Add GetPackDimension to StateDescriptor

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Current develop
 
 ### Added (new features/APIs/variables/...)
+- [[PR 1148]](https://github.com/parthenon-hpc-lab/parthenon/pull/1148) Add `GetPackDimension` to `StateDescriptor` for calculating pack sizes before `Mesh` initialization
 - [[PR 1143]](https://github.com/parthenon-hpc-lab/parthenon/pull/1143) Add tensor indices to VariableState, add radiation constant to constants, add TypeLists, allow for arbitrary containers for solvers
 - [[PR 1140]](https://github.com/parthenon-hpc-lab/parthenon/pull/1140) Allow for relative convergence tolerance in BiCGSTAB solver.
 - [[PR 1047]](https://github.com/parthenon-hpc-lab/parthenon/pull/1047) General three- and four-valent 2D forests w/ arbitrary orientations.

src/interface/state_descriptor.cpp

@@ -13,6 +13,7 @@
 
 #include <iomanip>
 #include <iostream>
+#include <numeric>
 #include <sstream>
 #include <string>
 #include <unordered_map>
@@ -566,4 +567,35 @@ StateDescriptor::GetVariableNames(const Metadata::FlagCollection &flags) {
   return GetVariableNames({}, flags, {});
 }
 
+// Get the total length of this StateDescriptor's variables when packed
+int StateDescriptor::GetPackDimension(const std::vector<std::string> &req_names,
+                                      const Metadata::FlagCollection &flags,
+                                      const std::vector<int> &sparse_ids) {
+  std::vector<std::string> names = GetVariableNames(req_names, flags, sparse_ids);
+  int dimension = 0;
+  for (auto name : names) {
+    const auto &meta = metadataMap_[VarID(name)];
+    // if meta.Shape().size() < 1, then 'accumulate' will return the initialization value,
+    // which is 1. Otherwise, this multiplies all elements present in 'Shape' to obtain
+    // total length
+    dimension += std::accumulate(meta.Shape().begin(), meta.Shape().end(), 1,
+                                 [](auto a, auto b) { return a * b; });
+  }
+  return dimension;
+}
+int StateDescriptor::GetPackDimension(const std::vector<std::string> &req_names,
+                                      const std::vector<int> &sparse_ids) {
+  return GetPackDimension(req_names, Metadata::FlagCollection(), sparse_ids);
+}
+int StateDescriptor::GetPackDimension(const Metadata::FlagCollection &flags,
+                                      const std::vector<int> &sparse_ids) {
+  return GetPackDimension({}, flags, sparse_ids);
+}
+int StateDescriptor::GetPackDimension(const std::vector<std::string> &req_names) {
+  return GetPackDimension(req_names, Metadata::FlagCollection(), {});
+}
+int StateDescriptor::GetPackDimension(const Metadata::FlagCollection &flags) {
+  return GetPackDimension({}, flags, {});
+}
+
 } // namespace parthenon

src/interface/state_descriptor.hpp

@@ -276,6 +276,16 @@ class StateDescriptor {
   std::vector<std::string> GetVariableNames(const std::vector<std::string> &req_names);
   std::vector<std::string> GetVariableNames(const Metadata::FlagCollection &flags);
 
+  int GetPackDimension(const std::vector<std::string> &req_names,
+                       const Metadata::FlagCollection &flags,
+                       const std::vector<int> &sparse_ids);
+  int GetPackDimension(const std::vector<std::string> &req_names,
+                       const std::vector<int> &sparse_ids);
+  int GetPackDimension(const Metadata::FlagCollection &flags,
+                       const std::vector<int> &sparse_ids);
+  int GetPackDimension(const std::vector<std::string> &req_names);
+  int GetPackDimension(const Metadata::FlagCollection &flags);
+
   std::size_t
   RefinementFuncID(const refinement::RefinementFunctions_t &funcs) const noexcept {
     return refinementFuncMaps_.funcs_to_ids.at(funcs);

tst/unit/test_state_descriptor.cpp

@@ -121,6 +121,22 @@ TEST_CASE("Test Associate in StateDescriptor", "[StateDescriptor]") {
   }
 }
 
+TEST_CASE("Test GetPackDimension in StateDescriptor", "[StateDescriptor]") {
+  GIVEN("Some flags and state descriptors") {
+    StateDescriptor state("state");
+    WHEN("We add some fields with various shapes and total size") {
+      state.AddField("foo", Metadata(std::vector<MetadataFlag>{}, std::vector<int>{4}));
+      state.AddField("bar",
+                     Metadata(std::vector<MetadataFlag>{}, std::vector<int>{4, 4}));
+      state.AddField("baz",
+                     Metadata(std::vector<MetadataFlag>{}, std::vector<int>{4, 4, 4}));
+      THEN("The total length is identified correctly") {
+        REQUIRE(state.GetPackDimension(Metadata::GetUserFlag("state")) == 84);
+      }
+    }
+  }
+}
+
 TEST_CASE("Test dependency resolution in StateDescriptor", "[StateDescriptor]") {
   GIVEN("Some empty state descriptors and metadata") {
     // metadata