cluster.tex

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% Cluster - Gareth Moore        23/01/02 (updated 27/03/02)
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\newpage
\mysect{Cluster}{Cluster}

\mysubsect{Function}{Cluster-Function}

\index{cluster@\htool{Cluster}|(}
This program is used to statistically cluster words into deterministic
classes.  The main purpose of \htool{Cluster} is to optimise a class
map on the basis of the training text likelihood, although it can also
import an existing class map and generate one of the files necessary
for creating a class-based language model from the \HTK\ language
modelling tools.

Class-based language models use a reduced number of classes relative
to the number of words, with each class containing one or more words,
to allow a language model to be able to generalise to unseen training
contexts.  Class-based models also typically require less training
text to produce a well-trained model than a similar complexity word
model, and are often more compact due to the much reduced number of
possible distinct history contexts that can be encountered in the
training data.

\htool{Cluster} takes as input a set of one or more training text gram
files, which may optionally be weighted on input, and their associated
word map.  It then clusters the words in the word map into classes
using a bigram likelihood measure.  Due to the computational
complexity of this task a sub-optimal greedy algorithm is used, but
multiple iterations of this algorithm may be performed in order to
further refine the class map, although at some point a local maximum
will be reached where the class map will not change
further.\footnote{On a 65,000 word vocabulary test set with 170 million
words of training text this was found to occur after around 45
iterations}  In practice as few as two iterations may be perfectly
adequate, even with large training data sets.

The algorithm works by considering each word in the vocabulary in turn
and calculating the change in bigram training text likelihood if the
word was moved from its default class (see below) to each other class
in turn.  The word is then moved to the class which increases the
likelihood the most, or it is left in its current class if no such
increase is found.  Each iteration of the algorithm considers each
word exactly once.  Because this can be a slow process, with typical
execution times measured in terms of a few hours, not a few minutes,
the \htool{Cluster} tool also allows \textit{recovery} files to be written
at regular intervals, which contain the current class map part-way
through an iteration along with associated files detailing at what
point in the iteration the class map was exported.  These files are
not essential for operation, but might be desirable if there is a risk
of a long-running process being killed via some external influence.
During the execution of an iteration the tool claims no new
memory,\footnote{other than a few small local variables taken from the
stack as functions are called} so it cannot crash
in the middle of an iteration due to a lack of memory (it can,
however, fail to start an iteration in the first place).

Before beginning an iteration, \htool{Cluster} places each word either
into a default class or one specified via the \texttt{-l}, import
classmap, or \texttt{-x}, use recovery, options.  The default
distribution, given $m$ classes, is to place the most frequent $(m-1)$
words into singleton classes and then the remainder into the remaining
class.  \htool{Cluster} allows words to be considered in either
decreasing frequency of occurrence order, or the order they are
encountered in the word map.  The popular choice is to use the former
method, although in experiments it was found that the more random
second approach typically gave better class maps after fewer
iterations in practice.\footnote{Note that these schemes are
approximately similar, since the most frequent words are most likely
to be encountered sooner in the training text and thus occur higher up
in the word map} The \texttt{-w} option specifies this choice.

During execution \htool{Cluster} will always write a logfile
describing the changes it makes to the classmap, unless you explicitly
disable this using the \texttt{-n} option.  If the \texttt{-v} switch
is used then this logfile is written in explicit English, allowing you
to easily trace the execution of the clusterer; without \texttt{-v}
then similar information is exported in a more compact format.

Two or three special classes are also defined.  The sentence start and
sentence end word tokens are always kept in singleton classes, and
optionally the unknown word token can be kept in a singleton class too
-- pass the \texttt{-k} option.\footnote{The author always uses this
option but has not empirically tested its efficaciousness} These
tokens are placed in these classes on initialisation and no moves to
or from these classes are ever considered.

Language model files are built using either the \texttt{-p} or
\texttt{-q} options, which are effectively equivalent if using
the \HTK\ language modelling tools as black boxes.  The former creates
a word-given-class probabilities file, whilst the latter stores word
counts and lets the language model code itself calculate the same
probabilities.

