Incorrect Wrap in nonNegativeDerivative

[Tony Bunce]

I think the wrapping/rollover calculation used in nonNegativeDerivative is incorrect.

This is the current formula that is being used:
newValues.append( prev + (maxValue - val) )

I think the correct formula is:
 newValues.append( val+ (maxValue - prev) + 1 )
or
 newValues.append( maxValue + (prev- val) + 1 )

Look at "Counter Wraps" here:
http://www.mrtg.org/rrdtool/tut/rrdtutorial.en.html

For example on a 32 bit snmp counter:
MAX: 4294967295
Reading 1: 429496700
Reading 2: 100
Delta = -429496600
graphite value = ( 429496700 + (4294967295 - 100) ) = 4724463895
RRD Value = 429496700 + (4294967295 - 100) + 1 = 3865470696

I think 3865470696 is the correct value

[chrismd (chrismd)]

You're absolutely right, committing this fix to trunk.