Implement minus 8 day rule

lib/sympto/minus-8-day-rule.js

@@ -0,0 +1,26 @@
+import { LocalDate } from 'js-joda'
+import getNfpStatus from './index'
+
+export default function (previousCycles) {
+  const fhms = previousCycles
+    .map(cycle => {
+      const status = getNfpStatus({ cycle })
+      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
+}