\mysubsect{Use}{Cluster-Use}

\htool{Cluster} is invoked by the command line
\begin{verbatim}
   Cluster [options] mapfile [mult] gramfile [[mult] gramfile ...]
\end{verbatim}
The given word map is loaded and then each of the specified gram files
is imported.  The list of input gram files can be interspersed with
multipliers. These are floating-point format numbers which must begin
with a plus or minus character (e.g. \texttt{+1.0}, \texttt{-0.5},
etc.). The effect of a multiplier \texttt{mult} is to scale the $n$-gram
counts in the following gram files by the factor \texttt{mult}. The
resulting scaled counts are rounded to the nearest integer when
actually used in the clustering algorithm. A multiplier stays in
effect until it is redefined.

The allowable options to \htool{Cluster} are as follows
\begin{optlist}

  \ttitem{-c n} Use {\tt n} classes. This specifies the number of
        classes that should be in the resultant class map.

  \ttitem{-i n} Perform {\tt n} iterations. This is the number of
        iterations of the clustering algorithm that should be
        performed. (If you are using the {\tt -x} option then
        completing the current iteration does not count towards
        the total number, so use {\tt -i 0} to complete it and
        then finish)

  \ttitem{-k} Keep the special unknown word token in its own
        singleton class.  If not passed it can be moved to or from
        any class.

  \ttitem{-l fn} Load the classmap {\tt fn} at start up and when
        performing any further iterations do so from this starting
        point.

  \ttitem{-m} Record the running value of the maximum likelihood
        function used by the clusterer to optimised the training
        text likelihood in the log file.  This option is principally
        provided for debugging purposes.

  \ttitem{-n} Do not write any log file during execution of an
        iteration.

  \ttitem{-o fn} Specify the prefix of all output files.  All output
        class map, logfile and recovery files share the same filename
        prefix, and this is specified via the {\tt -o} switch.  The
        default is {\tt cluster}.

  \ttitem{-p fn} Write a word-given-class probabilities file. Either
        this or the {\tt -q} switch are required to actually build a
        class-based language model. The \HTK\ language model library,
        \htool{LModel}, supports both probability and count-based
        class files.  There is no difference in use, although each
        allows different types of manual manipulation of the file.
        Note that if you do not pass {\tt -p} or {\tt -q} you may
        run \htool{Cluster} at a later date using the {\tt -l} and
        {\tt -i 0} options to just produce a language model file.

  \ttitem{-q fn} Write a word-given-class counts file. See the
        documentation for {\tt -p}.

  \ttitem{-r n} Write recovery files after moving {\tt n} words
        since the previous recovery file was written or an iteration
        began.  Pass {\tt -r n} to disable writing of recovery files.

  \ttitem{-s tkn} Specify the sentence start token.

  \ttitem{-t tkn} Specify the sentence end token.

  \ttitem{-u tkn} Specify the unknown word token.

  \ttitem{-v} Use verbose log file format.

  \ttitem{-w [WMAP/FREQ]} Specify the order in which word moves are
        considered. Default is {\tt WMAP} in which words are
        considered in the order they are encountered in the word map.
        Specifying {\tt FREQ} will consider the most frequent word
        first and then the remainder in decreasing order of frequency.

  \ttitem{-x fn} Continue execution from recovery file {\tt fn}.

\end{optlist}
\stdopts{Cluster}

\mysubsect{Tracing}{Cluster-Tracing}

\htool{Cluster} supports the following trace options, where each trace flag is 
given using an octal base:
\begin{optlist}
  \ttitem{00001} basic progress reporting. 
  \ttitem{00002} report major file operations - good for following start-up.
  \ttitem{00004} more detailed progress reporting.
  \ttitem{00010} trace memory usage during execution and at end.
\end{optlist}
Trace flags are set using the \texttt{-T} option or the \texttt{TRACE}
configuration variable.
\index{cluster@\htool{Cluster}|)}