[rrd-developers] Unexpected behaviour of PREDICT and PREDICTSIGMA

[Steve Shipway]

So, I'm delving into the PREDICT operations in the RPN, and have found
something unexpected.

 

There are two ways of specifying the shift offsets - as a (positive) count
of shifts and a list; or as a negative count and a base shift.

 

CDEF:predict1=172800,86400,0,3,3600,x,PREDICT

CDEF:predict2=86400,-3,3600,x,PREDICT

 

My tests (using 1.4.trunk) indicate that the two above have identical
behaviour.  My question is, is this intended?  In particular, when giving a
negative  shift count, should the resulting list of shifts be based at 0?  

 

Given this:

 

CDEF:predict=s,-3,w,x,PREDICT

 

I had expected to get shifts of s, 2s and 3s.  Instead I get 0, s and 2s.
Is this intentional, or a feature?

 

As a corollary, it means that these two lines are also equal for ANY value
of s

 

CDEF:predict1=s,-1,3600,x,PREDICT

CDEF:predict2=x,3600,TRENDNAN

 

Thanks,

 

Steve

 

 

Steve Shipway

University of Auckland

UNIX Systems Design Team Lead

s.shipway at auckland.ac.nz

+64 (9) 3737 599 ext 86487

 

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