Question about extending Similarity

[Georgios Georgiadis]

Hi,
I would like to extend Similarity to have the following functionality: if the 
query is "A B C" and a field contains "B C" then I would like to call that a 
"match" and return a score of 1 (2/2). If the query is "A B C" and the field 
contains "B D" then I would like to call that a partial match and give a score 
of 0.5 (1/2). Is this possible?
Best,
Georgios