drip/lib/cycle-length.js

export function getCycleLengthStats(cycleLengths) {
  throwIfArgsAreNotInRequiredFormat(cycleLengths)
  const cycleLengthStats = {}
  const sortedCycleLengths = cycleLengths.sort((a, b) => {
    return a - b
  })
  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) {
    cycleLengthStats.median =
      sortedCycleLengths[(cycleLengths.length + 1) / 2 - 1]
  } else {
    const middle = cycleLengths.length / 2
    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 - cycleLengthStats.mean, 2)
      })
      .reduce(getSum)
    cycleLengthStats.stdDeviation =
      Math.round(Math.sqrt(sumOfSquares / (cycleLengths.length - 1)) * 100) /
      100
  } else {
    cycleLengthStats.stdDeviation = null
  }
  return cycleLengthStats
}

function getSum(total, num) {
  return total + num
}

function throwIfArgsAreNotInRequiredFormat(cycleLengths) {
  if (!Array.isArray(cycleLengths)) throw new Error('Input should be an array.')
  if (cycleLengths.length < 1)
    throw new Error('Input array should not be empty.')
  cycleLengths.forEach((cycleLength) => {
    if (typeof cycleLength !== 'number' || Number.isNaN(cycleLength))
      throw new Error('Elements in the array should be of type number.')
  })
}