number_encoder.rb

MAP = []
('A'...'Z').each do |c|
  MAP << c
end
('a'..'z').each do |c|
  MAP << c
end
('0'..'9').each do |c|
  MAP << c.to_s
end

BASE = MAP.length

def encode(number)
  res = []

  while number > 0
    rem = number % BASE
    res << MAP[rem]
    number = (number -rem)/BASE
  end

  res.join
end


def decode(str)
  number = 0
  factor = 1
  str.split(//).each do |d|
    number += factor*MAP.index(d)
    factor *= BASE
  end
  number
end

#
# Time.now.to_i
#  => 1342583287
# encode(Time.now.to_i)
#  => "Ff77kB"
#
# decode("Ff77kB")
#  => 1342583287