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Mark Harris
Opus
Commits
9c3e22c4
Commit
9c3e22c4
authored
Dec 07, 2007
by
JeanMarc Valin
Browse files
Moving to nonmultiplyfree entropy coder
parent
fc43dbb7
Changes
3
Hide whitespace changes
Inline
Sidebyside
libentcode/Makefile.am
View file @
9c3e22c4
...
...
@@ 2,7 +2,7 @@ INCLUDES =
METASOURCES
=
AUTO
lib_LTLIBRARIES
=
libentcode.la
libentcode_la_SOURCES
=
bitrdec.c bitree.c bitrenc.c ecintrin.h entcode.c
\
entdec.c entenc.c laplace.c
mf
rngdec.c
mf
rngenc.c probdec.c probenc.c probmod.c
entdec.c entenc.c laplace.c r
a
ng
e
dec.c r
a
nge
e
nc.c probdec.c probenc.c probmod.c
bin_PROGRAMS
=
ectest
ectest_SOURCES
=
ectest.c
ectest_LDADD
=
$(top_builddir)
/libentcode/libentcode.la
...
...
libentcode/rangedec.c
0 → 100644
View file @
9c3e22c4
#include <stddef.h>
#include "entdec.h"
#include "mfrngcod.h"
/*A multiplyfree range decoder.
This is an entropy decoder based upon \cite{Mar79}, which is itself a
rediscovery of the FIFO arithmetic code introduced by \cite{Pas76}.
It is very similar to arithmetic encoding, except that encoding is done with
digits in any base, instead of with bits, and so it is faster when using
larger bases (i.e.: a byte).
The author claims an average waste of $\frac{1}{2}\log_b(2b)$ bits, where $b$
is the base, longer than the theoretical optimum, but to my knowledge there
is no published justification for this claim.
This only seems true when using nearinfinite precision arithmetic so that
the process is carried out with no rounding errors.
IBM (the author's employer) never sought to patent the idea, and to my
knowledge the algorithm is unencumbered by any patents, though its
performance is very competitive with proprietary arithmetic coding.
The two are based on very similar ideas, however.
An excellent description of implementation details is available at
http://www.arturocampos.com/ac_range.html
A recent work \cite{MNW98} which proposes several changes to arithmetic
encoding for efficiency actually rediscovers many of the principles
behind range encoding, and presents a good theoretical analysis of them.
The coder is made multiplyfree by replacing the standard multiply/divide
used to partition the current interval according to the total frequency
count.
The new partition function scales the count so that it differs from the size
of the interval by no more than a factor of two and then assigns each symbol
one or two code words in the interval.
For details see \cite{SM98}.
This coder also handles the end of the stream in a slightly more graceful
fashion than most arithmetic or range coders.
Once the final symbol has been encoded, the coder selects the code word with
the shortest number of bits that still falls within the final interval.
This method is not novel.
Here, by the length of the code word, we refer to the number of bits until
its final 1.
Any trailing zeros may be discarded, since the encoder, once it runs out of
input, will pad its buffer with zeros.
But this means that no encoded stream would ever have any zero bytes at the
end.
Since there are some coded representations we cannot produce, it implies that
there is still some redundancy in the stream.
In this case, we can pick a special byte value, RSV1, and should the stream
end in a sequence of zeros, followed by the RSV1 byte, we can code the
zeros, and discard the RSV1 byte.
The decoder, knowing that the encoder would never produce a sequence of zeros
at the end, would then know to add in the RSV1 byte if it observed it.
Now, the encoder would never produce a stream that ended in a sequence of
zeros followed by a RSV1 byte.
So, if the stream ends in a nonempty sequence of zeros, followed by any
positive number of RSV1 bytes, the last RSV1 byte is discarded.
The decoder, if it encounters a stream that ends in nonempty sequence of
zeros followed by any nonnegative number of RSV1 bytes, adds an additional
RSV1 byte to the stream.
With this strategy, every possible sequence of input bytes is transformed to
one that could actually be produced by the encoder.
The only question is what nonzero value to use for RSV1.
We select 0x80, since it has the nice property of producing the shortest
possible byte streams when using our strategy for selecting a number within
the final interval to encode.
Clearly if the shortest possible code word that falls within the interval has
its last one bit as the most significant bit of the final byte, and the
previous bytes were a nonempty sequence of zeros followed by a nonnegative
number of 0x80 bytes, then the last byte would be discarded.
If the shortest code word is not so formed, then no other code word in the
interval would result in any more bytes being discarded.
Any longer code word would have an additional one bit somewhere, and so would
require at a minimum that that byte would be coded.
If the shortest code word has a 1 before the final one that is preventing the
stream from ending in a nonempty sequence of zeros followed by a
nonnegative number of 0x80's, then there is no code word of the same length
which contains that bit as a zero.
If there were, then we could simply leave that bit a 1, and drop all the bits
after it without leaving the interval, thus producing a shorter code word.
In this case, RSV1 can only drop 1 bit off the final stream.
Other choices could lead to savings of up to 8 bits for particular streams,
but this would produce the odd situation that a stream with more nonzero
bits is actually encoded in fewer bytes.
@PHDTHESIS{Pas76,
author="Richard Clark Pasco",
title="Sorce coding algorithms for fast data compression",
school="Dept. of Electrical Engineering, Stanford University",
address="Stanford, CA",
month=May,
year=1976
}
@INPROCEEDINGS{Mar79,
author="Martin, G.N.N.",
title="Range encoding: an algorithm for removing redundancy from a digitised
message",
booktitle="Video & Data Recording Conference",
year=1979,
address="Southampton",
month=Jul
}
@ARTICLE{MNW98,
author="Alistair Moffat and Radford Neal and Ian H. Witten",
title="Arithmetic Coding Revisited",
journal="{ACM} Transactions on Information Systems",
year=1998,
volume=16,
number=3,
pages="256294",
month=Jul,
URL="http://dev.acm.org/pubs/citations/journals/tois/1998163/p256moffat/"
}
@INPROCEEDINGS{SM98,
author="Lang Stuiver and Alistair Moffat",
title="Piecewise Integer Mapping for Arithmetic Coding",
booktitle="Proceedings of the {IEEE} Data Compression Conference",
pages="110",
address="Snowbird, UT",
month="Mar./Apr.",
year=1998
}*/
/*Gets the next byte of input.
After all the bytes in the current packet have been consumed, and the extra
end code returned if needed, this function will continue to return zero each
time it is called.
Return: The next byte of input.*/
static
int
ec_dec_in
(
ec_dec
*
_this
){
int
ret
;
ret
=
ec_byte_read1
(
_this
>
buf
);
if
(
ret
<
0
){
unsigned
char
*
buf
;
long
bytes
;
bytes
=
ec_byte_bytes
(
_this
>
buf
);
buf
=
ec_byte_get_buffer
(
_this
>
buf
);
/*Breaking abstraction: don't do this at home, kids.*/
if
(
_this
>
buf
>
storage
==
bytes
){
ec_byte_adv1
(
_this
>
buf
);
if
(
bytes
>
0
){
unsigned
char
*
p
;
p
=
buf
+
bytes
;
/*If we end in a string of 0 or more EC_FOF_RSV1 bytes preceded by a
zero, return an extra EC_FOF_RSV1 byte.*/
do
p

