Select immediate predecessor by date - drools

Question

I am new to Drools and I'm using Drools 7.12.0 to try and validate a set of meter readings, which look like
public class MeterReading() {
private long id;
private LocalDate readDate;
private int value;
private String meterId
private boolean valid;
/* Getters & Setters omitted */
}
As part of the validation I need to compare the values of each MeterReading with its immediate predecessor by readDate.
I first tried using 'accumulate'
when $mr: MeterReading()
$previousDate: LocalDate() from accumulate(MeterReading($pdate: readDate < $mr.readDate ), max($pdate))
then
System.out.println($mr.getId() + ":" + $previousDate);
end
but then discovered that this only returns the date of the previous meter read, not the object that contains it. I then tried a custom accumulate with
when
$mr: MeterReading()
$previous: MeterReading() from accumulate(
$p: MeterReading(id != $mr.id),
init( MeterReading prev = null; ),
action( if( prev == null || $p.readDate < prev.readDate) {
prev = $p;
}),
result(prev))
then
System.out.println($mr.getId() + ":" + $previous.getId() + ":" + $previous.getReadDate());
end
but this selects the earliest read in the set of meter readings, not the immediate predecessor. Can someone point me in the right direction as to what I should be doing or reading to be able to select the immediate predecessor to each individual meter read.
Regards

Answer 1

After further research I found this article http://planet.jboss.org/post/how_to_implement_accumulate_functions which I used to write my own accumulate function;\
public class PreviousReadFinder implements AccumulateFunction {
#Override
public Serializable createContext() {
return new PreviousReadFinderContext();
}
#Override
public void init(Serializable context) throws Exception {
PreviousReadFinderContext prfc = (PreviousReadFinderContext) context;
prfc.list.clear();
}
#Override
public void accumulate(Serializable context, Object value) {
PreviousReadFinderContext prfc = (PreviousReadFinderContext) context;
prfc.list.add((MeterReading) value);
}
#Override
public void reverse(Serializable context, Object value) throws Exception {
PreviousReadFinderContext prfc = (PreviousReadFinderContext) context;
prfc.list.remove((MeterReading) value);
}
#Override
public Object getResult(Serializable context) throws Exception {
PreviousReadFinderContext prfc = (PreviousReadFinderContext) context;
return prfc.findLatestReadDate();
}
#Override
public boolean supportsReverse() {
return true;
}
#Override
public Class<?> getResultType() {
return MeterReading.class;
}
#Override
public void writeExternal(ObjectOutput out) throws IOException {
}
#Override
public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
}
private static class PreviousReadFinderContext implements Serializable {
List<MeterReading> list = new ArrayList<>();
public Object findLatestReadDate() {
Optional<MeterReading> optional = list.stream().max(Comparator.comparing(MeterReading::getReadDate));
if (optional.isPresent()) {
MeterReading to = optional.get();
return to;
}
return null;
}
}
}
and my rule is now
rule "Opening Read With Previous"
dialect "mvel"
when $mr: MeterReading()
$pmr: MeterReading() from accumulate($p: MeterReading(readDate < $mr.readDate ), previousReading($p))
then
System.out.println($mr.getId() + ":" + $pmr.getMeterReadDate());
end
How do I write a rule to select the eatliest meter reading in the set which does not have a previous read?