feat: Implement %rowIndex environment variable

[piotrszul]

Summary

Implement %rowIndex environment variable for ViewDefinition iterations, as defined in the SQL on FHIR ViewDefinition spec.

%rowIndex provides the 0-based index of the current element within the iterated collection. At the top level (no iteration), it evaluates to 0. Each nesting level maintains independent values.

forEach / forEachOrNull support

Uses Spark's indexed transform(array, (element, index) -> ...) to track element positions during unnesting, threading the index through ProjectionContext into FHIRPath evaluation as a supplied variable.

File	Change
ColumnRepresentation	Added transformWithIndex() using Spark's indexed transform
SingleResourceEvaluator	Added withVariable() for creating evaluators with additional variables
ProjectionContext	Added rowIndex field, withRowIndex(), and %rowIndex injection in evalExpression()
UnnestingSelection	Rewrote evaluate() to use indexed transform, passing element index as %rowIndex

repeat support

Implements %rowIndex for repeat clauses using a RowIndexCounter that assigns monotonically increasing indices across breadth-first iteration levels, resetting per resource. This required new encoder-level support for injecting index values during repeat expansion.

File	Change
RowIndexCounter	New thread-safe counter that assigns sequential indices across repeat levels, resetting per resource
ValueFunctions	New withRowIndex() UDF that pairs each element with its assigned index
Expressions.scala	New repeatWithRowIndex() that threads RowIndexCounter through the repeat expansion loop
RepeatSelection	Updated to use repeatWithRowIndex() and pass index into ProjectionContext
ExpressionsBothModesTest	Unit tests for repeatWithRowIndex() covering linear chains, branching trees, and counter reset

Test coverage (15 test cases in viewTests/rowindex.json)

forEach / forEachOrNull (8 tests):

• 3 spec examples: element position, nested iteration with independent indices, unionAll with inherited/independent indices
• Top-level %rowIndex defaults to 0
• forEach and forEachOrNull with %rowIndex
• Nested forEach with independent %rowIndex values
• forEachOrNull with empty collection (null row)
• Arithmetic expression (%rowIndex + 1)

repeat (7 tests):

• Linear chain with sequential indices
• Arithmetic expression with repeat %rowIndex
• Repeat with nested forEach (independent indices)
• Branching tree with breadth-first ordering
• Counter resets per resource
• Repeat nested inside repeat (independent indices)
• Repeat nested inside forEach (independent indices)

Test plan

• All 15 new %rowIndex tests pass via FhirViewExtraTest
• All 18 existing view tests pass (no regressions)
• All existing FhirViewExecutorTest + FhirViewBuilderUnitTest tests pass
• ExpressionsBothModesTest unit tests pass for repeat index logic
• Pre-existing compliance test failures unchanged (2 failures unrelated to this change)
• SonarCloud thread-safety warning addressed with @SuppressWarnings annotation

Closes #2560

🤖 Generated with Claude Code

[johngrimes]

• Integrate variant/custom schema unification approach to repeat/repeatAll functions
• Investigate: change tree-traversing expression to keep track of index

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud