Encontrar taxis cercanos usando la fórmula Great Circle Distance

// C++ code to find cabs nearby
#include <bits/stdc++.h>
using namespace std;
  
// Latitude of customer who needs a cab.
#define lat1d 12.9611159
  
// Longitude of customer who needs a cab.
#define lon1d 77.6362214
  
#define pi 3.14159265358979323846
#define earth_radius 6371.0
  
ifstream customer_list ("customers.json");
ofstream out ("answer.json");
  
// Function to convert degree to radian.
double degtorad(double deg)
{
    return ( deg * pi / 180);
}
  
// Function to calculate distance
// between 2 given locations 
// using Great Circle Distance Formula.
double distanceEarth(double lat2d, double lon2d)
{                 
    double lat1, lon1, lat2, lon2, 
           delta_lon, central_ang;
  
    lat1 = degtorad(lat1d);
    lon1 = degtorad(lon1d);
    lat2 = degtorad(lat2d);
    lon2 = degtorad(lon2d);
  
    delta_lon = lon2 - lon1;
      
    // great circle distance formula.
    central_ang = acos ( sin(lat1) *
                  sin(lat2) + cos(lat1) *
                  cos(lat2) * cos(delta_lon) ); 
                    
    return (earth_radius * central_ang);
}
  
// Structure which contains data and
// functions for accessing and processing
// data from the given customers.json file.
struct json
{
    /* i and j are used to access various
    elements of the char arrays. x is used
    to measure the size of the element of
    latitude_as_string array. y is used to
    measure the size of the element of
    longitude_as_string array. m is used
    to measure the size of the element
    of id_as_string array. n is used to
    measure the size of the element of
    name array. f keeps count of " " "
    symbol. fi keeps count of " : " symbol.
    */
    long long int length, i, j, x, y, m,
                  n, f, fi, id[100000];
      
    char latitude_as_string[1000], 
         longitude_as_string[1000], 
         id_as_string[1000], name[1000];
      
    double lat2d, lon2d;
      
    // To get each line of customers.json
    // file as string.
    string line;
  
    // Function to check whether distance between
    // 2 points is less than 50km or not.
    void distance_calculator()
    {
        if (distanceEarth(lat2d, lon2d) <= 50.0000)
        {
            // Converting id to int format.
            id[i] = atoll(id_as_string);
            i++;
            out << "{\"user_id\": " << id[i - 1] << 
                ", \"name\": " << name << "}" << endl;
        }
    }
  
    // Function to read various attributes
    // like latitude, longitude, name , id,
    // etc, from customers.json file. simplistic
    // approach is used to get JSON attributes.
    void json_parser()
    {                     
        if (customer_list.is_open())
        {
              
            while (getline(customer_list, line))
            {
                  
                f = 0; x = 0; y = 0; fi = 0; m = 0, n = 0;
                length = line.size();
  
                for (j = 0; j < length; j++)
                {
                      
                    if (line[j] == '"')
                        f++;
  
                    else if (line[j] == ':')
                        fi++;
                          
                    // To get latitude of the location.    
                    if (f == 3)
                    {
                        j++;
  
                        while (line[j] != '"')
                        {
                            latitude_as_string[x] = line[j];
                            x++; j++;
                        }
  
                        j--; latitude_as_string[x] = '\0';
                    }
                      
                    // To get longitude of the location.
                    else if (f == 13)
                    {
                        j++;
  
                        while (line[j] != '"')
                        {
                            longitude_as_string[y] = line[j];
                            y++; j++;
                        }
  
                        j--; longitude_as_string[y] = '\0';
                    }
                      
                    // To get id of the friend.
                    if (fi == 2)
                    {
                        j += 2;
  
                        while (line[j] != ',')
                        {
                            id_as_string[m] = line[j];
                            m++; j++;
                        }
  
                        j--; id_as_string[m] = '\0';
                        fi++;
                    }
                      
                    // To get name of the friend.
                    else if (fi == 4)
                    {
                        j += 2;
  
                        while (line[j] != ',')
                        {
                            name[n] = line[j];
                            n++; j++;
                        }
  
                        j--; name[n] = '\0';
                        fi++; f += 2;
                    }
                }
  
                // Converting latitude and longitude
                // in string to float.
                lat2d = atof(latitude_as_string);
                lon2d = atof(longitude_as_string);
                distance_calculator();
            }
        }
          
        // closing stream of customer's file.
        customer_list.close();
          
        // closing stream of answer's file.
        out.close();
    }
};
  
int main()
{
    // Creating object of the structure json
    json obj;
      
    // To read customers.json file.
    obj.json_parser();
    return 0;
}