module OTP
  module Base32
    ENCODE_CHARS = %w(
      A B C D E F G H I J K L M N O P
      Q R S T U V W X Y Z 2 3 4 5 6 7
    )

    DECODE_MAP = {
      "A"=>0,  "B"=>1,  "C"=>2,  "D"=>3,  "E"=>4,  "F"=>5,  "G"=>6,  "H"=>7,
      "I"=>8,  "J"=>9,  "K"=>10, "L"=>11, "M"=>12, "N"=>13, "O"=>14, "P"=>15,
      "Q"=>16, "R"=>17, "S"=>18, "T"=>19, "U"=>20, "V"=>21, "W"=>22, "X"=>23,
      "Y"=>24, "Z"=>25, "2"=>26, "3"=>27, "4"=>28, "5"=>29, "6"=>30, "7"=>31,
      "="=>-1,
    }

    DECODE_LENGTH = {
      1 => 1,  #  5 bits -> 1 byte (irregular)
      2 => 1,  # 10 bits -> 1 byte
      3 => 2,  # 15 bits -> 2 bytes (irregular)
      4 => 2,  # 20 bits -> 2 bytes
      5 => 3,  # 25 bits -> 3 bytes
      6 => 4,  # 30 bits -> 4 bytes (irregular)
      7 => 4,  # 35 bits -> 4 bytes
      8 => 5,  # 40 bits -> 5 bytes
    }

    module_function

    def encode(bytes, padding: true)
      return nil unless bytes
      pad = padding ? "=" : ""
      ret = ""
      bytes = bytes.dup.force_encoding("binary")
      off = 0
      while off < bytes.length
        n = 0
        bits = bytes[off, 5]
        off += 5
        l = (bits.length * 8.0 / 5.0).ceil
        bits << "\0" while bits.length < 5
        bits.each_byte{|b| n = (n << 8) | b }
        (1..8).each do |i|
          ret << ((i > l) ? pad : ENCODE_CHARS[(n >> (8-i)*5) & 0x1f])
        end
      end
      return ret
    end

    def decode(chars)
      return nil unless chars
      ret = ""
      chars = chars.upcase
      ret.force_encoding("binary")
      off = 0
      while off < chars.length
        n = l = 0
        bits = chars[off, 8]
        off += 8
        bits << "=" while bits.length < 8
        bits.each_char.with_index do |c, i|
          d = DECODE_MAP[c]
          if d.nil?
            raise ArgumentError, "invalid char: #{c}"
          elsif d < 0
            n <<= 5 * (8-i)
            off = chars.length
            break
          else
            n <<= 5
            n |= d
            l = DECODE_LENGTH[i+1]
          end
        end
        ret << (1..l).map{|i| (n >> 40 - i * 8) & 0xff }.pack("c*")
      end
      return ret
    end
  end
end