ref: d710177d0a10703443162449c9996494dc353b42
parent: 9d35ccdaea23f2b46411eeff815d99e6c4679dc5
author: Timothy B. Terriberry <[email protected]>
date: Tue Dec 11 08:25:00 EST 2007
Documentation fixes and a couple of other minor edits.
--- a/libentcode/entdec.h
+++ b/libentcode/entdec.h
@@ -49,6 +49,7 @@
_fl: The cumulative frequency of all symbols that come before the symbol
decoded.
_fh: The cumulative frequency of all symbols up to and including the symbol
+ decoded.
Together with _fl, this defines the range [_fl,_fh) in which the value
returned above must fall.
_ft: The total frequency of the symbols in the alphabet the symbol decoded
--- a/libentcode/entenc.h
+++ b/libentcode/entenc.h
@@ -15,7 +15,7 @@
ec_byte_buffer *buf;
/*A buffered output symbol, awaiting carry propagation.*/
int rem;
- /*Number of extra carry propogating symbols.*/
+ /*Number of extra carry propagating symbols.*/
size_t ext;
/*The number of values in the current range.*/
ec_uint32 rng;
@@ -34,10 +34,14 @@
It is allowable to change the frequency information, or even the entire
source alphabet, so long as the decoder can tell from the context of the
previously encoded information that it is supposed to do so as well.
- _fl: The sum of the frequencies of symbols before the one to be encoded.
- _fs: The frequency of the symbol to be encoded.
+ _fl: The cumulative frequency of all symbols that come before the one to be
+ encoded.
+ _fh: The cumulative frequency of all symbols up to and including the one to
+ be encoded.
+ Together with _fl, this defines the range [_fl,_fh) in which the
+ decoded value will fall.
_ft: The sum of the frequencies of all the symbols*/
-void ec_encode(ec_enc *_this,unsigned _fl,unsigned _fs,unsigned _ft);
+void ec_encode(ec_enc *_this,unsigned _fl,unsigned _fh,unsigned _ft);
/*Encodes a sequence of raw bits in the stream.
_fl: The bits to encode.
_ftb: The number of bits to encode.
--- a/libentcode/mfrngcod.h
+++ b/libentcode/mfrngcod.h
@@ -16,7 +16,7 @@
# define EC_CODE_TOP (1U<<EC_CODE_BITS-1)
/*Low-order bit of the high-order range symbol.*/
# define EC_CODE_BOT (EC_CODE_TOP>>EC_SYM_BITS)
-/*Code for which propogating carries are possible.*/
+/*Code for which propagating carries are possible.*/
# define EC_CODE_CARRY (EC_SYM_MAX<<EC_CODE_SHIFT)
/*The number of bits available for the last, partial symbol in the code field.*/
# define EC_CODE_EXTRA ((EC_CODE_BITS-2)%EC_SYM_BITS+1)
--- a/libentcode/mfrngdec.c
+++ b/libentcode/mfrngdec.c
@@ -90,7 +90,7 @@
@PHDTHESIS{Pas76,author="Richard Clark Pasco",
- title="Sorce coding algorithms for fast data compression",
+ title="Source coding algorithms for fast data compression",
school="Dept. of Electrical Engineering, Stanford University",
address="Stanford, CA",
month=May,
@@ -159,8 +159,8 @@
else return ret;
}
-/*Normalizes the contents of low and rng so that rng is contained in the
- high-order symbol of low.*/
+/*Normalizes the contents of dif and rng so that rng lies entirely in the
+ high-order symbol.*/
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){@@ -176,7 +176,7 @@
/*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;
+ _this->dif^=_this->dif&_this->dif-1&EC_CODE_TOP;
}
}
@@ -248,7 +248,7 @@
msk=EC_CODE_TOP-1;
do{msk>>=1;
- end=(low+msk)&~msk|msk+1;
+ end=low+msk&~msk|msk+1;
}
while(end-low>=_this->rng);
}
--- a/libentcode/mfrngenc.c
+++ b/libentcode/mfrngenc.c
@@ -41,15 +41,18 @@
/*Outputs a symbol, with a carry bit.
- If there is a potential to propogate a carry over several symbols, they are
+ If there is a potential to propagate 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.*/
+ If the counter for the buffered symbols overflows, then the stream becomes
+ undecodable.
+ This gives a theoretical limit of a few billion symbols in a single packet on
+ 32-bit systems.
+ The alternative is to truncate the range in order to force a carry, but
+ requires similar carry tracking in the decoder, needlessly slowing it down.*/
static void ec_enc_carry_out(ec_enc *_this,int _c){ if(_c!=EC_SYM_MAX){- /*No further carry propogation possible, flush buffer.*/
+ /*No further carry propagation possible, flush buffer.*/
int carry;
carry=_c>>EC_SYM_BITS;
/*Don't output a byte on the first write.
@@ -129,7 +132,7 @@
msk=EC_CODE_TOP-1;
do{msk>>=1;
- end=(_this->low+msk)&~msk|msk+1;
+ end=_this->low+msk&~msk|msk+1;
}
while(end-_this->low>=_this->rng);
}
--- a/libentcode/rangedec.c
+++ b/libentcode/rangedec.c
@@ -90,7 +90,7 @@
@PHDTHESIS{Pas76,author="Richard Clark Pasco",
- title="Sorce coding algorithms for fast data compression",
+ title="Source coding algorithms for fast data compression",
school="Dept. of Electrical Engineering, Stanford University",
address="Stanford, CA",
month=May,
@@ -159,8 +159,8 @@
else return ret;
}
-/*Normalizes the contents of low and rng so that rng is contained in the
- high-order symbol of low.*/
+/*Normalizes the contents of dif and rng so that rng lies entirely in the
+ high-order symbol.*/
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){@@ -176,7 +176,7 @@
/*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;
+ _this->dif^=_this->dif&_this->dif-1&EC_CODE_TOP;
}
}
--- a/libentcode/rangeenc.c
+++ b/libentcode/rangeenc.c
@@ -41,15 +41,18 @@
/*Outputs a symbol, with a carry bit.
- If there is a potential to propogate a carry over several symbols, they are
+ If there is a potential to propagate 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.*/
+ If the counter for the buffered symbols overflows, then the stream becomes
+ undecodable.
+ This gives a theoretical limit of a few billion symbols in a single packet on
+ 32-bit systems.
+ The alternative is to truncate the range in order to force a carry, but
+ requires similar carry tracking in the decoder, needlessly slowing it down.*/
static void ec_enc_carry_out(ec_enc *_this,int _c){ if(_c!=EC_SYM_MAX){- /*No further carry propogation possible, flush buffer.*/
+ /*No further carry propagation possible, flush buffer.*/
int carry;
carry=_c>>EC_SYM_BITS;
/*Don't output a byte on the first write.
@@ -110,7 +113,7 @@
msk=EC_CODE_TOP-1;
do{msk>>=1;
- end=(_this->low+msk)&~msk|msk+1;
+ end=_this->low+msk&~msk|msk+1;
}
while(end-_this->low>=_this->rng);
}