eVPack/VelocyPack.md

VelocyPack (VPack)
==================

    Version 1

VelocyPack (VPack) is a fast and compact serialization format


## Generalities

VPack is (unsigned) byte oriented, so VPack values are simply sequences
of bytes and are platform independent. Values are not necessarily
aligned, so all access to larger subvalues must be properly organised to
avoid alignment assumptions of the CPU.


## Value types

We describe a single VPack value, which is recursive in nature, but
resides (with two exceptions, see below) in a single contiguous block of
memory. Assume that the value starts at address A, the first byte V
indicates the type (and often the length) of the VPack value at hand:

We first give an overview with a brief but accurate description for
reference, for arrays and objects see below for details:

- `0x00`        : none - this indicates absence of any type and value, this is not allowed in VPack values
- `0x01`        : empty array
- `0x02`        : array without index table (all subitems have the same byte length), 1-byte byte length
- `0x03`        : array without index table (all subitems have the same byte length), 2-byte byte length
- `0x04`        : array without index table (all subitems have the same byte length), 4-byte byte length
- `0x05`        : array without index table (all subitems have the same byte length), 8-byte byte length
- `0x06`        : array with 1-byte index table offsets, bytelen and # subvals
- `0x07`        : array with 2-byte index table offsets, bytelen and # subvals
- `0x08`        : array with 4-byte index table offsets, bytelen and # subvals
- `0x09`        : array with 8-byte index table offsets, bytelen and # subvals
- `0x0a`        : empty object
- `0x0b`        : object with 1-byte index table offsets, sorted by attribute name, 1-byte bytelen and # subvals
- `0x0c`        : object with 2-byte index table offsets, sorted by attribute name, 2-byte bytelen and # subvals
- `0x0d`        : object with 4-byte index table offsets, sorted by attribute name, 4-byte bytelen and # subvals
- `0x0e`        : object with 8-byte index table offsets, sorted by attribute name, 8-byte bytelen and # subvals
- `0x0f`        : object with 1-byte index table offsets, not sorted by attribute name, 1-byte bytelen and # subvals
- `0x10`        : object with 2-byte index table offsets, not sorted by attribute name, 2-byte bytelen and # subvals
- `0x11`        : object with 4-byte index table offsets, not sorted by attribute name, 4-byte bytelen and # subvals
- `0x12`        : object with 8-byte index table offsets, not sorted by attribute name, 8-byte bytelen and # subvals
- `0x13`        : compact array, no index table
- `0x14`        : compact object, no index table
- `0x15`-`0x16` : reserved
- `0x17`        : illegal - this type can be used to indicate a value that is illegal in the embedding application
- `0x18`        : null
- `0x19`        : false
- `0x1a`        : true
- `0x1b`        : double IEEE-754, 8 bytes follow, stored as little endian uint64 equivalent
- `0x1c`        : UTC-date in milliseconds since the epoch, stored as 8 byte signed int, little endian, two's complement
- `0x1d`        : external (only in memory): a char* pointing to the actual place in memory, where another VPack item
  resides, not allowed in VPack values on disk or on the network
- `0x1e`        : minKey, nonsensical value that compares < than all other values
- `0x1f`        : maxKey, nonsensical value that compares > than all other values
- `0x20`-`0x27` : signed int, little endian, 1 to 8 bytes, number is V - `0x1f`, two's complement
- `0x28`-`0x2f` : uint, little endian, 1 to 8 bytes, number is V - `0x27`
- `0x30`-`0x39` : small integers 0, 1, ... 9
- `0x3a`-`0x3f` : small negative integers -6, -5, ..., -1
- `0x40`-`0xbe` : UTF-8-string, using V - `0x40` bytes (not Unicode characters!), length 0 is possible, so `0x40` is the
  empty string, maximal length is 126, note that strings here are not zero-terminated and may contain NUL bytes
- `0xbf`        : long UTF-8-string, next 8 bytes are length of string in bytes (not Unicode characters) as little
  endian unsigned integer, note that long strings are not zero-terminated and may contain NUL bytes
- `0xc0`-`0xc7` : binary blob, next V - `0xbf` bytes are the length of blob in bytes, note that binary blobs are not
  zero-terminated
- `0xc8`-`0xcf` : positive long packed BCD-encoded float, V - `0xc7` bytes follow that encode in a little endian way the
  length of the mantissa in bytes. Directly after that follow 4 bytes encoding the (power of 10) exponent, by which the
  mantissa is to be multiplied, stored as little endian two's complement signed 32-bit integer. After that, as many
  bytes follow as the length information at the beginning has specified, each byte encodes two digits in big-endian
  packed BCD. Example: 12345 decimal can be encoded as
  `c8 03 00 00 00 00 01 23 45` or
  `c8 03 ff ff ff ff 12 34 50`
- `0xd0`-`0xd7` : negative long packed BCD-encoded float, V - `0xcf` bytes follow that encode in a little endian way the
  length of the mantissa in bytes. After that, same as positive long packed BCD-encoded float above.
- `0xd8`-`0xed` : reserved
- `0xee`-`0xef` : value tagging for logical types
- `0xf0`-`0xff` : custom types


