fix: bugs-04

backend/app/rules/transit_rules.py

@@ -4,9 +4,13 @@ Identifies transactions that are internal transfers between the victim's
 own accounts (e.g. bank -> Alipay -> WeChat) and should NOT be counted
 as fraud loss.
 """
+from datetime import datetime
+
 from app.models.transaction import TransactionRecord
 
 SELF_KEYWORDS = ["本人", "自己", "余额", "充值", "提现", "银行卡转入", "银行卡充值"]
+FEE_TRANSIT_WINDOW_SECONDS = 120
+FEE_TOLERANCE_RATIO = 0.02
 
 
 def is_self_transfer(tx: TransactionRecord, known_self_accounts: list[str]) -> bool:
@@ -33,3 +37,27 @@ def is_self_transfer(tx: TransactionRecord, known_self_accounts: list[str]) -> b
                 return True
 
     return False
+
+
+def is_fee_tolerant_transit_pair(tx_a: TransactionRecord, tx_b: TransactionRecord) -> bool:
+    """Two-way transfer pattern: opposite direction, close time, similar amount."""
+    if tx_a.direction.value == tx_b.direction.value:
+        return False
+
+    amount_a = float(tx_a.amount or 0)
+    amount_b = float(tx_b.amount or 0)
+    if amount_a <= 0 or amount_b <= 0:
+        return False
+
+    time_a = tx_a.trade_time
+    time_b = tx_b.trade_time
+    if not isinstance(time_a, datetime) or not isinstance(time_b, datetime):
+        return False
+    if abs((time_a - time_b).total_seconds()) > FEE_TRANSIT_WINDOW_SECONDS:
+        return False
+
+    amount_base = max(amount_a, amount_b)
+    if amount_base <= 0:
+        return False
+    diff_ratio = abs(amount_a - amount_b) / amount_base
+    return diff_ratio <= FEE_TOLERANCE_RATIO