Extract getLastMensesStart

lib/get-last-menses-start.js

@@ -0,0 +1,34 @@
+import * as joda from 'js-joda'
+
+const LocalDate = joda.LocalDate
+
+export default function getLastMensesStart(targetDateString, bleedingDaysSortedByDate, maxBreakInBleeding) {
+  const targetDate = LocalDate.parse(targetDateString)
+  const withWrappedDates = bleedingDaysSortedByDate
+    .filter(day => !day.bleeding.exclude)
+    .map(day => {
+      day.wrappedDate = LocalDate.parse(day.date)
+      return day
+    })
+
+  const firstBleedingDayBeforeTargetDayIndex = withWrappedDates.findIndex(day => {
+    return (
+      day.wrappedDate.isEqual(targetDate) ||
+      day.wrappedDate.isBefore(targetDate)
+    )
+  })
+
+  if (firstBleedingDayBeforeTargetDayIndex < 0) return null
+  const previousBleedingDays = withWrappedDates.slice(firstBleedingDayBeforeTargetDayIndex)
+
+  const lastPeriodStart = previousBleedingDays.find((day, i) => {
+    return thereIsNoPreviousBleedingDayWithinTheThreshold(day, previousBleedingDays.slice(i + 1), maxBreakInBleeding)
+  })
+
+  return lastPeriodStart
+}
+
+function thereIsNoPreviousBleedingDayWithinTheThreshold(bleedingDay, earlierCycleDays, allowedBleedingBreak) {
+  const periodThreshold = bleedingDay.wrappedDate.minusDays(allowedBleedingBreak + 1)
+  return !earlierCycleDays.some(({ wrappedDate }) => wrappedDate.equals(periodThreshold) || wrappedDate.isAfter(periodThreshold))
+}