FML transform getting very slow for growing/large lists

[mrunibe]

We have noticed that the FML transform is getting very slow if it is used with a large number of elements (target has an element with many children). I could reproduce it with a simple dummy testcase: A bundle with a large number of observations.

Runtimes for the transformations (time measured just for the transform() without loading resources):

# Elements	Time [s]
10	0.2
100	0.5
1000	2
10000	64
20000	250
100000	5316

The approx. quadratic increase of runtime is caused by the recursive call to numberChildren() for every child that is created with makeElement(). For every element that is added, the entire list of children is recursively re-numbered every time.

With a workaround by disabling the numberChildren (mrunibe@a0f67d4), the performance becomes linear again:

# Elements	Time [s]	Time with Workaround [s]
10	0.2	0.2
100	0.5	0.5
1000	2	2
10000	64	11
20000	250	23
100000	5316	126

The issue/numbers can be reproduced with the attached testcase.zip:

java org.hl7.fhir.validation.ValidatorCli testcase/bundle-10000.json -transform http://test.ch/DummyBundleToBundle -version 4.0 -ig testcase/map -output result-10000.json

Not sure if numberChildren() is required in the makeElement() (the index seems to be used for the $index FHIRPath expression)? At least I have not observed any issues for the transform with the workaround.

CC @oliveregger