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🛠️A library for creating byte level data structures.
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readme.md

construct-js

construct-js is a library for creating byte level data structures. It focuses on a declarative API and simplicity of use.

npm i construct-js

Example

The following example builds a completely valid zip archive with one file inside - helloworld.txt.

const fs = require('fs');
const {
  RawString,
  U16,
  U32,
  Struct,
} = require('construct-js');

const data = RawString('helloworldhelloworldhelloworldhelloworldhelloworldhelloworldhelloworldhelloworld');
const filename = RawString('helloworld.txt');

const sharedHeaderInfo = Struct('sharedHeaderInfo')
  .field('minVersion', U16(10))
  .field('gpFlag', U16(0))
  .field('compressionMethod', U16(0))
  .field('lastModifiedTime', U16(0))
  .field('lastModifiedDate', U16(0))
  .field('crc32', U32(0))
  .field('compressedSized', U32(data.byteLength))
  .field('uncompressedSized', U32(data.byteLength))
  .field('filenameSize', U16(filename.byteLength))
  .field('extraFieldLength', U16(0));

const localHeader = Struct('localHeader')
  .field('header', U32(0x04034b50))
  .field('sharedHeaderInfo', sharedHeaderInfo)
  .field('filename', filename);

const centralDirectory = Struct('centralDirectory')
  .field('header', U32(0x02014b50))
  .field('madeByVersion', U16(10))
  .field('sharedHeaderInfo', sharedHeaderInfo)
  .field('fileCommentSize', U16(0))
  .field('diskNumber', U16(0))
  .field('internalFileAttributes', U16(0))
  .field('externalFileAttributes', U32(0))
  .field('relativeOffset', U32(0))
  .field('filename', filename);

const endOfCentralDirectory = Struct('endOfCentralDirectory')
  .field('header', U32(0x06054b50))
  .field('diskNumber', U16(0))
  .field('centralDirDiskStart', U16(0))
  .field('numberOfCentralDirsOnDisk', U16(1))
  .field('totalNumberOfCentralDirs', U16(1))
  .field('centralDirSize', U32(0))
  .field('offsetToStart', U32(0))
  .field('commentLength', U16(0));


const zipFile = Struct('ZipFile')
  .field('localHeader', localHeader)
  .field('data', data)
  .field('centralDirectory', centralDirectory)
  .field('endOfCentralDirectory', endOfCentralDirectory);

const offset = zipFile.getOffset('centralDirectory');
endOfCentralDirectory.get('offsetToStart').set(offset);

const fileBuffer = zipFile.toBuffer();

fs.writeFile('./test.zip', fileBuffer, () => {});

API

Struct

Struct(name, littleEndian = true)

Creates a Struct object. If littleEndian is set to true, the Struct will be considered to be little endian ordering.

forceEndianess

forceEndianess

forceEndianess is true by default, which means fields created will be forced into the endianess set when creating the Struct.

field

.field(name, value)

Adds a field to the struct. name is used to lookup the field using struct.get(name). value must be either a Struct or one of the other data types provided by construct-js.

get

.get(name)

Returns the field with that name.

getOffset

.getOffset(name)

Returns the byte offset within the struct of the field with that name.

getDeep

.getDeep(path)

Returns the field within multiple structs, where path is a . separated string. For example:

const struct = Struct('firstStruct')
  .field('aRawString', RawString('ABC'));

const struct2 = Struct('secondStruct')
  .field('deeperStruct', struct);

struct2.getDeep('deeperStruct.aRawString');

getDeepOffset

.getDeepOffset(path)

Returns the byte offset within multiple structs, where path is a . separated string.

computeBufferSize

.computeBufferSize()

Returns the size of the struct in bytes.

toBuffer

.toBuffer()

Returns a Buffer representation of the Struct.

BitStruct

BitStruct(name)

Creates a BitStruct object, for storing and addressing data on the sub-byte level.