;
while
(
p
>
buf
&&
p
[
0
]
==
EC_FOF_RSV1
);
if
(
!
p
[
0
])
return
EC_FOF_RSV1
;
}
}
return
0
;
}
else
return
ret
;
}
/*Normalizes the contents of low and rng so that rng is contained in the
highorder symbol of low.*/
static
void
ec_dec_normalize
(
ec_dec
*
_this
){
/*If the range is too small, rescale it and input some bits.*/
while
(
_this
>
rng
<=
EC_CODE_BOT
){
int
sym
;
_this
>
rng
<<=
EC_SYM_BITS
;
/*Use up the remaining bits from our last symbol.*/
sym
=
_this
>
rem
<<
EC_CODE_EXTRA
&
EC_SYM_MAX
;
/*Read the next value from the input.*/
_this
>
rem
=
ec_dec_in
(
_this
);
/*Take the rest of the bits we need from this new symbol.*/
sym
=
_this
>
rem
>>
EC_SYM_BITS

EC_CODE_EXTRA
;
_this
>
dif
=
(
_this
>
dif
<<
EC_SYM_BITS
)

sym
&
EC_CODE_MASK
;
/*dif can never be larger than EC_CODE_TOP.
This is equivalent to the slightly more readable:
if(_this>dif>EC_CODE_TOP)_this>dif=EC_CODE_TOP;*/
_this
>
dif
^=
(
_this
>
dif
&
_this
>
dif

