computes the stats about the period lengths, started tests

lib/periode-length.js

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+export default function getPeriodLengthStats(cycleLengthArray) {
+  if (Array.isArray(cycleLengthArray) && (cycleLengthArray.length > 0) && cycleLengthArray.every(cycleLength => {
+    return (typeof cycleLength === 'number') && !(isNaN(cycleLength))
+  })) {
+    const PeriodLengthStats = {}
+    const sortedCycleLegthArray = cycleLengthArray.sort((a, b) => {
+      return a - b
+    })
+    PeriodLengthStats.minimum = sortedCycleLegthArray[0]
+    PeriodLengthStats.maximum = sortedCycleLegthArray[cycleLengthArray.length - 1]
+    PeriodLengthStats.mean = cycleLengthArray.reduce(getSum) / cycleLengthArray.length
+    // median
+    if (cycleLengthArray.length % 2 == 1) {
+      PeriodLengthStats.median = sortedCycleLegthArray[(cycleLengthArray.length + 1) / 2 - 1]
+    }
+    else {
+      const middle = cycleLengthArray.length / 2
+      PeriodLengthStats.median = (sortedCycleLegthArray[middle - 1] + sortedCycleLegthArray[middle]) / 2
+    }
+
+    PeriodLengthStats.stdDeviation = 0 // default
+    if (cycleLengthArray.length > 1) {
+      const sumOfSquares = cycleLengthArray.map(cycleLength => {
+        return Math.pow(cycleLength - PeriodLengthStats.mean, 2)
+      }).reduce(getSum)
+      PeriodLengthStats.stdDeviation = Math.sqrt(sumOfSquares / (cycleLengthArray.length - 1 ))
+    }
+    return PeriodLengthStats
+  }
+  console.error('getPeriodLengthStats requiers an array of numbers with length > 0.')
+}
+
+function getSum(total, num) {
+  return total + num
+}

test/periode-length.spec.js

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+import chai from 'chai'
+import periodInfo from '../lib/period-length'
+
+const expect = chai.expect
+
+describe('it calculates the median correctly', () => {
+  it('works for an odd-numbered array', () => {
+    const periodLengths = [1, 2, 5, 99, 100]
+    const result = periodInfo(periodLengths).median
+    expect(result).to.eql(5)
+  })
+
+  /* it('works for an even-numbered array', () => {
+
+  }) */
+})