packet

KO | EN

gitlite — search

#javascript #python #ai #machine-learning #deep-learning #docker #nodejs #windows #cli #kubernetes #vue #awesome-list #pytorch #openai

(★ 190)

Incremental binary parsers and serializers for Node.js.

File Explorer

Download Latest Version (.zip)

- - nodejs.yml
- parser.js
- - objects-vs-closures.js
- - objects-vs-closures.js
- args.js
- buffer.js
- case.js
- indexof.js
- switch.js
- pack.js
- unpack.js
- unsign.js
- examples.c
- utf8.js
- mqtt.js
- mqtt.packet.js
- parser.js
- serializer.js
- - packetize.bin.t.js
- - function.t.js
  - require.t.js
- - absent.t.js
  - accumulator.t.js
  - array.t.js
  - assert.t.js
  - bigint.t.js
  - conditional.t.js
  - cycle.js
  - fixed.t.js
  - ieee.t.js
  - include.t.js
  - inline.t.js
  - integer.t.js
  - literal.t.js
  - lookup.t.js
  - packed.t.js
  - require.t.js
  - structure.t.js
  - switch.t.js
  - terminated.t.js
  - twiddle.js
- - lookup.lookup.js
  - lookup.parser.all.js
  - lookup.parser.bff.js
  - lookup.parser.chk.js
  - lookup.parser.inc.js
  - lookup.serializer.all.js
  - lookup.serializer.bff.js
  - lookup.serializer.chk.js
  - lookup.serializer.inc.js
  - lookup.sizeof.js
  - mqtt.lookup.js
  - mqtt.parser.all.js
  - mqtt.parser.bff.js
  - mqtt.parser.chk.js
  - mqtt.parser.inc.js
  - mqtt.serializer.all.js
  - mqtt.serializer.bff.js
  - mqtt.serializer.chk.js
  - mqtt.serializer.inc.js
  - mqtt.sizeof.js
  - required.js
- - empty-array.t.js
- - absent.t.js
  - accumulator.t.js
  - arguments.t.js
  - array.t.js
  - bigint.t.js
  - conditional.t.js
  - fixed.t.js
  - ieee.t.js
  - include.t.js
  - inline.t.js
  - integer.t.js
  - literal.t.js
  - lookup.t.js
  - minimal.t.js
  - packed.t.js
  - print.t.js
  - require.t.js
  - simplified.t.js
  - structure.t.js
  - switch.t.js
  - terminated.t.js
  - vivify.t.js
- - signage.t.js
  - unpack.t.js
- - mqtt.t.js
- - assertion-mirrored.js
  - assertion-named.js
  - assertion-not-assertion.js
  - assertion-parameter-named-reuse.js
  - assertion-parameter-reuse.js
  - assertion-parameter.js
  - assertion.js
  - checksum.js
  - conditional-packed.js
  - conditional.js
  - constant.js
  - constants.js
  - fixed-calculated.js
  - fixed.js
  - float.js
  - length-encoded-buffer.js
  - length-encoded-nested.js
  - length-encoded-structures.js
  - length-encoded.js
  - literal-repeat.js
  - little-endian-64.js
  - little-endian-twos-compliment-64.js
  - little-endian-twos-compliment.js
  - little-endian.js
  - negative-integer-64.js
  - negative-integer.js
  - nested-structures.js
  - packed-integer-little-endian.js
  - packed-integer.js
  - parameters.js
  - partial.js
  - readme.t.js
  - require.js
  - string-value-map.js
  - switch.js
  - terminated-multibyte.js
  - terminated.js
  - transform-basic.js
  - transform-by-name.js
  - transform-dollar-under.js
  - transform-mask-array-full-named.js
  - transform-mask-array-named.js
  - transform-mask-array-positional.js
  - transform-mask-function-syntax.js
  - transform-mask-long-named.js
  - transform-mask-named.js
  - transform-mask-positional.js
  - transform-mask-same.js
  - transform-pre-only.js
  - unamed-literal-little-endian-explicit.js
  - unamed-literal-little-endian-repeat.js
  - unamed-literal-little-endian.js
  - unamed-literal-repeat-once.js
  - unamed-literal-repeat.js
  - unnamed-literal-after.js
  - unnamed-literals-before-and-after.js
  - unnamed-literals.js
  - whole-integer-64.js
  - whole-integer.js
- - packed.t.js
- - parser.t.js
  - serializer.t.js
- generated.js
.gitignore
.npmignore
.travis.yml
arguments.js
countdown.js
declare.js
design.md
diary.md
edify.md
example.js
function.js
generate.js
hash.js
hex.js
homogeneous.js
inline.js
inlines.js
join.js
language.js
language.md
LICENSE
lookup.js
Makefile
map.js
orphaned.md
pack.js
package.json
packetize.bin.js
packetize.js
parse.all.js
parse.inc.js
print.js
README.md
require.js
required.js
serialize.all.js
serialize.inc.js
sizeof.js
tour.js
unpack.js
vivify.js

// repository documentation

Was this content helpful?

★ 0 (0 ratings)

[![Actions Status](https://github.com/bigeasy/packet/workflows/Node%20CI/badge.svg)](https://github.com/bigeasy/packet/actions)
[![codecov](https://codecov.io/gh/bigeasy/packet/branch/master/graph/badge.svg)](https://codecov.io/gh/bigeasy/packet)
[![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)

Incremental binary parsers and serializers for Node.js.

| What          | Where                                         |
| --- | --- |
| Discussion    | https://github.com/bigeasy/packet/issues/1    |
| Documentation | https://bigeasy.github.io/packet              |
| Source        | https://github.com/bigeasy/packet             |
| Issues        | https://github.com/bigeasy/packet/issues      |
| CI            | https://travis-ci.org/bigeasy/packet          |
| Coverage:     | https://codecov.io/gh/bigeasy/packet          |
| License:      | MIT                                           |

Packet creates **pre-compiled**, **pure-JavaScript**, **binary parsers** and
**serializers** that are **incremental** through a packet definition pattern
language that is **declarative** and very **expressive**.

Packet simplifies the construction and maintenance of libraries that convert
binary to JavaScript and back. The name Packet may make you think that it is
designed solely for binary network protocols, but it is also great for reading
and writing binary file formats.

**Incremental** &mdash; Node packet creates incremental parsers and serializers
that are almost as fast as the parser you'd write by hand, but a easier to
define and maintain.

**Declarative** &mdash; Packet defines a binary structure using a syntax-bashed
JavaScript definition langauge. A single definition is used to define both
parsers and serializers. If you have a protocol specification, or even just a C
header file with structures that define your binary data, you can probably
translate that directly into a Packet definition.

**Expressive** &mdash; The pattern language can express

* signed and unsigned integers,
  * endianess of singed and unsigned integers,
  * floats and doubles,
  * fixed length arrays of characters or numbers,
  * length encoded strings of characters or numbers,
  * zero terminated strings of characters or numbers,
  * said strings terminated any fixed length terminator you specify,
  * padding of said strings with any padding value you specify,
  * signed and unsigned integers extracted from bit packed integers,
  * string to integer value mappings,
  * if/then/else conditionals,
  * switch conditions,
  * character encodings,
  * custom transformations,
  * assertions,
  * and pipelines of custom transformations and assertions.

