class ROTP::OTP

def byte_secret
  Base32.decode(@secret)
end

def encode_params(uri, params)
  params_str = "?"
  params.each do |k,v|
    if v
      params_str << "#{k}=#{CGI::escape(v.to_s)}&"
    end
  end
  params_str.chop!
  uri + params_str
end

def generate_otp(input, padded=true)
  hmac = OpenSSL::HMAC.digest(
    OpenSSL::Digest.new(digest),
    byte_secret,
    int_to_bytestring(input)
  )
  offset = hmac[-1].ord & 0xf
  code = (hmac[offset].ord & 0x7f) << 24 |
    (hmac[offset + 1].ord & 0xff) << 16 |
    (hmac[offset + 2].ord & 0xff) << 8 |
    (hmac[offset + 3].ord & 0xff)
  if padded
    (code % 10 ** digits).to_s.rjust(digits, '0')
  else
    code % 10 ** digits
  end
end

def initialize(s, options = {})
  @digits = options[:digits] || 6
  @digest = options[:digest] || "sha1"
  @secret = s
end

def int_to_bytestring(int, padding = 8)
  result = []
  until int == 0
    result << (int & 0xFF).chr
    int >>=  8
  end
  result.reverse.join.rjust(padding, 0.chr)
end

def time_constant_compare(a, b)
  return false if a.empty? || b.empty? || a.bytesize != b.bytesize
  l = a.unpack "C#{a.bytesize}"
  res = 0
  b.each_byte { |byte| res |= byte ^ l.shift }
  res == 0
end

def verify(input, generated)
  unless input.is_a?(String) && generated.is_a?(String)
    raise ArgumentError, "ROTP only verifies strings - See: https://github.com/mdp/rotp/issues/32"
  end
  time_constant_compare(input, generated)
end