845dfa1986
This renames ec_dec_cdf() to ec_dec_icdf(), and changes the functionality to use an "inverse" CDF table, where icdf[i]=ft-cdf[i+1]. The first entry is omitted entirely. It also adds a corresonding ec_enc_icdf() to the encoder, which uses the same table. One could use ec_encode_bin() by converting the values in the tables back to normal CDF values, but the icdf[] table already has them in the form ec_encode_bin() wants to use them, so there's no reason to translate them and then translate them back. This is done primarily to allow SILK to use the range coder with 8-bit probability tables containing cumulative frequencies that span the full range 0...256. With an 8-bit table, the final 256 of a normal CDF becomes 0 in the "inverse" CDF. It's the 0 at the start of a normal CDF which would become 256, but this is the value we omit, as it already has to be special-cased in the encoder, and is not used at all in the decoder.
145 lines
6.3 KiB
C
145 lines
6.3 KiB
C
/* Copyright (c) 2001-2008 Timothy B. Terriberry
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Copyright (c) 2008-2009 Xiph.Org Foundation */
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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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- Neither the name of the Xiph.org Foundation nor the names of its
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contributors may be used to endorse or promote products derived from
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this software without specific prior written permission.
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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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#if !defined(_entdec_H)
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# define _entdec_H (1)
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# include <limits.h>
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# include "entcode.h"
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typedef struct ec_dec ec_dec;
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/*The entropy decoder.*/
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struct ec_dec{
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/*The buffer to decode.*/
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ec_byte_buffer *buf;
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/*The remainder of a buffered input symbol.*/
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int rem;
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/*The number of values in the current range.*/
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ec_uint32 rng;
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/*The difference between the top of the current range and the input value,
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minus one.*/
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ec_uint32 dif;
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/*Normalization factor.*/
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ec_uint32 nrm;
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/*Bits that were written at the end.*/
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ec_window end_window;
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/*Number of valid bits in end_window.*/
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int nend_bits;
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/*The total number of whole bits read.*/
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int nbits_total;
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/*Nonzero if an error occurred.*/
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int error;
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};
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/*Initializes the decoder.
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_buf: The input buffer to use.
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Return: 0 on success, or a negative value on error.*/
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void ec_dec_init(ec_dec *_this,ec_byte_buffer *_buf);
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/*Calculates the cumulative frequency for the next symbol.
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This can then be fed into the probability model to determine what that
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symbol is, and the additional frequency information required to advance to
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the next symbol.
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This function cannot be called more than once without a corresponding call to
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ec_dec_update(), or decoding will not proceed correctly.
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_ft: The total frequency of the symbols in the alphabet the next symbol was
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encoded with.
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Return: A cumulative frequency representing the encoded symbol.
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If the cumulative frequency of all the symbols before the one that
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was encoded was fl, and the cumulative frequency of all the symbols
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up to and including the one encoded is fh, then the returned value
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will fall in the range [fl,fh).*/
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unsigned ec_decode(ec_dec *_this,unsigned _ft);
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unsigned ec_decode_bin(ec_dec *_this,unsigned _bits);
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/*Advance the decoder past the next symbol using the frequency information the
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symbol was encoded with.
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Exactly one call to ec_decode() must have been made so that all necessary
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intermediate calculations are performed.
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_fl: The cumulative frequency of all symbols that come before the symbol
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decoded.
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_fh: The cumulative frequency of all symbols up to and including the symbol
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decoded.
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Together with _fl, this defines the range [_fl,_fh) in which the value
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returned above must fall.
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_ft: The total frequency of the symbols in the alphabet the symbol decoded
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was encoded in.
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This must be the same as passed to the preceding call to ec_decode().*/
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void ec_dec_update(ec_dec *_this,unsigned _fl,unsigned _fh,
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unsigned _ft);
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/*Extracts a sequence of raw bits from the stream.
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The bits must have been encoded with ec_enc_bits().
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No call to ec_dec_update() is necessary after this call.
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_ftb: The number of bits to extract.
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This must be between 0 and 25, inclusive.
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Return: The decoded bits.*/
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ec_uint32 ec_dec_bits(ec_dec *_this,unsigned _ftb);
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/*Extracts a raw unsigned integer with a non-power-of-2 range from the stream.
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The bits must have been encoded with ec_enc_uint().
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No call to ec_dec_update() is necessary after this call.
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_ft: The number of integers that can be decoded (one more than the max).
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This must be at least one, and no more than 2**32-1.
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Return: The decoded bits.*/
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ec_uint32 ec_dec_uint(ec_dec *_this,ec_uint32 _ft);
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/*Decodes a symbol given an "inverse" CDF table.
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No call to ec_dec_update() is necessary after this call.
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_icdf: The "inverse" CDF, such that symbol s falls in the range
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[s>0?ft-_icdf[s-1]:0,ft-_icdf[s]), where ft=1<<_ftb.
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The values must be monotonically non-increasing, and the last value
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must be 0.
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_ftb: The number of bits of precision in the cumulative distribution.
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Return: The decoded symbol s.*/
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int ec_dec_icdf(ec_dec *_this,const unsigned char *_icdf,unsigned _ftb);
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/* Decode a bit that has a _prob/65536 probability of being a one */
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int ec_dec_bit_prob(ec_dec *_this,unsigned _prob);
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/* Decode a bit that has a 1/(1<<_logp) probability of being a one */
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int ec_dec_bit_logp(ec_dec *_this,unsigned _logp);
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/*Returns the number of bits "used" by the encoded symbols so far.
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This same number can be computed by the encoder, and is suitable for making
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coding decisions.
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_b: The number of extra bits of precision to include.
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At most 16 will be accurate.
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Return: The number of bits scaled by 2**_b.
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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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ec_uint32 ec_dec_tell(ec_dec *_this,int _b);
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/*Return: A nonzero value if any error has been detected during decoding.*/
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int ec_dec_get_error(ec_dec *_this);
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#endif
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