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

solution/logger.py

@@ -0,0 +1,68 @@
+import sys
+import time
+
+LEVELS = ("DEBUG", "INFO", "WARNING", "ERROR")
+
+
+class Record:
+    __slots__ = ("ts", "level", "msg", "context")
+
+    def __init__(self, ts, level, msg, context):
+        self.ts = ts
+        self.level = level
+        self.msg = msg
+        self.context = context
+
+    def render(self):
+        ctx = " ".join(f"{k}={v}" for k, v in sorted(self.context.items()))
+        return f"{self.ts:.6f} {self.level:<7} {self.msg} {ctx}".rstrip()
+
+
+class BufferedLogger:
+    def __init__(self, name, level="INFO", stream=None, capacity=64):
+        self.name = name
+        self.stream = stream if stream is not None else sys.stderr
+        self.capacity = capacity
+        self._context = {}
+        self._buffer = []
+        self.set_level(level)
+
+    def set_level(self, level):
+        level = level.upper()
+        if level not in LEVELS:
+            raise ValueError(f"unknown level {level!r}")
+        self.level = level
+
+    def push(self, **fields):
+        self._context.update(fields)
+
+    def clear(self):
+        self._context.clear()
+
+    def _enabled(self, level):
+        return LEVELS.index(level) >= LEVELS.index(self.level)
+
+    def _log(self, level, msg):
+        if not self._enabled(level):
+            return
+        self._buffer.append(Record(time.time(), level, msg, dict(self._context)))
+        if len(self._buffer) >= self.capacity:
+            self.flush()
+
+    def debug(self, msg):
+        self._log("DEBUG", msg)
+
+    def info(self, msg):
+        self._log("INFO", msg)
+
+    def warning(self, msg):
+        self._log("WARNING", msg)
+
+    def error(self, msg):
+        self._log("ERROR", msg)
+
+    def flush(self):
+        for rec in self._buffer:
+            self.stream.write(f"[{self.name}] {rec.render()}\n")
+        self.stream.flush()
+        self._buffer.clear()