Add mucus shift detection

lib/sympto/index.js

@@ -4,4 +4,9 @@ import getMucusStatus from './mucus'
 export default function (cycleDays) {
   const temperatureStatus = getTemperatureStatus(cycleDays)
   const mucusStatus = getMucusStatus(cycleDays)
+  return {
+    assumeFertility: true,
+    temperatureStatus,
+    mucusStatus
+  }
 }

lib/sympto/mucus.js

@@ -0,0 +1,19 @@
+export default function (cycleDays) {
+  const mucusDays = cycleDays.filter(day => day.mucus && !day.mucus.exclude)
+  const bestQuality = Math.max(...mucusDays.map(day => day.mucus.value))
+  const mucusPeak = mucusDays.find((day, i) => {
+    if (day.mucus.value !== bestQuality) return false
+
+    const threeFollowingDays = cycleDays.slice(i + 1, i + 4)
+    if (threeFollowingDays.length < 3) return false
+
+    return threeFollowingDays.every(day => day.mucus.value < bestQuality)
+  })
+
+  if (!mucusPeak) return { detected: false }
+
+  return {
+    detected: true,
+    mucusPeak
+  }
+}