Extract GeohashParticipantTracker from ChatViewModel

Refactor participant tracking logic into a dedicated, testable class:

- Create GeohashParticipantTracker with GeohashParticipantContext protocol
- Move GeoPerson struct to new file
- Extract participant recording, refresh, and count logic
- Add 18 comprehensive tests for the tracker
- Update ChatViewModel to use tracker via dependency injection
- Subscribe to tracker's objectWillChange for SwiftUI observation

This reduces coupling in ChatViewModel and makes participant tracking
testable in isolation.

bitchatTests/GeohashParticipantTrackerTests.swift

@@ -0,0 +1,293 @@
+//
+// GeohashParticipantTrackerTests.swift
+// bitchatTests
+//
+// Tests for GeohashParticipantTracker.
+// This is free and unencumbered software released into the public domain.
+//
+
+import Testing
+import Foundation
+@testable import bitchat
+
+/// Mock context for testing
+@MainActor
+final class MockParticipantContext: GeohashParticipantContext {
+    var blockedPubkeys: Set<String> = []
+    var nicknameMap: [String: String] = [:]
+    var selfPubkey: String?
+
+    func displayNameForPubkey(_ pubkeyHex: String) -> String {
+        let suffix = String(pubkeyHex.suffix(4))
+        if let self = selfPubkey, pubkeyHex.lowercased() == self.lowercased() {
+            return "me#\(suffix)"
+        }
+        if let nick = nicknameMap[pubkeyHex.lowercased()] {
+            return "\(nick)#\(suffix)"
+        }
+        return "anon#\(suffix)"
+    }
+
+    func isBlocked(_ pubkeyHexLowercased: String) -> Bool {
+        blockedPubkeys.contains(pubkeyHexLowercased.lowercased())
+    }
+}
+
+@MainActor
+struct GeohashParticipantTrackerTests {
+
+    // MARK: - Basic Recording Tests
+
+    @Test func recordParticipant_addsToActiveGeohash() async {
+        let tracker = GeohashParticipantTracker()
+        let context = MockParticipantContext()
+        tracker.configure(context: context)
+        tracker.setActiveGeohash("abc123")
+
+        tracker.recordParticipant(pubkeyHex: "deadbeef1234")
+
+        #expect(tracker.participantCount(for: "abc123") == 1)
+    }
+
+    @Test func recordParticipant_noActiveGeohash_noOp() async {
+        let tracker = GeohashParticipantTracker()
+        let context = MockParticipantContext()
+        tracker.configure(context: context)
+        // No active geohash set
+
+        tracker.recordParticipant(pubkeyHex: "deadbeef1234")
+
+        // Should not throw or crash
+        #expect(tracker.participantCount(for: "abc123") == 0)
+    }
+
+    @Test func recordParticipant_specificGeohash() async {
+        let tracker = GeohashParticipantTracker()
+        let context = MockParticipantContext()
+        tracker.configure(context: context)
+
+        tracker.recordParticipant(pubkeyHex: "pubkey1", geohash: "geo1")
+        tracker.recordParticipant(pubkeyHex: "pubkey2", geohash: "geo2")
+
+        #expect(tracker.participantCount(for: "geo1") == 1)
+        #expect(tracker.participantCount(for: "geo2") == 1)
+    }
+
+    @Test func recordParticipant_updatesLastSeen() async {
+        let tracker = GeohashParticipantTracker()
+        let context = MockParticipantContext()
+        tracker.configure(context: context)
+        tracker.setActiveGeohash("abc123")
+
+        tracker.recordParticipant(pubkeyHex: "pubkey1")
+        // Small delay and record again
+        try? await Task.sleep(nanoseconds: 10_000_000) // 10ms
+        tracker.recordParticipant(pubkeyHex: "pubkey1")
+
+        // Should still count as 1 participant (updated, not duplicated)
+        #expect(tracker.participantCount(for: "abc123") == 1)
+    }
+
