[rrd4j] Reuse the state for identical values

[joerg1985]

Description

A lot of items report the same value for some time, the same State can be used to avoid creating the identical State instances again and again.

[J-N-K]

Do you have an idea how much memory/time is saved by this code?

[joerg1985]

@J-N-K sure, when locking at a graph with one week of data, there are ~ 10000 datapoints.
Here are some numbers based on items in my openHab instance:

Item	Same as previouse value	Different than previouse value
Temperatur outside item	8113	1941
Temperatur inside item	8097	1957
Solar power item	9350	706
Radon item	9281	773
Raspi Temperature item	6154	3900

For these items 60% to 90% of the state's are duplicates compared to the previouse value.
Creating a state is quite heavy, as it creates a String from the double and parses it back into a BigDecimal.
It's hard to tell the concrete memory / time savings here, i think the high percentages of duplicates speaks for them self.

The effect does depend on the data, so when looking at a random number item, this would be more like 0%.
But the overhead of comparing two doubles is minimal an should not harm in theses cases.