Version 5.0 The LEDA User Manual

9.25 Block Coder ( BlockCoder )
1. Definition
An instance of type BlockCoder encodes a source stream as follows: It divides
the stream into equally sized blocks and encodes each block seperately with an instance
of type Coder. If you later want to decode only parts of the encoded data then you do
not have to decode the entire data but only the appropriate blocks, which is usually much
faster. (The block size can be specified by the user; please note that the main memory of
your computer has to be large enough to hold one block.)
The class BlockCoder has been designed to speed up the seek operations of the class
decoding ifstream (see Section 9.22). In order to gain the speed up simply replace
the template parameter Coder in decoding ifstream by BlockCoder (see also the
example below).
There is an important precondition for BlockCoder: In encoding mode the target stream
must support seek operations which query the current position (i.e. seek(0, ios :: cur,
ios :: out)). In decoding mode the source stream must be capable of random seek operations.
These conditions are surely fullfilled if the respective stream is a file. Moreover, if
BlockCoder is used within a coder pipe the pipe will make sure that it operates properly.
However, this configuration only makes sense if all coders in the pipe optimize seek
operations. Then the whole pipe will offer fast seek operations.
#include < LEDA/coding/block coder.h >
2. Creation
BlockCoder C(streambuf ∗ src stream = 0, streambuf ∗ tgt stream = 0,
bool own streams = false,
uint32 block size = DefaultBlockSize);
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. The initial block size is
block size.
BlockCoder C(const char ∗ src file name, const char ∗ tgt file name,
uint32 block size = DefaultBlockSize);
creates an instance C which uses file-streams for input and output.
The block size is set to block size.