void pointer duck typing

programs/base.kl

@@ -1,12 +1,18 @@
-Os :: #import "os.kl"
-
+Os    :: #import "os.kl"
 SizeU :: #strict U64
 
+arena_di: U64
+
 Arena :: struct
+  di: U64 // @debug_id
   memory: Os.Memory
   alignment: U64
   len: U64
 
+  ADDITIONAL_COMMIT_SIZE :: 1024*1024
+  DEFAULT_RESERVE_SIZE   :: 1024*1024*1024
+  DEFAULT_ALIGNMENT      :: 8
+
 clamp_top_sizeu :: (val: SizeU, max: SizeU): SizeU
   if val > max
     return max
@@ -22,3 +28,22 @@ get_align_offset :: (size: SizeU, align: SizeU): SizeU
 align_up :: (size: SizeU, align: SizeU): SizeU
   result := size + get_align_offset(size, align)
   return result
+
+arena_init :: (a: *Arena)
+  a.memory    = Os.reserve(a.DEFAULT_RESERVE_SIZE)
+  a.alignment = a.DEFAULT_ALIGNMENT
+  a.di        = arena_di++
+
+arena_push_size :: (a: *Arena, size: SizeU): *void
+  generous_size := size + a.alignment
+  if a.len + generous_size > a.memory.commit
+    if a.memory.reserve == 0
+      arena_init(a)
+    result := Os.commit(&a.memory, generous_size + a.ADDITIONAL_COMMIT_SIZE)
+    assert(result == true)
+  a.len = align_up(a.len, a.alignment)
+  assert(a.memory.reserve  > a.len + a.memory.commit)
+  result: *void = a.memory.data + a.len
+  a.len += size
+  return result
+