# WORK IN PROGRESS (c) A.R.R. JSTN 2017

class RadarImageParser
  attr_reader :data

  CODES_WITH_ELEVATION_ANGLES = [ 19, 20, 32, 37, 38, 65, 66, 67, 90, 94, 159 ]
  CODES_WITH_NONSTANDARD_THRESHOLDS = [ 32, 81, 93, 94, 99, 134, 135, 138, 153, 154, 155, 159, 161, 163, 177, 195 ]

  def initialize(io_or_string)
    @io = io_or_string.is_a?(String) ? StringIO.new(io_or_string, "rb") : io
    parse
  end

  private

    def parse
      @data = Hash.new

      # BLOCK: Message Header (ICD 3-7)
      @data[:message_header] = {
        wmo_header: string, # like "SDUS55 KABQ 031229"
        awips_header: string, # like "N0RABX"
        message_code: halfword, # -131 to -16, 0 to 211 (ICD Table II)
        message_time: unixtime,
        length_of_message: fullword, # number of bytes including header
        source_id: halfword, # 0-999
        destination_id: halfword, # 0-999
        number_of_blocks: halfword, #1-51, including this header
      }

      # BLOCK: Product Description (ICD 3-27, 3-32)
      raise "no product description block" unless halfword == -1
      @data[:product_description] = {
        radar_latitude: fullword.to_f / 1000,
        radar_longitude: fullword.to_f / 1000,
        radar_height: halfword, # -100 to 11000, feet above mean sea level
        product_code: halfword, # 16 to 299, -16 to -299 (ICD Table III)
        operational_mode: halfword, # 0-2
        volume_coverage_pattern: halfword, # 1-767
        sequence_number: halfword, # -13, 0 to 32767
        volume_scan_number: halfword, # 1 to 80
        volume_scan_time: unixtime,
        generation_time: unixtime,
        product_dependent_param_1: halfword, # ICD Table V
        product_dependent_param_2: halfword, # ICD Table V
        elevation_number: halfword, # 0-20 (for elevation based products)
        product_dependent_param_3: halfword # ICD Table V, NOTE 3
      }

      if CODES_WITH_ELEVATION_ANGLES.include? @data[:product_description][:product_code]
        param = @data[:product_description].delete(:product_dependent_param_3)
        @data[:product_description][:elevation_angle] = param.to_f / 10
      end

      if CODES_WITH_NONSTANDARD_THRESHOLDS.include? @data[:product_description][:product_code]
        raise "nonstandard thresholds TKTKTK"
      else
        16.times do |i|
          msb = @io.getbyte
          lsb = @io.getbyte
          threshold = ""

          if msb & 0b10000000 > 0
            threshold += case lsb
              when  1 then "TH"
              when  2 then "ND"
              when  3 then "RF"
              when  4 then "BI" # Biological
              when  5 then "GC" # AP/Ground Clutter
              when  6 then "IC" # Ice Crystals
              when  7 then "GR" # Graupel
              when  8 then "WS" # Wet Snow
              when  9 then "DS" # Dry Snow
              when 10 then "RA" # Light and Moderate Rain
              when 11 then "HR" # Heavy Rain
              when 12 then "BD" # Big Drops
              when 13 then "HA" # Hail and Rain Mixed
              when 14 then "UK" # Unknown
              when 15 then "LH" # Large Hail
              when 16 then "GH" # Giant Hail
              else ""
            end
          else
            if    msb & 0b00001000 > 0    # bit 4
              threshold += ">"
            elsif msb & 0b00000100 > 0    # bit 5
              threshold += "<"
            elsif msb & 0b00000010 > 0    # bit 6
              threshold += "+"
            elsif msb & 0b00000001 > 0    # bit 7
              threshold += "-"
            end

            if    msb & 0b01000000 > 0    # bit 1
              threshold += lsb.to_f / 100
            elsif msb & 0b00100000 > 0    # bit 2
              threshold += lsb.to_f / 20
            elsif msb & 0b00010000 > 0    # bit 3
              threshold += lsb.to_f / 10
            else
              threshold += "#{lsb}"
            end
          end

          @data[:product_description]["threshold_#{i + 1}".to_sym] = threshold
        end
      end

      @data
    end

    def string
      @io.gets.strip
    end

    def halfword
      # 16 bit signed integer, big-endian (INT*2)
      @io.read(2).unpack("s>").first
    end

    def fullword
      # 32 bit signed integer, big-endian (INT*4)
      @io.read(4).unpack("l>").first
    end

    def unixtime
      # modified julian, offset by 1 day
      days = halfword.days - 1.day
      seconds = fullword.seconds
      Time.zone.at(days + seconds).iso8601
    end
end

# HC SVNT DRACONES