Fragment: decoding weather strings in a weather.gov grib2 file

grib2-local.scala

// convert a byte into a list of ones and zeros

def bits(num: Int): List[Int] = (for (i <- Range(0,8)) yield (((num << i) & 0xff) >>> 7)).toList

// given a list, take the first N bits

def next(bits: List[Int], numBits: Int) = {

  val filler = List[Int](8 - numBits) map (_ => 0)
  val bitList = filler ++ bits.take(numBits)
  
  def convert (accumulator: Int, val_pos: Tuple2[Int, Int]) =
    accumulator + (val_pos._1 << val_pos._2)

  bitList.reverse.zipWithIndex.foldLeft(0) { convert }
}

def localStrings(len: Int, data: DataInputStream): List[String] = {

  // Works for the NDFD data files as I've found them, but doesn't
  // account for more than one group of data (say), or bits that
  // might be stored as floating point numbers.

  val numGroups = readInteger(2, data)
  val numValues = readInteger(4, data)
  val refValue  = data.readFloat()
  val scale     = readInteger(2, data)
  val recScale10 = 1 / math.pow(10, scale)
  val numBits    = data.readUnsignedByte()
  val dataType   = data.readUnsignedByte()

  // TODO: This really needs to be "streamed" in some
  // way so we don't have to keep such a massive list
  // in memory. Okay for local WX strings, but is
  // unusable on huge CONUS data sets.

  // Convert the data into a list of ones and zeros

  var packed: List[Int] = List()

  for (i <- 1 to (len - 14))
    packed = packed ++ bits(data.readUnsignedByte)

  // Expand the packed bit list into bytes

  var unpacked: List[Char] = List()

  for (i <- 1 to numValues) {
    unpacked = unpacked ++ List(((refValue + next(packed, numBits)) * recScale10).toChar)
    packed = packed.drop(numBits)
  }

  // Convert the zero-delimited strings into a list of strings

  unpacked.mkString.split(0.toChar.toString).toList
}