xo-reader2 scaffold (fomo+arena version of xo-reader/) [WIP]

include/xo/reader2/ParserStack.hpp

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+/** @file ParserStack.hpp
+ *
+ *  @author Roland Conybeare, Jan 2026
+ **/
+
+#pragma once
+
+#include "SyntaxStateMachine.hpp"
+#include <xo/alloc2/Allocator.hpp>
+#include <xo/facet/obj.hpp>
+
+namespace xo {
+    namespace scm {
+
+        /** @brief A stack of expression state machines
+         *
+         *  Each state machine is dedicated to a particular syntax instance.
+         *  The innermost machine is in xsm; machines for surrounding expressions
+         *  are in progressively removed frames reached via parent links.
+         **/
+        class ParserStack {
+        public:
+            using AAllocator = xo::mm::AAllocator;
+
+        public:
+            ParserStack(obj<ASyntaxStateMachine> ssm, ParserStack * parent);
+
+            /** create new top of stack for syntax @p ssm, using memory from @p mm.
+             *  previous stack given by @p parent
+             **/
+            ParserStack * push(obj<AAllocator> mm,
+                               obj<ASyntaxStateMachine> ssm);
+
+            obj<ASyntaxStateMachine> top() const noexcept { return ssm_; }
+            ParserStack * parent() const noexcept { return parent_; }
+
+        private:
+            /** top of parsing stack: always non-null **/
+            obj<ASyntaxStateMachine> ssm_;
+            /** remainder of parsing stack excluding top **/
+            ParserStack * parent_ = nullptr;
+        };
+
+    } /*namespace scm*/
+} /*namespace xo*/
+
+/* end ParserStack.hpp */