Add answer key and fixed solution

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

solution/ledger.py

@@ -0,0 +1,86 @@
+from dataclasses import dataclass
+
+SIDES = ("BUY", "SELL")
+
+
+@dataclass
+class Trade:
+    trade_id: str
+    symbol: str
+    side: str
+    qty: int
+    price: float
+
+    def signed_qty(self):
+        return -self.qty if self.side == "SELL" else self.qty
+
+
+@dataclass
+class Position:
+    symbol: str
+    qty: int = 0
+    avg_price: float = 0.0
+    realized: float = 0.0
+
+    def update(self, qty, price):
+        new_qty = self.qty + qty
+        if self.qty == 0 or (self.qty > 0) == (qty > 0):
+            self.avg_price = (self.avg_price * self.qty + price * qty) / new_qty
+        else:
+            closed = min(abs(qty), abs(self.qty))
+            direction = 1 if self.qty > 0 else -1
+            self.realized += closed * (price - self.avg_price) * direction
+            if new_qty == 0:
+                self.avg_price = 0.0
+            elif (new_qty > 0) != (self.qty > 0):
+                self.avg_price = price
+        self.qty = new_qty
+
+
+class Ledger:
+    def __init__(self, logger):
+        self.log = logger
+        self.positions = {}
+        self.history = []
+
+    def _validate(self, trade):
+        if trade.side not in SIDES:
+            raise ValueError(f"bad side {trade.side!r}")
+        if trade.qty <= 0:
+            raise ValueError(f"bad qty {trade.qty!r}")
+        if trade.price <= 0:
+            raise ValueError(f"bad price {trade.price!r}")
+
+    def apply(self, trade):
+        self.log.push(trade_id=trade.trade_id, symbol=trade.symbol)
+        try:
+            self._validate(trade)
+        except ValueError as exc:
+            self.log.error(f"rejected: {exc}")
+            raise
+        pos = self.positions.setdefault(trade.symbol, Position(trade.symbol))
+        pos.update(trade.signed_qty(), trade.price)
+        self.history.append(trade)
+        self.log.info(f"applied {trade.side} {abs(trade.qty)}@{trade.price:.2f} -> {pos.qty}")
+        return pos
+
+    def position(self, symbol):
+        return self.positions.get(symbol, Position(symbol))
+
+    @classmethod
+    def rebuild(cls, trades, logger):
+        ledger = cls(logger)
+        for trade in trades:
+            try:
+                ledger.apply(trade)
+            except ValueError:
+                continue
+        return ledger
+
+    def reconcile(self, other):
+        breaks = []
+        for symbol in sorted(set(self.positions) | set(other.positions)):
+            mine, theirs = self.position(symbol), other.position(symbol)
+            if mine.qty != theirs.qty or abs(mine.avg_price - theirs.avg_price) > 1e-9:
+                breaks.append((symbol, mine.qty, theirs.qty))
+        return breaks