winnow/lib/calculate-class-breaks/index.js

Apply sql-like filters to GeoJSON

const Classifier = require('classybrew')
const normalizeClassificationValues = require('./normalize-classification-values')
const calculateStdDevIntervals = require('./calculate-std-dev-intervals')
const transformClassBreaksToRanges = require('./transform-class-breaks-to-ranges')
const filterAndValidateClassificationFeatures = require('./filter-and-validate-classification-features')

function calculateClassBreaks (features, classification) {
  if (!classification.method) throw new Error('must supply classification.method')
  if (!classification.breakCount) throw new Error('must supply classification.breakCount')

  const values = classification.normType ? normalizeClassificationValues(features, classification) : getFieldValues(features, classification.field)
  // limit break count to num values
  if (classification.breakCount > values.length) classification.breakCount = values.length

  // calculate break ranges [ [a-b], [b-c], ...] from input values
  const classBreakArray = transformValuesToClassBreaksArray(values, classification)
  return transformClassBreaksToRanges(classBreakArray, classification)
}

function transformValuesToClassBreaksArray (values, classification) {
  const {
    breakCount,
    method
  } = classification
  const classifier = new Classifier()
  classifier.setSeries(values)
  classifier.setNumClasses(breakCount)

  switch (classification.method) {
    case 'equalInterval': return classifier.classify('equal_interval')
    case 'naturalBreaks': return classifier.classify('jenks')
    case 'quantile': return classifier.classify('quantile')
    case 'geomInterval': throw new Error('Classification method not yet supported')
    case 'stddev': return calculateStdDevIntervals(values, classification)
    default: throw new Error('invalid classificationMethod: ' + method)
  }
}

function getFieldValues (features, classificationField) {
  const values = filterAndValidateClassificationFeatures(features, classificationField).map(value => {
    return value <= 0 ? 0 : value
  })

  if (!values || values.length === 0) {
    throw new Error(`"${classificationField}" was not found on any feature.`)
  }
  return values
}

module.exports = calculateClassBreaks