Dissolve connected GeoJSON LineStrings

connected-line-fixture.geojson

dissolve-lines.js

var meta = require('@turf/meta')
var lineString = require('@turf/helpers').lineString
var roundTo = require('round-to')

module.exports = function dissolve (fc) {
  var endPoints = {}
  var index = []
  var dissolvedFeatures = []

  meta.featureEach(fc, indexPoints)

  function indexPoints (f) {
    if (f.geometry.type === 'MultiLineString') {
      return f.geometry.coordinates.forEach(function (_) {
        indexPoints(lineString(_, f.properties))
      })
    }
    if (f.geometry.type !== 'LineString') return
    index.push(f)
    var id = index.length - 1
    var coords = getCoords(f)
    var p1 = getId(coords[0])
    var p2 = getId(coords[coords.length - 1])
    // Closed loops and single-point lines should not be dissolved
    if (p1 === p2) return
    if (!endPoints[p1]) endPoints[p1] = []
    endPoints[p1].push(id)
    if (!endPoints[p2]) endPoints[p2] = []
    endPoints[p2].push(id)
  }

  Object.keys(endPoints).forEach(function (pointId) {
    var ids = endPoints[pointId]
    if (ids.length !== 2) return
    var merged = merge(index[ids[0]], index[ids[1]])
    if (!merged) return
    index[ids[0]] = index[ids[1]] = merged
  })

  for (var i = 0; i < index.length; i++) {
    if (dissolvedFeatures.indexOf(index[i]) > -1) continue
    dissolvedFeatures.push(index[i])
  }

  return {
    type: 'FeatureCollection',
    features: dissolvedFeatures
  }
}

function getId (coord, precision) {
  if (!precision) return coord.join('/')
  return roundTo(coord[0], precision) + '/' + roundTo(coord[1])
}

function getCoords (f) {
  return f.geometry.coordinates
}

function merge (f1, f2) {
  var coords1 = getCoords(f1)
  var coords2 = getCoords(f2)
  var coords
  var s1 = coords1[0]
  var e1 = coords1[coords1.length - 1]
  var s2 = coords2[0]
  var e2 = coords2[coords2.length - 1]

  // TODO: Lines that both start or end at the same point are randomly
  // reversed to merge, leaving no guarantee on final line direction
  if (coordsEq(e1, s2)) {
    coords = coords1.concat(coords2.slice(1))
  } else if (coordsEq(e2, s1)) {
    coords = coords2.concat(coords1.slice(1))
  } else if (coordsEq(e1, e2)) {
    coords = coords1.concat(coords2.slice(0, -1).reverse())
  } else if (coordsEq(s1, s2)) {
    coords = coords1.concat(coords2.slice(1).reverse())
  } else {
    return null
  }

  var mergedProps = {}
  // If props are equal or undefined in one of the input features, then include in merged props
  for (var prop in f1.properties) {
    if (f2.properties.hasOwnProperty(prop) && f1.properties[prop] !== f2.properties[prop]) continue
    mergedProps[prop] = f1.properties[prop]
  }
  for (prop in f2.properties) {
    if (f1.properties.hasOwnProperty(prop)) continue
    mergedProps[prop] = f2.properties[prop]
  }
  return lineString(coords, mergedProps)
}

// Needs to check precision
function coordsEq (c1, c2) {
  return c1[0] === c2[0] && c1[1] === c2[1]
}