feat(fusion_accounting_assets): prorate service for partial-period depreciation

Made-with: Cursor

fusion_accounting_assets/services/__init__.py

@@ -1 +1,2 @@
 from . import depreciation_methods
+from . import prorate

fusion_accounting_assets/services/prorate.py

@@ -0,0 +1,34 @@
+"""Prorating helpers for first-period and last-period depreciation.
+
+When an asset starts mid-month, the first period charges only a fraction
+of the full period_amount. Three conventions:
+- 'full_month': always charge full month (no proration)
+- 'days_365': pro-rate by actual days / 365
+- 'days_period': pro-rate by actual days in period / total days in period
+"""
+
+from datetime import date
+from typing import Literal
+
+
+ProrateConvention = Literal['full_month', 'days_365', 'days_period']
+
+
+def prorate_factor(*, period_start: date, period_end: date,
+                   asset_start: date,
+                   convention: ProrateConvention = 'days_period') -> float:
+    """Return a 0..1 factor for how much of `period`'s depreciation
+    applies to an asset that started on `asset_start`."""
+    if convention == 'full_month':
+        return 1.0
+    if asset_start <= period_start:
+        return 1.0
+    if asset_start > period_end:
+        return 0.0
+    actual_days = (period_end - asset_start).days + 1
+    if convention == 'days_365':
+        return actual_days / 365.0
+    if convention == 'days_period':
+        period_days = (period_end - period_start).days + 1
+        return actual_days / period_days
+    raise ValueError(f"Unknown convention: {convention}")