Packet installs from NPM.

```text
npm install packet
```

## Living `README.md`

This `README.md` is also a unit test using the
[Proof](https://github.com/bigeasy/proof) unit test framework. We'll use the
Proof `okay` function to assert out statements in the readme. A Proof unit test
generally looks like this.

```javascript
require('proof')(4, okay => {
    okay('always okay')
    okay(true, 'okay if true')
    okay(1, 1, 'okay if equal')
    okay({ value: 1 }, { value: 1 }, 'okay if deep strict equal')
})
```

You can run this unit test yourself to see the output from the various
code sections of the readme.

```text
git clone git@github.com:bigeasy/packet.git
cd packet
npm install --no-package-lock --no-save
node --allow-natives-syntax test/readme/readme.t.js
```

Be sure to run the unit test with the `--allow-natives-syntax` switch. The
`--allow-natives-syntax` switch allows us to test that when we parse we are
creating objects that have JavaScript "fast properties."

## Parsers and Serializers

**TODO** Here you need your incremental and whole parser interface with a simple
example. Would be an over view. In the next section we get into the weeds.

## Packet Definition Language

To test our examples below we are going to use the following function.

```javascript
const fs = require('fs')
const path = require('path')

const packetize = require('packet/packetize')
const SyncParser = require('packet/sync/parser')
const SyncSerializer = require('packet/sync/serializer')

// Generate a packet parser and serializer mechnaics module.

// Please ignore all the synchronous file operations. They are for testing
// only. You will not generate packet parsers at runtime. You will use the
// `packetizer` executable to generate your packet parser and serializer
// mechanics modules and ship them.

//
function compile (name, definition, options) {
    const source = packetize(definition, options)
    const file = path.resolve(__dirname, '..', 'readme', name + '.js')
    fs.writeFileSync(file, source)
    return file
}

// Load mechanics and run a synchronous serialize and parse.

// This looks more like production code, except for the part where you call
// our for-the-sake-of-testing runtime compile.

//
function test (name, definition, object, expected, options = {}) {
    const moduleName = compile(name, definition, options)

const mechanics = require(moduleName)

const serializer = new SyncSerializer(mechanics)
    const buffer = serializer.serialize('object', object)

okay(buffer.toJSON().data, expected, `${name} correctly serialized`)

const parser = new SyncParser(mechanics)
    const actual = parser.parse('object', buffer)

okay(actual, object, `${name} correctly parsed`)
}
```

### Whole Integers

Integers are specified as multiples of 8 bits. For integers less than 48 bits
you can define the integer field as a JavaScript `typeof === 'number'`. If the
integer is larger than 48 bits you should define the field as JavaScript
`BigInt`.

**Mnemonic**: A count of bits to serialize or parse defined as a JavaScript
`'number'` or `BigInt` since that's the type it will produce. We use a count of
bits as opposed to a count of bytes so that our code looks consistent when when
we define bit packed integers which need to be defined in bits and not bytes.

In the following definition `value` is a 16-bit `'number'` with valid integer
values from 0 to 65,535. Serializes objects with `'number'` fields must provide
a `'number'` type value and the number must be in range. No type or range
checking is performed.

```javascript
const definition = {
    object: {
        value: 16
    }
}

const object = {
    value: 0xabcd
}

test('whole-integer', definition, object, [ 0xab, 0xcd ])
```

Integers smaller than 48 bits _should_ be defined using a `'number'` to specify
the count of bits. Integers larger than 48 bits _must_ be defined as `BigInt`.

In the following definition `value` is a 64-bit `BigInt` with a valid integer
values from 0 to 18,446,744,073,709,551,616. Serializes objects with `BigInt`
fields must provide a `BigInt` type value and the number must be in range. No
type or range checking is performed.

**Mnemonic**: The `n` suffix is the same suffix used to indicate a `BigInt`
literal in JavaScript.

```javascript
const definition = {
    object: {
        value: 64n
    }
}

const object = {
    value: 0xfedcba9876543210n
}

test('whole-integer-64', definition, object, [
    0xfe, 0xdc, 0xba, 0x98, 0x76, 0x54, 0x32, 0x10
])
```

### Negative Integers

Integers with negative values are generally represented as two's compliment
integers on most machines. To parse and serialize as two's compliment you
precede the bit length of an integer field with a `-` negative symbol.

In the following definition `value` is a two's compliment 16-bit integer with
valid values from -32768 to 32767. Two's compliment is a binary representation
of negative numbers.

**Mnemonic**: Negative symbol to indicate a potentially negative value.

```javascript
const definition = {
    object: {
        value: -16
    }
}

const object = {
    value: -1
}

test('negative-integer', definition, object, [ 0xff, 0xff ])
```

As with whole integers, you _must_ define an integer larger than 32-bits as a
`BitInt`.

In the following definition `value` is a two's compliment 16-bit integer with
valid values from -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807.

```javascript
const definition = {
    object: {
        value: -64n
    }
}

const object = {
    value: -1n
}

test('negative-integer-64', definition, object, [
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff
])
```

### Endianness

By default, all numbers are written out big-endian, where the bytes are written
from the most significant to the least significant. The same order in which
you'd specify the value as a hexadecimal literal in JavaScript.

Little-endian means that the bytes are serialized from the least significant
byte to the most significant byte. Note that this is the order of _bytes_ and
not _bits_. This would be the case if you wrote an integer out directly to a
file from a C program on an Intel machine.

To parse and serialize an integer as little-endian you precede the bit length of
an integer field with a `~` tilde.

In the following definition `value` is a 16-bit `number` field with valid
integer values from 0 to 65,535. A value of `0xabcd` would be serialized in
little-endian order as `[ 0xcd, 0xab ]`.

**Mnemonic**: The tilde is curvy and we're mixing things up, turning them
around, vice-versa like.

```javascript
const definition = {
    object: {
        value: ~16
    }
}

const object = {
    value: 0xabcd
}

test('little-endian', definition, object, [ 0xcd, 0xab ])
```

If you want a little-endian negative number combine both `-` and `~`. You can
combine the `-` and `~` as `-~` and `~-`.

