Initial extraction from monorepo

fire_planner/strategies/trinity.py

@@ -0,0 +1,24 @@
+"""Trinity 4% Safe Withdrawal Rate.
+
+Bengen's seminal 1994 paper + the Trinity Study (Cooley/Hubbard/Walz,
+1998) — withdraw 4% of the starting balance in year 1, then keep the
+real withdrawal constant for the rest of retirement. In our real-GBP
+internal frame this is just "the same number every year".
+"""
+from __future__ import annotations
+
+from fire_planner.strategies.base import StrategyState, WithdrawalStrategy
+
+DEFAULT_INITIAL_RATE = 0.04
+
+
+class TrinityStrategy(WithdrawalStrategy):
+    name = "trinity"
+
+    def __init__(self, initial_rate: float = DEFAULT_INITIAL_RATE) -> None:
+        self.initial_rate = initial_rate
+
+    def propose_withdrawal(self, state: StrategyState) -> float:
+        if state.year_idx == 0:
+            return state.initial_portfolio * self.initial_rate
+        return state.last_withdrawal