sshkarupa · March 19, 2021 08:54 · Jul 10, 2011 · Jul 9, 2011
diff --git a/bijective.rb b/bijective.rb
@@ -30,8 +30,14 @@ def bijective_decode(s)
   i
 end
 
-# A little demo:
+# 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")
+
diff --git a/bijective.rb b/bijective.rb
@@ -0,0 +1,37 @@
+# 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
+
+# A little demo:
+num = 125
+(num..(num+10)).each do |i|
+  print i, " ", bijective_encode(i), " ", bijective_decode(bijective_encode(i)), "\n"
+end