Deserializer :: struct { // downcasts to []u8
  count: s64;
  data: *u8;
  cursor: s64;
  ok := true;   // cleared by any read that would run past the end; never set back
}

to_deserializer :: (view: $T/.[[]u8, string]) -> Deserializer #expand {
  return .{view.count, view.data, 0, true};
}

deserializer_can_read :: inline (d: *Deserializer, byte_count: s64) -> bool {
  if byte_count < 0 || d.cursor < 0 || d.cursor > d.count || byte_count > d.count - d.cursor {
    d.ok = false;
    return false;
  }
  return true;
}

Read :: inline (d: *Deserializer, $T: Type) -> T {
  mem: T = ---;
  if !deserializer_can_read(d, size_of(T)) {
    Initialize(*mem);   // a failed read yields a zero value
    return mem;
  }
  
  memcpy(*mem, d.data + d.cursor, size_of(T));
  
  d.cursor += size_of(T);
  
  return mem;
}

ReadArrayView :: inline (d: *Deserializer, view_count: s64, $T: Type) -> []T {
  // Range-check the count before multiplying, so a hostile count cannot overflow it.
  if view_count < 0 || d.cursor < 0 || d.cursor > d.count
  || view_count > (d.count - d.cursor) / size_of(T) {
    d.ok = false;
    return .[];
  }
  
  array: []T = .{view_count, cast(*T)(d.data + d.cursor)};
  
  d.cursor += view_count * size_of(T);
  
  return array;
}

ReadSizedArray :: inline (d: *Deserializer, $T: Type, $count_type: Type = s64) -> []T {
  view_count := xx Read(d, count_type);
  
  return ReadArrayView(d, view_count, T);
}

ReadStringView :: inline (d: *Deserializer, view_count: s64) -> string {
  if !deserializer_can_read(d, view_count)  return "";
  
  s := string.{view_count, d.data + d.cursor};
  
  d.cursor += view_count;
  
  return s;
}

ReadSizedString  :: inline (d: *Deserializer, $count_type: Type = s64) -> string {
  view_count : s64 = xx Read(d, count_type);
  
  return ReadStringView(d, view_count);
}

// Use Resizable_Array APIs directly: e.g. array_reset_keeping_memory, array_free, array_reset...
Serializer :: [..]u8;

new_serializer :: inline (reserve_count: s64) -> Serializer {
  serializer: Serializer;
  array_reserve(*serializer, reserve_count);
  
  return serializer;
}

Add :: inline (s: *Serializer, item: $T) {
  // we use the offset, do NOT use s.data, because the pointer 
  // can move on resize, so it is no longer valid!
  starting_offset := s.count; 
  array_resize(s, s.count + size_of(T));
  memcpy(s.data + starting_offset, *item, size_of(T));
}

AddArray_NoSize :: inline (s: *Serializer, view: []$T) {
  starting_offset := s.count;
  array_resize(s, s.count + (view.count * size_of(T)));
  memcpy(s.data + starting_offset, view.data, view.count * size_of(T));
}

AddSizedArray :: inline (s: *Serializer, view: []$T, $count_type: Type = s64) -> bool {
  // A count that does not survive count_type would be written truncated and then followed by
  // the full payload, so every field after it reads from the wrong offset. The round trip
  // catches that for any count_type; cast,no_check keeps a debug build from aborting on the
  // cast before this can report it.
  view_count := cast,no_check(count_type) view.count;
  if cast(s64) view_count != view.count {
    log_error("[AddSizedArray] a % element array does not fit a % length prefix.", view.count, count_type);
    return false;
  }

  Add(s, view_count);
  AddArray_NoSize(s, view);
  return true;
}

AddString :: inline (s: *Serializer, view: string, $count_type: Type = s64) -> bool {
  view_count := cast,no_check(count_type) view.count;
  if cast(s64) view_count != view.count {
    log_error("[AddString] a % byte string does not fit a % length prefix.", view.count, count_type);
    return false;
  }

  Add(s, view_count);
  AddArray_NoSize(s, cast([]u8)view);
  return true;
}

// This has specialized use cases where the offsets are saved in a separate array. e.g. SoA filesystem string storage.
AddString_NoSize :: inline (s: *Serializer, data: *u8, count: $T, $offset_type: Type = s32) -> offset_type {
  offset := cast(offset_type) s.count;

  AddArray_NoSize(s, []u8.{count, data});

  return offset;
}

AddString_PaddedTo8ByteAlignment :: (s: *Serializer, view: string) {
  // The purpose of this function is to zero-pad a string to
  // 8-bytes so we don't run into annoying unaligned copy issues.
  Add(s, view.count);
  AddArray_NoSize(s, cast([]u8)view);
  
  starting_offset := s.count;
  remainder := align_forward(view.count, size_of(s64)) - view.count;
  array_resize(s, s.count + remainder);
  
  // Fill remaining space with zeros
  memset(s.data + starting_offset, 0, remainder);
}