decoder-ring

node-decoder-ring

Decoder Ring allows you to use a Javascript object as a specification to decode Node.js Buffers into a Javascript object.

Installation

npm install decoder-ring

Usage

Javascript Object Specification

The Javascript object specification is used to specify endianness and a description of the fields present in the buffer.

{
    bigEndian: true,
    fields: [
        {name: "field1", start: 0,  type: 'int8'  },
        {name: "field2", start: 1,  type: 'uint8' },
        {name: "field3", start: 2,  type: 'int16' },
        {name: "field4", start: 4,  type: 'uint16'},
        {name: "field5", start: 6,  type: 'float' },
        {name: "field6", start: 10, type: 'double'},
        {name: "field7", start: 18, type: 'ascii', length: 10 },
        {name: "field8", start: 28, type: 'utf8',  length: 9  },
        {name: "field9", start: 37, type: 'bit', position: 7},
        {name: "field10", start: 37, type: 'bit', position: 6},
        {name: "field11", start: 37, type: 'bit', position: 0},
        {name: "field12", start: 38, type: 'uint32'},
        {name: "field13", start: 42, type: 'int32'}
        {name: "field14", start: 46, type: 'int8', default: 42 }
        {name: "field15", start: 50, type: 'buffer', length: 4 }
    ]
}
 
 

All fields must have a name, a starting byte, and a type. The name is used for assigning the property in the resulting javascript object. Additionally, fields can have a default value which is used when encoding an object's properties that have null or undefined values.

Types

• buffer - Raw buffer

• int8 - Signed 8-bit integer

• uint8 - Unsigned 8-bit integer

• int16 - Signed 16-bit integer

• uint16 - Unsigned 16-bit integer

• int32 - Signed 32-bit integer

• uint32 - Unsigned 32-bit integer

• float - 4-bit floating point number

• double - 8-bit double precision floating point number

• ascii - 8-bit per character ASCII encoded text

  This field type must also have a length property which is a count of the number of characters.

• utf8 - 8-bit per character UTF8 encoded text

  This field type must also have a length property which is a count of the number of characters.

• bit - true/false values

  Bit fields are pieces of a 1-byte unsigned integer. Given a big endian, unsigned integer of 129, it will appear as the following when broken down into bits:

  128(2^7)	64(2^6)	32(2^5)	16(2^4)	8(2^3)	4(2^2)	2(2^1)	1(2^0)
  1	0	0	0	0	0	0	1

  Bit fields must have a position property which is used to check if a specific bit is on or off in the 1-byte unsigned integer. The position of the bit of interest, is defined as which power of two the bit falls in the integer. For the bit in the 128th's place, the position would be 7, for the bit in the 1's place the position would be 0.

API

decode: (buffer, spec, { noAssert = false })

• set noAssert to true to ignore validation of offsets

encode: (obj, spec, { noAssert = false, padding = null })

• set padding to a character to right pad ascii and utf8 strings instead of using null termination
• set noAssert to true to ignore validation of written values and offsets

Example

var DecoderRing = require("decoder-ring");
var decoderRing = new DecoderRing();
 
var bufferBE = Buffer.alloc(51);
bufferBE.writeInt8(-127, 0);
bufferBE.writeUInt8(254, 1);
bufferBE.writeInt16BE(5327, 2);
bufferBE.writeUInt16BE(5328, 4);
bufferBE.writeFloatBE(-15.33, 6);
bufferBE.writeDoubleBE(-1534.98, 10);
bufferBE.write("ascii", 18, 10,'ascii');
bufferBE.write("utf8 text", 28, 9, 'utf8');
bufferBE.writeUInt8(129, 37);
bufferBE.writeUInt32BE(79001, 38);
bufferBE.writeInt32BE(-79001, 42);
bufferBE.writeInt8(1, 46);
 
var testBuffer = new Buffer("test");
testBuffer.copy(bufferBE, 47, 0, 4);
 
var spec = {
  bigEndian: true,
  fields: [
    { name: "field1", start: 0,   type: 'int8'  },
    { name: "field2", start: 1,   type: 'uint8' },
    { name: "field3", start: 2,   type: 'int16' },
    { name: "field4", start: 4,   type: 'uint16'},
    { name: "field5", start: 6,   type: 'float' },
    { name: "field6", start: 10,  type: 'double'},
    { name: "field7", start: 18,  type: 'ascii', length: 10 },
    { name: "field8", start: 28,  type: 'utf8', length: 9  },
    { name: "field9", start: 37,  type: 'bit', position: 7 },
    { name: "field10", start: 37, type: 'bit', position: 6 },
    { name: "field11", start: 37, type: 'bit', position: 0 },
    { name: "field12", start: 38, type: 'uint32' },
    { name: "field13", start: 42, type: 'int32' },
    { name: "field14", start: 46, type: 'int8', default: 42 },
    { name: "field15", start: 47, type: 'buffer', length: 4 }
  ]
};
 
// Decode the buffer into a javascript object
var result = decoderRing.decode(bufferBE, spec);
console.log(result);
 
// Assign field14 to undefined to test default value on encoding
result.field14 = undefined;
 
// Encode the object to a buffer
var buffer = decoderRing.encode(result, spec);
console.log(buffer);
 
// Decode buffer to object and check field14 for default value
var resultWithDefaultValue = decoderRing.decode(buffer, spec);
console.log("Field14 default value: " + resultWithDefaultValue.field14);

Result of the above example

{ 
  field1: -127,
  field2: 254,
  field3: 5327,
  field4: 5328,
  field5: -15.329999923706055,
  field6: -1534.98,
  field7: 'ascii\u0000\u0000\u0000\u0000\u0000',
  field8: 'utf8 text',
  field9: true,
  field10: false,
  field11: true,
  field12: 79001,
  field13: -79001,
  field14: 1,
  field15: <Buffer 74 65 73 74> 
}
 
<Buffer 81 fe 14 cf 14 d0 c1 75 47 ae c0 97 fb eb 85 1e b8 52 61 73 63 69 69 20 20 20 20 20 75 74 66 38 20 74 65 78 74 81 00 01 34 99 ff fe cb 67 2a>
 
Field14 42

Development

Running tests

npm test

Testing the package locally

npm pack
npm install decoder-ring-0.4.0.tgz
node example.js