littlebson

A local BSON database like sqlite.

The following info has been gathered from the official BSON specifications located on https://bsonspec.org/spec.html.

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BSON is a binary format in which zero or more ordered key/value pairs are stored as a single entity. We call this entity a document.

The following grammar specifies version 1.1 of the BSON standard. We've written the grammar using a pseudo-BNF syntax. Valid BSON data is represented by the document non-terminal.

Binary

Basic Types

The following basic types are used as terminals in the rest of the grammar. Each type must be serialized in little-endian format.

Type	Bytes	Bits
byte	1	8-bit
int32	4	32-bit signed, two's compliment
int64	8	64-bit signed, two's compliment
uint64	8	64-bit unsigned integer
double	8	64-bit IEEE 754-2008 binary floating point
decimal128	16	128-bit IEEE 754-2008 decimal floating point

Non-terminals

The following specifies the rest of the BSON grammar. Note that quoted strings represent terminals, and should be interpreted with C semantics (e.g. "\x01" represents the byte 0000 0001). Also note that we use the * operator as shorthand for repetition (e.g. ("\x01"*2) is "\x01\x01"). When used as a unary operator, * means that the repetition can occur 0 or more times.

document structure

document ::= int32 e_list "\x00" BSON 

BSON Document. int32 is the total number of bytes comprising the document.

e_list structure

e_list ::= element e_list
         | ""

element structure

element ::= "\x01" e_name double    64-bit binary floating point
          | "\x02" e_name string        UTF-8 string
          | "\x03" e_name document      Embedded document  
          | "\x04" e_name document      Array - see notes
          | "\x05" e_name binary        Binary data
          | "\x07" e_name uint64        Lil Bson Object Id
          | "\x08" e_name "\x00"        Boolean false
          | "\x08" e_name "\x01"        Boolean true
          | "\x09" e_name int64         UTC datetime (TODO)
          | "\x0A" e_name               Null value
          | "\x10" e_name int32         32-bit integer
          | "\x11" e_name uint64        64-bit unsigned integer (Timestamp)
          | "\x12" e_name int64         64-bit integer
          | "\x13" e_name decimal128    128-bit decimal floating point (TODO)

e_name structure

e_name ::= cstring

string structure

string ::= int32 (byte*) "\x00   

The int32 is the number bytes in the (byte*) + 1 (for the trailing '\x00'). The (byte*) is zero or more UTF-8 encoded characters.

cstring structure

cstring ::= (byte*) "\x00"

Zero or more modified UTF-8 encoded characters followed by '\x00'. The (byte*) MUST NOT contain '\x00', hence it is not full UTF-8.

binary structure

binary ::= int32 subtype (byte*)

The int32 is the number of bytes in the (byte*).

subtype structure

TODO - subtypes are not currently supported.

subtype ::= "\x00"  Generic binary subtype
          | "\x01"  Function
          | "\x02"  Binary (Old)
          | "\x03"  UUID (Old)
          | "\x04"  UUID
          | "\x05"  MD5
          | "\x06"  Encrytped BSON value
          | "\x80"  User Defined

Notes

• Array - The document for an array is a normal BSON document with integer values for the keys, starting with 0 and continuing sequentially. For example, the array ['red', 'blue'] would be encoded as the document {'0': 'red', '1': 'blue'}. The keys must be in ascending numerical order.
• Lil Bson ID - Is a 64 bit unsigned integer. The bits are determined as follows: [10 bit machine id][41 bits of time][13 bit rand num]. This differs from the objectId because I wanted this ID to be simple to use, read, compare, and generate.
• UTC datetime - The int64 is UTC milliseconds since the Unix epoch.
• Timestamp - Special internal type used by MongoDB replication and sharding. First 4 bytes are an increment, second 4 are a timestamp.
• Min key - Special type which compares lower than all other possible BSON element values.
• Max key - Special type which compares higher than all other possible BSON element values.
• Generic binary subtype - This is the most commonly used binary subtype and should be the 'default' for drivers and tools.
• The BSON "binary" or "BinData" datatype is used to represent arrays of bytes. It is somewhat analogous to the Java notion of a ByteArray. BSON binary values have a subtype. This is used to indicate what kind of data is in the byte array. Subtypes from zero to 127 are predefined or reserved. Subtypes from 128-255 are user-defined. \x02 Binary (Old) - This used to be the default subtype, but was deprecated in favor of \x00. Drivers and tools should be sure to handle \x02 appropriately. The structure of the binary data (the byte* array in the binary non-terminal) must be an int32 followed by a (byte*). The int32 is the number of bytes in the repetition.
  • \x03 UUID (Old) - This used to be the UUID subtype, but was deprecated in favor of \x04. Drivers and tools for languages with a native UUID type should handle \x03 appropriately.
  • \x80-\xFF "User defined" subtypes. The binary data can be anything.