1
)
&
EC_CODE_TOP
;
}
}
void
ec_dec_init
(
ec_dec
*
_this
,
ec_byte_buffer
*
_buf
){
_this
>
buf
=
_buf
;
_this
>
rem
=
ec_dec_in
(
_this
);
_this
>
rng
=
1U
<<
EC_CODE_EXTRA
;
_this
>
dif
=
_this
>
rng

(
_this
>
rem
>>
EC_SYM_BITS

EC_CODE_EXTRA
);
/*Normalize the interval.*/
ec_dec_normalize
(
_this
);
}
unsigned
ec_decode
(
ec_dec
*
_this
,
unsigned
_ft
){
unsigned
s
;
_this
>
nrm
=
_this
>
rng
/
_ft
;
s
=
(
unsigned
)((
_this
>
dif

1
)
/
_this
>
nrm
);
return
_ft

EC_MINI
(
s
+
1
,
_ft
);
}
void
ec_dec_update
(
ec_dec
*
_this
,
unsigned
_fl
,
unsigned
_fh
,
unsigned
_ft
){
ec_uint32
s
;
s
=
_this
>
nrm
*
(
_ft

_fh
);
_this
>
dif
=
s
;
_this
>
rng
=
_fl
>
0
?
_this
>
nrm
*
(
_fh

_fl
)
:
_this
>
rng

s
;
ec_dec_normalize
(
_this
);
}
#if 0
int ec_dec_done(ec_dec *_this){
unsigned low;
int ret;
/*Check to make sure we've used all the input bytes.
This ensures that no more ones would ever be inserted into the decoder.*/
if(_this>buf>ptrec_byte_get_buffer(_this>buf)<=
ec_byte_bytes(_this>buf)){
return 0;
}
/*We compute the smallest finitely odd fraction that fits inside the current
range, and write that to the stream.
This is guaranteed to yield the smallest possible encoding.*/
/*TODO: Fix this line, as it is wrong.
It doesn't seem worth being able to make this check to do an extra
subtraction for every symbol decoded.*/
low=/*What we want: _this>top_this>rng; What we have:*/_this>dif
if(low){
unsigned end;
end=EC_CODE_TOP;
/*Ensure that the next free end is in the range.*/
if(endlow>=_this>rng){
unsigned msk;
msk=EC_CODE_TOP1;
do{
msk>>=1;
end=(low+msk)&~mskmsk+1;
}
while(endlow>=_this>rng);
}
/*The remaining input should have been the next free end.*/
return endlow!=_this>dif;
}
return 1;
}
#endif
libentcode/rangeenc.c
0 → 100644
View file @
9c3e22c4
#include <stddef.h>
#include "entenc.h"
#include "mfrngcod.h"
/*A multiplyfree range encoder.
See mfrngdec.c and the references for implementation details
\cite{Mar79,MNW98,SM98}.
@INPROCEEDINGS{Mar79,
author="Martin, G.N.N.",
title="Range encoding: an algorithm for removing redundancy from a digitised
message",
booktitle="Video \& Data Recording Conference",
year=1979,
address="Southampton",
month=Jul
}
@ARTICLE{MNW98,
author="Alistair Moffat and Radford Neal and Ian H. Witten",
title="Arithmetic Coding Revisited",
journal="{ACM} Transactions on Information Systems",
year=1998,
volume=16,
number=3,
pages="256294",
month=Jul,
URL="http://dev.acm.org/pubs/citations/journals/tois/1998163/p256moffat/"
}
@INPROCEEDINGS{SM98,
author="Lang Stuiver and Alistair Moffat",
title="Piecewise Integer Mapping for Arithmetic Coding",
booktitle="Proceedings of the {IEEE} Data Compression Conference",
pages="110",
address="Snowbird, UT",
month="Mar./Apr.",
year=1998
}*/
/*Outputs a symbol, with a carry bit.
If there is a potential to propogate a carry over several symbols, they are
buffered until it can be determined whether or not an actual carry will
occur.
If the counter for the buffered symbols overflows, then the range is
truncated to force a carry to occur, towards whichever side maximizes the
remaining range.*/
static
void
ec_enc_carry_out
(
ec_enc
*
_this
,
int
_c
){
if
(
_c
!=
EC_SYM_MAX
){
/*No further carry propogation possible, flush buffer.*/
int
carry
;
carry
=
_c
>>
EC_SYM_BITS
;
/*Don't output a byte on the first write.
This compare should be taken care of by branchprediction thereafter.*/
if
(
_this
>
rem
>=
0
)
ec_byte_write1
(
_this
>
buf
,
_this
>
rem
+
carry
);
if
(
_this
>
ext
>
0
){
unsigned
sym
;
sym
=
EC_SYM_MAX
+
carry
&
EC_SYM_MAX
;
do
ec_byte_write1
(
_this
>
buf
,
sym
);
while
(

