first commit

backend/app/rules/transit_rules.py

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+"""Transit (self-transfer) detection rules.
+
+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 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"]
+
+
+def is_self_transfer(tx: TransactionRecord, known_self_accounts: list[str]) -> bool:
+    """Check if a transaction is an inter-account transfer by the victim."""
+    counterparty = (tx.counterparty_name or "").lower()
+    remark = (tx.remark or "").lower()
+
+    # Rule 1: counterparty matches known self accounts
+    for acct in known_self_accounts:
+        if acct and acct.lower() in counterparty:
+            return True
+
+    # Rule 2: counterparty contains self-transfer keywords
+    for kw in SELF_KEYWORDS:
+        if kw in counterparty or kw in remark:
+            return True
+
+    # Rule 3: counterparty references another payment app owned by victim
+    app_keywords = ["支付宝", "微信", "银行卡", "数字钱包"]
+    victim_patterns = [f"{app}-" for app in app_keywords] + app_keywords
+    for pat in victim_patterns:
+        if pat in counterparty:
+            if tx.direction.value == "out":
+                return True
+
+    return False