fix: avoid websocket snapshot hash allocation

src/book.rs

@@ -208,11 +208,11 @@ pub struct OrderBook {
     /// Snapshot generation used to retain book levels without rescanning input payloads.
     snapshot_generation: u64,
 
-    /// Last accepted full-book snapshot hash, when the feed provides one.
+    /// Last accepted full-book snapshot hash fingerprint, when the feed provides one.
     ///
     /// This is used as a tie-breaker for same-millisecond snapshots. If the timestamp is equal
-    /// but the hash differs, the snapshot can still represent a newer book state.
-    last_snapshot_hash: Option<String>,
+    /// but the hash fingerprint differs, the snapshot can still represent a newer book state.
+    last_snapshot_hash_fingerprint: Option<u64>,
 
     /// Minimum tick size for this market in ticks (like 10 for $0.001 increments)
     /// Some markets only allow certain price increments
@@ -255,7 +255,7 @@ impl OrderBook {
             bids: BookSide::new(BookSideKind::Bid, max_depth), // Empty to start
             asks: BookSide::new(BookSideKind::Ask, max_depth), // Empty to start
             snapshot_generation: 0,
-            last_snapshot_hash: None,
+            last_snapshot_hash_fingerprint: None,
             tick_size_ticks: None, // We'll set this later when we learn about the market
             max_depth,
         }
@@ -620,7 +620,7 @@ impl OrderBook {
         }
 
         self.last_snapshot_timestamp_ms = update.timestamp;
-        self.last_snapshot_hash = update.hash.clone();
+        self.last_snapshot_hash_fingerprint = update.hash.as_deref().map(snapshot_hash_fingerprint);
         self.timestamp = chrono::DateTime::<Utc>::from_timestamp_millis(update.timestamp as i64)
             .unwrap_or_else(Utc::now);
         self.begin_snapshot();
@@ -730,7 +730,9 @@ impl OrderBook {
             return false;
         }
 
-        hash.is_some_and(|hash| self.last_snapshot_hash.as_deref() != Some(hash))
+        hash.is_some_and(|hash| {
+            self.last_snapshot_hash_fingerprint != Some(snapshot_hash_fingerprint(hash))
+        })
     }
 
     #[inline]
@@ -769,7 +771,7 @@ impl OrderBook {
         }
 
         self.last_snapshot_timestamp_ms = timestamp;
-        self.last_snapshot_hash = hash.map(str::to_owned);
+        self.last_snapshot_hash_fingerprint = hash.map(snapshot_hash_fingerprint);
         self.timestamp = chrono::DateTime::<Utc>::from_timestamp_millis(timestamp as i64)
             .unwrap_or_else(Utc::now);
         self.begin_snapshot();
@@ -1005,6 +1007,16 @@ struct BookShard {
     books: RwLock<HashMap<String, OrderBook>>,
 }
 
+#[inline]
+fn snapshot_hash_fingerprint(hash: &str) -> u64 {
+    let mut fingerprint = 0xcbf2_9ce4_8422_2325u64;
+    for &byte in hash.as_bytes() {
+        fingerprint ^= byte as u64;
+        fingerprint = fingerprint.wrapping_mul(0x0000_0100_0000_01b3);
+    }
+    fingerprint
+}
+
 #[inline]
 fn shard_index(token_id: &str, shard_count: usize) -> usize {
     debug_assert!(shard_count > 0);

tests/no_alloc_hot_paths.rs

@@ -182,6 +182,50 @@ fn ws_book_message(
     json.into_bytes()
 }
 
+fn ws_book_message_with_hash(
+    asset_id: &str,
+    timestamp: u64,
+    hash: &str,
+    bid_ticks: &[i64],
+    ask_ticks: &[i64],
+) -> Vec<u8> {
+    let mut json =
+        String::with_capacity(190 + hash.len() + (bid_ticks.len() + ask_ticks.len()) * 40);
+    write!(
+        &mut json,
+        "{{\"event_type\":\"book\",\"asset_id\":\"{asset_id}\",\"market\":\"0xabc\",\"timestamp\":{timestamp},\"hash\":\"{hash}\",\"bids\":["
+    )
+    .unwrap();
+
+    for (idx, price_ticks) in bid_ticks.iter().enumerate() {
+        if idx > 0 {
+            json.push(',');
+        }
+        write!(
+            &mut json,
+            "{{\"price\":\"{}\",\"size\":\"100.0000\"}}",
+            Decimal::new(*price_ticks, 4)
+        )
+        .unwrap();
+    }
+
+    json.push_str("],\"asks\":[");
+    for (idx, price_ticks) in ask_ticks.iter().enumerate() {
+        if idx > 0 {
+            json.push(',');
+        }
+        write!(
+            &mut json,
+            "{{\"price\":\"{}\",\"size\":\"100.0000\"}}",
+            Decimal::new(*price_ticks, 4)
+        )
+        .unwrap();
+    }
+
+    json.push_str("]}");
+    json.into_bytes()
+}
+
 fn contiguous_ticks(start: i64, len: usize, step: i64) -> Vec<i64> {
     (0..len).map(|idx| start + (idx as i64 * step)).collect()
 }
@@ -410,6 +454,30 @@ fn no_alloc_ws_book_update_processor_apply_existing_levels() {
     guard.assert_no_heap_traffic();
 }
 
+#[test]
+fn no_alloc_ws_book_update_processor_same_timestamp_hash_change() {
+    let asset_id = "test_asset_id";
+    let manager = OrderBookManager::new(100);
+    manager.get_or_create_book(asset_id).unwrap();
+    seed_book_levels(&manager, asset_id, &[7500], &[7600]);
+
+    let mut processor = WsBookUpdateProcessor::new(4096);
+    let mut warmup_msg = ws_book_message_with_hash(asset_id, 10, "hash_a", &[7500], &[7600]);
+    processor
+        .process_bytes(warmup_msg.as_mut_slice(), &manager)
+        .unwrap();
+
+    let mut msg = ws_book_message_with_hash(asset_id, 10, "hash_b", &[7500], &[7600]);
+
+    let _ = heap_operation_count();
+
+    let guard = NoHeapTrafficGuard::new();
+    processor
+        .process_bytes(msg.as_mut_slice(), &manager)
+        .unwrap();
+    guard.assert_no_heap_traffic();
+}
+
 #[test]
 fn no_alloc_ws_book_update_processor_one_new_level_with_reserved_capacity() {
     let asset_id = "test_asset_id";