Implement 5-day-rule

lib/sympto/pre-ovulatory.js

@@ -0,0 +1,21 @@
+export default function(cycle) {
+  const fiveDayRuleDays = cycle.slice(0, 5)
+  const preOvulatoryDays = getDaysUntilFertileMucus(fiveDayRuleDays)
+  return {
+    cycleDays: preOvulatoryDays,
+    start: {
+      date: preOvulatoryDays[0].date
+    },
+    end: {
+      date: preOvulatoryDays[preOvulatoryDays.length - 1].date,
+    }
+  }
+}
+
+function getDaysUntilFertileMucus(days) {
+  const firstFertileMucusDayIndex = days.findIndex(day => day.mucus && day.mucus.value > 1)
+  if (firstFertileMucusDayIndex > -1) {
+    return days.slice(0, firstFertileMucusDayIndex)
+  }
+  return days
+}