Adding the ability to specify a negative index when loading a db (#964)

armi/bookkeeping/db/database3.py

@@ -79,6 +79,7 @@
 from armi.reactor import grids
 from armi.bookkeeping.db.typedefs import History, Histories
 from armi.reactor import systemLayoutInput
+from armi.utils import getNodesPerCycle
 from armi.utils.textProcessors import resolveMarkupInclusions
 from armi.nucDirectory import nuclideBases
 
@@ -681,19 +682,20 @@ def load(
         Providing these from the client could be useful when performing snapshot runs
         or where it is expected to use results from a run using different settings and
         continue with new settings (or if blueprints are not on the database).
-        Geom is read from the database itself.
+        Geometry is read from the database itself.
 
         Parameters
         ----------
         cycle : int
-            cycle number
+            Cycle number
         node : int
-            time node
-        cs : armi.settings.Settings, optional
-            if not provided one is read from the database
-        bp : armi.reactor.Blueprints, optional
-            if not provided one is read from the database
-        statePointName : str, optional
+            Time node. If value is negative, will be indexed from EOC backwards
+            like a list.
+        cs : armi.settings.Settings (optional)
+            If not provided one is read from the database
+        bp : armi.reactor.Blueprints (optional)
+            If not provided one is read from the database
+        statePointName : str
             Optional arbitrary statepoint name (e.g., "special" for "c00n00-special/")
         allowMissing : bool, optional
             Whether to emit a warning, rather than crash if reading a database
@@ -720,6 +722,14 @@ def load(
             settings.setMasterCs(cs)
         bp = bp or self.loadBlueprints()
 
+        if node < 0:
+            numNodes = getNodesPerCycle(cs)[cycle]
+            if (node + numNodes) < 0:
+                raise ValueError(
+                    f"Node {node} specified does not exist for cycle {cycle}"
+                )
+            node = numNodes + node
+
         h5group = self.h5db[getH5GroupName(cycle, node, statePointName)]
 
         layout = Layout((self.versionMajor, self.versionMinor), h5group=h5group)

armi/bookkeeping/db/tests/test_database3.py

@@ -326,6 +326,15 @@ def test_load(self):
 
         _r = self.db.load(0, 0, allowMissing=True)
 
+        # show that we can use negative indices to load
+        r = self.db.load(0, -2, allowMissing=True)
+        self.assertEqual(r.p.timeNode, 1)
+
+        with self.assertRaises(ValueError):
+            # makeShuffleHistory only populates 2 nodes, but the case settings
+            # defines 3, so we must check -4 before getting an error
+            self.db.load(0, -4, allowMissing=True)
+
         del self.db.h5db["c00n00/Reactor/missingParam"]
         _r = self.db.load(0, 0, allowMissing=False)
 

doc/release/0.2.rst

@@ -13,6 +13,7 @@ What's new in ARMI
 #. Add pinQuantities parameter category for blockParams that have spatial distribution within a block.
 #. Use r.core.p.axialMesh instead of r.core.refAssem.getAxialMesh() for the uniform mesh converter. (`PR#959 <https://github.com/terrapower/armi/pull/959>`_)
 #. Split algorithms specific to hex assemblies out of ``FuelHandler``. (`PR#962 <https://github.com/terrapower/armi/pull/962>`_)
+#. Add ability to load from a db using negative node index
 
 Bug fixes
 ---------