DragonFly On-Line Manual Pages

ENTROPY(1)             DragonFly General Commands Manual            ENTROPY(1)

NAME
       entropy - calculate data entropy

SYNOPSIS
entropy  [ -sv ] [ -tstring ]

DESCRIPTION
       This manual describes the operating procedure for the entropy program.
       This program will calculate the specific entropy for a given chunk of
       binary or ascii data that was previously read from stdin or supplied on
       the command line via -t.

       The most common use for this program is to gauge the efficiency of a
       compression algorithm. In theory, based on the data's entropy, the
       maximum theoretical compression rate can be calculated.

       To understand how compression works, we must understand that all data
       can be characterized by informational content, called its entropy (the
       term is borrowed from Thermo-Dynamics).  Compression is possible
       because most data is represented with more bits then its entropy
       suggests is optimal.

OPTIONS
       -tstring
               Rather then read the data from the stdin input stream, use the
               string string as the source of the data. This is useful when
               you would like to calculate for a specific word or sentence.

       -s      Sorted output based on symbol frequencies

       -v      Print entropy version.

THEORY
       A data set's entropy is the sum of each distinct symbol's entropies.  A
       symbol is a distinct set of contiguous bits. For the sake of
       simplicity, I use 8 (number of bits making up a byte). It is important
       to note that a symbol does NOT have to consist of 8 bits.  A distinct
       symbols entropy `S' of symbol 'z' can be defined as:

                             Sz = -log2(Pz)

       Where `Pz' is the probability of symbol 'z' being found in the set.  By
       storing each distinct symbol as a node in a list, we can calculate
       exactly how many times the symbol occurs in the set. This is also known
       as solving for the symbol's frequency.

       In order to calculate the overall entropy of the data; we sum the total
       entropies contributed by each distinct symbol and divide that by a
       coefficient known as the data size. This coefficient is calculated by
       taking the number of bits per symbol and multiplying it by the total
       quantity of symbols.

DIAGNOSTICS
       The entropy utility exits 0 on success, and non-zero if an error
       occurs.

SEE ALSO
       gzip(1), bzip2(1), tar(1), minigzip(1), gzexe(1), kgzip(8)
       compress(1), zcmp(1), zdiff(1), zlib(3)

AUTHOR
        o Chris S.J. Peron

BUGS
       None known. This does not mean they do not exist though.  Please send
       bug reports and source code patches to (bugs@sqrt.ca).

entropy version 1.0.0             9 Aug 2002                        ENTROPY(1)