moves getCycleLength to corresponding files, adds function to get next menses (not finished) with first tests

lib/cycle.js

@@ -1,4 +1,5 @@
 import * as joda from 'js-joda'
+import {getCycleLengthStats, getCycleLength} from './cycle-length'
 const LocalDate = joda.LocalDate
 const DAYS = joda.ChronoUnit.DAYS
 
@@ -143,11 +144,41 @@ export default function config(opts) {
     }
   }
 
+  function getPredictedMenses(maxCycleLength, minCyclesForPrediction) {
+    maxCycleLength = maxCycleLength || 99
+    minCyclesForPrediction = minCyclesForPrediction || 3
+    const allMensesStarts = getAllMensesStarts()
+
+    const atLeastOneCycle = allMensesStarts.length > 1
+    if (!atLeastOneCycle ||
+      allMensesStarts.length < minCyclesForPrediction ||
+      getCycleDayNumber(LocalDate.now().toString()) > maxCycleLength
+    ) {
+      return {}
+    }
+    const cycleLengths = getCycleLength(allMensesStarts)
+    const cycleInfo = getCycleLengthStats(cycleLengths)
+    const periodDistance = Math.round(cycleInfo.mean)
+    const periodStartVariation = (cycleInfo.stdDeviation < 1.5) ? 1 : 2 // threshold is choosen a little arbitrarily
+    var lastStart = allMensesStarts[0]
+    const predictedMenses = []
+    for (let i = 0; i < 3; i++) {
+      lastStart = LocalDate.parse(lastStart).plusDays(periodDistance).toString()
+      const nextPredictedRange = {
+        'startDate': LocalDate.parse(lastStart).minusDays(periodStartVariation).toString(),
+        'endDate': LocalDate.parse(lastStart).plusDays(periodStartVariation).toString()
+      }
+      predictedMenses.push(nextPredictedRange)
+    }
+    return predictedMenses
+  }
+
   return {
     getCycleDayNumber,
     getCycleForDay,
     getPreviousCycle,
     getCyclesBefore,
-    getAllMensesStarts
+    getAllMensesStarts,
+    getPredictedMenses
   }
 }