## Arrays

Empty arrays are simply a single byte `0x01`.

We next describe the type cases `0x02` to `0x09`, see below for the special compact type `0x13`.

Non-empty arrays look like one of the following:

    one of 0x02 to 0x05
    BYTELENGTH
    OPTIONAL UNUSED: padding
    sub VPack values

or

    0x06
    BYTELENGTH in 1 byte
    NRITEMS in 1 byte
    OPTIONAL UNUSED: 6 bytes of padding
    sub VPack values
    INDEXTABLE with 1 byte per entry

or

    0x07
    BYTELENGTH in 2 bytes
    NRITEMS in 2 bytes
    OPTIONAL UNUSED: 4 bytes of padding
    sub VPack values
    INDEXTABLE with 4 byte per entry

or

    0x08
    BYTELENGTH in 4 bytes
    NRITEMS in 4 bytes
    sub VPack values
    INDEXTABLE with 4 byte per entry

or

    0x09
    BYTELENGTH in 8 bytes
    sub VPack values
    INDEXTABLE with 8 byte per entry
    NRITEMS in 8 bytes

If any optional padding is allowed for a type, the padding must consist of exactly that many bytes that the length of
the padding, the length of BYTELENGTH and the length of NRITEMS (if present) sums up to 8. If the length of BYTELENGTH
is already 8, there is no padding allowed. The entire padding must consist of zero bytes (ASCII NUL).

Numbers (for byte length, number of subvalues and offsets in the INDEXTABLE) are little endian unsigned integers, using
1 byte for types `0x02` and `0x06`, 2 bytes for types `0x03` and `0x07`, 4 bytes for types
`0x04` and `0x08`, and 8 bytes for types `0x05` and `0x09`.

NRITEMS is a single number as described above.

The INDEXTABLE consists of:

- for types `0x06`-`0x09` an array of offsets (unaligned, in the number format described above) earlier offsets reside
  at lower addresses. Offsets are measured from the start of the VPack value.

Non-empty arrays of types `0x06` to `0x09` have a small header including their byte length, the number of subvalues,
then all the subvalues and finally an index table containing offsets to the subvalues. To find the index table, find the
number of subvalues, then the end, and from that the base of the index table, considering how wide its entries are.

For types `0x02` to `0x05` there is no offset table and no number of items. The first item begins at address A+2, A+3,
A+5 or respectively A+9, depending on the type and thus the width of the byte length field. Note the following special
rule: The actual position of the first subvalue is allowed to be further back, after some run of padding zero bytes.

For example, if 2 bytes are used for both the byte length (BYTELENGTH), then an optional padding of 4 zero bytes is then
allowed to follow, and the actual VPack subvalues can start at A+9. This is to give a program that builds a VPack value
the opportunity to reserve 8 bytes in the beginning and only later find out that fewer bytes suffice to write the byte
length. One can determine the number of subvalues by finding the first subvalue, its byte length, and dividing the
amount of available space by it.

For types `0x06` to `0x09` the offset table describes where the subvalues reside. It is not necessary for the subvalues
to start immediately after the number of subvalues field.

