=head1 NAME

rrdgraph_rpn - About RPN Math in rrdtool graph

=head1 SYNOPSIS

I<RPN expression>:=I<vname>|I<operator>|I<value>[,I<RPN expression>]

=head1 DESCRIPTION

If you have ever used a traditional HP calculator you already know
B<RPN> (Reverse Polish Notation).
The idea behind B<RPN> is that you have a stack and push
your data onto this stack. Whenever you execute an operation, it
takes as many elements from the stack as needed. Pushing is done
implicitly, so whenever you specify a number or a variable, it gets
pushed onto the stack automatically.

At the end of the calculation there should be one and only one value left on
the stack.  This is the outcome of the function and this is what is put into
the I<vname>.  For B<CDEF> instructions, the stack is processed for each
data point on the graph. B<VDEF> instructions work on an entire data set in
one run. Note, that currently B<VDEF> instructions only support a limited
list of functions.

Example: C<VDEF:maximum=mydata,MAXIMUM>

This will set variable "maximum" which you now can use in the rest
of your RRD script.

Example: C<CDEF:mydatabits=mydata,8,*>

This means:  push variable I<mydata>, push the number 8, execute
the operator I<*>. The operator needs two elements and uses those
to return one value.  This value is then stored in I<mydatabits>.
As you may have guessed, this instruction means nothing more than
I<mydatabits = mydata * 8>.  The real power of B<RPN> lies in the
fact that it is always clear in which order to process the input.
For expressions like C<a = b + 3 * 5> you need to multiply 3 with
5 first before you add I<b> to get I<a>. However, with parentheses
you could change this order: C<a = (b + 3) * 5>. In B<RPN>, you
would do C<a = b, 3, +, 5, *> without the need for parentheses.

=head1 OPERATORS

=over 4

=item Boolean operators

B<LT, LE, GT, GE, EQ, NE>

Pop two elements from the stack, compare them for the selected condition
and return 1 for true or 0 for false. Comparing an I<unknown> or an
I<infinite> value will always result in 0 (false).

B<UN, ISINF>

Pop one element from the stack, compare this to I<unknown> respectively
to I<positive or negative infinity>. Returns 1 for true or 0 for false.

B<IF>

Pops three elements from the stack.  If the element popped last is 0
(false), the value popped first is pushed back onto the stack,
otherwise the value popped second is pushed back. This does, indeed,
mean that any value other than 0 is considered to be true.

Example: C<A,B,C,IF> should be read as C<if (A) then (B) else (C)>

Z<>

=item Comparing values

B<MIN, MAX>

Pops two elements from the stack and returns the smaller or larger,
respectively.  Note that I<infinite> is larger than anything else.
If one of the input numbers is I<unknown> then the result of the operation will be
I<unknown> too.

B<LIMIT>

Pops two elements from the stack and uses them to define a range.
Then it pops another element and if it falls inside the range, it
is pushed back. If not, an I<unknown> is pushed.

The range defined includes the two boundaries (so: a number equal
to one of the boundaries will be pushed back). If any of the three
numbers involved is either I<unknown> or I<infinite> this function
will always return an I<unknown>

Example: C<CDEF:a=alpha,0,100,LIMIT> will return I<unknown> if
alpha is lower than 0 or if it is higher than 100.

Z<>

=item Arithmetics

B<+, -, *, /, %>

Add, subtract, multiply, divide, modulo

B<ADDNAN>

NAN-safe addition. If one parameter is NAN/UNKNOWN it'll be treated as
zero. If both parameters are NAN/UNKNOWN, NAN/UNKNOWN will be returned.

B<SIN, COS, LOG, EXP, SQRT>

Sine and cosine (input in radians), log and exp (natural logarithm),
square root.

B<ATAN>

Arctangent (output in radians).

B<ATAN2>

Arctangent of y,x components (output in radians).
This pops one element from the stack, the x (cosine) component, and then
a second, which is the y (sine) component.
It then pushes the arctangent of their ratio, resolving the ambiguity between
quadrants.

Example: C<CDEF:angle=Y,X,ATAN2,RAD2DEG> will convert C<X,Y>
components into an angle in degrees.

B<FLOOR, CEIL>

Round down or up to the nearest integer.

B<DEG2RAD, RAD2DEG>

Convert angle in degrees to radians, or radians to degrees.

B<ABS>

Take the absolute value.

=item Set Operations

B<SORT, REV>

Pop one element from the stack.  This is the I<count> of items to be sorted
(or reversed).  The top I<count> of the remaining elements are then sorted
(or reversed) in place on the stack.

Example: C<CDEF:x=v1,v2,v3,v4,v5,v6,6,SORT,POP,5,REV,POP,+,+,+,4,/> will
compute the average of the values v1 to v6 after removing the smallest and
largest.

B<AVG>

Pop one element (I<count>) from the stack. Now pop I<count> elements and build the
average, ignoring all UNKNOWN values in the process.

Example: C<CDEF:x=a,b,c,d,4,AVG>

B<TREND, TRENDNAN>

Create a "sliding window" average of another data series.

