bitfield-rle

A run-length-encoder (RLE) that compresses bitfields.

npm install bitfield-rle

Usage

var rle = require('bitfield-rle')
var bitfield = require('bitfield')
 
var bits = bitfield(1024)
 
bits.set(400, true) // set bit 400
 
var enc = rle.encode(bits.buffer) // rle encode the bitfield
console.log(enc.length) // 6 bytes
var dec = rle.decode(enc) // decode the rle encoded buffer
console.log(dec.length) // 128 bytes (like the old bitfield)
 
bits = bitfield(dec)
console.log(bits.get(400)) // still returns true

The encoder uses a compact format and will only run length encode sequences of bits if it compresses the bitfield. The encoded bitfield should therefore always be smaller or the same size as the original bitfield with the exception of a 1-6 byte header.

Since this uses run-length-encoding, you'll get the best compression results if you have longer sequences of the same bit in your bitfield.

API

The API is abstract-encoding compliant

buffer = rle.encode(bitfield, [buffer], [offset])

Run-length-encode a bitfield. Optionally you can pass in a buffer to encode it to.

length = rle.encodingLength(bitfield)

Returns how many bytes are needed to encode the bitfield

bitfield = rle.decode(buffer, [offset])

Decode an encoded bitfield.

length = rle.decodingLength(buffer, [offset])

Returns how many bytes a decoded bitfield will use.

Format

The encoded bitfield is a series of compressed and uncompressed bit sequences. All sequences start with a header that is a varint.

If the last bit is set in the varint (it is an odd number) then a header represents a compressed bit sequence.

compressed-sequence = varint(byte-length-of-sequence << 2 | bit << 1 | 1)

If the last bit is not set then a header represents an non compressed sequence

uncompressed-sequence = varint(byte-length-of-bitfield << 1 | 0) + (bitfield)

License

MIT