winnow/lib/calculate-class-breaks/transform-class-breaks-to-ranges.js

Apply sql-like filters to GeoJSON

const _ = require('lodash')

function transformClassBreaksToRanges (classBreakArray, { method } = {}) {
  const arrayLength = classBreakArray.length
  return _.range(0, arrayLength - 1).map(index => {
    return [classBreakArray[index], classBreakArray[index + 1]]
  }).map((range, index) => {
    if (method === 'stddev' && index < arrayLength) {
      return stddevRangeAdjustments({ range, index, arrayLength })
    }
    if (index === 0) return range

    return rangeAdjustments(range)
  })
}

function stddevRangeAdjustments ({ range, index, arrayLength }) {
  const divisor = Math.pow(10, getPrecision(range[0]))
  const midpointIndex = Math.ceil(arrayLength / 2)
  if (index === midpointIndex - 1) {
    return range
  }
  if (index <= midpointIndex - 1) {
    const max = Math.round((range[1] - (1 / divisor)) * divisor) / divisor
    return [range[0], max]
  }

  const min = Math.round((range[0] + (1 / divisor)) * divisor) / divisor
  return [min, range[1]]
}

function rangeAdjustments (range) {
  const divisor = Math.pow(10, getPrecision(range[0]))
  const min = Math.round((range[0] + (1 / divisor)) * divisor) / divisor
  return [min, range[1]]
}

function getPrecision (value) {
  if (!isFinite(value)) return 0
  let e = 1
  let precision = 0
  while (Math.round(value * e) / e !== value) {
    e *= 10
    precision++
  }
  return precision
}

module.exports = transformClassBreaksToRanges