From 62c2a81eac7862d526e5861ef2befc00b7f5b759 Mon Sep 17 00:00:00 2001
From: Weiqun Zhang <WeiqunZhang@lbl.gov>
Date: Fri, 11 Oct 2024 20:10:51 -0500
Subject: [PATCH] SmallMatrix: Structured binding support (#4189)

---
 Src/Base/AMReX_SmallMatrix.H | 19 +++++++++++++++++++
 Tests/SmallMatrix/main.cpp   | 31 +++++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+)

diff --git a/Src/Base/AMReX_SmallMatrix.H b/Src/Base/AMReX_SmallMatrix.H
index 4fe673816d..05305d1839 100644
--- a/Src/Base/AMReX_SmallMatrix.H
+++ b/Src/Base/AMReX_SmallMatrix.H
@@ -11,6 +11,7 @@
 #include <algorithm>
 #include <initializer_list>
 #include <iostream>
+#include <tuple>
 #include <type_traits>
 
 namespace amrex {
@@ -388,6 +389,14 @@ namespace amrex {
             return r;
         }
 
+        template <int N, std::enable_if_t<(N<NRows*NCols),int> = 0>
+        [[nodiscard]] AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
+        constexpr T const& get () const { return m_mat[N]; }
+
+        template <int N, std::enable_if_t<(N<NRows*NCols),int> = 0>
+        [[nodiscard]] AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
+        constexpr T& get () { return m_mat[N]; }
+
     private:
         T m_mat[NRows*NCols];
     };
@@ -447,6 +456,16 @@ namespace amrex {
     using SmallRowVector = SmallMatrix<T,1,N,Order::F,StartIndex>;
 }
 
+template <class T, int NRows, int NCols, amrex::Order ORDER, int StartIndex>
+struct std::tuple_size<amrex::SmallMatrix<T,NRows,NCols,ORDER,StartIndex> >
+    : std::integral_constant<std::size_t,NRows*NCols> {};
+
+template <std::size_t N, class T, int NRows, int NCols, amrex::Order ORDER, int StartIndex>
+struct std::tuple_element<N, amrex::SmallMatrix<T,NRows,NCols,ORDER,StartIndex> >
+{
+    using type = T;
+};
+
 #endif
 
 /*
diff --git a/Tests/SmallMatrix/main.cpp b/Tests/SmallMatrix/main.cpp
index de5732101f..505ae50bd6 100644
--- a/Tests/SmallMatrix/main.cpp
+++ b/Tests/SmallMatrix/main.cpp
@@ -141,6 +141,21 @@ int main (int argc, char* argv[])
         b.setVal(-1);
         AMREX_ALWAYS_ASSERT(a.dot(b) == -30);
     }
+    {
+        SmallVector<int, 3> v{10,20,30};
+        auto const& [x,y,z] = v;
+        AMREX_ALWAYS_ASSERT(x == 10 && y == 20 && z == 30);
+
+        auto& [a,b,c] = v;
+        a = 100; b = 200; c = 300;
+        AMREX_ALWAYS_ASSERT(v[0] == 100 && v[1] == 200 && v[2] == 300);
+
+        auto const [i,j,k] = v;
+        AMREX_ALWAYS_ASSERT(i == 100 && j == 200 && k == 300);
+
+        auto [d,e,f] = v;
+        AMREX_ALWAYS_ASSERT(d == 100 && e == 200 && f == 300);
+    }
 
     // 1-based indexing
     {
@@ -271,5 +286,21 @@ int main (int argc, char* argv[])
         b.setVal(-1);
         AMREX_ALWAYS_ASSERT(a.dot(b) == -30);
     }
+    {
+        SmallVector<int, 3, 1> v{10,20,30};
+        auto const& [x,y,z] = v;
+        AMREX_ALWAYS_ASSERT(x == 10 && y == 20 && z == 30);
+
+        auto& [a,b,c] = v;
+        a = 100; b = 200; c = 300;
+        AMREX_ALWAYS_ASSERT(v[1] == 100 && v[2] == 200 && v[3] == 300);
+
+        auto const [i,j,k] = v;
+        AMREX_ALWAYS_ASSERT(i == 100 && j == 200 && k == 300);
+
+        auto [d,e,f] = v;
+        AMREX_ALWAYS_ASSERT(d == 100 && e == 200 && f == 300);
+    }
+
     amrex::Finalize();
 }