[rrd-developers] Patch: Prediction CDEF functions for rrdtool 1.3

[Martin Sperl]

Hi!

As promised in my libDBI Patch announcement a few days ago, I have now 
created a patch with 2 new CDEFS that allow for some 
prediction/extrapolation of values into the future by calculating 
averages of values from the previous days/weeks.

Here an example image of a series with the "prediction" and its 
uncertainty borders:
example image for prediction

The graph creation is achieved with:
rrdtool graph image.png --imgformat=PNG \
 --start=-7days --end=+3days --width=1000 --height=200 --alt-autoscale-max \
 DEF:value=value.rrd:value:AVERAGE:start=-14days \
 LINE1:value#ff0000:value \
 CDEF:predict=86400,-7,1800,value,PREDICT \
 CDEF:sigma=86400,-7,1800,value,PREDICTSIGMA \
 CDEF:upper=predict,sigma,3,*,+ \
 CDEF:lower=predict,sigma,3,*,- \
 LINE1:predict#00ff00:prediction \
 LINE1:upper#0000ff:upper\ certainty\ limit \
 LINE1:lower#0000ff:lower\ certainty\ limit \
 CDEF:exceeds=value,UN,0,value,lower,upper,LIMIT,UN,IF \
 TICK:exceeds#aa000080:1

The prediction is only valid for a few days into the future.
For longer term predictions another patch with a set of VDEFS/CDEFS is 
planned that will use least square fitting of polynomials plus sums of 
sinuses to get a reasonable prediction for time periods of 3 to 6 month 
into the future. This will be based on some code that was written for my 
astronomy thesis several years ago...

Attached the patch, which only touches: src/rrd_rpncalc.h 
src/rrd_rpncalc.c doc/rrdgraph_rpn.pod
It also contains the documentation for the 2 new CDEFS in rrdgraph_rpn - 
so in case of questions regarding usage, please read the documentation 
in the patch.

Please review and give feedback, so that it may get included into one of 
the next releases of rrdtool.

Thanks,
             Martin
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