refactor(furvm): use an arena allocator for thing data

Refs: #10

furvm/src/thing.cpp

@@ -5,6 +5,8 @@
 #include "furvm/context.hpp" // IWYU pragma: keep
 
 #include <cstdlib>
+#include <cstring>
+#include <stdexcept>
 
 namespace furvm {
 
@@ -16,10 +18,29 @@ std::size_t thing_type_size(thing_t type) {
 }
 
 thing::thing(rzecz, thing_handle id, thing_t type, const context_p& context)
-  : m_id(id), m_type(type), m_context(context), m_data(std::calloc(1, thing_type_size(type))) {}
+  : m_id(id), m_type(type), m_context(context) {
+    std::size_t size = thing_type_size(type);
+    std::byte*  data = nullptr;
+    if (!m_context->m_deadThings.empty()) {
+        thing_t itType{};
+        for (auto it = m_context->m_deadThings.rbegin(); it != m_context->m_deadThings.rend(); ++it) {
+            std::memcpy(&itType, static_cast<std::byte*>(*it) - sizeof(itType), sizeof(itType));
+            if (size == thing_type_size(itType)) {
+                data = static_cast<std::byte*>(*it);
+                break;
+            }
+        }
+    }
+    if (data == nullptr) data = m_context->m_thingArena.allocate<std::byte>(sizeof(type) + size);
+
+    if (data == nullptr) throw std::runtime_error("failed to allocate data for new thing");
+    std::memcpy(data, &type, sizeof(type));
+    m_data = data + sizeof(type);
+    std::memset(m_data, 0, size);
+}
 
 thing::~thing() {
-    std::free(m_data);
+    m_context->m_deadThings.push_back(m_data);
 }
 
 thing_p thing::create(const context_p& context, thing_t type) {