BrowserUk,
I don't understand the example. Should that t => 0 and t => 1 be 1 not 2?
Update: I am not going to have a chance to play with any of my ideas so I will just share them here in case they are of any help. I was hoping that there would be a way to "cancel out terms" such that there was less work to do. The two ideas I had for that would be performing bitwise operations on the strings to find out which characters were in common and only counting the remaining ones. The second idea I had would be to process the string in chunks rather than characters. If I were trying to do a generic solution though - I would like go with Inline::C (array vs hash) incrementing values for the first string, decrementing values for the second string and summing the 256 indices for the result in the end.