Version 5.0 The LEDA User Manual

9.1 Adaptive Arithmetic Coder ( A0Coder )
1. Definition
An instance C of A0Coder is an adaptive arithmetic coder of order zero (which explains
the “0” in the class name). When the coder encodes a stream it counts the frequencies
of the characters that have occured so far in order to obtain a model of the probabilities
for the future characters. The model is called order zero because it does not take into
account that the probability of a character c may depend on the characters that precede
it. (E.g., in an English text the probability that the next character is a “u” is very high if
the current character is a “q”. The PPMIICoder in Section 9.3 takes this into account.)
The advantage of arithmetic coding [92] is that a character in the source stream can be
encoded by a fractional number of bits in the target stream, which leads to good results
in practice.
The coder is called adaptive because the model evolves gradually while the coder scans
its input. In contrast to that a static coder reads the whole input once before it generates
a model. During the actual encoding (in a second scan) the model remains fixed. The
adaptive coder resets and/or scales its model when the number of characters read since
the last reset/scaling exceeds a certain threshold. Both thresholds can be controlled by
the user.
#include < LEDA/coding/arithmetic coding.h >
2. Creation
A0Coder C(streambuf ∗ src stream = 0, streambuf ∗ tgt stream = 0,
bool own streams = false);
creates an instance C which uses the given source and target
streams. If own streams is set, then C is responsible for the destruction
of the streams, otherwise the pointers src stream and tgt stream
must be valid during the life-time of C.
A0Coder
C(const char ∗ src file name, const char ∗ tgt file name);
creates an instance C which uses file-streams for input and output.
3. Operations
Standard Operations
void C.encode( ) encodes the source stream and writes the output to
the target stream.
void C.decode( ) decodes the source stream and writes the output to
the target stream.