Initial extraction from monorepo

fire_planner/glide_path.py

@@ -0,0 +1,46 @@
+"""Glide-path functions — stock-allocation as a function of years
+since retirement.
+
+Pfau & Kitces (2014) showed that *rising* equity glide paths
+(starting low and rising) reduce sequence-of-returns risk in the
+critical first decade of retirement. We default to that, with a
+classic static 60/40 also available.
+
+Each glide returns a fraction in [0, 1] for stock allocation — the
+remainder is bonds.
+"""
+from __future__ import annotations
+
+from collections.abc import Callable
+
+GlideFn = Callable[[int], float]
+
+
+def rising_equity(start: float = 0.30, end: float = 0.70, ramp_years: int = 15) -> GlideFn:
+    """Linear interpolation from `start` to `end` over `ramp_years`,
+    then constant at `end`."""
+    span = end - start
+
+    def fn(year: int) -> float:
+        if year >= ramp_years:
+            return end
+        return start + span * (year / ramp_years)
+
+    return fn
+
+
+def static(allocation: float) -> GlideFn:
+    """Constant allocation, e.g. 60/40 = static(0.60)."""
+    return lambda _year: allocation
+
+
+GLIDE_PATHS: dict[str, GlideFn] = {
+    "rising": rising_equity(),
+    "static_60_40": static(0.60),
+}
+
+
+def get(name: str) -> GlideFn:
+    if name not in GLIDE_PATHS:
+        raise KeyError(f"Unknown glide path: {name!r}. Known: {sorted(GLIDE_PATHS)}")
+    return GLIDE_PATHS[name]