In the following definition both `first` and `second` are 16-bit `number` fields
with valid integer values from -32768 to 32767. A value of `-0x2` would be
converted to the twos compliment representation 0xfffe and serialized in
little-endian as `[ 0xfe, 0xff ]`.

```javascript
const definition = {
    object: {
        first: ~-16,
        second: -~16
    }
}

const object = {
    first: -2,
    second: -2
}

test('little-endian-twos-compliment', definition, object, [
    0xfe, 0xff, 0xfe, 0xff
])
```

Just as with the default big-endian integer fields, little-endian integer fields
greater than 32-bits must be specified as `BigInt` fields using a `'bigint'`
literal.

```javascript
const definition = {
    object: {
        value: ~64n,
    }
}

const object = {
    value: 0xfedcba9876543210n
}

test('little-endian-64', definition, object, [
    0x10, 0x32, 0x54, 0x76, 0x98, 0xba, 0xdc, 0xfe
])
```

Similarly, for little-endian signed number fields greater than 32-bits you
combine the `-` and `~` with a `BigInt` literal. You can combine the `-` and `~`
as `-~` and `~-`.

```javascript
const definition = {
    object: {
        first: ~-64n,
        second: -~64n
    }
}

const object = {
    first: -2n,
    second: -2n
}

test('little-endian-twos-compliment-64', definition, object, [
    0xfe, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, // first
    0xfe, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff  // second
])
```

### Nested Structures

You can nest structures arbitrarily. The nesting of structures does not effect
the serialization or parsing, it will still be a series bytes in the stream, but
it may help the structure your programs group values in a meaningful way.

```javascript
const definition = {
    object: {
        header: {
            type: 8,
            length: 16
        },
        options: {
            encrypted: 8,
            checksum: 32
        }
    }
}

const object = {
    header: {
        type: 1,
        length: 64
    },
    options: {
        encrypted: 0,
        checksum: 0xaaaaaaaa
    }
}

test('nested-structures', definition, object, [
    0x01,                   // header.type
    0x00, 0x40,             // header.length
    0x00,                   // options.encrypted
    0xaa, 0xaa, 0xaa, 0xaa  // options.checksum
])
```

You can define a nested structure and then elide it by defining the structure
with a name that begins with a `_`. This is silly so don't do it. It is
available merely to be consistent with packet integers, accumulators and limits.

**TODO** Short example.

### Packed Integers

Packed integers are expressed as nested structures grouped in an `Array`
followed by an integer definition of the packed integer size. The byte lengths
in the packed integer must sum to the size of the packed integer.

Packed integer fields are always big-endian and cannot be made little endian.
Packed integer fields can be made two's compliment by preceding the field bit
length with a `-` negative symbol just like whole integers.

A packed 32-bit integer with a single two's compliment (potentially negative)
value named `volume`.

The bit length values of the packed values sum to 32. Note that we consider
`volume` to be 10 bits and not -10 bits in this summation of packed field
values. The `-` is used to indicate a two's compliment integer field.

```javascript
const definition = {
    object: {
        header: [{
            type: 7,
            encrypted: 1,
            volume: -10,
            length: 14
        }, 32 ]
    }
}

const object = {
    header: {
        type: 3,
        encrypted: 1,
        volume: -1,
        length: 1024
    }
}

test('packed-integer', definition, object, [
    0x7,    // type and encrypted packed into ne byte
    0xff,   // eight bytes of volume
    0xc4,   // two types of volume and top of length
    0x0     // rest of length
])
```

The packed integer will be serialized as big-endian by default. You can specify
that the packed integer is serialized as little-endian by proceeding the bit
length with a `~` tilde.

```javascript
const definition = {
    object: {
        header: [{
            type: 7,
            encrypted: 1,
            volume: -10,
            length: 14
        }, ~32 ]
    }
}

const object = {
    header: {
        type: 3,
        encrypted: 1,
        volume: -1,
        length: 1024
    }
}

test('packed-integer-little-endian', definition, object, [
    0x0,    // rest of length
    0xc4,   // two types of volume and top of length
    0xff,   // eight bytes of volume
    0x7     // type and encrypted packed into ne byte
])
```

You many not want the nested structure of the packed header to appear in your
parsed object. You can elide the nested structure by giving it a name that
begins with an `_`.

**TODO** Example.

Elide only works for packet integers, accumulators, limits and structures. If
you name a field of any other type it will be defined with the underscore.

**TODO** Example.

### Literals

Literals are bytes written on serialization that are constant and not based on a
value in the serialized structure.

You define a constant with an array that contains a `String` that describes the
constant value in hexadecimal digits. There should be two hexadecimal digits for
every byte. The length of the field is determined by the number of bytes
necessary to hold the value.

**Mnemonic**: A string literal reminds us this is literal and stands out because
it is not numeric. Hexadecimal helps distinguish these constant values from
field sizes and other aspects of the definition language expressed with numbers.

```javascript
const definition = {
    object: {
        constant: [ 'fc' ],
        value: 16
    }
}

const object = {
    value: 0xabcd
}

test('constant', definition, object, [
    0xfc, 0xab, 0xcd
])
```

Generated parsers skip the constant bytes and do not validate the parsed value.
If you want to perform validation you can define the field as an integer field
and inspect the parsed field value. This means you will also have to
consistently set the serialized field value on your own.

A literal is ignored on serialization if it exists. It is not set in the
generated structure on parsing. In our example the `contsant` property of the
object is not generated on parse.

**TODO** How about an explicit example that doesn't require as much exposition
as our `test` definition.

Not much point in naming a literal, is there? The literal it will not be read
from the serialized object nor will the named literal property be set in a
parsed parsed object. What if you have multiple literals? Now you have to have
`constant1` and `constant2`. It starts to look ugly as follows.

```javascript
const definition = {
    object: {
        constant1: [ 'fc' ],
        key: 16,
        constant2: [ 'ab' ],
        value: 16
    }
}

const object = {
    key: 1,
    value: 0xabcd
}

test('constants', definition, object, [
    0xfc, 0x0, 01,
    0xab, 0xab, 0xcd
])
```

You can forgo naming a literal by defining it as padding before or after a
field.

To prepend a literal to a field definition in an array where the literal
definition is the first element and field definition is the second. The literal
will be written before writing the field value and skipped when parsing the
field value.

```javascript
const definition = {
    object: {
        key: [[ 'fc' ], 16 ],
        value: [[ 'ab' ], 16 ]
    }
}

const object = {
    key: 1,
    value: 0xabcd
}

test('unnamed-literals', definition, object, [
    0xfc, 0x0, 01,
    0xab, 0xab, 0xcd
])
```

You can specify an unnamed literal that follows a field. Enclose the field
definition in an array with the field definition as the first element and the
literal definition as the second element.

```javascript
const definition = {
    object: {
        value: [ 16, [ 'ea' ] ],
    }
}

const object = {
    value: 0xabcd
}

test('unnamed-literal-after', definition, object, [
    0xab, 0xcd, 0xea
])
```

You can specify an unnamed literal both before and after a field. Enclose the
field definition in an array and define preceding literal as the first element
and following literal as the last element.

The example above can be defined using literals around the `key` property alone.

```javascript
const definition = {
    object: {
        key: [[ 'fc' ], 16, [ 'ab' ] ],
        value: 16
    }
}

const object = {
    key: 1,
    value: 0xabcd
}

test('unnamed-literals-before-and-after', definition, object, [
    0xfc, 0x0, 01,
    0xab, 0xab, 0xcd
])
```

You can define a literal that repeats its value. The constant value is defined
using an array that contains a `String` with the literal value as the first
element and the number of times to repeat the value as the second element.

