allocator: add arena_allocator

This allocator uses Lua userdatas for dynamic allocation
that is automatically freed when the current scope exits.

src/arena_allocator.c

@@ -0,0 +1,44 @@
+#include "arena_allocator.h"
+
+#include <string.h>
+#include <lauxlib.h>
+
+void lxl_arena_init(lua_State *L, lxl_arena *arena) {
+  lua_newtable(L);
+  arena->L = L; arena->ref = lua_gettop(L);
+}
+
+void *lxl_arena_malloc(lxl_arena *arena, size_t size) {
+  if (!arena || !arena->L) return NULL;
+  if (!lua_istable(arena->L, arena->ref)) luaL_error(arena->L, "invalid arena reference");
+  void *data = lua_newuserdata(arena->L, size);
+  lua_pushlightuserdata(arena->L, data);
+  lua_pushvalue(arena->L, -2);
+  lua_settable(arena->L, arena->ref);
+  lua_pop(arena->L, 1);
+  return data;
+}
+
+void *lxl_arena_zero(lxl_arena *arena, size_t size) {
+  void *v = lxl_arena_malloc(arena, size);
+  return v ? memset(v, 0, size) : NULL;
+}
+
+char *lxl_arena_strdup(lxl_arena *arena, const char *str) {
+  if (!str) return NULL;
+  return lxl_arena_copy(arena, (void *) str, (strlen(str) + 1) * sizeof(char));
+}
+
+char *lxl_arena_copy(lxl_arena *arena, void *ptr, size_t len) {
+  if (!ptr) return NULL;
+  char *output = lxl_arena_malloc(arena, sizeof(char) * len);
+  return output ? memcpy(output, ptr, len) : NULL;
+}
+
+void lxl_arena_free(lxl_arena *arena, void *ptr) {
+  if (!arena || !arena->L || !ptr) return;
+  if (!lua_istable(arena->L, arena->ref)) luaL_error(arena->L, "invalid arena reference");
+  lua_pushlightuserdata(arena->L, ptr);
+  lua_pushnil(arena->L);
+  lua_settable(arena->L, arena->ref);
+}

src/arena_allocator.h

@@ -0,0 +1,25 @@
+/**
+ * An arena allocator using the Lua state; similar to luaL_Buffer.
+ * Initialize the arena with lxl_arena_init(), and you can use lxl_arena_malloc(),
+ * lxl_arena_zero() to allocate (and optionally zero) the memory.
+ * lxl_arena_free() can be optionally used to free memory, but this is generally not needed.
+ */
+
+#ifndef LUA_ALLOCATOR_H
+#define LUA_ALLOCATOR_H
+
+#include <lua.h>
+
+typedef struct lxl_arena {
+  lua_State *L;
+  int ref;
+} lxl_arena;
+
+void lxl_arena_init(lua_State *L, lxl_arena *arena);
+void *lxl_arena_malloc(lxl_arena *arena, size_t size);
+void *lxl_arena_zero(lxl_arena *arena, size_t size);
+char *lxl_arena_copy(lxl_arena *arena, void *ptr, size_t len);
+char *lxl_arena_strdup(lxl_arena *arena, const char *str);
+void lxl_arena_free(lxl_arena *arena, void *ptr);
+
+#endif

src/meson.build

@@ -6,6 +6,7 @@ lite_sources = [
     'api/system.c',
     'api/process.c',
     'api/utf8.c',
+    'arena_allocator.c',
     'renderer.c',
     'renwindow.c',
     'rencache.c',