From d7e7f90d93cc754d8ff0181297ae6cd39ffae74f Mon Sep 17 00:00:00 2001
From: Roland Conybeare <rconybeare@gmail.com>
Date: Tue, 6 Jan 2026 00:17:13 -0500
Subject: [PATCH] xo-arena: annex ArenaConfig.*pp from xo-alloc2/

---
 include/xo/arena/ArenaConfig.hpp | 70 ++++++++++++++++++++++++++++++++
 src/arena/ArenaConfig.cpp        | 18 ++++++++
 src/arena/CMakeLists.txt         |  1 +
 3 files changed, 89 insertions(+)
 create mode 100644 include/xo/arena/ArenaConfig.hpp
 create mode 100644 src/arena/ArenaConfig.cpp

diff --git a/include/xo/arena/ArenaConfig.hpp b/include/xo/arena/ArenaConfig.hpp
new file mode 100644
index 0000000..7d3f347
--- /dev/null
+++ b/include/xo/arena/ArenaConfig.hpp
@@ -0,0 +1,70 @@
+/** @file ArenaConfig.hpp
+ *
+ *  @author Roland Conybeare, Dec 2025
+ **/
+
+#pragma once
+
+#include <string>
+#include <cstdint>
+
+namespace xo {
+    namespace mm {
+
+        /** @class ArenaConfig
+         *  @brief configuration for a @ref DArena instance
+         **/
+        struct ArenaConfig {
+            /** @defgroup mm-arenaconfig-ctors ArenaConfig ctors **/
+            ///@{
+
+            /** create anonymous arena with size @p z **/
+            static ArenaConfig simple(std::size_t z);
+
+            ///@}
+
+            /** @defgroup mm-arenaconfig-instance-vars ArenaConfig members **/
+            ///@{
+
+            /** optional name, for diagnostics **/
+            std::string name_;
+            /** desired arena size -- hard max = reserved virtual memory **/
+            std::size_t size_;
+            /** hugepage size -- using huge pages relieves some TLB pressure
+             *  (provided you use their full extent :)
+             **/
+            std::size_t hugepage_z_ = 2 * 1024 * 1024;
+            /** if non-zero, allocate extra space between allocs, and fill
+             *  with fixed test-pattern contents. Allows for simple
+             *  runtime arena sanitizing checks.
+             *  Will be rounded up to multiple of @ref padding::c_alloc_alignment
+             **/
+            std::size_t guard_z_ = 0;
+            /** if guard_z_ > 0, write at least that many copies
+             *  of this guard byte following each complete allocation
+             **/
+            std::uint8_t guard_byte_ = 0xfd;
+            /** if store_header_flag_ is true: mask bits for allocation size.
+             *  remaining bits can be stolen for other purposes
+             *  otherwise ignored
+             **/
+            /** true to store header (8 bytes) at the beginning of each allocation.
+             *  necessary and sufficient to allows iterating over allocs
+             *  present in arena
+             **/
+            bool store_header_flag_ = false;
+            /** mask applied to 8-byte alloc header.
+             *  bits set to 1 store alloc size; remaining
+             *  bits in alloc header can be used for other purposes.
+             **/
+            std::uint64_t header_size_mask_ = 0xffffffff;
+            /** true to enable debug logging **/
+            bool debug_flag_ = false;
+
+            ///@}
+        };
+
+    } /*namespace mm*/
+} /*namespace xo*/
+
+/* end ArenaConfig.hpp */
diff --git a/src/arena/ArenaConfig.cpp b/src/arena/ArenaConfig.cpp
new file mode 100644
index 0000000..d08feb4
--- /dev/null
+++ b/src/arena/ArenaConfig.cpp
@@ -0,0 +1,18 @@
+/** @file ArenaConfig.cpp
+ *
+ *  @author Roland Conybeare, Dec 2025
+ **/
+
+#include "ArenaConfig.hpp"
+
+namespace xo {
+    namespace mm {
+        ArenaConfig
+        ArenaConfig::simple(std::size_t z)
+        {
+            return ArenaConfig { .name_ = "anonymous", .size_ = z };
+        }
+    }
+}
+
+/* end ArenaConfig.cpp */
diff --git a/src/arena/CMakeLists.txt b/src/arena/CMakeLists.txt
index 9a12405..345e9cc 100644
--- a/src/arena/CMakeLists.txt
+++ b/src/arena/CMakeLists.txt
@@ -4,6 +4,7 @@ set(SELF_LIB xo_arena)
 set(SELF_SRCS
     AllocError.cpp
     AllocInfo.cpp
+    ArenaConfig.cpp
 )
 
 xo_add_shared_library4(${SELF_LIB} ${PROJECT_NAME}Targets ${PROJECT_VERSION} 1 ${SELF_SRCS})