was not distributed with this source code in the LICENSE file, you can
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* obtain it at www.aomedia.org/license/software. If the Alliance for Open
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* Media Patent License 1.0 was not distributed with this source code in the
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* PATENTS file, you can obtain it at www.aomedia.org/license/patent.
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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 "aom_dsp/entcode.h"
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/*Given the current total integer number of bits used and the current value of
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rng, computes the fraction number of bits used to OD_BITRES precision.
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This is used by od_ec_enc_tell_frac() and od_ec_dec_tell_frac().
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nbits_total: The number of whole bits currently used, i.e., the value
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returned by od_ec_enc_tell() or od_ec_dec_tell().
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rng: The current value of rng from either the encoder or decoder state.
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Return: The number of bits scaled by 2**OD_BITRES.
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This will always be slightly larger than the exact value (e.g., all
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rounding error is in the positive direction).*/
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uint32_t od_ec_tell_frac(uint32_t nbits_total, uint32_t rng) {
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uint32_t nbits;
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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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od_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 = nbits_total << OD_BITRES;
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l = 0;
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for (i = OD_BITRES; i-- > 0;) {
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int b;
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rng = rng * rng >> 15;
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b = (int)(rng >> 16);
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l = l << 1 | b;
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rng >>= b;
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}
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return nbits - l;
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}
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