Add trie-based user search cache to replace non-performant linear scans

Changelog-Added: Speed up user search
Tested-by: William Casarin <jb55@jb55.com>
Fixes: #1219
Closes: #1342

damus/Models/UserSearchCache.swift

@@ -0,0 +1,107 @@
+//
+//  UserSearchCache.swift
+//  damus
+//
+//  Created by Terry Yiu on 6/27/23.
+//
+
+import Foundation
+
+/// Cache of searchable users by name, display_name, NIP-05 identifier, or own contact list petname.
+/// Optimized for fast searches of substrings by using a Trie.
+/// Optimal for performing user searches that could be initiated by typing quickly on a keyboard into a text input field.
+class UserSearchCache {
+    private let trie = Trie<String>()
+
+    func search(key: String) -> [String] {
+        let results = trie.find(key: key)
+        return results
+    }
+
+    /// Computes the differences between an old profile, if it exists, and a new profile, and updates the user search cache accordingly.
+    func updateProfile(id: String, profiles: Profiles, oldProfile: Profile?, newProfile: Profile) {
+        // Remove searchable keys tied to the old profile if they differ from the new profile
+        // to keep the trie clean without empty nodes while avoiding excessive graph searching.
+        if let oldProfile {
+            if let oldName = oldProfile.name, newProfile.name?.caseInsensitiveCompare(oldName) != .orderedSame {
+                trie.remove(key: oldName.lowercased(), value: id)
+            }
+            if let oldDisplayName = oldProfile.display_name, newProfile.display_name?.caseInsensitiveCompare(oldDisplayName) != .orderedSame {
+                trie.remove(key: oldDisplayName.lowercased(), value: id)
+            }
+            if let oldNip05 = oldProfile.nip05, newProfile.nip05?.caseInsensitiveCompare(oldNip05) != .orderedSame {
+                trie.remove(key: oldNip05.lowercased(), value: id)
+            }
+        }
+
+        addProfile(id: id, profiles: profiles, profile: newProfile)
+    }
+
+    /// Adds a profile to the user search cache.
+    private func addProfile(id: String, profiles: Profiles, profile: Profile) {
+        // Searchable by name.
+        if let name = profile.name {
+            trie.insert(key: name.lowercased(), value: id)
+        }
+
+        // Searchable by display name.
+        if let displayName = profile.display_name {
+            trie.insert(key: displayName.lowercased(), value: id)
+        }
+
+        // Searchable by NIP-05 identifier.
+        if let nip05 = profiles.is_validated(id) {
+            trie.insert(key: "\(nip05.username.lowercased())@\(nip05.host.lowercased())", value: id)
+        }
+    }
+
+    /// Computes the diffences between an old contacts event and a new contacts event for our own user, and updates the search cache accordingly.
+    func updateOwnContactsPetnames(id: String, oldEvent: NostrEvent?, newEvent: NostrEvent) {
+        guard newEvent.known_kind == .contacts && newEvent.pubkey == id else {
+            return
+        }
+
+        var petnames: [String: String] = [:]
+
+        // Gets all petnames from our new contacts list.
+        newEvent.tags.forEach { tag in
+            guard tag.count >= 4 && tag[0] == "p" else {
+                return
+            }
+
+            let pubkey = tag[1]
+            let petname = tag[3]
+
+            petnames[pubkey] = petname
+        }
+
+        // Compute the diff with the old contacts list, if it exists,
+        // mark the ones that are the same to not be removed from the user search cache,
+        // and remove the old ones that are different from the user search cache.
+        if let oldEvent, oldEvent.known_kind == .contacts && oldEvent.pubkey == id {
+            oldEvent.tags.forEach { tag in
+                guard tag.count >= 4 && tag[0] == "p" else {
+                    return
+                }
+
+                let pubkey = tag[1]
+                let oldPetname = tag[3]
+
+                if let newPetname = petnames[pubkey] {
+                    if newPetname.caseInsensitiveCompare(oldPetname) == .orderedSame {
+                        petnames.removeValue(forKey: pubkey)
+                    } else {
+                        trie.remove(key: oldPetname, value: pubkey)
+                    }
+                } else {
+                    trie.remove(key: oldPetname, value: pubkey)
+                }
+            }
+        }
+
+        // Add the new petnames to the user search cache.
+        for (pubkey, petname) in petnames {
+            trie.insert(key: petname, value: pubkey)
+        }
+    }
+}