As above, it is allowed to include optional padding. Again here, any padding must consist of a run of consecutive zero
bytes (ASCII NUL) and must be as long that it fills up the length of BYTELENGTH and the length of NRITEMS to 8.

For example, if both BYTELENGTH and NRITEMS can be expressed using 2 bytes each, the sum of their lengths is 4. It is
therefore allowed to add 4 bytes of padding here, so that the first subvalue could be at address A+9.

There is one exception for the 8-byte numbers case (type `0x05`):
In this case the number of elements is moved behind the index table. This is to get away without moving memory when one
has reserved 8 bytes in the beginning and later noticed that all 8 bytes are needed for the byte length. For this case
it is not allowed to include any padding.

All offsets are measured from base A.


*Example*:

`[1,2,3]` has the hex dump

    02 05 31 32 33

in the most compact representation, but the following are equally
possible, though not necessarily advised to use:

*Examples*:

    03 06 00 31 32 33

    04 08 00 00 00 31 32 33

    05 0c 00 00 00 00 00 00 00 31 32 33

    06 09 03 31 32 33 03 04 05

    07 0e 00 03 00 31 32 33 05 00 06 00 07 00

    08 18 00 00 00 03 00 00 00 31 32 33 09 00 00 00 0a 00 00 00 0b 00 00 00

    09
    2c 00 00 00 00 00 00 00
    31 32 33
    09 00 00 00 00 00 00 00
    0a 00 00 00 00 00 00 00
    0b 00 00 00 00 00 00 00
    03 00 00 00 00 00 00 00

Note that it is not recommended to encode short arrays in too long a format.

We now describe the special type `0x13`, which is useful for a particularly compact array representation. Note that to
some extent this goes against the principles of the VelocyPack format, since quick access to subvalues is no longer
possible, all items in the array must be scanned to find a particular one. However, there are certain use cases for
VelocyPack which only require sequential access (for example JSON dumping) and have a particular need for compactness.

The overall format of this array type is

    0x13 as type byte
    BYTELENGTH
    sub VPack values
    NRITEMS

There is no index table at all, although the sub VelocyPack values can
have different byte sizes. The BYTELENGTH and NRITEMS are encoded in a
special format, which we describe now.

The BYTELENGTH consists of 1 to 8 bytes, of which all but the last one
have their high bit set. Thus, the high bits determine, how many bytes
are actually used. The lower 7 bits of all these bits together comprise
the actual byte length in a little endian fashion. That is, the byte at
address A+1 contains the least significant 7 bits (0 to 6) of the byte length,
the following byte at address A+2 contains the bits 7 to 13, and so on.
Since the total number of bytes is limited to 8, this encodes unsigned
integers of up to 56 bits, which is the overall limit for the size of
such a compact array representation.

The NRITEMS entry is encoded essentially the same, except that it is
laid out in reverse order in memory. That is, one has to use the
BYTELENGTH to find the end of the array value and go back bytes until
one finds a byte with high bit reset. The last byte (at the highest
memory address) contains the least significant 7 bits of the NRITEMS
value, the second one bits 7 to 13 and so on.

Here is an example, the array [1, 16] can be encoded as follows:

    13 06
    31 28 10
    02

## Objects

Empty objects are simply a single byte `0x0a`.

We next describe the type cases `0x0b` to `0x12`, see below for the special compact type `0x14`.

Non-empty objects look like this:

    one of 0x0b - 0x12
    BYTELENGTH
    optional NRITEMS
    sub VPack values as pairs of attribute and value
    optional INDEXTABLE
    NRITEMS for the 8-byte case

Numbers (for byte length, number of subvalues and offsets in the INDEXTABLE) are little endian unsigned integers, using
1 byte for types `0x0b` and `0x0f`, 2 bytes for types `0x0c` and `0x10`, 4 bytes for types
`0x0d` and `0x11`, and 8 bytes for types `0x0e` and `0x12`.

NRITEMS is a single number as described above.

The INDEXTABLE consists of:
  - an array of offsets (unaligned, in the number format described
    above) earlier offsets reside at lower addresses.
    Offsets are measured from the beginning of the VPack value.

Non-empty objects have a small header including their byte length, the number of subvalues, then all the subvalues and
finally an index table containing offsets to the subvalues. To find the index table, find number of subvalues, then the
end, and from that the base of the index table, considering how wide its entries are.

For all types the offset table describes where the subvalues reside. It is not necessary for the subvalues to start
immediately after the number of subvalues field. For performance reasons when building the value, it could be desirable
to reserve 8 bytes for the byte length and the number of subvalues and not fill the gap, even though it turns out later
that offsets and thus the byte length only uses 2 bytes, say.

There is one special case: the empty object is simply stored as the single byte `0x0a`.

There is another exception: For 8-byte numbers (`0x12`) the number of subvalues is stored behind the INDEXTABLE. This is
to get away without moving memory when one has reserved 8 bytes in the beginning and later noticed that all 8 bytes are
needed for the byte length.

All offsets are measured from base A.

Each entry consists of two parts, the key and the value, they are encoded as normal VPack values as above, the first is
always a short or long UTF-8 string starting with a byte `0x40`-`0xbf` as described below. The second is any other VPack
value.

There is one extension: For the key it is possible to use the positive small integer values `0x30`-`0x39` or an unsigned
integer starting with a type byte of `0x28`-`0x2f`. Any such integer value is an index into an outside-given table of
attribute names. These are convenient when only very few attribute names occur or some are repeated very often. The
standard way to encode such an attribute name table is as a VPack array of strings as specified here.

Objects are always stored with sorted key/value pairs, sorted by bytewise comparisons of the keys on each nesting level.
Sorting has some overhead but will allow looking up keys in logarithmic time later. Note that only the index table needs
to be sorted, it is not required that the offsets in these tables are increasing. Since the index table resides after
the actual subvalues, one can build up a complex VPack value by writing linearly.

Example: the object `{"a": 12, "b": true, "c": "xyz"}` can have the hexdump:

    0b
    13 03
    41 62 1a
    41 61 28 0c
    41 63 43 78 79 7a
    06 03 0a

The same object could have been done with an index table with longer
entries, as in this example:

    0d
    22 00 00 00
    03 00 00 00
    41 62 1a
    41 61 28 0c
    41 63 43 78 79 7a
    0c 00 00 00 09 00 00 00 10 00 00 00

Similarly with type `0x0c` and 2-byte offsets, byte length and number of subvalues, or with type `0x0e` and 8-byte
numbers.

Note that it is not recommended to encode short objects with too long index tables.

### Special compact objects

We now describe the special type `0x14`, which is useful for a particularly compact object representation. Note that to
some extent this goes against the principles of the VelocyPack format, since quick access to subvalues is no longer
possible, all key/value pairs in the object must be scanned to find a particular one. However, there are certain use
cases for VelocyPack which only require sequential access
(for example JSON dumping) and have a particular need for compactness.

The overall format of this object type is

    0x14 as type byte
    BYTELENGTH
    sub VPack key/value pairs
    NRPAIRS

There is no index table at all, although the sub VelocyPack values can have different byte sizes. The BYTELENGTH and
NRPAIRS are encoded in a special format, which we describe now. It is the same as for the special compact array
type `0x13`, which we repeat here for the sake of completeness.

The BYTELENGTH consists of 1 to 8 bytes, of which all but the last one have their high bit set. Thus, the high bits
determine, how many bytes are actually used. The lower 7 bits of all these bits together comprise the actual byte length
in a little endian fashion. That is, the byte at address A+1 contains the least significant 7 bits (0 to 6) of the byte
length, the following byte at address A+2 contains the bits 7 to 13, and so on. Since the total number of bytes is
limited to 8, this encodes unsigned integers of up to 56 bits, which is the overall limit for the size of such a compact
array representation.

The NRPAIRS entry is encoded essentially the same, except that it is laid out in reverse order in memory. That is, one
has to use the BYTELENGTH to find the end of the array value and go back bytes until one finds a byte with high bit
reset. The last byte (at the highest memory address) contains the least significant 7 bits of the NRPAIRS value, the
second one bits 7 to 13 and so on.

Here is an example, the object `{"a":1, "b":16}` can be encoded as follows:

    14 0a
    41 61 31 42 62 28 10
    02


## Doubles

Type `0x1b` indicates a double IEEE-754 value using the 8 bytes following the type byte. To guarantee
platform-independentness the details of the byte order are as follows. Encoding is done by using memcpy to copy the
internal double value to an uint64\_t. This 64-bit unsigned integer is then stored as little endian 8 byte integer in
the VPack value. Decoding works in the opposite direction. This should sort out the undetermined byte order in IEEE-754
in practice.


## Dates

Type `0x1c` indicates a signed 64-int integer stored in 8 bytes little endian two's complement notation directly after
the type. The value means a universal UTC-time measured in milliseconds since the epoch, which is 00:00 on 1 January
1970 UTC.


## External VPack values

This type is only for use within memory, not for data exchange over disk
or network. Therefore, we only need to specify that the following k
bytes are the memcpy of a char* on the current architecture. That char*
points to the actual VPack value elsewhere in memory.

## Artificial minimal and maximal keys

These values of types `0x1e` and `0x1f` have no meaning other than comparing smaller or greater respectively than any
other VPack value. The idea is that these can be used in systems that define a total order on all VPack values to
specify left or right ends of infinite intervals.


## Integer types

There are different ways to specify integers. For small values -6 to 9 inclusively there are specific type bytes in the
range `0x30` to `0x3f` to allow for storage in a single byte. After that there are signed and unsigned integer types
that can code in the type byte the number of bytes used (ranges `0x20`-`0x27` for signed and `0x28`-`0x2f` for unsigned)
.

## Null and boolean values

These three values use a single byte to store the corresponding JSON values.

## Strings

Strings are stored as UTF-8 encoded byte sequences. There are two variants, a short one and a long one. In the short
one, the byte length
(not the number of UTF-8 characters) is directly encoded in the type, and this works up to and including byte length

126. Types `0x40` to `0xbe`
     are used for this and the byte length is V - `0x3f`, if V is the type byte. For strings longer than 126 bytes, the
     type byte is `0xbf` and the byte length of the string is stored in the first 8 bytes after the type byte, using a
     little endian unsigned integer representation. The actual string follows after these 8 bytes. There is no
     terminating zero byte in either case and the string may contain zero bytes.

## Binary data

The type bytes `0xc0` to `0xc7` allow to store arbitrary binary byte sequences as a VPack value. The format is as
follows: If V is the type byte, then V - `0xbf` bytes follow it to make a little endian unsigned integer representing
the length of the binary data, which directly follows these length bytes. No alignment is guaranteed. The content is
entirely up to the user.

## Packed BCD long floats

These types are used to represent arbitrary precision decimal numbers.
There are different types for positive and negative numbers. The overall
format of these values is:

    one of 0xc8 - 0xcf (positive) or of 0xd0 - 0xd7 (negative)
    LENGTH OF MANTISSA in bytes
    EXPONENT (as 4-byte little endian signed two's complement integer)
    MANTISSA (as packed BCD-encoded integer, big-endian)

The type byte describes the sign of the number as well as the number of bytes used to specify the byte length of the
mantissa. As usual, if V is the type byte, then V - `0xc7` (in the positive case) or V - `0xcf` (in the negative case)
bytes are used for the length of the mantissa, stored as little endian unsigned integer directly after the byte length.
After this follow exactly 4 bytes (little endian signed two's complement integer) to specify the exponent. After the
exponent, the actual mantissa bytes follow.

Packed BCD is used, so that each byte stores exactly 2 decimal digits as in `0x34` for the decimal digits 34. Therefore,
the mantissa always has an even number of decimal digits. Note that the mantissa is stored in big endian form, to make
parsing and dumping efficient. This leads to the
"unholy nibble problem": When a JSON parser sees the beginning of a longish number, it does not know whether an even or
odd number of digits follow. However, for efficiency reasons it wants to start writing bytes to the output as it reads
the input. This is, where the exponent comes to the rescue, which is illustrated by the following example. 12345 decimal
can be encoded as:

    c8 03 00 00 00 00 01 23 45
    c8 03 ff ff ff ff 12 34 50

The former encoding puts a leading 0 in the first byte and uses exponent 0, the latter encoding directly starts putting
two decimal digits in one byte and then in the end has to "erase" the trailing 0 by using exponent -1, encoded by the 4
byte sequence `ff ff ff ff`.

Therefore, the unholy nibble problem is solved and parsing (and indeed dumping) can be efficient.


## Tagging

Types `0xee`-`0xef` are used for tagging of values to implement logical types.

For example, if type `0x1c` did not exist, the database driver could serialize a timestamp object (Date in JavaScript,
Instant in Java, etc)
into a Unix timestamp, a 64-bit integer. Assuming the lack of schema, upon deserialization it would not be possible to
tell an integer from a timestamp and deserialize the value accordingly.

Type tagging resolves this by attaching an integer tag to values that can then be read when deserializing the value,
e.g. that tag=1 is a timestamp and the relevant timestamp class should be used.

The tag values are specified separately and applications can also specify their own to have the database driver
deserialize their specific data types into the appropriate classes (including models).

Essentially this is object-relational mapping for parts of documents.

The format of the type is:

    0xee
    TAG number in 1 byte
    sub VPack value

or

    0xef
    TAG number in 8 bytes, little-endian encoding
    sub VPack value


## Custom types

Note that custom types should usually not be used for data exchange but
only internally in systems. Nevertheless, the design of this part of
the specification is made such that it is possible by generic methods
to derive the byte length of each custom data type.

The following user-defined types exist:

- `0xf0`        : 1 byte payload, directly following the type byte
- `0xf1`        : 2 bytes payload, directly following the type byte
- `0xf2`        : 4 bytes payload, directly following the type byte
- `0xf3`        : 8 bytes payload, directly following the type byte
- `0xf4`-`0xf6` : length of the payload is described by a single further unsigned byte directly following the type byte,
  the payload of that many bytes follows
- `0xf7`-`0xf9` : length of the payload is described by two bytes (little endian unsigned integer) directly following
  the type byte, the payload of that many bytes follows
- `0xfa`-`0xfc` : length of the payload is described by four bytes (little endian unsigned integer) directly following
  the type byte, the payload of that many bytes follows
- `0xfd`-`0xff` : length of the payload is described by eight bytes (little endian unsigned integer) directly following
  the type byte, the payload of that many bytes follows

Note: In types `0xf4` to `0xff` the "payload" refers to the actual data not including the length specification.

## Portability

Serialized booleans, integers, strings, arrays, objects etc. all have a defined endianess and length, which is
platform-independent. These types are fully portable in serialized VelocyPack.

There are still a few caveats when it comes to portability:

It is possible to build up very large values on a 64 bit system, but it may not be possible to read them back on a 32
bit system. This is because the maximum memory allocation size on a 32 bit system may be severely limited compared to a
64 bit system, i.e. a 32 bit OS may simply not allow to allocate buffers larger than 4 GB. This is not a limitation of
VelocyPack, but a limitation of 32 bit architectures. If all VelocyPack values are kept small enough so that they are
well below the 32 bit length boundaries, this should not matter though.

The VelocyPack type *External* contains just a raw pointer to memory, which should only be used during the buildup of
VelocyPack values in memory. The *External* type is not supposed to be used in VelocyPack values that are serialized and
stored persistently, and then later read back from persistence. Doing it anyway is not portable and will also pose a
security risk. Not using the *External* type for any data that is serialized will avoid this problem entirely.

The VelocyPack type *Custom* is completely user-defined, and there is no default implementation for them. So it is up to
the embedder to make these custom type bindings portable if portability of them is a concern.

VelocyPack *Double* values are serialized as integer equivalents in a specific way, and unserialized back into integers
that overlay a IEEE-754 double-precision floating point value in memory. We found this to be sufficiently portable for
our needs, although at least in theory there may be portability issues with some systems.

The [following](https://en.wikipedia.org/wiki/Endianness#Floating_point) was used as a backing for our "reasonably
portable in the real world" assumptions:

> It may therefore appear strange that the widespread IEEE 754 floating-point standard does not specify endianness.[17] Theoretically, this means that even standard IEEE floating-point data written by one machine might not be readable by another. However, on modern standard computers (i.e., implementing IEEE 754), one may in practice safely assume that the endianness is the same for floating-point numbers as for integers, making the conversion straightforward regardless of data type.