Usage:
CDEF:smoothed=x,1800,TREND

This will create a half-hour (1800 second) sliding window average of x.  The
average is essentially computed as shown here:

                 +---!---!---!---!---!---!---!---!--->
                                                     now
                       delay     t0
                 <--------------->
                         delay       t1
                     <--------------->
                              delay      t2
                         <--------------->


     Value at sample (t0) will be the average between (t0-delay) and (t0)
     Value at sample (t1) will be the average between (t1-delay) and (t1)
     Value at sample (t2) will be the average between (t2-delay) and (t2)

TRENDNAN is - in contrast to TREND - NAN-safe. If you use TREND and one 
source value is NAN the complete sliding window is affected. The TRENDNAN 
operation ignores all NAN-values in a sliding window and computes the 
average of the remaining values.


=item Special values

B<UNKN>

Pushes an unknown value on the stack

B<INF, NEGINF>

Pushes a positive or negative infinite value on the stack. When
such a value is graphed, it appears at the top or bottom of the
graph, no matter what the actual value on the y-axis is.

B<PREV>

Pushes an I<unknown> value if this is the first value of a data
set or otherwise the result of this B<CDEF> at the previous time
step. This allows you to do calculations across the data.  This
function cannot be used in B<VDEF> instructions.

B<PREV(vname)>

Pushes an I<unknown> value if this is the first value of a data
set or otherwise the result of the vname variable at the previous time
step. This allows you to do calculations across the data. This
function cannot be used in B<VDEF> instructions.

B<COUNT>

Pushes the number 1 if this is the first value of the data set, the
number 2 if it is the second, and so on. This special value allows
you to make calculations based on the position of the value within
the data set. This function cannot be used in B<VDEF> instructions.

=item Time

Time inside RRDtool is measured in seconds since the epoch. The
epoch is defined to be S<C<Thu Jan  1 00:00:00 UTC 1970>>.

B<NOW>

Pushes the current time on the stack.

B<TIME>

Pushes the time the currently processed value was taken at onto the stack.

B<LTIME>

Takes the time as defined by B<TIME>, applies the time zone offset
valid at that time including daylight saving time if your OS supports
it, and pushes the result on the stack.  There is an elaborate example
in the examples section below on how to use this.

=item Processing the stack directly

B<DUP, POP, EXC>

Duplicate the top element, remove the top element, exchange the two
top elements.

Z<>

=back

=head1 VARIABLES

These operators work only on B<VDEF> statements. Note that currently ONLY these work for B<VDEF>.

=over 4

=item MAXIMUM, MINIMUM, AVERAGE

Return the corresponding value, MAXIMUM and MINIMUM also return
the first occurrence of that value in the time component.

Example: C<VDEF:avg=mydata,AVERAGE>

=item STDEV

Returns the standard deviation of the values.

Example: C<VDEF:stdev=mydata,STDEV>

=item LAST, FIRST

Return the last/first value including its time.  The time for
FIRST is actually the start of the corresponding interval, whereas
LAST returns the end of the corresponding interval.

Example: C<VDEF:first=mydata,FIRST>

=item TOTAL

Returns the rate from each defined time slot multiplied with the
step size.  This can, for instance, return total bytes transfered
when you have logged bytes per second. The time component returns
the number of seconds.

Example: C<VDEF:total=mydata,TOTAL>

=item PERCENT

This should follow a B<DEF> or B<CDEF> I<vname>. The I<vname> is popped,
another number is popped which is a certain percentage (0..100). The
data set is then sorted and the value returned is chosen such that
I<percentage> percent of the values is lower or equal than the result.
I<Unknown> values are considered lower than any finite number for this
purpose so if this operator returns an I<unknown> you have quite a lot
of them in your data.  B<Inf>inite numbers are lesser, or more, than the
finite numbers and are always more than the I<Unknown> numbers.
(NaN E<lt> -INF E<lt> finite values E<lt> INF)

Example: C<VDEF:perc95=mydata,95,PERCENT>

=item LSLSLOPE, LSLINT, LSLCORREL

Return the parameters for a B<L>east B<S>quares B<L>ine I<(y = mx +b)> 
which approximate the provided dataset.  LSLSLOPE is the slope I<(m)> of
the line related to the COUNT position of the data.  LSLINT is the 
y-intercept I<(b)>, which happens also to be the first data point on the 
graph. LSLCORREL is the Correlation Coefficient (also know as Pearson's 
Product Moment Correlation Coefficient).  It will range from 0 to +/-1 
and represents the quality of fit for the approximation.   

Example: C<VDEF:slope=mydata,LSLSLOPE>

=back

=head1 SEE ALSO

L<rrdgraph> gives an overview of how B<rrdtool graph> works.
L<rrdgraph_data> describes B<DEF>,B<CDEF> and B<VDEF> in detail.
L<rrdgraph_rpn> describes the B<RPN> language used in the B<?DEF> statements.
L<rrdgraph_graph> page describes all of the graph and print functions.

Make sure to read L<rrdgraph_examples> for tipsE<amp>tricks.

=head1 AUTHOR

Program by Tobias Oetiker E<lt>tobi@oetiker.chE<gt>

This manual page by Alex van den Bogaerdt E<lt>alex@vandenbogaerdt.nlE<gt>
with corrections and/or additions by several people
