A CIF file (example) contains:
- One or more data blocks
- Each data block has one or more category
- Each category has one or more field
- Each field contains data
To represent this hierarchy, the basic shape of the BinaryCIF file defines the following interfaces:
File {
version: string
encoder: string
dataBlocks: DataBlock[]
}
DataBlock {
header: string
categories: Category[]
}
Category {
name: string
rowCount: number
columns: Column[]
}
Column {
name: string
data: Data
mask: Data
}
Data {
data: Uint8Array
encoding: Encoding[]
}
The most interesting part is the Data
interface where the actual data is stored.
The interface has two properties: data
which is just an array of bytes (the binary data)
and an array of encodings that describes the transformations that were applied to the
source data to obtain the final binary result stored in the data
field.
Additionally, the Column
interface defines a mask
property used to determine
if a certain value is present, not present (.
token in CIF), or unknown (?
token in CIF).
Currently, BinaryCIF supports these encoding methods:
type Encoding =
| ByteArray
| FixedPoint
| IntervalQuantization
| RunLength
| Delta
| IntegerPacking
| StringArray
Encodes an array of numbers of specified types as raw bytes.
ByteArray {
kind = "ByteArray"
type: Int8 | Int16 | Int32 | Uint8 | Uint16 | Uint32 | Float32 | Float64
}
Converts an array of floating point numbers to an array of 32-bit integers multiplied by a given factor.
FixedPoint {
kind = "FixedPoint"
factor: number
srcType: Float32 | Float64
}
[1.2, 1.23, 0.123]
---FixedPoint--->
{ factor = 100 } [120, 123, 12]
Converts an array of floating point numbers to an array of 32-bit integers where the values are quantized within a given interval into specified number of discrete steps. Values lower than the minimum value or greater than the maximum are reprented by the respective boundary values.
IntervalQuantization {
kind = "IntervalQuantization"
min: number,
max: number,
numSteps: number,
srcType: Float32 | Float64
}
[0.5, 1, 1.5, 2, 3, 1.345 ]
---IntervalQuantization--->
{ min = 1, max = 2, numSteps = 3 } [0, 0, 1, 2, 2, 1]
Represents each integer value in the input as a pair of (value, number of repeats)
and stores the result sequentially as an array of 32-bit integers. Additionally,
stores the size of the original array to make decoding easier.
RunLength {
kind = "RunLength"
srcType: int[]
srcSize: number
}
[1, 1, 1, 2, 3, 3]
---RunLength--->
{ srcSize = 6 } [1, 3, 2, 1, 3, 2]
Stores the input integer array as an array of consecutive differences.
Delta {
kind = "Delta"
origin: number
srcType: int[]
}
Because delta encoding is often used in conjuction with integer packing,
the origin
property is present. This is to optimize the case
where the first value is large, but the differences are small.
[1000, 1003, 1005, 1006]
---Delta--->
{ origin = 1000, srcType = Int32 } [0, 3, 2, 1]
Stores a 32-bit integer array using 8- or 16-bit values. Includes the size of the input array for easier decoding. The encoding is more effective when only unsigned values are provided.
If a value exceeds the maximum value for the 8- or 16- bit type, the maximum value may be output numerous times to add up the value.
IntegerPacking {
kind = "IntegerPacking"
byteCount: number
srcSize: number
isUnsigned: boolean
}
[1, 2, -3, 128]
---IntgerPacking--->
{ byteCount = 1, srcSize = 4, isUnsigned = false } [1, 2, -3, 127, 1]
Stores an array of strings as a concatenation of all unique strings, an array of offsets describing substrings, and indices into the offset array. indices to corresponding substrings.
StringArray {
kind = "StringArray"
dataEncoding: Encoding[]
stringData: string
offsetEncoding: Encoding[]
offsets: Uint8Array
}
['a','AB','a']
---StringArray--->
{ stringData = 'aAB', offsets = [0, 1, 3] } [0, 1, 0]
To encode the data, a sequence of encoding transformations needs to be specified
for each input column. For example, to encode the _atoms.id
column
from the background section, we could specify the encoding as [Delta, RunLength, IntegerPacking]
:
[1, 2, 3, 4]
---Delta--->
{ srcType = Int32 } [1, 1, 1, 1]
---RunLength--->
{ srcSize = 4 } [1, 4]
---IntegerPacking--->
{ byteCount = 1, srcSize = 2 } [1, 4]
Little endian is used everywhere to encode the data.
Once each column has been encoded and the File
data structure built, the
MessagePack format (which is more or less a binary encoding of the standard
JSON format) is used to produce the final binary result.
Optionally, the data can be compressed using standard methods such as Gzip to achieve further compression.
To decode the BinaryCIF data, first the MessagePack data are decoded and then
for each column, the binary data are decoded applying inverses of the transformations
specified in the encoding
array backwards. So to decode the encoding specified by
[Delta, RunLength, IntegerPacking]
we would first apply the decoding
of IntegerPacking
, then RunLength
, and finally Delta
.