define "Initial Population":
  AgeInYearsAt(start of "Measurement Period") >= 18

// Likely some condition or other relevant characteristic such as a depression diagnosis
define "Measure Population":
  true

/*
Measure observation is a function defined in terms of the types of members of the measure
population, Patient in this case. In a population evaluation scenario, the measure
observation is evaluated for every member of the population.

In an individual report scenario, however, the measure observation is evaluated for
the current patient. Note that the patient context is flowing through the call here
*/
define function "MeasureObservation"(patient Patient):
  PHQ9Logic.CalculateTotalScore

define "Initial Population":
  AgeInYearsAt(start of "Measurement Period") >= 18

// Likely some condition or other relevant characteristic such as a depression diagnosis
define "Measure Population":
  true

/*
Measure observation is a function defined in terms of the types of members of the measure
population, Patient in this case. In a population evaluation scenario, the measure
observation is evaluated for every member of the population.

In an individual report scenario, however, the measure observation is evaluated for
the current patient. Note that the patient context is flowing through the call here
*/
define function "MeasureObservation"(patient Patient):
  PHQ9Logic.CalculateTotalScore

// Use the QuestionnaireResponse provided as a parameter if available
// Otherwise look it up from the record
define "QuestionnaireResponse":
  if response is not null then
    response
  else
    Last(
      [QuestionnaireResponse] QR
        where QR.questionnaire = 'http://somewhere.org/fhir/uv/mycontentig/Questionnaire/phq-9-questionnaire'
        sort by (authored as FHIR.dateTime) desc
    )

// NOTE: This ties this logic directly to this particular rendering of the Questionnaire.
// Ideally the items would be filtered based on Code of the Item, rather than the LinkId,
// but that would require pulling in the Questionnaire because the QuestionnaireResponse
// only has the LinkId.
define "Responses":
  "QuestionnaireResponse" QR
    return QR.item I where I.linkId in {
      'LittleInterest',
      'FeelingDown',
      'TroubleSleeping',
      'FeelingTired',
      'BadApetite',
      'FeelingBadAboutSelf',
      'TroubleConcetrating',
      'MovingSpeaking'
    }

define "CalculateTotalScore":
  Sum("Responses" R return all AnswerCardinality(First(R.answer.value)))

/*
@source: https://www.med.umich.edu/1info/FHP/practiceguides/depress/score.pdf
*/
define function "AnswerCardinality"(answer Coding):
  case
    when answer ~ "Not at all" then 0
    when answer ~ "Several days" then 1
    when answer ~ "More than half the days" then 2
    when answer ~ "Nearly every day" then 3
    else null
  end

define function ToString(value uri): value.value

define function ToCode(coding FHIR.Coding):
    if coding is null then
        null
    else
        System.Code {
          code: coding.code.value,
          system: coding.system.value,
          version: coding.version.value,
          display: coding.display.value
        }