Chunk home filters to avoid hitting max filter item limits

When a user is following several accounts, they may get a stale feed
caused by the subscription request being rejected by relays (due to max filter item limits).

This commit implements a fix that gets around the issue by
creating several chunked filters for the home feed event and contact
metadata subscriptions.

This is a short to medium-term practical fix, where we get around the
practical limitations imposed by most relays. In the future we should
work on longer-term solutions, which will likely require protocol improvements

Main Test
---------

Procedure:
1. Login with Elsat's npub (Or some account that follows about 2K people)
2. Check the home feed. There should be fresh notes.

REPRO:
Device: iPhone 15 simulator
iOS: 17.4
Damus: 1.9 (3) (0d9954290a)
Results:
- No fresh notes, most recent post is from several hours ago (Feed is stale)

FIX TEST:
Device: iPhone 15 simulator
iOS: 17.4
Damus: This commit
Results:
- Fresh notes appear, most recent post is from a few seconds ago.

Other testing:
--------------

- New automated test passing
- All other automated tests passing
- Tested scrolling down the feed on these conditions:
  - Device: iPhone 13 Mini
  - iOS: 17.4.1
  - Accounts:
    - One with about 160 contacts and 10 relays (Daniel D’Aquino)
    - One with about 1K+ contacts and 9 relays (Freedom Smuggler)
    - One with about 981 contacts and 6 relays (jb55)
    - Elsat's account (2K+ accounts and 8 relays)
  - Result: None of those were stale

Changelog-Fixed: Fix stale feed issue when follow list is too big
Closes: https://github.com/damus-io/damus/issues/2194
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>
Reviewed-by: William Casarin <jb55@jb55.com>

damus/Nostr/NostrFilter.swift

@@ -54,4 +54,68 @@ struct NostrFilter: Codable, Equatable {
     public static func filter_hashtag(_ htags: [String]) -> NostrFilter {
         NostrFilter(hashtag: htags.map { $0.lowercased() })
     }
+    
+    /// Splits the filter on a given filter path/axis into chunked filters
+    ///
+    /// - Parameter path: The path where chunking should be done
+    /// - Parameter chunk_size: The maximum size of each chunk.
+    /// - Returns: An array of arrays, where each contained array is a chunk of the original array with up to `size` elements.
+    func chunked(on path: ChunkPath, into chunk_size: Int) -> [Self] {
+        let chunked_slices = self.get_slice(from: path).chunked(into: chunk_size)
+        var chunked_filters: [NostrFilter] = []
+        for chunked_slice in chunked_slices {
+            var chunked_filter = self
+            chunked_filter.apply_slice(chunked_slice)
+            chunked_filters.append(chunked_filter)
+        }
+        return chunked_filters
+    }
+    
+    /// Gets a slice from a NostrFilter on a given path/axis
+    ///
+    /// - Parameter path: The path where chunking should be done
+    /// - Parameter chunk_size: The maximum size of each chunk.
+    /// - Returns: An array of arrays, where each contained array is a chunk of the original array with up to `size` elements.
+    func get_slice(from path: ChunkPath) -> Slice {
+        switch path {
+            case .pubkeys:
+                return .pubkeys(self.pubkeys)
+            case .authors:
+                return .authors(self.authors)
+        }
+    }
+    
+    /// Overrides one member/axis of a NostrFilter using a specific slice
+    /// - Parameter slice: The slice to be applied on this NostrFilter
+    mutating func apply_slice(_ slice: Slice) {
+        switch slice {
+            case .pubkeys(let pubkeys):
+                self.pubkeys = pubkeys
+            case .authors(let authors):
+                self.authors = authors
+        }
+    }
+    
+    
+    /// A path to one of the axes of a NostrFilter.
+    enum ChunkPath {
+        case pubkeys
+        case authors
+        // Other paths/axes not supported yet
+    }
+    
+    /// Represents the value of a single axis of a NostrFilter
+    enum Slice {
+        case pubkeys([Pubkey]?)
+        case authors([Pubkey]?)
+        
+        func chunked(into chunk_size: Int) -> [Slice] {
+            switch self {
+                case .pubkeys(let array):
+                    return (array ?? []).chunked(into: chunk_size).map({ .pubkeys($0) })
+                case .authors(let array):
+                    return (array ?? []).chunked(into: chunk_size).map({ .authors($0) })
+            }
+        }
+    }
 }