(
_this
>
ext
)
>
0
);
}
_this
>
rem
=
_c
&
EC_SYM_MAX
;
}
else
_this
>
ext
++
;
}
static
void
ec_enc_normalize
(
ec_enc
*
_this
){
/*If the range is too small, output some bits and rescale it.*/
while
(
_this
>
rng
<=
EC_CODE_BOT
){
ec_enc_carry_out
(
_this
,(
int
)(
_this
>
low
>>
EC_CODE_SHIFT
));
/*Move the nexttohighorder symbol into the highorder position.*/
_this
>
low
=
_this
>
low
<<
EC_SYM_BITS
&
EC_CODE_TOP

1
;
_this
>
rng
<<=
EC_SYM_BITS
;
}
}
void
ec_enc_init
(
ec_enc
*
_this
,
ec_byte_buffer
*
_buf
){
_this
>
buf
=
_buf
;
_this
>
rem
=
1
;
_this
>
ext
=
0
;
_this
>
low
=
0
;
_this
>
rng
=
EC_CODE_TOP
;
}
void
ec_encode
(
ec_enc
*
_this
,
unsigned
_fl
,
unsigned
_fh
,
unsigned
_ft
){
unsigned
r
;
unsigned
s
;
r
=
_this
>
rng
/
_ft
;
if
(
_fl
>
0
){
s
=
r
*
(
_ft

_fl
);
_this
>
low
+=
_this
>
rng

s
;
_this
>
rng
=
r
*
(
_fh

_fl
);
}
else
_this
>
rng
=
r
*
(
_ft

_fh
);
ec_enc_normalize
(
_this
);
}
void
ec_enc_done
(
ec_enc
*
_this
){
/*We compute the integer in the current interval that has the largest number
of trailing zeros, and write that to the stream.
This is guaranteed to yield the smallest possible encoding.*/
if
(
_this
>
low
){
unsigned
end
;
end
=
EC_CODE_TOP
;
/*Ensure that the end value is in the range.*/
if
(
end

_this
>
low
>=
_this
>
rng
){
unsigned
msk
;
msk
=
EC_CODE_TOP

1
;
do
{
msk
>>=
1
;
end
=
(
_this
>
low
+
msk
)
&~
msk

msk
+
1
;
}
while
(
end

_this
>
low
>=
_this
>
rng
);
}
/*The remaining output is the next free end.*/
while
(
end
){
ec_enc_carry_out
(
_this
,
end
>>
EC_CODE_SHIFT
);
end
=
end
<<
EC_SYM_BITS
&
EC_CODE_TOP

1
;
}
}
/*If we have a buffered byte...*/
if
(
_this
>
rem
>=
0
){
unsigned
char
*
p
;
unsigned
char
*
buf
;
/*Flush it into the output buffer.*/
ec_enc_carry_out
(
_this
,
0
);
/*We may be able to drop some redundant bytes from the end.*/
buf
=
ec_byte_get_buffer
(
_this
>
buf
);
p
=
buf
+
ec_byte_bytes
(
_this
>
buf
)

1
;
/*Strip trailing zeros.*/
while
(
p
>=
buf
&&!
p
[
0
])
p

;
/*Strip one trailing EC_FOF_RSV1 byte if the buffer ends in a string of
consecutive EC_FOF_RSV1 bytes preceded by one (or more) zeros.*/
if
(
p
>
buf
&&
p
[
0
]
==
EC_FOF_RSV1
){
unsigned
char
*
q
;
q
=
p
;
do
q

;
while
(
q
>
buf
&&
q
[
0
]
==
EC_FOF_RSV1
);
if
(
!
q
[
0
])
p

;
}
ec_byte_writetrunc
(
_this
>
buf
,
p
+
1

buf
);
}
}
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