fix(orderbook): use pl.len() for trade counts (polars 2.0 compat, refs #162)

CHANGELOG.md

@@ -16,6 +16,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Orderbook `get_statistics()` failed with "Analysis failed" under polars 2.0
+  because `pl.count()` no longer accepts zero arguments. Trade-side counts now
+  use `pl.len()` (compatible with polars 1.31+ and 2.0).
+
 ### Internal
 
 - Normalize Polars `Series.sum()` aggregates to `float`/`int` before

src/project_x_py/orderbook/analytics.py

@@ -592,7 +592,7 @@ async def get_statistics(self) -> dict[str, Any]:
                 # Optimized: Single pass through data with aggregation
                 trade_stats = self.orderbook.recent_trades.group_by("side").agg(
                     [
-                        pl.count().alias("count"),
+                        pl.len().alias("count"),
                         pl.col("volume").sum().alias("total_volume"),
                     ]
                 )

tests/orderbook/test_orderbook_analytics.py

@@ -447,6 +447,9 @@ async def test_get_statistics(self, market_analytics):
         assert result["total_bid_size"] == 100
         assert result["total_ask_size"] == 100
         assert result["total_trades"] == 5
+        # Fixture sides: buy, sell, buy, sell, buy
+        assert result["buy_trades"] == 3
+        assert result["sell_trades"] == 2
 
 
 class TestErrorHandling: