--- no_site_title: true title: "Preserves: Zero-copy Binary Syntax" --- Tony Garnock-Jones {{ site.version_date }}. Version {{ site.version }}. *Preserves* is a data model, with associated serialization formats. This document defines one of those formats: a binary syntax for `Value`s from the [Preserves data model](preserves.html) that avoids, in many cases, use of intermediate data structures during reading and writing. This makes it suitable for use for representation of very large values whose fully-decoded representations may not fit in working memory. ## Zero-Copy Binary Syntax A `Buf` is a zero-copy syntax encoding, or representation, of a non-immediate `Value`. A `Ref` is either a type-tagged representation of a small immediate `Value` or a type-tagged pointer to a `Buf`. Each `Ref` is a 64-bit unsigned value. Its tag appears in the low 4 bits. The remaining 60 bits encode either an unsigned offset pointing to a previously-encoded `Buf`, or an immediate value. Pointers always point backwards to earlier positions. Each `Buf` is prefixed with a 64-bit payload length, counted in units of bytes, and is zero-padded to the nearest multiple of 16 bytes. Neither the length of the padding nor the length of the length itself are included in the length. Offsets in pointer `Ref`s are counted in 16-byte units, measuring from the beginning of the length indicator of the `Buf` in which the `Ref` appears. A zero offset is special: it denotes an *empty value* of the type associated with the tag in the `Ref`. All multi-byte quantities are encoded using little-endian byte order. ### Header. Because `Ref`s are typed, but `Buf`s are not, the outermost `Value` in e.g. a file or network stream is always encoded preceded by a special header. Offset Length Description -------- ------ ----------- 00000000 1 Marker byte 0xFF 00000001 1 Version number 0x00 00000002 6 Reserved, 0x00 00000008 8 Special Ref 00000010 8 Length ("n") of encoded data, in bytes 00000018 n Encoded data - - Zero-padding to next 16-byte boundary The `Ref` in the header at offset 8 is special. If it encodes an immediate `Value`, that `Value` is the encoded value, and the length field and encoded data are omitted. The entire encoded value is exactly 16 bytes long in this case. However, if the special `Ref` is an encoding of a pointer to a `Buf`, the offset is interpreted as counting back from the very end of the padding at the end of the encoded data. The entire encoded value is the length of the encoded data, plus 24, rounded up to the next multiple of 16. Either way, the tag on the special `Ref` is the type of the encoded value. ### Tags and Refs. Tag Type Interpretation of 60-bit payload --- ------------- -------------------------------- 0 Boolean 0 = False, 1 = True 1 IEEE 754 Offset to Buf holding little-endian 32/64-bit float 2 SignedInteger Signed 60-bit integer 3 SignedInteger Offset to Buf holding little-endian signed integer 4 String 0-7 bytes of UTF-8; length in lower 4 bits 5 String Offset to Buf holding UTF-8 data 6 ByteString 0-7 bytes of raw binary; length in lower 4 bits 7 ByteString Offset to Buf holding raw binary data 8 Symbol 0-7 bytes of UTF-8; length in lower 4 bits 9 Symbol Offset to Buf holding UTF-8 data A Record Offset to Buf holding Refs (label, fields) B Sequence Offset to Buf holding Refs (sequence values) C Set Offset to Buf holding Refs (elements in arbitrary order) D Dictionary Offset to Buf holding Refs (key/value pairs) E Embedded Offset to Buf holding a single Ref F - (reserved) ### Records, Sequences, Sets and Dictionaries. Offset Length Description -------- ------ ----------- 00000000 8 n*8: length of following sequence of n Refs, in bytes 00000008 8 Ref 0 ... ... ... n*8 8 Ref n-1 (n+1)*8 8 Padding, only if n is even Each compound datum is represented as a sequence of `Ref`s representing the contained `Value`s. Each `Record`'s sequence represents the label, followed by the fields in order. Each `Sequence`'s representation is just its contained values in order. `Set`s are ordered arbitrarily into a sequence. The key-value pairs in a `Dictionary` are ordered arbitrarily, alternating between keys and their matching values. There is *no* ordering requirement on the elements of `Set`s or the key-value pairs in a `Dictionary`. They may appear in any order. However, the elements and keys *MUST* be pairwise distinct according to the [Preserves equivalence relation](preserves.html#equivalence). ### SignedIntegers. Integers between -259 and 259-1, inclusive, are represented as immediate values in a `Ref` with tag 2. Integers outside this range are represented with a `Ref` with tag 3 pointing to a `Buf` containing exactly as many 64-bit words as needed to unambiguously identify the value and its sign, in little-endian byte and word ordering. Every `SignedInteger` *MUST* be represented with its shortest possible encoding. For example, Number (decimal) Ref (64-bit) Buf (hex bytes) ----------------------------------------- ---------------- ---------------- -576460752303423488 8000000000000002 - -257 FFFFFFFFFFFFEFF2 - -1 FFFFFFFFFFFFFFF2 - 0 0000000000000002 - 1 0000000000000012 - 257 0000000000001012 - 576460752303423487 7FFFFFFFFFFFFFF2 - 1000000000000000000000000000000 ...............3 1000000000000000 00000040EAED7446 D09C2C9F0C000000 0000000000000000 -1000000000000000000000000000000 ...............3 1000000000000000 000000C015128BB9 2F63D360F3FFFFFF 0000000000000000 87112285931760246646623899502532662132736 ...............3 1800000000000000 0000000000000000 0000000000000000 0001000000000000 ### Strings, ByteStrings and Symbols. Syntax for these three types varies only in the tag used. For `String` and `Symbol`, the encoded data is a UTF-8 encoding of the `Value`'s code points, while for `ByteString` it is the raw data contained within the `Value` unmodified. Encoded data of length 7 bytes or shorter is represented as an immediate `Ref` with tag 4 (`String`), 6 (`ByteString`) or 8 (`Symbol`). The lower 4 bits of the 60-bit payload are the length of the encoded data; the upper 56 bits are 7 bytes of data, with the first data byte in the lowest byte, so that the order of data bytes in memory in an immediate encoding matches the order in a `Buf` encoding. Data longer than 7 bytes is represented with a `Ref` with tag 5, 7 or 9 pointing to a `Buf` containing the bytes of encoded data. Empty values (length 0) *MUST* be encoded using pointer `Ref` form with special offset zero. For example, Value Ref (64-bit) Buf (hex bytes) ----------------------------------------- ---------------- ---------------- "" 0000000000000005 - #"" 0000000000000007 - || 0000000000000009 - "Hello" 48656C6C6F000054 - "a\0a" 6100610000000034 - "Hello, world!" ...............5 0D00000000000000 48656C6C6F2C2077 6F726C6421000000 0000000000000000 ### Booleans. Value Ref (64-bit) Buf (hex bytes) ----------------------------------------- ---------------- ---------------- #f 0000000000000000 - #t 0000000000000010 - ### Floats and Doubles. Each IEEE 754 4- and 8-byte binary representation is encoded into a `Buf`, pointed to with a `Ref` with tag 1. The length of the `Buf` disambiguates between 32-bit floats and 64-bit doubles. ((This is a very sparse encoding! Each float/double takes up 24 bytes split across the `Buf` and `Ref`.)) ### Embeddeds. To encode an `Embedded`, first choose a `Value` to represent the denoted object, and encode that, producing a `Ref`. Place that ref in a `Buf` all of its own (with length 8). Finally, point to the `Buf` with a `Ref` with tag 15. ### Annotations. ((Not sure: put them as a trailer after a Header?)) ## Security Considerations ((TBD)) ## Appendix. Autodetection of textual or binary syntax The first byte of a Header is 0xFF, which may not appear in any UTF-8 string. ((...))