How do you geofence 500 stores when an app may monitor only 20 regions at a time?

Predict first

What changes between refreshes?

import Foundation
import CoreLocation

struct Store { let id: Int; let loc: CLLocation }
var seed: UInt64 = 42
func rnd() -> Double { seed = seed &* 6364136223846793005 &+ 1442695040888963407; return Double(seed >> 33) / Double(1 << 31) }
let stores = (0..<500).map { Store(id: $0, loc: CLLocation(latitude: 18.3 + rnd() * 0.5, longitude: 73.6 + rnd() * 0.5)) }

func nearest(_ n: Int, to p: CLLocation) -> [Store] { Array(stores.sorted { $0.loc.distance(from: p) < $1.loc.distance(from: p) }.prefix(n)) }
var monitored = Set<Int>()
func refresh(at p: CLLocation) {
    let want = Set(nearest(20, to: p).map(\.id))
    let stop = monitored.subtracting(want), start = want.subtracting(monitored)
    print("monitoring \(want.count): stop \(stop.count), start \(start.count)")
    monitored = want
}
let a = CLLocation(latitude: 18.52, longitude: 73.85)
refresh(at: a)
refresh(at: a)                                              // nothing moved
refresh(at: CLLocation(latitude: 18.53, longitude: 73.86))   // moved about 1.5 km
let far = nearest(20, to: a).last!
print("20th nearest store is", Int(far.loc.distance(from: a)), "m away")

• 20 start, then nothing, then 2 stop and 2 start
• 20 start every time
• Nothing starts the first time