From 2818b8a3a05bd3792cb13620dfb1e8316b45e776 Mon Sep 17 00:00:00 2001
From: David Andrs <andrsd@gmail.com>
Date: Thu, 8 Oct 2026 09:07:18 -0600
Subject: [PATCH] Adding `File::get_elemental_var_table()`

---
 include/exodusIIcpp/file.h        |  7 ++++++
 include/exodusIIcpp/truth_table.h | 40 +++++++++++++++++++++++++++++++
 python/src/exodusIIcpp.cpp        |  3 +++
 src/file.cpp                      | 15 ++++++++++++
 test/File_test.cpp                |  3 +++
 5 files changed, 68 insertions(+)
 create mode 100644 include/exodusIIcpp/truth_table.h

diff --git a/include/exodusIIcpp/file.h b/include/exodusIIcpp/file.h
index d4841ec..f1ac611 100644
--- a/include/exodusIIcpp/file.h
+++ b/include/exodusIIcpp/file.h
@@ -11,6 +11,7 @@
 #include "exodusIIcpp/error.h"
 #include "exodusIIcpp/node_set.h"
 #include "exodusIIcpp/side_set.h"
+#include "exodusIIcpp/truth_table.h"
 
 namespace fs = std::filesystem;
 
@@ -148,6 +149,12 @@ class File {
     /// @return The number of side sets
     int get_num_side_sets() const;
 
+    /// Get elemental variable table
+    ///
+    /// @return Truth table. Rows are block indices (not IDs), columns are
+    ///         variable indices. Both are 1-based indices!
+    TruthTable get_elemental_var_table() const;
+
     /// Get the x-coordinates of nodes
     ///
     /// @return The x-coordinates of nodes
diff --git a/include/exodusIIcpp/truth_table.h b/include/exodusIIcpp/truth_table.h
new file mode 100644
index 0000000..efa8e8f
--- /dev/null
+++ b/include/exodusIIcpp/truth_table.h
@@ -0,0 +1,40 @@
+// SPDX-FileCopyrightText: 2026 David Andrs <andrsd@gmail.com>
+// SPDX-License-Identifier: MIT
+
+#pragma once
+
+#include <cstddef>
+#include <cstring>
+
+namespace exodusIIcpp {
+
+class TruthTable {
+public:
+    ~TruthTable() { delete[] this->data; }
+
+    /// Get value from row `i` and column `j`
+    ///
+    /// @param i Row (1-based)
+    /// @param j Column (1-based)
+    bool
+    operator()(int i, int j)
+    {
+        int ofst = (i - 1) * this->n_cols + (j - 1);
+        return this->data[ofst] == 1;
+    }
+
+private:
+    TruthTable(int rows, int cols) : n_cols(cols)
+    {
+        std::size_t n_elems = static_cast<std::size_t>(rows) * static_cast<std::size_t>(cols);
+        this->data = new int[n_elems];
+        std::memset(this->data, 0, n_elems * sizeof(int));
+    }
+
+    int n_cols;
+    int * data;
+
+    friend class File;
+};
+
+} // namespace exodusIIcpp
diff --git a/python/src/exodusIIcpp.cpp b/python/src/exodusIIcpp.cpp
index 031a07e..07b7c25 100644
--- a/python/src/exodusIIcpp.cpp
+++ b/python/src/exodusIIcpp.cpp
@@ -21,6 +21,8 @@ PYBIND11_MODULE(exodusIIcpp, m)
         .value("WRITE", exodusIIcpp::FileAccess::WRITE)
         .value("APPEND", exodusIIcpp::FileAccess::APPEND);
 
+    py::class_<exodusIIcpp::TruthTable>(m, "TruthTable");
+
     py::class_<exodusIIcpp::ElementBlock>(m, "ElementBlock")
         .def(py::init())
         .def("get_id", &ElementBlock::get_id)
@@ -78,6 +80,7 @@ PYBIND11_MODULE(exodusIIcpp, m)
         .def("get_num_element_blocks", &File::get_num_element_blocks)
         .def("get_num_node_sets", &File::get_num_node_sets)
         .def("get_num_side_sets", &File::get_num_side_sets)
+        .def("get_elemental_var_table", &File::get_elemental_var_table)
         .def("get_x_coords", &File::get_x_coords)
         .def("get_y_coords", &File::get_y_coords)
         .def("get_z_coords", &File::get_z_coords)
diff --git a/src/file.cpp b/src/file.cpp
index 77f8300..d5b679b 100644
--- a/src/file.cpp
+++ b/src/file.cpp
@@ -247,6 +247,21 @@ File::get_num_side_sets() const
     return this->n_side_sets;
 }
 
+TruthTable
+File::get_elemental_var_table() const
+{
+    int num_elem_var = 0;
+    EXODUSIICPP_CHECK_ERROR(ex_get_variable_param(exoid, EX_ELEM_BLOCK, &num_elem_var));
+
+    TruthTable truth_table(this->n_elem_blks, num_elem_var);
+    EXODUSIICPP_CHECK_ERROR(ex_get_truth_table(this->exoid,
+                                               EX_ELEM_BLOCK,
+                                               this->n_elem_blks,
+                                               num_elem_var,
+                                               truth_table.data));
+    return truth_table;
+}
+
 const std::vector<double> &
 File::get_x_coords() const
 {
diff --git a/test/File_test.cpp b/test/File_test.cpp
index aa269cb..765ba81 100644
--- a/test/File_test.cpp
+++ b/test/File_test.cpp
@@ -387,6 +387,9 @@ TEST(FileTest, test)
         EXPECT_THAT(ss_cnts, ElementsAre(2, 2));
         EXPECT_THAT(ss_nodes, ElementsAre(2, 3, 7, 8));
 
+        auto ev_tab = f.get_elemental_var_table();
+        EXPECT_TRUE(ev_tab(1, 3));
+
         f.close();
     }
 }