Yes, I agree that emoncms should have more process/functions. There are some things I wanted to be able to do, and found that the standard set of functions available in Input processing were not quite flexible enough, or in one or two instances, a perfectly sensible calculation was impossible to perform.
I have a similar process function to yours, one of several I have written. It is called “Limit to n”, with n being a user-set value. It replaces the current value with “n”, if the value is higher. It was quite possible to do this with several standard functions, but boy, it was clumsy! A single one is more efficient.
Another one is “Now”, which forwards the current time to the next process.
A third is “Store to n”, which saves the current value to an array variable, the user providing the index “n”. There is a corresponding “Recall from n”, though I haven’t needed it yet.
I needed the Store to allow my “Log to Rfeed” to retrieve a duration from earlier in the processlist while still letting it receive the current value (Watts). It needs the duration because its speciality over the standard one is updating earlier datapoints with the newly calculated Watts value, and the duration tells it how far back in the feed to go. The “Log to Rfeed” expects to see the Stored duration in Store, using its own feed id as the index. This is designed for pulse-based data, where the value for earlier datapoints could not be calculated until the next pulse arrived. I use this in my gas processlist.
“Inverse” just replaces the value with 1/value, so I could then use the standard “x” with a constant C set by the user, to effectively divide that value by the one that Inverse had received. Ie, 1 / V * C is equivalent to C / V. Trying to do that with the standard functions was doing my head in. It could be done, but again it was going to rely on feeds that I would create for no other purpose than to permit a calculation using only the most recent value.
Another useful one is “Difference n”, which is a feedless version of “Rate of Change”. It seemed silly and wasteful, especially in a low-write system, to use a continually written-to feed just to compare to the previous value, and never any earlier value. Again “n” is a user-set index to another array variable, as long as it is unique in all of the processlists, you can use it as often as you like. In conjunction with “Now”, I use it to derive the duration between the current increment in pulsecount and its previous increment (in my system, I never see more than one pulse in 32 seconds, so any increment is always 1). Without “Now”, there is no way to derive a duration.
By the way, just to be clear I am not trying to badmouth Emoncms at all, I am solidly impressed with its capabilities, and with the depth of thinking that has clearly gone into it. But it isn’t perfect. The process functions can be made more flexible than they are, and means exist within emoncms to allow for new functions to be written and added, and that is itself testament to forward-thinking.
My functions are all working, but I am aware that some of them need some sanitation applied before they are ready for the emoncms world. They were all written over the last few weeks, and some are a bit raw. Maybe in a couple of days or so I can post some.
