Printable Data: a data de/serialization
Because its not easy to find a good one that is:
- Standardized
- Human readable/printable
- Fast encoding/decoding
- Light weighted
- Easy toolchain
On the contrary, data size and streaming capability are traded off.
Finally, S-Expression by Ron Rivest is choosed as the starting point of pd.
A token is an octet-string. A list is an ordered collection noted by a pair
of bracket '(' and ')'. An S-Expression is a list of token or list.
S-Expression has various ways of representation. Pd limits it into three ways:
- Canonical representation: Each pd has a unique canonical representation in binary form. It can be used as data representation for digital signature. It is easy and fast in marshalling / unmarshalling.
- Transport representation:
Transport is a base64 encoded Canonical representation in pair of
'{''}'for text mode or canonical representation itself in binary mode. - Printable representation: The printable representation is pretty-printted human readable in text mode. It is encoded in utf-8 by default.
Token in Canonical Representation is a byte length followed by a column and the data itself:
(4:text)
(1:06:xxxxxx(3:abc2:ok))
The byte length and column is in the ASCII encoding (binary 0x30-0x3A).
The list notion '(' and ')' is in the ASCII encoding (binary 0x28, 0x29).
the data is 'verbatim' without encoding.
Empty value is represented as 0:, empty string is '()'.
Binary mode Transport Representation is the same as Canonical Representation.
Text mode Transport Representation is the
base64 encoded Canonical Representation
without outer bracket and wrapped in '{' '}' (binary 0x7B, 0x7D) pairs.
{NDp0ZXh0Cg==}
{MTowNjp4eHh4eHgoMzphYmMyOm9rKQo=}
Printable representation is a human readble version of canonical representation.
It omits the prefixed byte length and the column.
It prints out nested list in a indented way.
It prints out the item in list as individual rows.
It prints out the binary octet-string as is if all of them falls into the 7-bit printable ascii (0x20-0x7E) and '\t' 0x09 '\n' 0x0A '\r' 0x0D. Or the one byte spaced hex encoding.
Empty value is showen as qouted 'null'. Empty List is ()
(test)
(0
XX XX XX XX XX XX
(abc
ok))