From 3a34576c993037bb3c826a224cafb9e7f2def5b7 Mon Sep 17 00:00:00 2001
From: Axel Huebl <axel.huebl@plasma.ninja>
Date: Mon, 6 Jan 2025 18:28:48 -0800
Subject: [PATCH] SmallMatrix: Assert Lower Bound Index (#4283)

## Summary

With 1-based indices, it is easy to access `0` and be out-of-bounds. Add
an assert for the lower bound of the index range as well.

## Additional background

Follow-up to #4188

## Checklist

The proposed changes:
- [ ] fix a bug or incorrect behavior in AMReX
- [x] add new capabilities to AMReX
- [ ] changes answers in the test suite to more than roundoff level
- [ ] are likely to significantly affect the results of downstream AMReX
users
- [ ] include documentation in the code and/or rst files, if appropriate
---
 Src/Base/AMReX_SmallMatrix.H | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/Src/Base/AMReX_SmallMatrix.H b/Src/Base/AMReX_SmallMatrix.H
index 05305d1839c..b1b6ee30405 100644
--- a/Src/Base/AMReX_SmallMatrix.H
+++ b/Src/Base/AMReX_SmallMatrix.H
@@ -100,6 +100,7 @@ namespace amrex {
                 --i;
                 --j;
             }
+            AMREX_ASSERT(i >= 0 && j >= 0);
             AMREX_ASSERT(i < NRows && j < NCols);
             if constexpr (ORDER == Order::F) {
                 return m_mat[i+j*NRows];
@@ -116,6 +117,7 @@ namespace amrex {
                 --i;
                 --j;
             }
+            AMREX_ASSERT(i >= 0 && j >= 0);
             AMREX_ASSERT(i < NRows && j < NCols);
             if constexpr (ORDER == Order::F) {
                 return m_mat[i+j*NRows];
@@ -132,6 +134,7 @@ namespace amrex {
             if constexpr (StartIndex == 1) {
                 --i;
             }
+            AMREX_ASSERT(i >= 0);
             AMREX_ASSERT(i < NRows*NCols);
             return m_mat[i];
         }
@@ -144,6 +147,7 @@ namespace amrex {
             if constexpr (StartIndex == 1) {
                 --i;
             }
+            AMREX_ASSERT(i >= 0);
             AMREX_ASSERT(i < NRows*NCols);
             return m_mat[i];
         }
@@ -156,6 +160,7 @@ namespace amrex {
             if constexpr (StartIndex == 1) {
                 --i;
             }
+            AMREX_ASSERT(i >= 0);
             AMREX_ASSERT(i < NRows*NCols);
             return m_mat[i];
         }
@@ -168,6 +173,7 @@ namespace amrex {
             if constexpr (StartIndex == 1) {
                 --i;
             }
+            AMREX_ASSERT(i >= 0);
             AMREX_ASSERT(i < NRows*NCols);
             return m_mat[i];
         }