resolves some of the suggested improvements, renames the term period with cycle

components/stats.js

 import React, { Component } from 'react'
 import {
   Text,
+  View,
   ScrollView
 } from 'react-native'
 import { LocalDate, ChronoUnit } from 'js-joda'
 import styles from '../styles/index'
 import cycleModule from '../lib/cycle'
-import getPeriodInfo from '../lib/period-length'
+import getCycleInfo from '../lib/cycle-length'
 
 export default class Stats extends Component {
-  constructor(props) {
-    super(props)
-    const allMensesStarts = cycleModule().getAllMensesStarts()
-    this.test = allMensesStarts
-    this.state = {
-      text: determineStatsText(allMensesStarts)
-    }
-  }
-
   render() {
+    const allMensesStarts = cycleModule().getAllMensesStarts()
+    const statsText = determineStatsText(allMensesStarts)
     return (
       <ScrollView>
-        <Text style={styles.welcome}>{this.state.text}</Text>
+        <View>
+          <Text style={styles.stats}>{statsText}</Text>
+        </View>
       </ScrollView>
     )
   }
 }
 
 function getCycleLength(cycleStartDates) {
-  const periodLengths = []
+  const cycleLengths = []
   for (let i = 0; i < cycleStartDates.length - 1; i++) {
-    const nextPeriodStart = LocalDate.parse(cycleStartDates[i])
-    const periodStart = LocalDate.parse(cycleStartDates[i + 1])
-    periodLengths.push(periodStart.until(nextPeriodStart, ChronoUnit.DAYS))
+    const nextCycleStart = LocalDate.parse(cycleStartDates[i])
+    const cycleStart = LocalDate.parse(cycleStartDates[i + 1])
+    cycleLengths.push(cycleStart.until(nextCycleStart, ChronoUnit.DAYS))
   }
-  return periodLengths
+  return cycleLengths
 }
 
 function determineStatsText(allMensesStarts) {
-  const emptyStats = 'At least one completed period is needed to present you with stats here.'
+  const emptyStats = 'At least one completed cycle is needed to present you with stats here.'
   if (allMensesStarts.length < 2) {
     return emptyStats
   } else {
     const cycleLengths = getCycleLength(allMensesStarts)
     const numberOfCycles = cycleLengths.length
-    const periodInfo = getPeriodInfo(cycleLengths)
     if (numberOfCycles === 1) {
-      return `You have documented one period of ${cycleLengths[0]} days.`
-    } else {
-      const statsText = `Stats are based on ${numberOfCycles} completed 
-        periods.\n\n
-        Average period length: ${periodInfo.mean} days\n\n
-        shortest period: ${periodInfo.minimum} days\n
-        longest period: ${periodInfo.maximum} days\n
-        median length (meaning 50% of periods are of this length or shorter):
-         ${periodInfo.median} days\n
-        standard deviation: ${periodInfo.stdDeviation}`
-      return statsText
+      return `You have documented one cycle of ${cycleLengths[0]} days.`
     }
+    const cycleInfo = getCycleInfo(cycleLengths)
+    const statsText = `Stats are based on ${numberOfCycles} completed cycles.\n\n\
+    Average cycle length: ${cycleInfo.mean} days\n\nShortest cycle: ${cycleInfo.minimum} days\nLongest cycle: ${cycleInfo.maximum} days\nMedian length (meaning 50% of cycles are of this length or shorter): ${cycleInfo.median} days\nStandard deviation: ${cycleInfo.stdDeviation}`
+    return statsText
   }
 }
\ No newline at end of file

lib/period-length.js → lib/cycle-length.js

 import assert from 'assert'
 
