2021-08-26

2021-09-18

data as lists or not as lists?

In lisp, any structured data can be expressed as lists of lists or values.

In dabric the data structure can be anything. Each value is prefixed with a type that tells the size of the following value in bytes so that if the reader does not know how to interpret the value it can skip it in the stream.

Dabric file structure

dabric file headers

In a Dabric file values are often prefixed with a number that tells how many bytes the following value takes. This way the file can be parsed even if the readers does not know how to interpret some of the values. The byte count is always expressed as a Variable length quantity (VLQ). This kind of values are called here "byte size prefixed".

Dabric file has a header that consists of

• UTF-8 string "DABRIC" as a file signature
• file format version as a VLQ
• byte size prefixed sequence of 128 bit stream ids. Their position in the sequence are used in the entity ids later in the file.
• byte size prefixed sequence of value type definitions.
• Each value type definition consists of byte size / entity id pair. Byte size is expressed as a VLQ. Zero means the value will be byte size prefixed rather than of fixed length.
• Entity id:s are encoded as byte size prefixed array of VLQ:s. The first vlq referes to a position in the preceding stream id sequence.

After the header comes the body as a type value pair.

Value types in Dabric prelude

• entity id as an array of vlq:s where the first vlq referes to a stream id in the header
• Length prefixed array of type tagged values
• array of entity id / type tagged value pairs

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