Version 5.0 The LEDA User Manual

9.11 Move-To-Front Coder II ( MTF2Coder )
1. Definition
This is a variant of the Move-To-Front coder from the previous section. It was proposed
by Balkenhol and Shtarkov [7] and sometimes yields better compression than MTFCoder
(when used with BWTCoder as preprocessor and A0Coder as postprocessor). The idea
is not to exchange the symbol at the head of MTF list L whenever a new symbol is
encountered.
The details are as follows: Let us call the position of a symbol c in L the rank of c where
the rank of the head symbol is 0. Whenever a symbol of rank greater than 1 appears we
move it to position 1, i.e. behind the head. When a symbol of rank 1 is encountered it
is moved to position 0 only if the previous symbol was not the head of L. Otherwise it
remains at position 1.
#include < LEDA/coding/MTF2.h >
2. Creation
MTF2Coder 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.
MTF2Coder C(const char ∗ src file name, const char ∗ tgt file name);
3. Operations
Standard Operations
creates an instance C which uses file-streams for input and output.
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.
uint32 C.encode memory chunk(const char ∗ in buf , uint32 in len, char ∗ out buf ,
uint32 out len)
encodes the memory chunk starting at in buf with
size in len into the buffer starting at out buf with size
out len. The function returns actual length of the
encoded chunk which may be smaller than out len. If
the output buffer is too small for the encoded data
the failure flag will be set (see below).