+    @Test func recordParticipant_lowercasesPubkey() async {
+        let tracker = GeohashParticipantTracker()
+        let context = MockParticipantContext()
+        tracker.configure(context: context)
+        tracker.setActiveGeohash("abc123")
+
+        tracker.recordParticipant(pubkeyHex: "DEADBEEF")
+        tracker.recordParticipant(pubkeyHex: "deadbeef")
+
+        // Should be treated as same participant
+        #expect(tracker.participantCount(for: "abc123") == 1)
+    }
+
+    // MARK: - Visible People Tests
+
+    @Test func getVisiblePeople_returnsActiveGeohashParticipants() async {
+        let tracker = GeohashParticipantTracker()
+        let context = MockParticipantContext()
+        tracker.configure(context: context)
+        tracker.setActiveGeohash("abc123")
+
+        tracker.recordParticipant(pubkeyHex: "pubkey1")
+        tracker.recordParticipant(pubkeyHex: "pubkey2")
+
+        let people = tracker.getVisiblePeople()
+        #expect(people.count == 2)
+    }
+
+    @Test func getVisiblePeople_excludesBlockedParticipants() async {
+        let tracker = GeohashParticipantTracker()
+        let context = MockParticipantContext()
+        context.blockedPubkeys = ["pubkey2"]
+        tracker.configure(context: context)
+        tracker.setActiveGeohash("abc123")
+
+        tracker.recordParticipant(pubkeyHex: "pubkey1")
+        tracker.recordParticipant(pubkeyHex: "pubkey2")
+
+        let people = tracker.getVisiblePeople()
+        #expect(people.count == 1)
+        #expect(people.first?.id == "pubkey1")
+    }
+
+    @Test func getVisiblePeople_usesDisplayNameFromContext() async {
+        let tracker = GeohashParticipantTracker()
+        let context = MockParticipantContext()
+        context.nicknameMap = ["pubkey1234": "alice"]
+        tracker.configure(context: context)
+        tracker.setActiveGeohash("abc123")
+
+        tracker.recordParticipant(pubkeyHex: "pubkey1234")
+
+        let people = tracker.getVisiblePeople()
+        #expect(people.count == 1)
+        #expect(people.first?.displayName == "alice#1234")
+    }
+
+    @Test func getVisiblePeople_sortedByLastSeen() async {
+        let tracker = GeohashParticipantTracker()
+        let context = MockParticipantContext()
+        tracker.configure(context: context)
+        tracker.setActiveGeohash("abc123")
+
+        tracker.recordParticipant(pubkeyHex: "older")
+        try? await Task.sleep(nanoseconds: 10_000_000) // 10ms
+        tracker.recordParticipant(pubkeyHex: "newer")
+
+        let people = tracker.getVisiblePeople()
+        #expect(people.count == 2)
+        #expect(people.first?.id == "newer")
+        #expect(people.last?.id == "older")
+    }
+
+    @Test func getVisiblePeople_emptyWhenNoActiveGeohash() async {
+        let tracker = GeohashParticipantTracker()
+        let context = MockParticipantContext()
+        tracker.configure(context: context)
+
+        tracker.recordParticipant(pubkeyHex: "pubkey1", geohash: "abc123")
+
+        let people = tracker.getVisiblePeople()
+        #expect(people.isEmpty)
+    }
+
+    // MARK: - Activity Cutoff Tests
+
+    @Test func participantCount_excludesExpiredEntries() async {
+        // Use a very short cutoff for testing
+        let tracker = GeohashParticipantTracker(activityCutoff: -0.05) // 50ms cutoff
+        let context = MockParticipantContext()
+        tracker.configure(context: context)
+        tracker.setActiveGeohash("abc123")
+
+        tracker.recordParticipant(pubkeyHex: "pubkey1")
+
+        // Should be counted immediately
+        #expect(tracker.participantCount(for: "abc123") == 1)
+
+        // Wait for expiry
