Serialize Composite Objects

For composite objects, the individual elements in the sequence are recursively serialized and packed together, separated by an offset of 4 bytes, into 32 Byte chunks, with the length (and type for Unions) "mixed-in". The final serialized object is a concatenation of the fixed parts and the variable parts.

Fixed-size and variable-size

An object is considered fixed-size if it is:

• a basic-type
• a fixed composition of fixed-size types

An object is considered variable-size if it is not fixed-size

Serialize and Deserialize

Representation of sequences can be thought of as two parts: the fixed part, and the variable part.

Fixed-size types do not have a variable part.

• Fixed part

  • For each of the elements in order, if the element type is:

    • fixed-size:

      • Serialize the element and append it to the fixed-size part.
        • Lists of fixed-size elements effectively concatenate all elements, the naming of the fixed-size part as whole does not apply to the list, but to element-type.

    • variable-size:

      • Append an offset to the fixed-size part, pointing to the start of the element data in the variable-size part.
      • Serialize the element and append it to the variable size part.

• Variable part

  • For variable-size elements, the elements are serialized, tightly packed, then appended in order to the variable-size part.

• Offsets

  • Within the fixed-size part offsets may be encoded to locate elements in the variable-size part.
  • Offsets are 4 bytes each, typed as uint32, and can range from [bytelen(fixed_part), bytelen(fixed_part) + bytelen(variable_part)]. I.e. the fixed-part byte length is included as part of the offset.
  • Each offset is pointing to the start of the serialized data, the index of the first byte of the element.
  • For each offset, it MUST hold that offsets[i-1] <= offsets[i] <= offsets[i+1], so that elements can be read from the byte stream following the offsets in order.
  • It also MUST hold that the first offset aligns correctly:
  • In a List this means that the first offset MUST be an exact multiple of the offset size, i.e. a multiple of 4.
  • In a Container, this means that the first offset equals the fixed-size part of the container.