prob.3

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.\" $Id: Prob.3,v 1.1 1996/07/13 01:35:40 stolcke Exp $
.TH Prob 3 "$Date: 1996/07/13 01:35:40 $" SRILM
.SH NAME
Prob \- Probabilities for SRILM
.SH SYNOPSIS
.B "#include <Prob.h>"
.SH DESCRIPTION
.B Prob
is a collection of types, constants and utility functions for handling
probabilities in the SRILM library.
.SH TYPES
.TP
.B Prob
A floating point number representing a probability.
.TP
.B LogP
Logarithm to base 10 of a probability.
.SH CONSTANTS
.TP
.B LogP_Zero
Log of probability 0.
.TP
.B LogP_Inf
Log of probability infinity (not a legal probability, of course).
.TP
.B LogP_One
Log of probability 1.
.TP
.B LogP_Precision
The number of significant digits in a LogP
.TP 
.B Prob_Epsilon
A positive value close to 0; probability sums less than this should be
considered effectively zero.
.SH FUNCTIONS
.TP
.B "Boolean parseLogP(const char *\fIstring\fP, LogP &\fIprob\fP)"
Converts a floating point string representation into a LogP.
Returns
.B true
iff the number was parsed correctly.
This function should be much faster than generic C library functions
for floating point parsing.
Also, it parses singular LogP's (plus/minus infinity) correctly.
.TP
.B "Prob LogPtoPPL(LogP \fIprob\fP)"
Converts a LogP into a perplexity (PPL).
.TP
.B "ProbToLogP(Prob \fPprob\fP)"
Converts a probability into a LogP.
.TP
.B "LogP MixLogP(LogP \fIprob1\fP, LogP \fIprob2\fP, double \fIlambda\fP)"
Computes the LogP resulting from interpolating two LogP's.
If \fIp1\fP and \fIp2\fP are probabilities corresponding to \fIprob1\fP
and \fIprob2\fP, respectively, then the result is the LogP corresponding
to \fIlambda\fP * \fIp1\fP + (1 - \fIlambda\fP) * \fIp2\fP.
.PP
The following functions deal with \fIbytelogs\fP.
Bytelogs are logarithms scaled to represent probabilties and likelihoods
as a short integer in SRI's DECIPHER(TM) recognizer
(bytelog(\fIp\fP) = log(\fIp\fP) * 10000.5 / 1024).
.TP
.B "double ProbToBytelog(Prob \fIprob\fP)"
Converts a probability to a bytelog.
.TP
.B "double LogPtoBytelog(LogP \fIprob\fP)"
Convert a LogP to a bytelog.
.TP
.B "LogP BytelogToLogP(double \fIbytelog\fP)"
Convert a bytelog to a LogP.
.SH "SEE ALSO"
.SH BUGS
.SH AUTHOR
Andreas Stolcke <[e-mail]>.
.br
Copyright 1995, 1996 SRI International