**Mnemonic**: The repeat count follows the hexadecimal definition, its relation
to the definition is expressed by its containment in an array.

```javascript
const definition = {
    object: {
        constant: [ 'beaf', 3 ],
        value: 16
    }
}

const object = { value: 0xabcd }

test('literal-repeat', definition, object, [
    0xbe, 0xaf, 0xbe, 0xaf, 0xbe, 0xaf,
    0xab, 0xcd
])
```

You can express repeated literals as unnamed literals by prepending or appending
them to a field definition.

```javascript
const definition = {
    object: {
        value: [[ 'beaf', 3 ], 16 ]
    }
}

const object = { value: 0xabcd }

test('unamed-literal-repeat', definition, object, [
    0xbe, 0xaf, 0xbe, 0xaf, 0xbe, 0xaf,
    0xab, 0xcd
])
```

Note that a literal definition without a repeat count is the same as a literal
definition with a repeat count of `1`.

```javascript
const definition = {
    object: {
        explicit: [[ 'beaf', 1 ], 16 ],
        implicit: [[ 'beaf' ], 16 ]
    }
}

const object = { explicit: 0xabcd, implicit: 0xabcd }

test('unamed-literal-repeat-once', definition, object, [
    0xbe, 0xaf, 0xab, 0xcd,
    0xbe, 0xaf, 0xab, 0xcd
])
```

Little endian serialization of literals seems like an unlikely use case. One
would imagine at a specification would specify the bytes in network byte order.
Often times filler bytes are a repeat of a single byte so endianness doesn't
matter.

If you want little-endian serialization of a literal value you could simply
reverse the bits yourself.

Here we write `0xbeaf` little-endian by explicitly flipping `0xbe` and `0xaf`.

```javascript
const definition = {
    object: {
        value: [[ 'afbe' ], 16 ]
    }
}

const object = { value: 0xabcd }

test('unamed-literal-little-endian-explicit', definition, object, [
    0xaf, 0xbe,
    0xab, 0xcd
])
```

Simple enough, however...

If specify a repeat count prepended by a `~` the pattern will be written
little-endian.

**Mnemonic**: Use use a tilde `~` because it's squiggly and we're swirling the
bytes around vice-versa. Same mnemonic for little-endian integer fields.

```javascript
const definition = {
    object: {
        value: [[ 'beaf', ~1 ], 16 ]
    }
}

const object = { value: 0xabcd }

test('unamed-literal-little-endian', definition, object, [
    0xaf, 0xbe,
    0xab, 0xcd
])
```

You can repeat the little-endian serialization more than once.

```javascript
const definition = {
    object: {
        value: [[ 'beaf', ~3 ], 16 ]
    }
}

const object = { value: 0xabcd }

test('unamed-literal-little-endian-repeat', definition, object, [
    0xaf, 0xbe, 0xaf, 0xbe, 0xaf, 0xbe,
    0xab, 0xcd
])
```

Unnamed little-endian literals can be appended or prepended. Any unnamed literal
definition can be appended, prepended or both.

### Length-Encoded Arrays

A common pattern in serialization formats is a series of repeated values
preceded by a count of those values.

**Mnemonic**: We enclose the definition in an array. The first element is an
integer field definition for the length. It's scalar appearance indicates that
it does not repeat. The repeated value is enclosed in an array indicating that
it will be the value that repeats. The ordering of the scalar followed by the
array mirrors the binary representation of a length/count followed by repeated
values.

```javascript
const definition = {
    object: {
        array: [ 16, [ 8 ] ]
    }
}

const object = {
    array: [ 0xaa, 0xbb, 0xcc, 0xdd ]
}

test('length-encoded', definition, object, [
    0x0, 0x4, 0xaa, 0xbb, 0xcc, 0xdd
])
```

The repeated value can be of any type including structures.

```javascript
const definition = {
    object: {
        array: [ 16, [{ key: 16, value: 16 }] ]
    }
}

const object = {
    array: [{ key: 0xaa, value: 0xbb }, { key: 0xcc, value: 0xdd }]
}

test('length-encoded-structures', definition, object, [
    0x0, 0x2,               // length encoding
    0x0, 0xaa, 0x0, 0xbb,   // first structure
    0x0, 0xcc, 0x0, 0xdd    // second structure
])
```

You can even nest length-encoded arrays inside length-encoded arrays.

```javascript
const definition = {
    object: {
        array: [ 16, [[ 16, [ 8 ]]] ]
    }
}

const object = {
    array: [[ 0xaa, 0xbb ], [ 0xcc, 0xdd ]]
}

test('length-encoded-nested', definition, object, [
    0x0, 0x2,               // length encoding
    0x0, 0x2, 0xaa, 0xbb,   // first array length encoding and values
    0x0, 0x2, 0xcc, 0xdd    // second array length encoding and values
])
```

Because pure binary data is a special case, instead of an array of `8` bit
bites, you can specify a length encoded binary data as a `Buffer`.

```javascript
const definition = {
    object: {
        array: [ 16, [ Buffer ] ]
    }
}

const object = {
    array: Buffer.from([ 0xaa, 0xbb, 0xcc, 0xdd ])
}

test('length-encoded-buffer', definition, object, [
    0x0, 0x4, 0xaa, 0xbb, 0xcc, 0xdd
])
```

### Inline Transforms and Assertions

Inline transforms are specified by wrapping a field definition in an array with
a pre-serialization function before or a post-parsing function after it or both.
The pre-serialization function and post-parsing function must be enclosed in an
array.

A pre-serialization transformation function takes the value from the JavaScript
object and returns the transformed that is then written to the stream. The
post-parsing transformation function takes a value extracted from the stream and
returns the transformed value that is assigned to the JavaScript object.

The following transform will convert a hexadecimal string to an integer on
serialization and back to a hexadecimal string on parse.

**Mnemonic**: A function is obviously a function, it does something to in the
midst of parsing. We used functions elsewhere in the language, so we enclose
them in arrays, The array brackets act as parenthesis, these are parenthetical
user actions on the stream.

```javascript
const definition = {
    object: {
        value: [[ $_ => parseInt($_, 16) ], 32, [ $_ => $_.toString(16) ]]
    }
}

const object = {
    value: '89abcdef'
}

test('transform-basic', definition, object, [
    0x89, 0xab, 0xcd, 0xef
])
```

Whoa, what's with the parameter names pal? `$_` violates everything I was ever
taught about naming variables. How would you even pronounce that?

Well, once upon a time I wrote me a lot of Perl. In Perl this variable is called
"dollar under." It is the default variable for an array value when you loop
through an array with `foreach`. I miss those days, so I thought I revive them.
You can name positional arguments anything you like, but I'll be using these
names to get you used to them, because they're available as named arguments as
well.

