MEDIUM: pattern: add indexation function.

MEDIUM: pattern: add indexation function.

Before this patch, the indexation function check the declared patttern
matching function and index the data according with this function. This
is not useful to add some indexation mode.

This commit adds dedicated indexation function. Each struct pattern is
associated with one indexation function. This function permit to index
data according with the type of pattern and with the type of match.