Merge branch '109-display-stats-about-period-length' into 'master'

Resolve "add stats button on home screen and show stats about period length"

Closes #109

See merge request bloodyhealth/drip!41

components/home.js

@@ -87,6 +87,12 @@ export default class Home extends Component {
               title="delete everything">
             </Button>
           </View>
+          <View style={styles.homeButton}>
+            <Button
+              onPress={() => navigate('stats')}
+              title="Go to stats">
+            </Button>
+          </View>
         </View>
       </ScrollView>
     )

components/stats.js

@@ -0,0 +1,51 @@
+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 getCycleInfo from '../lib/cycle-length'
+
+export default class Stats extends Component {
+  render() {
+    const allMensesStarts = cycleModule().getAllMensesStarts()
+    const statsText = determineStatsText(allMensesStarts)
+    return (
+      <ScrollView>
+        <View>
+          <Text style={styles.stats}>{statsText}</Text>
+        </View>
+      </ScrollView>
+    )
+  }
+}
+
+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
+}
+
+function determineStatsText(allMensesStarts) {
+  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
+    if (numberOfCycles === 1) {
+      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
+  }
+}