module PageRankr
  module Google
    class Checksum
      class << self
        def generate(site)
          bytes  = byte_array('info:' + site)
          length = bytes.length
          a = b = 0x9E3779B9
          c = 0xE6359A60
          k, len = 0, length
          while(len >= 12)
            a = m(a + bytes[k + 0] + (bytes[k + 1] << 8) + (bytes[k +  2] << 16) + (bytes[k +  3] << 24))
            b = m(b + bytes[k + 4] + (bytes[k + 5] << 8) + (bytes[k +  6] << 16) + (bytes[k +  7] << 24))
            c = m(c + bytes[k + 8] + (bytes[k + 9] << 8) + (bytes[k + 10] << 16) + (bytes[k + 11] << 24))

            a, b, c = mix(a, b, c)
            k += 12
            len -= 12
          end

          c = c + length

          c = mix(*toss(a, b, c, bytes, len, k))[2]
          "6" + c.to_s
        end

        private

        def byte_array(site)
          bytes = []
          site.each_byte {|b| bytes << b}
          bytes
        end

        # Need to keep numbers in the unsigned int 32 range
        def m(v)
          v % 0x100000000
        end

        def mix(a, b, c)
          a, b, c = m(a), m(b), m(c)

          a = m(a-b-c) ^ m(c >> 13)
          b = m(b-c-a) ^ m(a << 8)
          c = m(c-a-b) ^ m(b >> 13)

          a = m(a-b-c) ^ m(c >> 12)
          b = m(b-c-a) ^ m(a << 16)
          c = m(c-a-b) ^ m(b >> 5)

          a = m(a-b-c) ^ m(c >> 3)
          b = m(b-c-a) ^ m(a << 10)
          c = m(c-a-b) ^ m(b >> 15)

          [a, b, c]
        end

        def toss(a, b, c, bytes, len, k)
          case len
            when 9..11
              c = c + (bytes[k+len-1] << ((len % 8) * 8))
            when 5..8
              b = b + (bytes[k+len-1] << ((len % 5) * 8))
            when 1..4
              a = a + (bytes[k+len-1] << ((len - 1) * 8))
            else
              return [a, b, c]
          end
          toss(a, b, c, bytes, len-1, k)
        end
      end
    end
end
end