Added arena checkpointing

src/arena.h

@@ -17,21 +17,31 @@ struct Arena {
     Arena *current;
     U8 *base;
     U32 size;
-    U32 pos;
+    U32 pos;          // start of next allocation
+    U32 base_pos;     // absolute position of base relative to the first linked arena
     ArenaFlags flags;
     Arena *prev;
 };
 
+typedef struct ArenaCheckPoint ArenaCheckPoint;
+struct ArenaCheckPoint {
+    Arena *arena;
+    U32 absolute_pos;
+};
+
 StaticAssert(sizeof(Arena) <= 64);
-#define ARENA_HEADER_SZ 64
+#define ARENA_HEADER_SZ 64u
 
 
-INLINE U32 arena_available(const Arena *arena)
+static INLINE U32 arena_available(const Arena *arena)
 {
     return arena->size - arena->pos;
 }
 
-
 Arena *arena_alloc_(const ArenaParams *params);
 void arena_free(Arena *arena);
 void *arena_push(Arena *arena, U32 size, U32 align);
+
+ArenaCheckPoint arena_temp_begin(Arena *arena);
+void arena_temp_end(ArenaCheckPoint *checkpoint);
+void arena_pop_to(Arena *arena, U32 abs_pos);