From 48c6da113464eeadbbd54eb099ee51dcc74ab654 Mon Sep 17 00:00:00 2001
From: CHatingPython <chatingpython@gmail.com>
Date: Sun, 7 Jun 2026 12:02:36 +0200
Subject: [PATCH] feat(furvm): add stack underflow exception

---
 furvm/include/furvm/exceptions.hpp | 43 ++++++++++++++++++++++++++++++
 furvm/include/furvm/fwd.hpp        |  7 +++++
 furvm/src/executor.cpp             |  8 +++---
 furvm/src/main.cpp                 |  3 ++-
 4 files changed, 57 insertions(+), 4 deletions(-)
 create mode 100644 furvm/include/furvm/exceptions.hpp

diff --git a/furvm/include/furvm/exceptions.hpp b/furvm/include/furvm/exceptions.hpp
new file mode 100644
index 0000000..e9a0c58
--- /dev/null
+++ b/furvm/include/furvm/exceptions.hpp
@@ -0,0 +1,43 @@
+#ifndef FURVM_EXCEPTIONS_HPP
+#define FURVM_EXCEPTIONS_HPP
+
+#include <exception>
+
+namespace furvm {
+
+class stack_underflow : public std::exception {
+public:
+    stack_underflow()           = default;
+    ~stack_underflow() override = default;
+
+    /**
+     * @brief Move constructor.
+     */
+    stack_underflow(stack_underflow&&) noexcept = default;
+
+    /**
+     * @brief Move constructor.
+     */
+    stack_underflow& operator=(stack_underflow&&) noexcept = default;
+
+    /**
+     * @brief Copy constructor.
+     */
+    stack_underflow(const stack_underflow&) = default;
+
+    /**
+     * @brief Copy constructor.
+     */
+    stack_underflow& operator=(const stack_underflow&) = default;
+public:
+    /**
+     * @brief Returns a C-style string describing the cause of the error.
+     *
+     * @return The cause of the error.
+     */
+    const char* what() const noexcept override { return "stack underflow"; }
+};
+
+} // namespace furvm
+
+#endif // FURVM_EXCEPTIONS_HPP
\ No newline at end of file
diff --git a/furvm/include/furvm/fwd.hpp b/furvm/include/furvm/fwd.hpp
index 78a1a42..6d36280 100644
--- a/furvm/include/furvm/fwd.hpp
+++ b/furvm/include/furvm/fwd.hpp
@@ -140,6 +140,13 @@ class context;
  */
 using context_p = std::shared_ptr<context>;
 
+// exceptions.hpp:
+
+/**
+ * @brief Stack underflow exception.
+ */
+class stack_underflow;
+
 } // namespace furvm
 
 #endif // FURVM_FWD_HPP
\ No newline at end of file
diff --git a/furvm/src/executor.cpp b/furvm/src/executor.cpp
index 9101fb2..687cb00 100644
--- a/furvm/src/executor.cpp
+++ b/furvm/src/executor.cpp
@@ -1,6 +1,7 @@
 #include "furvm/executor.hpp"
 
 #include "furvm/context.hpp" // IWYU pragma: keep
+#include "furvm/exceptions.hpp"
 #include "furvm/instruction.hpp"
 #include "furvm/thing.hpp"
 
@@ -26,6 +27,7 @@ void executor::push_frame(const mod_p& mod, std::size_t position) {
 }
 
 struct executor::frame executor::pop_frame() {
+    if (m_frames.empty()) throw stack_underflow();
     struct executor::frame frame = m_frames.top();
     m_frames.pop();
     return frame;
@@ -40,14 +42,14 @@ void executor::push_thing(const thing_p& thing) {
 }
 
 thing_p executor::pop_thing() {
-    if (m_frames.top().stackBase >= m_stack.size()) return {};
+    if (m_frames.top().stackBase >= m_stack.size()) throw stack_underflow();
     thing_p top = std::move(m_stack.top());
     m_stack.pop();
     return top;
 }
 
 thing_p executor::thing() const {
-    if (m_frames.top().stackBase >= m_stack.size()) return {};
+    if (m_frames.top().stackBase >= m_stack.size()) throw stack_underflow();
     return m_stack.top();
 }
 
@@ -65,7 +67,7 @@ void executor::step() {
         m_stack.push(std::move(thing));
     } break;
     case instruction_t::Drop: {
-        m_stack.pop();
+        pop_thing();
     } break;
     case instruction_t::Return: {
         pop_frame();
diff --git a/furvm/src/main.cpp b/furvm/src/main.cpp
index f6b25a8..53474eb 100644
--- a/furvm/src/main.cpp
+++ b/furvm/src/main.cpp
@@ -8,9 +8,10 @@
 #include <cstddef>
 #include <memory>
 
-static constexpr std::array<furvm::byte, 3> s_bytecode = {
+static constexpr std::array<furvm::byte, 4> s_bytecode = {
     furvm::byte(furvm::instruction_t::PushB2I),
     67,
+    furvm::byte(furvm::instruction_t::Drop),
     furvm::byte(furvm::instruction_t::Return),
 };