bijective.rb

# Simple bijective function
#   Basically encodes any integer into a base(n) string,
#     where n is ALPHABET.length.
#   Based on pseudocode from http://stackoverflow.com/questions/742013/how-to-code-a-url-shortener/742047#742047

ALPHABET =
  "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".split(//)
  # make your own alphabet using:
  # (('a'..'z').to_a + ('A'..'Z').to_a + (0..9).to_a).shuffle.join

def bijective_encode(i)
  # from http://refactormycode.com/codes/125-base-62-encoding
  # with only minor modification
  return ALPHABET[0] if i == 0
  s = ''
  base = ALPHABET.length
  while i > 0
    s << ALPHABET[i.modulo(base)]
    i /= base
  end
  s.reverse
end

def bijective_decode(s)
  # based on base2dec() in Tcl translation 
  # at http://rosettacode.org/wiki/Non-decimal_radices/Convert#Ruby
  i = 0
  base = ALPHABET.length
  s.each_char { |c| i = i * base + ALPHABET.index(c) }
  i
end

# Two little demos:

# Encoding ints, decoding them back:
num = 125
(num..(num+10)).each do |i|
  print i, " ", bijective_encode(i), " ", bijective_decode(bijective_encode(i)), "\n"
end

# Decoding string mentioned in original SO question:
puts bijective_decode("e9a")