Reverse geocoding with big array is fastest way? - javascript and performance

Question

I have many points on Google Maps and I want to show for each point the nearest city (so a reverse geocoding). I have a multidimensional array like this:

citta_vicine = [];

var comuni = [
 ["Abano Terme (PD)",45.3594,11.7894],
 ["Abbadia Cerreto (LO)",45.3122,9.5928],
 ["Abbadia Lariana (LC)",45.8992,9.3336],
 ["Abbadia San Salvatore (SI)",42.8800,11.6775],
 ["Abbasanta (OR)",40.1250,8.8200]
]

//city, latitude, longitude

The problem is that my array has all city of Italy (8000 !) and is 300 Kb.

To get nearest city i can use this:

//this line will be inside for loop of points
 var city_near= estrapola_paese(50,lat,lng); //lat and lng are coordinates of these points
//


 function estrapola_paese(distanza,latB,longB){ 
  citta_vicine = [];
  for(var i= 0; i < comuni.length; i++){
    var dist_eqcity= dist_coords(comuni[i][1],comuni[i][2],latB,longB);
    if(dist_eqcity < distanza){
        citta_vicine.push([dist_eqcity, comuni[i][0]]);
    }
  }
  if(citta_vicine.length > 0){
    citta_vicine.sort(function(a, b) {
        return a - b;
    }); 
    return citta_vicine[0][1];
  }
  else{
    distanza = distanza+50;
    estrapola_paese(distanza,latB,longB);   
  }
 }

//calculate distance in Km between city and "point"
function dist_coords(latA,longA,latB,longB) {
 var R = 6372.795477598;
 var laA = latA * Math.PI/180;
 var laB = latB * Math.PI/180;
 var loA = longA * Math.PI/180;
 var loB = longB * Math.PI/180;
 var distanza = R * Math.acos(Math.sin(laA)*Math.sin(laB) + Math.cos(laA)*Math.cos(laB) * Math.cos(loA-loB));
 if(isNaN(distanza) == true){   
  distanza = 0;
 }
 return distanza;
} 

In short, for a question of performance, I consider (at first) only cities within a radius of 50 km from the point. If there are cities within 50 km, I add the city (and the distance) in the "citta_vicine" array and order the latter array from the lowest to the highest value. Therefore from the city closest to the most distant.

If instead there are no cities within 50 km then I perform the function "estrapola_paese" again but increase the radius to be considered of another 50 km.

---

I think CODE WORKS but I have many doubts:

1) The file weighs 459 KB: is it too much?

2) Is there a better way to do all this?

3) sorting of array citta_vicine is correct? If is not empty is like this:

   [
    ["tokyo",34],
    ["rome",24],
    ["paris",54]
   ]

using this:

   citta_vicine.sort(function(a, b) {
     return a - b;
   }); 

i will have this output:

   [        
    ["rome",24],
    ["tokyo",34],
    ["paris",54]
   ]

I hope you can help me and sorry for my English.

Answer 1

Step #1 Calculate distance of all locations.

Step #2 Sort the result by the distance value

Step #3 Find locations, repeat it at least one record is found.

Check the DEMO, to calculate a list with 7320 records took ~ 17.22119140625ms

const citites = [
  [`Abano Terme (PD)`, 45.3594, 11.7894],
  [`Abbadia Cerreto (LO)`, 45.3122, 9.5928],
  [`Abbadia Lariana (LC)`, 45.8992, 9.3336],
  [`Abbadia San Salvatore (SI)`, 42.8800, 11.6775],
  [`Abbasanta (OR)`, 40.1250, 8.8200]
]

function distance(lat, long) {
  const R = 6372.795477598
  const PI = Math.PI / 180
  return cities
    .map(city => {
      const laA = city[1] * PI
      const laB = lat * PI
      const loA = city[2] * PI
      const loB = long * PI
      const dist = R * Math.acos(
        Math.sin(laA) * Math.sin(laB) +
        Math.cos(laA) * Math.cos(laB) * Math.cos(loA - loB)
      ) || 0
      return { dist, city }
    })
    .sort((a, b) => a.dist - b.dist)
}


function nearest(dist, lat, long) {
  const locations = distance(lat, long)
  function find(delta) {
    const result = []
    for (let location of locations) {
      if (location.dist > delta) break
      result.push(location.city)
    }
    return result.length > 0
      ? result
      : find(delta + 50)
  }
  return find(dist)
}

const result = nearest(50, 41.89595563, 12.48325842)