Function fill_distance_matrix 

pub fn fill_distance_matrix(
    matrix: &mut [Vec<i64>],
    target: &[NoteHash],
    source: &[NoteHash],
) -> Result<()>

The cheapest edit onto every opening of the two, written into a matrix already set up: music21’s populateDistanceMatrix.

Inserting and deleting both cost what the source’s first hash says, which is music21’s reading and not a cost per note.

Errors

A source with nothing hashed, or a matrix not of the size setup_distance_matrix gives.