snappy-stream/lib/compress-stream.js

Compress data over a Stream using the snappy framing format

var Transform = require('stream').Transform
  , util = require('util')

  , snappy = require('snappy')
  , bufferFrom = require('buffer-from')

  , checksum = require('./checksum')

  , IDENTIFIER_FRAME = bufferFrom([
      0xff, 0x06, 0x00, 0x00, 0x73, 0x4e, 0x61, 0x50, 0x70, 0x59
    ])
  , COMPRESSED = bufferFrom([ 0x00 ])
  , UNCOMPRESSED = bufferFrom([ 0x01 ])

  , CompressStream = function () {
      if (!(this instanceof CompressStream))
        return new CompressStream()

      Transform.call(this)

      // first push the identifier frame
      this.push(IDENTIFIER_FRAME)
    }

util.inherits(CompressStream, Transform)

CompressStream.prototype._compressed = function (chunk, compressed) {
  var size = compressed.length + 4

  this.push(
    Buffer.concat([
        COMPRESSED
      , bufferFrom([ size, size >> 8, size >> 16 ])
      , checksum(chunk)
      , compressed
    ])
  )
}

CompressStream.prototype._uncompressed = function (chunk) {
  var size = chunk.length + 4

  this.push(
    Buffer.concat([
        UNCOMPRESSED
      , bufferFrom([ size, size >> 8, size >> 16 ])
      , checksum(chunk)
      , chunk
    ])
  )
}

CompressStream.prototype._transform = function (chunk, enc, callback) {
  var self = this

  snappy.compress(chunk, function (err, compressed) {
    if (err)
      return callback(err)

    if (compressed.length < chunk.length)
      self._compressed(chunk, compressed)
    else
      self._uncompressed(chunk)

    callback()
  })
}

module.exports = CompressStream