minus-8-day-rule.js

const { LocalDate } = require('js-joda')
const getNfpStatus = require('./index.js')

module.exports = function (previousCycles, secondarySymptom) {
  const fhms = previousCycles
    .map(cycle => {
      const status = getNfpStatus({
        cycle,
        secondarySymptom,
        excludePreOvu: true
      })
      if (status.temperatureShift) {
        const day = status.temperatureShift.firstHighMeasurementDay
        const firstCycleDayDate = LocalDate.parse(cycle[0].date)
        const fhmDate = LocalDate.parse(day.date)
        return fhmDate.compareTo(firstCycleDayDate) + 1
      }
      return null
    })
    .filter(val => typeof val === 'number')

  const preOvuLength = Math.min(...fhms) - 8

  // pre ovu length may only be lengthened if we have more than 12 previous fhms
  // if pre ovu length is less than 5, it shortened even with fewer prev fhms
  if (preOvuLength < 5) return preOvuLength
  if (fhms.length >= 12) return preOvuLength

  return null
}