Get nearby limit users location in dart code

✅ Main method

void main() {
double userLat = 19.0760;
double userLon = 72.8777;
allUsers.sort((a, b) {
GeoPointModel locA = a[‘current_location’] as GeoPointModel;
GeoPointModel locB = b[‘current_location’] as GeoPointModel;

double distanceA = calculateDistance(userLat, userLon, locA.latitude, locA.longitude);
double distanceB = calculateDistance(userLat, userLon, locB.latitude, locB.longitude);

return distanceA.compareTo(distanceB); // Sort nearest first
});
for (var user in allUsers) {
print(user[‘name’]);
}
}

✅ GeoPointModel is model class of GeoLocation

class GeoPointModel {
final double latitude;
final double longitude;
GeoPointModel(this.latitude, this.longitude);
}

List<Map<String, dynamic>> allUsers = [
{
‘name’: ‘User A’,
‘current_location’: GeoPointModel(12.9716, 77.5946), // Bangalore
},
{
‘name’: ‘User B’,
‘current_location’: GeoPointModel(28.7041, 77.1025), // Delhi
},
{
‘name’: ‘User C’,
‘current_location’: GeoPointModel(19.0760, 72.8777), // Mumbai
},
{
‘name’: ‘User D’,
‘current_location’: GeoPointModel(13.0827, 80.2707), // Chennai
},
];

✅ Haversine Distance Function

import ‘dart:math’;

double calculateDistance(double lat1, double lon1, double lat2, double lon2) {
const earthRadius = 6371; // Earth’s radius in km
double dLat = _degToRad(lat2 — lat1);
double dLon = _degToRad(lon2 — lon1);

double a = sin(dLat / 2) * sin(dLat / 2) +
cos(_degToRad(lat1)) * cos(_degToRad(lat2)) *
sin(dLon / 2) * sin(dLon / 2);

double c = 2 * atan2(sqrt(a), sqrt(1 — a));
return earthRadius * c;
}

double _degToRad(double deg) => deg * (pi / 180);

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The Haversine formula calculates the distance between two points on a sphere. The formula is broken down as follows:

double a = sin(dLat / 2) * sin(dLat / 2) +
           cos(_degToRad(lat1)) * cos(_degToRad(lat2)) *
           sin(dLon / 2) * sin(dLon / 2);

• sin(dLat / 2) * sin(dLat / 2): Square of the sine of half the difference in latitude.
• cos(_degToRad(lat1)) * cos(_degToRad(lat2)): Product of the cosines of the latitudes.
• sin(dLon / 2) * sin(dLon / 2): Square of the sine of half the difference in longitude.

double c = 2 * atan2(sqrt(a), sqrt(1 - a));

• atan2(sqrt(a), sqrt(1 - a)): This calculates the angular distance in radians between the two points.
• Multiply by 2 to get the central angle.

return earthRadius * c;

Multiply the central angle c by the Earth's radius (earthRadius) to get the distance in kilometers.

Summary of the Formula

The Haversine formula is:

Where:

• RR is the Earth’s radius.
• Δlat is the difference in latitude.
• Δlon is the difference in longitude.

This calculates the square of half the chord length between the two points.

✅ Sorting by Distance
Let’s assume the current user is in Mumbai (19.0760, 72.8777):

✅ Expected Output:

User C // Mumbai (self)
User A // Bangalore
User D // Chennai
User B // Delhi

✅ Summary
Use .sort() directly on the list.
Calculate distance inside the sort’s comparison function.
Use compareTo() to sort by nearest distance.