Function permutation_source 

fn permutation_source(
    permutation: &Permutation,
    direction: &ExchangeDirection,
    target_quarter: u8,
) -> u8

Compute the quarter number (starting at 0) of the source word from which the values for target_quarter (also starting at 0) would be taken if it were active.

The numbering is for the load direction (i.e. data is travelling downwards in the standard permutation diagram in table 7-2 of the Users Handbook).

Permutation behaviour is described in Volume 2 of the Technnical Manual. In particular see section 12-6.4 and figure 12-45 (page 12-75), and figures 13-12 and 13-13 (pages 13-24 and 13-25).