Multi-component stratifier loses components that share a scalar value

[lukedegruchy]

Fixes a silent data-loss bug in measure-report assembly where stratifier components could be dropped from the output stratum when two of them happened to evaluate to the same scalar value for the same subject. The bug was reported against a real-world RDM measure where a three-component RaceEthnicity stratifier (ProductLine, Race, Ethnicity) produced strata missing the Race component whenever Race and Ethnicity both resolved to AskedButNoAnswer for the same patient. The CQL output already contained all values; the loss happened inside MeasureMultiSubjectEvaluator because its accumulation table was keyed on the value alone, so the second put for the same (row, value) cell overwrote the first.

1. Root-cause fix in the multi-subject stratifier accumulator. Promote the table's column key from StratumValueWrapper to a small StratumTableColumnKey(component, value) record so that two distinct components emitting equal values for the same row key land in distinct cells instead of overwriting each other.
2. Failing-first regression coverage at three levels. Added an integration test in MeasureStratifierTest (driven by a new CohortBooleanStratSameValueComponents measure fixture), two evaluator-level unit tests in MeasureMultiSubjectEvaluatorTest, and a defense-in-depth check in MultiMeasureServiceTest to confirm the multi-measure orchestration path inherits the fix.

Example: the RDM-Reporting scenario end-to-end

The snippets below are the colleague's original reproduction for patient 2025.hpdi.0.115003, walking from CQL output → Measure definition → buggy MeasureReport → corrected MeasureReport. They show why the assembly-layer key change matters: the CQL already computes all three component values; the loss is purely in MeasureMultiSubjectEvaluator.

1. CQL results for the patient (input to assembly)

Initial population=true
Product lines=[Code { code: PPO, system: https://ncqa.org/fhir/CodeSystem/hedis-coverage-type },
               Code { code: POS, system: https://ncqa.org/fhir/CodeSystem/hedis-coverage-type }]
Race=AskedButNoAnswer
Ethnicity=AskedButNoAnswer

Note the collision: Race and Ethnicity are distinct stratifier components but both resolve to the same scalar AskedButNoAnswer for this patient. Product lines is multi-valued, so each value yields its own stratum (PPO and POS).

2. Measure stratifier definition (RDM-Reporting.json, three components)

"stratifier": [{
  "id": "RaceEthnicity-Stratifier",
  "code": { "text": "RaceEthnicity-Stratifier" },
  "component": [
    {
      "id": "RaceEthnicity-StratifierComponent-ProductLine",
      "code": { "text": "ProductLine" },
      "criteria": { "language": "text/cql-identifier", "expression": "Product lines" }
    },
    {
      "id": "RaceEthnicity-StratifierComponent-Race",
      "code": { "text": "Race" },
      "criteria": { "language": "text/cql-identifier", "expression": "Race" }
    },
    {
      "id": "RaceEthnicity-StratifierComponent-Ethnicity",
      "code": { "text": "Ethnicity" },
      "criteria": { "language": "text/cql-identifier", "expression": "Ethnicity" }
    }
  ]
}]

3. Before the fix — MeasureReport stratum is missing the Race component

"stratum": [
  {
    "component": [
      { "id": "RaceEthnicity-StratifierComponent-ProductLine",
        "code": { "text": "ProductLine" }, "value": { "text": "POS" } },
      { "id": "RaceEthnicity-StratifierComponent-Ethnicity",
        "code": { "text": "Ethnicity" },   "value": { "text": "AskedButNoAnswer" } }
    ],
    "population": [
      { "id": "RaceEthnicity-initial-population", "count": 1 }
    ]
  },
  {
    "component": [
      { "id": "RaceEthnicity-StratifierComponent-Ethnicity",
        "code": { "text": "Ethnicity" },   "value": { "text": "AskedButNoAnswer" } },
      { "id": "RaceEthnicity-StratifierComponent-ProductLine",
        "code": { "text": "ProductLine" }, "value": { "text": "PPO" } }
    ],
    "population": [
      { "id": "RaceEthnicity-initial-population", "count": 1 }
    ]
  }
]

Race is silently absent from both strata because, inside buildSubjectResultsTable, the cell (rowKey, "AskedButNoAnswer") → Race got overwritten by (rowKey, "AskedButNoAnswer") → Ethnicity when only the value was used as the column key.

4. After the fix — all three components survive in each stratum

"stratum": [
  {
    "component": [
      { "code": { "text": "ProductLine" }, "value": { "text": "PPO" } },
      { "code": { "text": "Race" },        "value": { "text": "AskedButNoAnswer" } },
      { "code": { "text": "Ethnicity" },   "value": { "text": "AskedButNoAnswer" } }
    ],
    "population": [
      { "code": { "coding": [{ "system": "http://hl7.org/fhir/CodeSystem/measure-population",
                               "code": "initial-population" }] },
        "count": 1 }
    ]
  },
  {
    "component": [
      { "code": { "text": "ProductLine" }, "value": { "text": "POS" } },
      { "code": { "text": "Race" },        "value": { "text": "AskedButNoAnswer" } },
      { "code": { "text": "Ethnicity" },   "value": { "text": "AskedButNoAnswer" } }
    ],
    "population": [
      { "code": { "coding": [{ "system": "http://hl7.org/fhir/CodeSystem/measure-population",
                               "code": "initial-population" }] },
        "count": 1 }
    ]
  }
]

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