You can also use named arguments via object deconstruction. When you do, you
must specify names that are in the current namespace. The namespace will contain
the object properties in the current path.

```javascript
const definition = {
    object: {
        value: [[ ({ value }) => parseInt(value, 16) ], 32, [ ({ value }) => value.toString(16) ]]
    }
}

const object = {
    value: '89abcdef'
}

test('transform-by-name', definition, object, [
    0x89, 0xab, 0xcd, 0xef
])
```

You can also refer to the current variable using the Perl-esque "dollar under"
variable. Perl-esque variables can make your code more concise. If used
consistently it will still be human readable.

```javascript
const definition = {
    object: {
        value: [[ ({ $_ }) => parseInt($_, 16) ], 32, [ ({ $_ }) => $_.toString(16) ]]
    }
}

const object = {
    value: '89abcdef'
}

test('transform-dollar-under', definition, object, [
    0x89, 0xab, 0xcd, 0xef
])
```

There are two Perl-esque variable names `$_` for the immediate property value,
and `$` for the root object. Any other system provided names such as `$i`,
`$buffer`, `$start` and `$end` will begin with a `$` do distinguish them from
user specified names and to avoid namespace collisions.

**Mnemonic**: Borrowed from Perl `foreach` loop, `$_` is the immediate property
value, useful for its brevity. `$` is the root variable, the shortest special
variable because if you're starting from the root, you have a path ahead of you.

A transform or assertion is always defined with an array with three elements. If
you only want to define a pre-serialization action, the last element will be an
empty array. If you only want to define a post-parsing action, the first element
will be an empty array.

In the following example we do not want to perform a post-parsing action, so we
leave the post-parsing array empty, but we do not neglect to add it.

```javascript
const definition = {
    object: {
        value: [[ $_ => typeof $_ == 'string' ? parseInt($_, 16) : $_ ], 32, []]
    }
}

const moduleName = compile('transform-pre-only', definition)
const mechanics = require(moduleName)

{
    const buffer = new SyncSerializer(mechanics).serialize('object', { value: '89abcdef' })
    const object = new SyncParser(mechanics).parse('object', buffer)
    okay(object, { value: 0x89abcdef }, 'transform-pre-only-convert')
}

{
    const buffer = new SyncSerializer(mechanics).serialize('object', { value: 0x89abcdef })
    const object = new SyncParser(mechanics).parse('object', buffer)
    okay(object, { value: 0x89abcdef }, 'transform-pre-only-no-convert')
}
```

Named arguments have limitations. We're using a simple regex based parser to
extract the arguments from the function source, not a complete JavaScript
parser. We are able to parse object destructuring, array destructuring, and
default argument values of numbers, single quoted strings and double quoted
strings.

Do not use regular expressions, interpolated strings or function calls, in your
default argument assignments. You can use any valid JavaScript in your function
bodies.

In the following definition we've added an unused named variable that is default
assigned a value extracted from a literal string by a regular expression. The
right curly brace in the literal string won't confuse our simple argument
parser, but the right curly brace in the regular expression will.

```javascript
const definition = {
    object: {
        value: [[
            ({ $_, packet: { extra = /^([}])/.exec("}")[1] } }) => parseInt($_, 16)
        ], 32, [
            ({ $_ }) => $_.toString(16)
        ]]
    }
}

const _definition = {
    object: {
        value: [[ $_ => typeof $_ == 'string' ? parseInt($_, 16) : $_ ], 32, []]
    }
}

try {
    packetize(definition)
} catch (error) {
    okay(error instanceof SyntaxError, 'unable to parse regex')
}
```

As you can see it's an unlikely use case. Basically, if you find yourself
writing logic in your named arguments, stop and place it in a function in a
module and invoke that module function from the inline function.

We'll continue to use `$_` and `$` in positional argument examples so we can all
get used to them.

The first argument to a transformation function with positional arguments is the
transformed value, the second argument is the root object being transformed.

The following WebSockets inspired example xors a value with a `mask` property in
the packet.

```javascript
const definition = {
    object: {
        mask: 16,
        value: [[ ($_, $) => $_ ^ $.mask ], 16, [ ($_, $) => $_ ^ $.mask ]]
    }
}

const object = {
    mask: 0xaaaa,
    value: 0xabcd
}

test('transform-mask-positional', definition, object, [
    0xaa, 0xaa, 0x1, 0x67
])
```

This can be expressed using named arguments. Note how we can order the arguments
any way we like.

```javascript
const definition = {
    object: {
        mask: 16,
        value: [[ ({ $, $_ }) => $_ ^ $.mask ], 16, [ ({ $_, $ }) => $_ ^ $.mask ]]
    }
}

const object = {
    mask: 0xaaaa,
    value: 0xabcd
}

test('transform-mask-named', definition, object, [
    0xaa, 0xaa, 0x1, 0x67
])
```

You can also name the names of the object properties in the current path. Again,
note that the order of names does not matter with named arguments.

```javascript
const definition = {
    object: {
        mask: 16,
        value: [[
            ({ object, value }) => value ^ object.mask
        ], 16, [
            ({ value, object }) => value ^ object.mask
        ]]
    }
}

const object = {
    mask: 0xaaaa,
    value: 0xabcd
}

test('transform-mask-long-named', definition, object, [
    0xaa, 0xaa, 0x1, 0x67
])
```

(Not to self: Seems like it might also be useful to be able to reference the
current object in a loop, which could be `$0` for the current object, `$1` for a
parent. This would be simpler than passing in the indices, but that would be
simple enough, just give them the already existing `$i`. Heh, no make them
suffer.)

The third argument passed to a transformation function is an array of indices
indicating the index of each array in the path to the object. **TODO** Move
fixed arrays above.

```javascript
const definition = {
    object: {
        array: [ 16, [{
            mask: 16,
            value: [[
                ($_, $, $i) => $_ ^ $.array[$i[0]].mask
            ], 16, [
                ($_, $, $i) => $_ ^ $.array[$i[0]].mask
            ]]
        }]]
    }
}

const object = {
    array: [{
        mask: 0xaaaa, value: 0xabcd
    }, {
        mask: 0xffff, value: 0x1234
    }]
}

test('transform-mask-array-positional', definition, object, [
    0x0, 0x2,                   // length encoded count of elements
    0xaa, 0xaa, 0x1, 0x67,      // first element
    0xff, 0xff, 0xed, 0xcb      // second element
])
```

We can use named arguments as well.

```javascript
const definition = {
    object: {
        array: [ 16, [{
            mask: 16,
            value: [[
                ({ $_, $, $i }) => $_ ^ $.array[$i[0]].mask
            ], 16, [
                ({ $_, $, $i }) => $_ ^ $.array[$i[0]].mask
            ]]
        }]]
    }
}

const object = {
    array: [{
        mask: 0xaaaa, value: 0xabcd
    }, {
        mask: 0xffff, value: 0x1234
    }]
}

test('transform-mask-array-named', definition, object, [
    0x0, 0x2,                   // length encoded count of elements
    0xaa, 0xaa, 0x1, 0x67,      // first element
    0xff, 0xff, 0xed, 0xcb      // second element
])
```

