Prioritize last piece. Dont ask me why :)

crates/librqbit/src/chunk_tracker.rs

@@ -72,6 +72,18 @@ impl ChunkTracker {
         self.needed_pieces.set(index.get() as usize, false)
     }
 
+    pub fn iter_needed_pieces(&self) -> impl Iterator<Item = usize> + '_ {
+        // Try the last piece first. Often important information is stored in the last piece.
+        // Then sequentially one by one. This is not super sophisticated but sometimes helps.
+        let last_piece_id = self.lengths.last_piece_id().get() as usize;
+        self.needed_pieces
+            .get(last_piece_id..)
+            .unwrap()
+            .iter_ones()
+            .map(move |n| n + last_piece_id)
+            .chain(self.needed_pieces.get(..last_piece_id).unwrap().iter_ones())
+    }
+
     // None if wrong chunk
     // true if did something
     // false if didn't do anything

crates/librqbit/src/torrent_state.rs

@@ -349,7 +349,7 @@ impl TorrentState {
     fn get_next_needed_piece(&self, peer_handle: PeerHandle) -> Option<ValidPieceIndex> {
         let g = self.locked.read();
         let bf = g.peers.get_live(peer_handle)?.bitfield.as_ref()?;
-        for n in g.chunks.get_needed_pieces().iter_ones() {
+        for n in g.chunks.iter_needed_pieces() {
             if bf.get(n).map(|v| *v) == Some(true) {
                 // in theory it should be safe without validation, but whatever.
                 return self.lengths.validate_piece_index(n as u32);
@@ -375,7 +375,7 @@ impl TorrentState {
         let n = {
             let mut n_opt = None;
             let bf = g.peers.get_live(peer_handle)?.bitfield.as_ref()?;
-            for n in g.chunks.get_needed_pieces().iter_ones() {
+            for n in g.chunks.iter_needed_pieces() {
                 if bf.get(n).map(|v| *v) == Some(true) {
                     n_opt = Some(n);
                     break;

crates/librqbit_core/src/lengths.rs

@@ -118,6 +118,9 @@ impl Lengths {
     pub const fn total_chunks(&self) -> u32 {
         ceil_div_u64(self.total_length, self.chunk_length as u64) as u32
     }
+    pub const fn last_piece_id(&self) -> ValidPieceIndex {
+        ValidPieceIndex(self.last_piece_id)
+    }
     pub const fn total_pieces(&self) -> u32 {
         self.last_piece_id + 1
     }