Declaratively encode and decode binary data
npm install @foliojs-fork/restructurejavascript
var r = require('restructure');
var Person = new r.Struct({
name: new r.String(r.uint8, 'utf8'),
age: r.uint8
});
// decode a person from a buffer
var stream = new r.DecodeStream(buffer);
Person.decode(stream); // returns an object with the fields defined above
// encode a person from an object
// pipe the stream to a destination, such as a file
var stream = new r.EncodeStream();
stream.pipe(fs.createWriteStream('out.bin'));
Person.encode(stream, {
name: 'Devon',
age: 21
});
stream.end();
`
API
All of the following types support three standard methods:
* decode(stream) - decodes an instance of the type from the given DecodeStream
* size(value) - returns the amount of space the value would take if encoded
* encode(stream, value) - encodes the given value into the given EncodeStream
Restructure supports a wide variety of types, but if you need to write your own for
some custom use that cannot be represented by them, you can do so by just implementing
the above methods. Then you can use your type just as you would any other type, in structures
and whatnot.
$3
The following built-in number types are available:
`javascript
uint8, uint16, uint24, uint32, int8, int16, int24, int32, float, double, fixed16, fixed32
`
Numbers are big-endian (network order) by default, but little-endian is supported, too:
`javascript
uint16le, uint24le, uint32le, int16le, int24le, int32le, floatle, doublele, fixed16le, fixed32le
`
To avoid ambiguity, big-endian may be used explicitly:
`javascript
uint16be, uint24be, uint32be, int16be, int24be, int32be, floatbe, doublebe, fixed16be, fixed32be
`
$3
Booleans are encoded as 0 or 1 using one of the above number types.
`javascript
var bool = new r.Boolean(r.uint32);
`
$3
The Reserved type simply skips data in a structure, where there are reserved fields.
Encoding produces zeros.
`javascript
// 10 reserved uint8s (default is 1)
var reserved = new r.Reserved(r.uint8, 10);
`
$3
The Optional type only encodes or decodes when given condition is truthy.
`javascript
// includes field
var optional = new r.Optional(r.uint8, true);
// excludes field
var optional = new r.Optional(r.uint8, false);
// determine whether field is to be included at runtime with a function
var optional = new r.Optional(r.uint8, function() {
return this.flags & 0x50;
});
`
$3
The Enum type maps a number to the value at that index in an array.
`javascript
var color = new r.Enum(r.uint8, ['red', 'orange', 'yellow', 'green', 'blue', 'purple']);
`
$3
The Bitfield type maps a number to an object with boolean keys mapping to each bit in that number,
as defined in an array.
`javascript
var bitfield = new r.Bitfield(r.uint8, ['Jack', 'Kack', 'Lack', 'Mack', 'Nack', 'Oack', 'Pack', 'Quack']);
bitfield.decode(stream);
var result = {
Jack: true,
Kack: false,
Lack: false,
Mack: true,
Nack: true,
Oack: false,
Pack: true,
Quack: true
};
bitfield.encode(stream, result);
`
$3
Extracts a slice of the buffer to a Node Buffer. The length can be a constant, or taken from
a previous field in the parent structure.
`javascript
// fixed length
var buf = new r.Buffer(2);
// length from parent structure
var struct = new r.Struct({
bufLen: r.uint8,
buf: new r.Buffer('bufLen')
});
`
$3
A String maps a JavaScript string to and from binary encodings. The length can be a constant, taken
from a previous field in the parent structure, or encoded using a number type immediately before the string.
Supported encodings include 'ascii', 'utf8', 'ucs2', 'utf16le', 'utf16be', and if you also install
iconv-lite, many other legacy codecs.
`javascript
// fixed length, ascii encoding by default
var str = new r.String(2);
// length encoded as number before the string, utf8 encoding
var str = new r.String(r.uint8, 'utf8');
// length from parent structure
var struct = new r.Struct({
len: r.uint8,
str: new r.String('len', 'utf16be')
});
// null-terminated string (also known as C string)
var str = new r.String(null, 'utf8')
`
$3
An Array maps to and from a JavaScript array containing instances of a sub-type. The length can be a constant,
taken from a previous field in the parent structure, encoded using a number type immediately
before the string, or computed by a function.
`javascript
// fixed length, containing numbers
var arr = new r.Array(r.uint16, 2);
// length encoded as number before the array containing strings
var arr = new r.Array(new r.String(10), r.uint8);
// length computed by a function
var arr = new r.Array(r.uint8, function() { return 5 });
// length from parent structure
var struct = new r.Struct({
len: r.uint8,
arr: new r.Array(r.uint8, 'len')
});
// treat as amount of bytes instead (may be used in all the above scenarios)
var arr = new r.Array(r.uint16, 6, 'bytes');
`
$3
The LazyArray type extends from the Array type, and is useful for large arrays that you do not need to access sequentially.
It avoids decoding the entire array upfront, and instead only decodes and caches individual items as needed. It only works when
the elements inside the array have a fixed size.
Instead of returning a JavaScript array, the LazyArray type returns a custom object that can be used to access the elements.
`javascript
var arr = new r.LazyArray(r.uint16, 2048);
var res = arr.decode(stream);
// get a single element
var el = res.get(2);
// convert to a normal array (decode all elements)
var array = res.toArray();
`
$3
A Struct maps to and from JavaScript objects, containing keys of various previously discussed types. Sub structures,
arrays of structures, and pointers to other types (discussed below) are supported.
`javascript
var Person = new r.Struct({
name: new r.String(r.uint8, 'utf8'),
age: r.uint8
});
`
$3
A VersionedStruct is a Struct that has multiple versions. The version is typically encoded at
the beginning of the structure, or as a field in a parent structure. There is an optional header
common to all versions, and separate fields listed for each version number.
`javascript
// the version is read as a uint8 in this example
// you could also get the version from a key on the parent struct
var Person = new r.VersionedStruct(r.uint8, {
// optional header common to all versions
header: {
name: new r.String(r.uint8, 'utf8')
},
0: {
age: r.uint8
},
1: {
hairColor: r.Enum(r.uint8, ['black', 'brown', 'blonde'])
}
});
`
$3
Pointers map an address or offset encoded as a number, to a value encoded elsewhere in the buffer.
There are a few options you can use: type, relativeTo, allowNull, and nullValue.
The type option has these possible values:
* local (default) - the encoded offset is relative to the start of the containing structure
* immediate - the encoded offset is relative to the position of the pointer itself
* parent - the encoded offset is relative to the parent structure of the immediate container
* global - the encoded offset is global to the start of the file
The relativeTo option accepts a function callback that should return the field on the containing structure which the encoded offset is relative to. The callback is called with the context as parameter.
By default, pointers are relative to the start of the containing structure (local).
The allowNull option lets you specify whether zero offsets are allowed or should produce null. This is
set to true by default. The nullValue option is related, and lets you override the encoded value that
represents null. By default, the nullValue is zero.
The lazy option allows lazy decoding of the pointer's value by defining a getter on the parent object.
This only works when the pointer is contained within a Struct, but can be used to speed up decoding
quite a bit when not all of the data is needed right away.
`javascript
var Address = new r.Struct({
street: new r.String(r.uint8),
zip: new r.String(5)
});
var Person = new r.Struct({
name: new r.String(r.uint8, 'utf8'),
age: r.uint8,
ptrStart: r.uint8,
address: new r.Pointer(r.uint8, Address)
});
`
If the type of a pointer is set to 'void', it is not decoded and the computed address in the buffer
is simply returned. To encode a void pointer, create a new r.VoidPointer(type, value)`.