We can also use the names of the object properties in the current path. The `$i`
array variable of is a special system property and it therefore retains its
dollar sign prepended name.

```javascript
const definition = {
    object: {
        array: [ 16, [{
            mask: 16,
            value: [[
                ({ value, object, $i }) => value ^ object.array[$i[0]].mask
            ], 16, [
                ({ value, object, $i }) => value ^ object.array[$i[0]].mask
            ]]
        }]]
    }
}

const object = {
    array: [{
        mask: 0xaaaa, value: 0xabcd
    }, {
        mask: 0xffff, value: 0x1234
    }]
}

test('transform-mask-array-full-named', definition, object, [
    0x0, 0x2,                   // length encoded count of elements
    0xaa, 0xaa, 0x1, 0x67,      // first element
    0xff, 0xff, 0xed, 0xcb      // second element
])
```

If your pre-serialization function and post-parsing function are the same you
can specify it once and use it for both serialization and parsing by surrounding
it with an additional array.

```javascript
const definition = {
    object: {
        mask: 16,
        value: [[[ ($_, $) => $_ ^ $.mask ]], 16 ]
    }
}

const object = {
    mask: 0xaaaa, value: 0xabcd
}

test('transform-mask-same', definition, object, [
    0xaa, 0xaa, 0x1, 0x67
])
```

Note that the above functions can also be defined using `function` syntax. Arrow
functions are generally more concise, however.

```javascript
const definition = {
    object: {
        mask: 16,
        value: [[[ function ({ value, object }) {
            return value ^ object.mask
        } ]], 16 ]
    }
}

const object = {
    mask: 0xaaaa, value: 0xabcd
}

test('transform-mask-function-syntax', definition, object, [
    0xaa, 0xaa, 0x1, 0x67
])
```

### Fixed Length Arrays

Fixed length arrays are arrays of a fixed length. They are specified by an array
containing the numeric length of the array.

**Mnemonic**: Like a length encoded definition the element definition is placed
inside an array because it is the array element. Like a length encoded
definition the length of the array precedes the element definition. It is the
length of the array enclosed in an array like C array declaration.

```javascript
const definition = {
    object: {
        fixed: [[ 2 ], [ 16 ]]
    }
}

const object = {
    fixed: [ 0xabcd, 0xdcba ]
}

test('fixed', definition, object, [
    0xab, 0xcd, 0xdc, 0xba
])
```

Calculated length arrays are fixed length arrays where the fixed length is
specified by function. The length is, therefore, not fixed at all. It is
calculated.

In the following example we have a length encoding that is in a header and there
is a field between the length encoding and the array so we can't use a
length-encoded array definition. We use a calculated length that references the
header's length field.

**Mnemonic** Same as fixed length arrays replacing the fixed length with a
function indicating that the function will calculate the length.

```javascript
const definition = {
    object: {
        header: {
            length: 16,
            type: 8
        },
        array: [[ $ => $.header.length ], [ 16 ]]
    }
}

const object = {
    header: {
        length: 2,
        type: 1
    },
    array: [ 0xabcd, 0xdcba ]
}

test('fixed-calculated', definition, object, [
    0x0, 0x2,               // header.length
    0x1,                    // header.type
    0xab, 0xcd, 0xdc, 0xba
])
```

### Requiring Modules

The functions in our packet parser may depend on external libraries. We can

```javascript
const definition = {
    object: {
        value: [[ value => ip.toLong(value) ], 32, [ value => ip.fromLong(value) ]]
    }
}

const source = packetize(definition, { require: { ip: 'ip' } })

const moduleName = path.resolve(__dirname, 'require.js')
fs.writeFileSync(moduleName, source)

const mechanics = require(moduleName)

const object = { value: '127.0.0.1' }

const buffer = new SyncSerializer(mechanics).serialize('object', object)

okay(buffer.toJSON().data, [
    127, 0, 0, 1
], 'require serialized')

const parsed = new SyncParser(mechanics).parse('object', buffer)
okay(parsed, object, 'require parsed')
```

When can also use modules local to the current project using relative paths, but
we face a problem; we're not going to ship language definition with our
completed project, we're going to ship the generated software. Therefore,
relative must be relative to the generated file. Your relative paths much be
relative to the output directory... (eh, whatever. Maybe I can fix that up for
you.)

```javascript
{
    ; ({
        packet: {
            value: [[ $value => ip.toLong($value) ], 32, [ $value => ip.fromLong($value) ]]
        }
    }, {
        require: { ip: '../ip' }
    })
}
```

### Assertions

**TODO** Needs examples of failed assertions.

We can also perform inline assertions. You specify an assertion the same way you
specify a transformation. You wrap your definition in an array.
A pre-serialization assertion is a function within an array in the element
before the definition. A post-parsing assertions is a function within an array
in the element after the definition.

When performing inline assertions, we are not transforming a value, we're simply
checking it's validity and raising an exception if a value is invalid. You could
use a transformation to do this, but you would end up returning the value as is.

With an assertion function the return value is ignored. It is not used as the
serialization or assignment value.

To declare an assertion function you assign a default value of `0` or `null` to
the immediate property argument.

In the following definition we use a `0` default value for the immediate
property argument which indicates that the value and should not be used for
serialization for the pre-serialization function nor assignment for the
post-parsing function.

```javascript
const definition = {
    object: {
        value: [[
            ($_ = 0) => assert($_ < 1000, 'excedes max value')
        ], 16, [
            ($_ = 0) => assert($_ < 1000, 'excedes max value')
        ]]
    }
}
const object = {
    value: 1
}
test('assertion', definition, object, [
    0x0, 0x1
], { require: { assert: 'assert' } })
```

(I assume I'll implement this in this way:) The exception will propagate to the
API caller so that you can catch it in your code and cancel the serialization or
parse. (However, if I do wrap the assertion in a try/catch and rethrow it
somehow, then the following example is moot.

If you where to use a transform, you would have to return the value and your
definition would be more verbose.

```javascript
const definition = {
    object: {
        value: [[
            $_ => {
                assert($_ < 1000, 'excedes max value')
                return $_
            }
        ], 16, [
            $_ => {
                assert($_ < 1000, 'execdes max value')
                return $_
            }
        ]]
    }
}
const object = {
    value: 1
}
test('assertion-not-assertion', definition, object, [
    0x0, 0x1
], { require: { assert: 'assert' } })
```

You can use the name function for both pre-serialization and post-parsing by
surrounding the function in an additional array.

```javascript
const definition = {
    object: {
        value: [[[ ($_ = 0) => assert($_ < 1000, 'excedes max value') ]], 16 ]
    }
}
const object = {
    value: 1
}
test('assertion-mirrored', definition, object, [
    0x0, 0x1
], { require: { assert: 'assert' } })
```

You can use named arguments to declare an assertion function.

```javascript
const definition = {
    object: {
        value: [[[ ({ $_ = 0 }) => assert($_ < 1000, 'excedes max value') ]], 16 ]
    }
}
const object = {
    value: 1
}
test('assertion-named', definition, object, [
    0x0, 0x1
], { require: { assert: 'assert' } })
```

### Assertion and Transformation Arguments

You can pass arguments to assertions and transforms. Any value in the array that
follows the function that is not itself a `function` is considered an argument
to the function. The arguments are passed in the order in which they are
specified preceding the immediate property value.

