dec-sixbit

Here is a pure JavaScript / TypeScript utility for using the DEC SIXBIT character encoding.

This package can be used to help construct the Control and Addressing field of the IL2P Protocol. However, it also contains basic functions so the user can perform a number of operations relating to the DEC SIXBIT encoding.

To install

npm install --save dec-sixbit

To use

import dec6b from 'dec-sixbit';

Methods

aToSix

Converts ASCII text to a DEC SIXBIT Buffer.

import { aToSix } from 'dec-sixbit';

const myMessage = 'A printable ASCII message';
const myD6bBuf = aToSix(myMessage);

console.log(myD6bBuf); // <Buffer 21 00 30 32 29 2e 34 21 22 2c 25 00 21 33 23 29 29 00 2d 25 33 33 21 27 25>

Replaces out-of-range codes with ? (0x1F)

import { aToSix } from 'dec-sixbit';

const myMessage = 'A 🤮 💥 message';
const myD6bBuf = aToSix(myMessage);

console.log(myD6bBuf); // <Buffer 21 00 1f 00 1f 00 2d 25 33 33 21 27 25>

sixToA

Converts bytes to an ASCII string

import { sixToA } from 'dec-sixbit';

const myBuf = Buffer.from('21003032292e3421222c25002133232929002d253333212725', 'hex');
const myString = sixToA(myBuf);

console.log(myString); // A PRINTABLE ASCII MESSAGE

Replaces out-of-range bytes with ?

import { sixToA } from 'dec-sixbit';

const myBuf = Buffer.from('21009900f3002d253333212725', 'hex');
const myString = sixToA(myBuf);

console.log(myString); // A ? ? MESSAGE

pack

Compresses a string or Buffer into the smallest 8-bit Buffer that it can. For example, a 4-character string of DEC SIXBIT can be packed into 3 bytes, since only 24 bits are required.

import { pack } from 'dec-sixbit';

const toPack = 'PACK';
const packed = pack(toPack);

console.log(packed.length); // 3
console.log(packed); // <Buffer c2 18 eb>

Packing is padded on the left by default when the number of bits of the DEC SIXBIT string is not evenly divisible by 8. For example, packing a 5-character string requires 30 bits, so the result needs to be padded with 2 zero bits.

import { pack } from 'dec-sixbit';

const toPack = 'PACK5';
const packed = pack(toPack);

console.log(packed.length); // 4
console.log(packed); // <Buffer 30 86 3a d5>

In this example, the Buffer 30 86 3a d5 can be written:

0x30    |0x86    |0x3a    |0xd5
00110000|10000110|00111010|11010101
  110000|100001|100011|101011|010101
  0x30  |0x21  |0x23  |0x2b  |0x15
  P      A      C      K      Z

Packing can also be padded on the right by passing false as the second parameter to pack:

import { pack } from 'dec-sixbit';

const toPack = 'PACK5';
const packed = pack(toPack, false);

console.log(packed.length); // 4
console.log(packed); // <Buffer c2 18 eb 54>

Notice how the result of this is the same as the result of calling pack('PACK') but with the addition of the byte 0x54, which is the DEX SIXBIT value for 5 0x15 shifted 2 bits to the left, (e.g. padded 2 bits on the right);

(0x15 << 2).toString(16); // 54
(0x15 << 2).toString(2); // 01010100

unPack

Reverses the pack operation described above on any Buffer. Requires a padding argument, either 'left' or 'right'.

import { unPack } from 'dec-sixbit';

const aBuffer = Buffer.from('024963033a788a9d008e1b808a5035ce59009afc80a52b30', 'hex');
const unpacked = unPack(aBuffer, 'left');