-export default function getPeriodLengthStats(cycleLengths) {
+export default function getCycleLengthStats(cycleLengths) {
   throwIfArgsAreNotInRequiredFormat(cycleLengths)
-  const periodLengthStats = {}
+  const cycleLengthStats = {}
   const sortedCycleLengths = cycleLengths.sort((a, b) => {
     return a - b
   })
-  periodLengthStats.minimum = sortedCycleLengths[0]
-  periodLengthStats.maximum = sortedCycleLengths[cycleLengths.length - 1]
-  periodLengthStats.mean = Math.round(
+  cycleLengthStats.minimum = sortedCycleLengths[0]
+  cycleLengthStats.maximum = sortedCycleLengths[cycleLengths.length - 1]
+  cycleLengthStats.mean = Math.round(
     cycleLengths.reduce(getSum) / cycleLengths.length * 100
   ) / 100
   // median
   if (cycleLengths.length % 2 == 1) {
-    periodLengthStats.median = sortedCycleLengths[
+    cycleLengthStats.median = sortedCycleLengths[
       (cycleLengths.length + 1) / 2 - 1
     ]
   } else {
     const middle = cycleLengths.length / 2
-    periodLengthStats.median = (sortedCycleLengths[middle - 1] +
+    cycleLengthStats.median = (sortedCycleLengths[middle - 1] +
        sortedCycleLengths[middle]) / 2
   }
   // corrected standard deviation (based on unbiased sample variance)
   if (cycleLengths.length > 1) {
     const sumOfSquares = cycleLengths.map(cycleLength => {
-      return Math.pow(cycleLength - periodLengthStats.mean, 2)
+      return Math.pow(cycleLength - cycleLengthStats.mean, 2)
     }).reduce(getSum)
-    periodLengthStats.stdDeviation = Math.round(
+    cycleLengthStats.stdDeviation = Math.round(
       Math.sqrt(sumOfSquares / (cycleLengths.length - 1 )) * 100
     ) / 100
   } else {
-    periodLengthStats.stdDeviation = null
+    cycleLengthStats.stdDeviation = null
   }
-  return periodLengthStats
+  return cycleLengthStats
 }
 
 function getSum(total, num) {

styles/index.js

@@ -82,5 +82,11 @@ export default StyleSheet.create({
     marginTop: 15,
     marginLeft: 'auto',
     marginRight: 'auto'
+  },
+  stats: {
+    fontSize: 18,
+    margin: 30,
+    textAlign: 'left',
+    textAlignVertical: 'center'
   }
 })
\ No newline at end of file

test/period-length.spec.js → test/cycle-length.spec.js

 import chai from 'chai'
 import { AssertionError } from 'assert'
 
-import periodInfo from '../lib/period-length'
+import cycleInfo from '../lib/cycle-length'
 
 const expect = chai.expect
 
-describe('getPeriodLengthStats', () => {
+describe('getCycleLengthStats', () => {
   it('works for a simple odd-numbered array', () => {
-    const periodLengths = [99, 5, 1, 2, 100]
-    const result = periodInfo(periodLengths)
+    const cycleLengths = [99, 5, 1, 2, 100]
+    const result = cycleInfo(cycleLengths)
     const expectedResult = {
       minimum: 1,
       maximum: 100,
@@ -20,8 +20,8 @@ describe('getPeriodLengthStats', () => {
   })
 
   it('works for a simple even-numbered array', () => {
-    const periodLengths = [4, 1, 15, 2, 20, 5]
-    const result = periodInfo(periodLengths)
+    const cycleLengths = [4, 1, 15, 2, 20, 5]
+    const result = cycleInfo(cycleLengths)
     const expectedResult = {
       minimum: 1,
       maximum: 20,
@@ -32,8 +32,8 @@ describe('getPeriodLengthStats', () => {
     expect(result).to.eql(expectedResult)
   })
   it('works for an one-element array', () => {
-    const periodLengths = [42]
-    const result = periodInfo(periodLengths)
+    const cycleLengths = [42]
+    const result = cycleInfo(cycleLengths)
     const expectedResult = {
       minimum: 42,
       maximum: 42,
@@ -45,20 +45,20 @@ describe('getPeriodLengthStats', () => {
   })
   describe('when args are wrong', () => {
     it('throws when arg object is an empty array', () => {
-      const periodLengths = []
-      expect(() => periodInfo(periodLengths).to.throw(AssertionError))
+      const cycleLengths = []
+      expect(() => cycleInfo(cycleLengths).to.throw(AssertionError))
     })
     it('throws when arg object is not in right format', () => {
       const wrongObject = { hello: 'world' }
-      expect(() => periodInfo(wrongObject).to.throw(AssertionError))
+      expect(() => cycleInfo(wrongObject).to.throw(AssertionError))
     })
     it('throws when arg array contains a string', () => {
       const wrongElement = [4, 1, 15, '2', 20, 5]
-      expect(() => periodInfo(wrongElement).to.throw(AssertionError))
+      expect(() => cycleInfo(wrongElement).to.throw(AssertionError))
     })
     it('throws when arg array contains a NaN', () => {
       const wrongElement = [4, 1, 15, NaN, 20, 5]
-      expect(() => periodInfo(wrongElement).to.throw(AssertionError))
+      expect(() => cycleInfo(wrongElement).to.throw(AssertionError))
     })
   })
 })
\ No newline at end of file