global const SizeU default_reserve_size   = gib(4);
global const SizeU default_alignment      = 8;
global const SizeU additional_commit_size = mib(1);

function void
memory_copy(U8 *dst, U8 *src, SizeU size){
  for(SizeU i = 0; i < size; i++){
    dst[i] = src[i];
  }
}

function void
memory_zero(void *p, SizeU size){
  U8 *pp = p;
  for(SizeU i = 0; i < size; i++)
    pp[i] = 0;
}

function void
arena_init(Arena *a){
  a->memory = os_reserve(default_reserve_size);
  a->alignment = default_alignment;
}

function void *
arena_push_size(Arena *a, SizeU size){
  SizeU generous_size = size + a->alignment;
  if(generous_size>a->memory.commit){
    if(a->memory.reserve == 0){
      arena_init(a);
    }
    os_commit(&a->memory, generous_size+additional_commit_size);
  }
  
  a->len = align_up(a->len, a->alignment);
  void *result = (U8*)a->memory.data + a->len;
  a->len += size;
  
  memory_zero(result, size);
  return result;
}

function String
arena_push_string_copy(Arena *arena, String string){
  U8 *copy = arena_push_array(arena, U8, string.len+1);
  memory_copy(copy, string.str, string.len);
  copy[string.len] = 0;
  return (String){copy, string.len};
}