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;
  if(a->memory.commit+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 void
arena_pop_pos(Arena *arena, SizeU pos){
  pos = clamp_top_sizeu(pos, arena->len);
  arena->len = pos;
}

function Arena_Checkpoint
arena_checkpoint(Arena *arena){
  Arena_Checkpoint result = {arena, arena->len};
  return result;
}

function void
arena_restore(Arena_Checkpoint checkpoint){
  arena_pop_pos(checkpoint.arena, checkpoint.pos);
}

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};
}

function String
string_fmtv(Arena *arena, const char *str, va_list args1) {
  va_list args2;
  va_copy(args2, args1);
  U64 len = vsnprintf(0, 0, str, args2);
  va_end(args2);
  
  char *result = (char *)arena_push_size(arena, len + 1);
  vsnprintf(result, len + 1, str, args1);
  if (arena->len > 0)
    arena->len -= 1;
  
  return (String){(U8 *)result, len};
}

function String 
string_fmt(Arena *arena, const char *str, ...) {
  va_list args1;
  va_start(args1, str);
  String result = string_fmtv(arena, str, args1);
  va_end(args1);
  return result;
}