Add generic indexer (#583)

* Add generic indexer

* Add test for GenericIndexer

* Clang format

* Update changelog

Author: jhp-lanl

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Added (new features/APIs/variables/...)
 - [[PR556]](https://github.com/lanl/singularity-eos/pull/556) Add introspection into types available in the variant
 - [[PR564]](https://github.com/lanl/singularity-eos/pull/564) Removed Get() function from IndexableTypes since it could have unexpected consequences when a type wasn't present
+- [[PR583]](https://github.com/lanl/singularity-eos/pull/583) Added GenericIndexer class to provide more complex array indirection
 
 ### Fixed (Repair bugs, etc)
 - [[PR567]](https://github.com/lanl/singularity-eos/pull/567) Fixed an OOB array access bug in the Fixed T PTE solver

singularity-eos/base/generic_indexer.hpp

@@ -0,0 +1,62 @@
+//------------------------------------------------------------------------------
+// © 2021-2025. Triad National Security, LLC. All rights reserved.  This
+// program was produced under U.S. Government contract 89233218CNA000001
+// for Los Alamos National Laboratory (LANL), which is operated by Triad
+// National Security, LLC for the U.S.  Department of Energy/National
+// Nuclear Security Administration. All rights in the program are
+// reserved by Triad National Security, LLC, and the U.S. Department of
+// Energy/National Nuclear Security Administration. The Government is
+// granted for itself and others acting on its behalf a nonexclusive,
+// paid-up, irrevocable worldwide license in this material to reproduce,
+// prepare derivative works, distribute copies to the public, perform
+// publicly and display publicly, and to permit others to do so.
+//------------------------------------------------------------------------------
+
+#ifndef SINGULARITY_EOS_BASE_GENERIC_INDEXER_HPP_
+#define SINGULARITY_EOS_BASE_GENERIC_INDEXER_HPP_
+
+#include <utility>
+
+namespace singularity {
+
+template <typename Arr, typename Map>
+struct GenericIndexer {
+  Arr arr_;
+  Map map_;
+
+  template <typename ArrT_, typename MapT_>
+  constexpr GenericIndexer(ArrT_ &&arr_in, MapT_ &&map_in)
+      : arr_(std::forward<ArrT_>(arr_in)), map_(std::forward<MapT_>(map_in)) {}
+
+  // & : non-const lvalue
+  template <class I>
+  constexpr decltype(auto) operator[](I i) & {
+    return arr_[map_[i]];
+  }
+
+  // const& : const lvalue
+  template <class I>
+  constexpr decltype(auto) operator[](I i) const & {
+    return arr_[map_[i]];
+  }
+
+  // && : rvalue (indexer is a temporary) — forward arr_’s value category
+  template <class I>
+  constexpr decltype(auto) operator[](I i) && {
+    return std::forward<Arr>(arr_)[map_[i]]; // move only if Arr is a value type
+  }
+
+  // const rvalue indexer
+  template <class I>
+  constexpr decltype(auto) operator[](I i) const && {
+    return std::forward<const Arr>(arr_)[map_[i]]; // preserves const
+  }
+};
+
+// CTAD: preserve references for lvalues, decay for rvalues
+template <class ArrT_, class MapT_>
+GenericIndexer(ArrT_ &&, MapT_ &&) -> GenericIndexer<ArrT_, MapT_>;
+
+} // namespace singularity
+
+#endif // #ifndef SINGULARITY_EOS_BASE_GENERIC_INDEXER_HPP_

test/CMakeLists.txt

@@ -40,6 +40,7 @@ add_executable(
   test_variadic_utils.cpp
   test_bounds.cpp
   test_indexable_types.cpp
+  test_generic_indxer.cpp
   )
 
 add_executable(

test/test_generic_indxer.cpp

@@ -0,0 +1,65 @@
+//------------------------------------------------------------------------------
+// © 2021-2025. Triad National Security, LLC. All rights reserved.  This
+// program was produced under U.S. Government contract 89233218CNA000001
+// for Los Alamos National Laboratory (LANL), which is operated by Triad
+// National Security, LLC for the U.S.  Department of Energy/National
+// Nuclear Security Administration. All rights in the program are
+// reserved by Triad National Security, LLC, and the U.S. Department of
+// Energy/National Nuclear Security Administration. The Government is
+// granted for itself and others acting on its behalf a nonexclusive,
+// paid-up, irrevocable worldwide license in this material to reproduce,
+// prepare derivative works, distribute copies to the public, perform
+// publicly and display publicly, and to permit others to do so.
+//------------------------------------------------------------------------------
+
+#include <array>
+
+#include <ports-of-call/portability.hpp>
+#include <singularity-eos/base/generic_indexer.hpp>
+
+#ifndef CATCH_CONFIG_FAST_COMPILE
+#define CATCH_CONFIG_FAST_COMPILE
+#include <catch2/catch_test_macros.hpp>
+#endif
+#include <catch2/matchers/catch_matchers_floating_point.hpp>
+
+SCENARIO("Generic Indexer", "[GenericIndexer]") {
+  GIVEN("An array of arays populated with data") {
+    constexpr std::size_t n = 5;
+    constexpr std::size_t m = 7;
+    auto data = std::array<std::array<Real, m>, n>{};
+    for (std::size_t i = 0; i < n; i++) {
+      for (std::size_t j = 0; j < m; j++) {
+        data[i][j] = (i + 1) * (j + 1);
+      }
+    }
+    WHEN("A flattened array is created with the same size and populated with "
+         "the same data") {
+      auto flat_data = std::array<Real, n * m>{};
+      for (std::size_t i = 0; i < n; i++) {
+        for (std::size_t j = 0; j < m; j++) {
+          flat_data[i * m + j] = data[i][j];
+        }
+      }
+      WHEN("We use the generic indexer to index into material data") {
+        struct constant_offset {
+          std::size_t offset;
+          constexpr constant_offset(std::size_t offset_) : offset{offset_} {}
+          constexpr std::size_t operator[](std::size_t i) { return i * offset; }
+        };
+        auto mat_indexer =
+            singularity::GenericIndexer(flat_data.data(), constant_offset{m});
+        THEN("The data returned should be same as if we accessed the array of "
+             "arrays") {
+          for (std::size_t i = 0; i < n; i++) {
+            auto *mat_array = &mat_indexer[i];
+            for (std::size_t j = 0; j < m; j++) {
+              INFO("i: " << i << " j: " << j);
+              CHECK_THAT(data[i][j], Catch::Matchers::WithinRel(mat_array[j], 1.0e-12));
+            }
+          }
+        }
+      }
+    }
+  }
+}