#!/usr/bin/env ruby

class Rdr
  def initialize(f)
    @f = f
    reset
  end

  def valid?
    @len == 10# && @csok
  end

  def read!
    v = @f.read(1)
    @len += 1

    case @len
    when 1
      reset if v != 0xAA
    when 2
      reset if v != 0xC0
    when 3
      @pm25 = v
    when 4
      @pm25 += v * 256
    when 5
      @pm10 = v
    when 6
      @pm10 += v * 256
    when 7
      @id = v
    when 8
      @id += v * 256
    when 9
      if checksum == v
        @csok = true
      else
        reset
      end
    when 10
      reset if v != 0xAB
    else
      reset
    end
  end

  def checksum
    ((@pm25 / 256) + (@pm25 % 256) +
     (@pm10 / 256) + (@pm10 % 256) +
     (@id   / 256) + (@id   % 256)) % 256
  end

  def reset
    @len = 0
    @pm25 = 0
    @pm10 = 0
    @id = 0
    @csok = false
  end
end

File.open("/dev/ttyUSB0", "rb") do |f|
  r = Rdr.new(f)
  r.read! until r.valid?
  puts "Hooray"
end