From ba224dcf1543b1208b3f1014e4bf2ea96c44c07a Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Mon, 10 Nov 2025 05:12:02 +0000
Subject: [PATCH 1/5] Initial plan


From fb004906d4c5cc5f3ad0b03effe2586b27b01ddc Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Mon, 10 Nov 2025 05:16:22 +0000
Subject: [PATCH 2/5] Initial plan for C++20 upgrade

Co-authored-by: chenpeizhi <8114085+chenpeizhi@users.noreply.github.com>
---
 deps/fbitset | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/deps/fbitset b/deps/fbitset
index c3ca474..765b2f7 160000
--- a/deps/fbitset
+++ b/deps/fbitset
@@ -1 +1 @@
-Subproject commit c3ca47404ac195a96f87ccf94ea66b3fc593cff5
+Subproject commit 765b2f740ca71e732d047aa65e067c1a9018feee

From 0397545c7ea15f0c575a7f525ed1efe1321de5dc Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Mon, 10 Nov 2025 05:23:59 +0000
Subject: [PATCH 3/5] Update to C++20 and add nodiscard attributes

Co-authored-by: chenpeizhi <8114085+chenpeizhi@users.noreply.github.com>
---
 .clang-format          | 2 +-
 CMakeLists.txt         | 5 +++--
 include/libparenth.hpp | 8 ++++----
 3 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/.clang-format b/.clang-format
index b1dc370..59e48d1 100644
--- a/.clang-format
+++ b/.clang-format
@@ -1,7 +1,7 @@
 ---
 Language: Cpp
 BasedOnStyle: WebKit
-Standard: Cpp11
+Standard: c++20
 ColumnLimit: 80
 ...
 
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 35c790c..b7ec966 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,11 +1,12 @@
-cmake_minimum_required(VERSION 3.8)
+cmake_minimum_required(VERSION 3.12)
 project(libparenth VERSION 0.1.0 LANGUAGES CXX)
 
 # OPTIONS
 option(BUILD_TESTS "Build unit tests" ON)
 
 # Set the building options.
-set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD 20)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 include_directories(
         "${PROJECT_SOURCE_DIR}/include"
diff --git a/include/libparenth.hpp b/include/libparenth.hpp
index 7e6e5e8..72122c9 100644
--- a/include/libparenth.hpp
+++ b/include/libparenth.hpp
@@ -125,12 +125,12 @@ class Parenther {
     /** The total number of summation and external indices.
      */
 
-    size_t n_dims() const noexcept { return dims_.size(); }
+    [[nodiscard]] size_t n_dims() const noexcept { return dims_.size(); }
 
     /** The total number of factors.
      */
 
-    size_t n_factors() const noexcept { return dims_on_.size(); }
+    [[nodiscard]] size_t n_factors() const noexcept { return dims_on_.size(); }
 
     /** The two operands of a pairwise contraction.
      *
@@ -218,7 +218,7 @@ class Parenther {
      * the traversed subproblems can be found, are returned.
      */
 
-    Mem opt(Mode mode, bool if_incl)
+    [[nodiscard]] Mem opt(Mode mode, bool if_incl)
     {
         Mem mem{};
 
@@ -839,7 +839,7 @@ class Parenther {
      * zero).
      */
 
-    Dim get_tot(const Dim_subset& dims) const noexcept
+    [[nodiscard]] Dim get_tot(const Dim_subset& dims) const noexcept
     {
         Dim res{ 1l };
         for (auto it = dims.begin(); it; ++it) {

From b542ba148fc1596b73d5b299ee19fcc94a0fdeb7 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Mon, 10 Nov 2025 05:25:14 +0000
Subject: [PATCH 4/5] Add clarifying comment about find_last behavior

Co-authored-by: chenpeizhi <8114085+chenpeizhi@users.noreply.github.com>
---
 include/libparenth.hpp | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/include/libparenth.hpp b/include/libparenth.hpp
index 72122c9..f79bb2a 100644
--- a/include/libparenth.hpp
+++ b/include/libparenth.hpp
@@ -331,6 +331,8 @@ class Parenther {
 
             auto has_top_idx = bsums.curr_sums.count() > 0;
             auto top_idx = bsums.curr_sums.find_last();
+            // Note: When count() == 0, find_last() returns SIZE_MAX.
+            // next_idx wraps to 0 via unsigned overflow, which is well-defined.
             auto next_idx = top_idx + 1;
             if (next_idx < sums_.size()) {
 

From a0ec058a6899d1b50037ea05f68645f0b68e14f0 Mon Sep 17 00:00:00 2001
From: Guo Chen <peizhi.chen@gmail.com>
Date: Mon, 10 Nov 2025 00:59:21 -0600
Subject: [PATCH 5/5] Update to CMake 3.15

---
 CMakeLists.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index b7ec966..35e2689 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.12)
+cmake_minimum_required(VERSION 3.15)
 project(libparenth VERSION 0.1.0 LANGUAGES CXX)
 
 # OPTIONS