In the following definition the function is followed by a `number` argument
which is passed as the first parameter to the function in serialize or parser.

```javascript
const definition = {
    object: {
        value: [[[ (max, $_ = 0) => assert($_ < max, `value excedes ${max}`), 1024 ]], 16 ]
    }
}
const object = {
    value: 1
}
test('assertion-parameter', definition, object, [
    0x0, 0x1
], { require: { assert: 'assert' } })
```

This is useful when defining a function that you use more than once in your
definition.

```javascript
const max = (max, $_ = 0) => assert($_ < max, `value excedes ${max}`)

const definition = {
    object: {
        length: [[[ max, 1024 ]], 16 ],
        type: [[[ max, 12 ]], 8 ]
    }
}
const object = {
    length: 256,
    type: 3
}
test('assertion-parameter-reuse', definition, object, [
    0x1, 0x0, 0x3
], { require: { assert: 'assert' } })
```

When using named arguments, the argument values are assigned to the named
parameters preceding the first variable that is defined in the current scope.
That is, the first occurrence of a variable name that is either the name of a
property in the current path or a system name beginning with `$` dollar sign.

In the following definition the first argument to the `max` function will be
assigned to the `max` named argument. The positional argument mapping stops at
the `$path` parameter since it is a system parameter beginning with `$` dollar
sign. The `'oops'` parameter of the `max` function call for the `type` property
will be ignored.

```javascript
const max = ({ max, $path, $_ = 0 }) => assert($_ < max, `${$path.pop()} excedes ${max}`)

const definition = {
    object: {
        length: [[[ max, 1024 ]], 16 ],
        type: [[[ max, 12, 'oops' ]], 8 ]
    }
}
const object = {
    length: 256,
    type: 3
}
test('assertion-parameter-named-reuse', definition, object, [
    0x1, 0x0, 0x3
], { require: { assert: 'assert' } })
```

### Terminated Arrays

In the following example, we terminate the array when we encounter a `0` value.
The `0` is not included in the array result.

```javascript
{
    const definition = {
        object: {
            array: [[ 8 ], 0x0 ]
        }
    }
    const object = {
        array: [ 0xab, 0xcd ]
    }
    test('terminated', definition, object, [
        0xab, 0xcd, 0x0
    ])
}
```

#### Multi-byte Terminators

You can specify multi-byte terminators by specifying the multi-byte terminator
byte by byte in the end of the definition array.

In the following example, we terminate the array when we encounter a `0xa` value
followed by a `0xd` value, carriage return followed by line feed.

The `0` is not included in the array result.

```javascript
const definition = {
    object: {
        array: [[ 8 ], 0xd, 0xa ]
    }
}
const object = {
    array: [ 0xab, 0xcd ]
}
test('terminated-multibyte', definition, object, [
    0xab, 0xcd, 0xd, 0xa
])
```

### String Value Maps

**TODO**: Need first draft.

```javascript
{
    const definition = {
        object: {
            header: [{ type: [ 8, [ 'off', 'on' ] ] }, 8 ]
        }
    }
    const object = {
        header: {
            type: 'on'
        }
    }
    test('string-value-map', definition, object, [
        0x1
    ])
}
```

```javascript
{
    const description = {
        packet: {
            type: [ 8, { 0: 'off', 1: 'on', null: 'unknown' } ]
        }
    }
}
```

### Floating Point Values

Packet supports serializing and parsing IEEE754 floating point numbers. This is
the representation common to C.

A floating point number is is specified by specifying the value as a floating
the point number as `number` with the bit size repeated in the decimal digits of
the number.

**TODO** Values of `1.1` and `-1.5` are not serializing and restoring correctly.
I can't remember if this is expected.

```javascript
const definition = {
    object: {
        doubled: 64.64,
        float: 32.32
    }
}
const object = {
    doubled: 1.2,
    float: -1.5
}
test('float', definition, object, [
    0x3f, 0xf3, 0x33, 0x33,
    0x33, 0x33, 0x33, 0x33,
    0xbf, 0xc0, 0x0, 0x0
])
```

There are only two sizes of floating point number available, 64-bit and 32-bit.
These are based on the IEEE 754 standard. As of 2008, the standard defines a
[128-bit quad precision floating
point](https://en.wikipedia.org/wiki/Quadruple-precision_floating-point_format)
but the JavaScript `number` is itself a 64-bit IEEE 754 double-precision float,
so we'd have to introduce one of the big decimal libraries from NPM to support
it, so it's probably best you sort out a solution for your application using
inline functions, maybe serializing to a byte array or `BigInt`. If you
encounter at 128-bit number in the wild, I'd be curious. Please let me know.

### Conditionals

**TODO**: Need first draft.

Basic conditionals are expressed as an array of boolean functions paired with
field definitions. The functions and definitions repeat creating an if/else if
conditional. The array can end with a field definition that acts as the `else`
condition.

If the function has positional arguments, the function is called with the root
object, followed by an array of indices into any arrays the current path,
followed by an array of names of the properties in the current path.

In the following definition the bit size of value is 8 bits of the `type`
property is `1`, 16 bits if the type property is `2`, 24 bits if the `type`
property `3` and `32` bits for any other value of `type`.

```javascript
{
    const definition = {
        object: {
            type: 8,
            value: [
                $ => $.type == 1, 8,
                $ => $.type == 2, 16,
                $ => $.type == 3, 24,
                true, 32
            ]
        }
    }
    const object = {
        type: 2,
        value: 1
    }
    test('conditional', definition, object, [
        0x2,
        0x0, 0x1
    ])
}
```

You can use conditionals in bit-packed integers as well.

```javascript
{
    const definition = {
        object: {
            header: [{
                type: 4,
                value: [
                    $ => $.header.type == 1, 28,
                    $ => $.header.type == 2, [{ first: 4, second: 24 }, 28 ],
                    $ => $.header.type == 3, [{ first: 14, second: 14 }, 28 ],
                    true, [[ 24, 'ffffff' ], 4 ]
                ]
            }, 32 ]
        }
    }
    const object = {
        header: {
            type: 2,
            value: { first: 0xf, second: 0x1 }
        }
    }
    test('conditional-packed', definition, object, [
        0x2f, 0x0, 0x0, 0x1
    ])
}
```

### Switch Conditionals

**TODO**: Need first draft. Also, example is wrong.

```javascript
const definition = {
    object: {
        type: 8,
        value: [
            ($) => $.type, [
                { $_: 1 },          8,
                { $_: [ 2, 3 ] },   16,
                { $_: [] },         32
            ]
        ]
    }
}
const object = {
    type: 2,
    value: 1
}
test('switch', definition, object, [
    0x2, 0x0, 0x1
])
```

### References to Parietals

**TODO**: First draft done.

If you have a complicated type that requires a complicated definition that is
tedious to repeat, you can reference that definition by name.

References can be used as types and can also be used as length encoding lengths
if they resolve to an integer type. If you create a type that is only used by
reference that you do not want available as a packet, prepend and underscore and
it will not be returned as a packet type.

