Hi Praveen! It's a nice shot! 
Couple of thoughts in addition:
1. Adding any specific separator into key sequence will help to avoid situations like:
ID=20 Dest=BB Source=YY against ID=20B Dest=B Source=YY.
2. Transferring key building mechanism from UDF to outside mapping might help to use this function with various keys with no limits. Just to build key via source values and Concat function and pass result to UDF.
Regards, Evgeniy.