Note: When bitStruct.toBuffer() is used, the resulting buffer will be byte aligned. This means if the size of the BitStruct is 12-bits, the resulting buffer will be 16-bits (2 bytes).

flag

.flag(name, value)

Sets a 1-bit flag in the structure.

multiBit

.multiBit(name, size, value)

Sets a multi-bit flag of size.

getOffset

.getOffset(name)

Gets the bit offset of the data with name.

computeBufferSize

.computeBufferSize()

Returns the size of the BitStruct in bytes.

toBuffer

.toBuffer()

Returns a byte-aligned Buffer representing the BitStruct.

Fields

All fields contain some common properties and methods. These are:

.set(value | values)

Which sets either the value or values of the field.

.setIsLittleEndian(trueOrFalse)

Manually sets this field to little or big endian.

The rest of the properties should be considered private and not modified directly.

U8

U8(value)

A single 8-bit unsigned value.

U16

U16(value)

A single 16-bit unsigned value.

U32

U32(value)

A single 32-bit unsigned value.

I8

I8(value)

A single 8-bit signed value.

I16

I16(value)

A single 16-bit signed value.

I32

I32(value)

A single 32-bit signed value.

RawString

RawString(string)

A collection of 8-bit unsigned values, interpreted directly from the string provided. The size of the field is the byte length of the string (which is not always the string.length when considering unicode).

U8s

U8s(array | number)

A collection of 8-bit unsigned values.

If the argument provided is an array, then the size of the field is array.length bytes, with each value corresponding to an 8-bit interpretation of that value.

U16s

U16s(array | number)

A collection of 16-bit unsigned values.

If the argument provided is an array, then the size of the field is array.length * 2 bytes, with each value corresponding to an 16-bit interpretation of that value.

U32s

U32s(array | number)

A collection of 32-bit unsigned values.

If the argument provided is an array, then the size of the field is array.length * 4 bytes, with each value corresponding to an 32-bit interpretation of that value.

I8s

I8s(array | number)

A collection of 8-bit signed values.

If the argument provided is an array, then the size of the field is array.length bytes, with each value corresponding to an 8-bit interpretation of that value.

I16s

I16s(array | number)

A collection of 16-bit signed values.

If the argument provided is an array, then the size of the field is array.length * 2 bytes, with each value corresponding to an 16-bit interpretation of that value.

I32s

I32s(array | number)

A collection of 32-bit signed values.

If the argument provided is an array, then the size of the field is array.length * 4 bytes, with each value corresponding to an 32-bit interpretation of that value.

Fields (Deprecated)

These fields are the same as those above, but using outdated and ambiguous terminology.

Byte

Byte(value)

A single 8-bit unsigned value.

Word

Word(value)

A single 16-bit unsigned value.

DoubleWord

DoubleWord(value)

A single 32-bit unsigned value.

SignedByte

SignedByte(value)

A single 8-bit signed value.

SignedWord

SignedWord(value)

A single 16-bit signed value.

SignedDoubleWord

SignedDoubleWord(value)

A single 32-bit signed value.

RawString

RawString(string)

A collection of 8-bit unsigned values, interpreted directly from the string provided. The size of the field is the byte length of the string (which is not always the string.length when considering unicode).

Bytes

Bytes(array | number)

A collection of 8-bit unsigned values.

If the argument provided is an array, then the size of the field is array.length bytes, with each value corresponding to an 8-bit interpretation of that value.

Words

Words(array | number)

A collection of 16-bit unsigned values.

If the argument provided is an array, then the size of the field is array.length * 2 bytes, with each value corresponding to an 16-bit interpretation of that value.

DoubleWords

DoubleWords(array | number)

A collection of 32-bit unsigned values.

If the argument provided is an array, then the size of the field is array.length * 4 bytes, with each value corresponding to an 32-bit interpretation of that value.

SignedBytes

SignedBytes(array | number)

A collection of 8-bit signed values.

If the argument provided is an array, then the size of the field is array.length bytes, with each value corresponding to an 8-bit interpretation of that value.

SignedWords

SignedWords(array | number)

A collection of 16-bit signed values.

If the argument provided is an array, then the size of the field is array.length * 2 bytes, with each value corresponding to an 16-bit interpretation of that value.

SignedDoubleWords

SignedDoubleWords(array | number)

A collection of 32-bit signed values.

If the argument provided is an array, then the size of the field is array.length * 4 bytes, with each value corresponding to an 32-bit interpretation of that value.

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