**Mnemonic**: A string name to name the referenced type.

In the following a definition an encoded integer is defined as a partial that
will not be presented as a packet due to the `_` prefix to the name. It is
referenced by the `series` property as a type and used for the length encoding
of the `data` property.

```javascript
const definition = {
    $encodedInteger: [
        [
            value => value <= 0x7f, 8,
            value => value <= 0x3fff, [ 16, [ 0x80, 7 ], [ 0x0, 7 ] ],
            value => value <= 0x1fffff, [ 24, [ 0x80, 7 ], [ 0x80, 7 ], [ 0x0, 7 ] ],
            true, [ 32, [ 0x80, 7 ], [ 0x80, 7 ], [ 0x80, 7 ], [ 0x0, 7 ] ]
        ],
        [ 8,
            sip => (sip & 0x80) == 0, 8,
            true, [ 8,
                sip => (sip & 0x80) == 0, [ 16, [ 0x80, 7 ], [ 0x0, 7 ] ],
                true, [ 8,
                    sip => (sip & 0x80) == 0, [ 24, [ 0x80, 7 ], [ 0x80, 7 ], [ 0x0, 7 ] ],
                    true, [ 32, [ 0x80, 7 ], [ 0x80, 7 ], [ 0x80, 7 ], [ 0x0, 7 ] ]
                ]
            ]
        ]
    ],
    object: {
        value: '$encodedInteger',
        // array: [ '$encodedInteger', [ 8 ] ] ~ I haven't done this, requires using conditionals in length encoded arrays or calculated arrays.
    }
}
const object = {
    value: 1,
      array: [ 1 ]
}
test('partial', definition, object, [
    0x1 // , 0x1, 0x1
])
```

### Checksums and Running Calculations

Some protocols perform checksums on the body of message. Others require tracking
the remaining bytes in a message based on a length property in a header and
making decisions about the contents of the message based on the bytes remaining.

To perform running calculations like buffers and remaining bytes we can use
accumulators, lexically scoped object variables that can be used to store the
state of a running calculation.

The following definition creates an MD5 checksum of the body of a packet and
stores the result in a checksum property that follows the body of the message.

```javascript
{
    const definition = {
        object: [{ hash: () => crypto.createHash('md5') }, {
            body: [[[
                ({ $buffer, $start, $end, hash }) => hash.update($buffer.slice($start, $end))
            ]], {
                number: 32,
                data: [[ 8 ], 0x0 ]
            }],
            checksum: [[
                ({ $_, hash }) => $_ = hash.digest()
            ], [[ 16 ], [ Buffer ]], [
                ({ checksum = 0, hash }) => {
                    assert.deepEqual(hash.digest().toJSON(), checksum.toJSON())
                }
            ]]
        }]
    }
    const object = {
        body: {
            number: 1,
            data: [ 0x41, 0x42, 0x43 ]
        },
        checksum: Buffer.from([ 0xc9, 0xd0, 0x87, 0xbd, 0x2f, 0x8f, 0x4a, 0x33, 0xd4, 0xeb, 0x2d, 0xe4, 0x47, 0xc0, 0x40, 0x28 ])
    }
    test('checksum', definition, object, [
        0x0, 0x0, 0x0, 0x1,
        0x41, 0x42, 0x43, 0x0,
        0xc9, 0xd0, 0x87, 0xbd,
        0x2f, 0x8f, 0x4a, 0x33,
        0xd4, 0xeb, 0x2d, 0xe4,
        0x47, 0xc0, 0x40, 0x28
    ], { require: { assert: 'assert', crypto: 'crypto' } })
}
```

Here we also introduce the concept of buffer inlines. These are inlines that
operate not on the serialized or parsed value, but instead on the underlying
buffer. In the above example the `hash.update()` inline is not called once for
each property in the `body`, it is called for each buffer chunk that contains
the binary data for the `body`.

Unlike ordinary inline functions, a buffer inline is not called prior to
serialization. Buffer inlines are called as late as possible to process as much
of the buffer continuously as possible. In the previous example, the
`hash.update()` inline is applied to the binary data that defines the entire
`body` which it encapsulates.

We use nested structures to group.

**TODO**: Simpler calculation example to start. Calculation is important because
it will allow us to talk about the difference between `sizeof`,
`offsetof`.

**TODO**: Come back and implement this by finding a way to extract sizeof and
offset of. Punting because I don't really know what inspired this example or
what it is supposed to illustrate.

```javascript
const definition = {
    packet: [{ counter: () => [] }, {
        header: {
            type: 8,
            length: [[
                ({ $, counter }) => {
                    return counter[0] = $sizeof.packet($) - $offsetof.packet($, 'body')
                }
            ], 16, [
                ({ $_, counter }) => {
                    return counter[0] = $_
                }
            ]]
        },
        body: [[[
            ({ $_ = 0, $start, $end, counter }) => counter[0] -= $end - $start
        ]], {
            value: 32,
            string: [[ 8 ], 0x0 ],
            variable: [
                ({ counter }) => counter[0] == 4, 32,
                ({ counter }) => counter[0] == 2, 16,
                8
            ]
        }],
    }]
}
```

### Parameters

**TODO**: Need first draft, or reread this and see if it is a real first draft.

Accumulators described in the preceding section also define parameters. Any
accumulator declared on the top most field will create parameters to the
generated serializes and parsers.

```javascript
const definition = {
    object: [{ counter: [ 0 ] }, [[[
        ({ $start, $end, counter }) => counter[0] += $end - $start
    ]], {
        number: 8,
        string: [ [ 8 ], 0x0 ]
    }]]
}
// **TODO**: API call to get counter.
```

The parameters are available as both arguments that can be passed to inline
functions as well as generally available in the program scope. Be careful not to
careful not to hide any module declarations you've declared.

```javascript
const definition = {
    object: [{ encoding: 'utf8' }, {
        string: [[
            value => Buffer.from(value, encoding)
        ], [ [ Buffer ], 0x0 ], [
            value => value.toString(encoding)
        ]]
    }]
}
const moduleName = compile('parameters', definition, {})
// *TODO*: API call to encode string ascii or something.
```

## What's Missing

A section of things that need to be written.

* **TODO** Did we support packed `BigInt` integers?
 * **TODO** Absent values.
 * **TODO** Switch statements.
 * **TODO** Composition.
 * **TODO** Parse or serialize conditionals, i.e. unconditionals.

## Outline

* Living `README.md`.
 * Parsers and Serializers
 * Packet Definition Language
    * Integers
        * Negative Integers
        * Endianness
    * Nested Structures
    * Packed Integers
    * Literals
    * Length-Encoded Arrays
    * Inline Transforms and Assertions
    * Fixed Length Arrays
    * Requiring Modules
    * Assertions
    * Assertion and Transformation Arguments
    * Terminated Arrays
        * Multi-byte Terminators
    * String Value Maps
    * Floating Point Values
    * Conditionals
    * Switch Conditionals
    * References to Partials
    * Accumulators
    * Parameters
 * What's Missing
 * Outline

packet

File Explorer

Today's picks

Was this content helpful?

Recent Feedback

packet

File Explorer

Today's picks

Was this content helpful?

Recent Feedback

Download README