FML transform performance tuning

[mrunibe]

Imrove performance of FML transform, especially for data with large lists.

• Skip numberChildren() when adding an element to a list: FML transform getting very slow for growing/large lists #1699
• Avoid creating new FHIRPathEngine in every new Property: FML transform performance - excessive creation of FHIRPathEngine in new Properties #1703

For large amount of input/output data, FML transform is > 1000x faster and using less memory.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 12.50%. Comparing base (33cb80d) to head (5a2bcf7).
Report is 21 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1704      +/-   ##
============================================
+ Coverage     12.49%   12.50%   +0.01%     
- Complexity    32218    32254      +36     
============================================
  Files          2203     2203              
  Lines        673493   673623     +130     
  Branches     198556   198592      +36     
============================================
+ Hits          84122    84213      +91     
- Misses       559399   559419      +20     
- Partials      29972    29991      +19     

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[grahamegrieve]

why is it ok to skip numberChildren() when changing the list?

[mrunibe]

why is it ok to skip numberChildren() when changing the list?

At least during the transform I have not seen where the index is used. But maybe we can just defer numberChildren() until the end of the transform? For example, do it when sorting:

org.hl7.fhir.core/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/utils/structuremap/StructureMapUtilities.java

Lines 1274 to 1277 in 1ad319e

	executeGroup("", context, map, vars, g, true);
	if (target instanceof Element)
	((Element) target).sort();

I've also noticed the numbering is not done in other place like forceElement or addElement, making me naively assume it might not be required in makeElement either.

Do you think deferring it to the end of transform() is ok?