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

lib/cycle-length.js

@@ -1,6 +1,8 @@
 import assert from 'assert'
+import { LocalDate, ChronoUnit } from 'js-joda'
+import cycleModule from '../lib/cycle'
 
-export default function getCycleLengthStats(cycleLengths) {
+export function getCycleLengthStats(cycleLengths) {
   throwIfArgsAreNotInRequiredFormat(cycleLengths)
   const cycleLengthStats = {}
   const sortedCycleLengths = cycleLengths.sort((a, b) => {
@@ -46,4 +48,14 @@ function throwIfArgsAreNotInRequiredFormat(cycleLengths) {
     assert.equal(typeof cycleLength, 'number', 'Elements in the array should be of type number.')
     assert.ok(!isNaN(cycleLength), 'Elements of array should not be NaN.')
   })
+}
+
+export function getCycleLength(cycleStartDates) {
+  const cycleLengths = []
+  for (let i = 0; i < cycleStartDates.length - 1; i++) {
+    const nextCycleStart = LocalDate.parse(cycleStartDates[i])
+    const cycleStart = LocalDate.parse(cycleStartDates[i + 1])
+    cycleLengths.push(cycleStart.until(nextCycleStart, ChronoUnit.DAYS))
+  }
+  return cycleLengths
 }