+        try? await Task.sleep(nanoseconds: 100_000_000) // 100ms
+
+        // Should be expired now
+        #expect(tracker.participantCount(for: "abc123") == 0)
+    }
+
+    // MARK: - Remove Participant Tests
+
+    @Test func removeParticipant_removesFromAllGeohashes() async {
+        let tracker = GeohashParticipantTracker()
+        let context = MockParticipantContext()
+        tracker.configure(context: context)
+
+        tracker.recordParticipant(pubkeyHex: "pubkey1", geohash: "geo1")
+        tracker.recordParticipant(pubkeyHex: "pubkey1", geohash: "geo2")
+        tracker.recordParticipant(pubkeyHex: "pubkey2", geohash: "geo1")
+
+        tracker.removeParticipant(pubkeyHex: "pubkey1")
+
+        #expect(tracker.participantCount(for: "geo1") == 1)
+        #expect(tracker.participantCount(for: "geo2") == 0)
+    }
+
+    // MARK: - Clear Tests
+
+    @Test func clear_removesAllData() async {
+        let tracker = GeohashParticipantTracker()
+        let context = MockParticipantContext()
+        tracker.configure(context: context)
+        tracker.setActiveGeohash("abc123")
+
+        tracker.recordParticipant(pubkeyHex: "pubkey1")
+        tracker.recordParticipant(pubkeyHex: "pubkey2", geohash: "other")
+
+        tracker.clear()
+
+        #expect(tracker.participantCount(for: "abc123") == 0)
+        #expect(tracker.participantCount(for: "other") == 0)
+        #expect(tracker.visiblePeople.isEmpty)
+    }
+
+    @Test func clearGeohash_removesOnlySpecificGeohash() async {
+        let tracker = GeohashParticipantTracker()
+        let context = MockParticipantContext()
+        tracker.configure(context: context)
+
+        tracker.recordParticipant(pubkeyHex: "pubkey1", geohash: "geo1")
+        tracker.recordParticipant(pubkeyHex: "pubkey2", geohash: "geo2")
+
+        tracker.clear(geohash: "geo1")
+
+        #expect(tracker.participantCount(for: "geo1") == 0)
+        #expect(tracker.participantCount(for: "geo2") == 1)
+    }
+
+    // MARK: - Set Active Geohash Tests
+
+    @Test func setActiveGeohash_clearsVisiblePeopleWhenNil() async {
+        let tracker = GeohashParticipantTracker()
+        let context = MockParticipantContext()
+        tracker.configure(context: context)
+        tracker.setActiveGeohash("abc123")
+        tracker.recordParticipant(pubkeyHex: "pubkey1")
+
+        #expect(!tracker.visiblePeople.isEmpty)
+
+        tracker.setActiveGeohash(nil)
+
+        #expect(tracker.visiblePeople.isEmpty)
+    }
+
+    @Test func setActiveGeohash_refreshesVisiblePeople() async {
+        let tracker = GeohashParticipantTracker()
+        let context = MockParticipantContext()
+        tracker.configure(context: context)
+
+        // Pre-populate a geohash
+        tracker.recordParticipant(pubkeyHex: "pubkey1", geohash: "abc123")
+
+        // Set it as active
+        tracker.setActiveGeohash("abc123")
+
+        #expect(tracker.visiblePeople.count == 1)
+    }
+
+    // MARK: - GeoPerson Tests
+
+    @Test func geoPerson_identifiable() async {
+        let person1 = GeoPerson(id: "abc", displayName: "alice", lastSeen: Date())
+        let person2 = GeoPerson(id: "abc", displayName: "alice", lastSeen: Date())
+        let person3 = GeoPerson(id: "xyz", displayName: "bob", lastSeen: Date())
+
+        #expect(person1.id == person2.id)
+        #expect(person1.id != person3.id)
+    }
+
+    @Test func geoPerson_equatable() async {
+        let date = Date()
+        let person1 = GeoPerson(id: "abc", displayName: "alice", lastSeen: date)
+        let person2 = GeoPerson(id: "abc", displayName: "alice", lastSeen: date)
+
+        #expect(person1 == person2)
+    }
+}