perf(as_of): fix quadratic reconstruction in forward simulations (×1.4–×5.4)

CHANGELOG.md

@@ -21,6 +21,7 @@
   - GET performance: a snapshot cursor makes forward-sequential reads O(k) (only new patches applied) instead of O(N); backward jumps degrade gracefully to O(N + k×P).
   - Retroactive `set_input` (out-of-order patches) is supported with automatic snapshot invalidation.
   - No change to the public API (`set_input`, `get_array`, `Variable.as_of`).
+- Fix quadratic reconstruction cost in `as_of` forward simulations: when the new patch is appended at the end (forward-sequential SET), the snapshot is updated instead of discarded so the next GET does not reconstruct from base through all patches; retroactive writes still invalidate correctly.
 
 #### Technical changes
 

openfisca_core/holders/holder.py

@@ -117,6 +117,10 @@ def get_array(self, period):
         value = self._memory_storage.get(period)
         if value is not None:
             return value
+        if self._as_of:
+            value = self._get_as_of(period)
+            if value is not None:
+                return value
         if self._disk_storage:
             return self._disk_storage.get(period)
         return None
@@ -430,11 +434,25 @@ def _set(self, period, value) -> None:
             >= self.simulation.memory_config.max_memory_occupation_pc
         )
 
+        if self._as_of:
+            # Reference sharing: reuse existing array object when value unchanged,
+            # otherwise store a read-only defensive copy so that callers cannot
+            # corrupt stored data by mutating their original array in-place.
+            prev = self._get_as_of(period)
+            if prev is not None and numpy.array_equal(value, prev):
+                value = prev  # prev is already read-only
+            else:
+                value = value.copy()
+                value.flags.writeable = False
+
         if should_store_on_disk:
             self._disk_storage.put(value, period)
         else:
             self._memory_storage.put(value, period)
 
+        if self._as_of:
+            self._register_instant(period)
+
     def put_in_cache(self, value, period) -> None:
         if self._do_not_store:
             return

setup.py

@@ -61,6 +61,7 @@
 
 dev_requirements = [
     "black >=24.8.0, <25.0",
+    "pytest-benchmark >=4.0.0, <5.0",
     "codespell >=2.3.0, <3.0",
     "colorama >=0.4.4, <0.5",
     "darglint >=1.8.1, <2.0",