[parsaeskandar]

SnarlDistanceIndex::get_snarl_tree_records() accepts several temporary indexes, one per connected component. Each temporary index numbers its root components from 0, and the function passed that per-index number directly to RootRecordWriter::add_component() and set_rank_in_parent(). With more than one temporary index:

• root components of later indexes overwrote the root-record slots of earlier ones;
• different components got the same connected component number, which zipcodes use to tell components apart.

This keeps a running component_offset across the temporary indexes, so each root component gets a distinct slot and number. With a single temporary index, which every current caller passes, the offset is 0 and the output is unchanged.

vg uses this to build the distance index one connected component at a time, in parallel (vgteam/vg PR: VG_PR_LINK):

• the full .dist of an HG002-sampled HPRC whole-genome graph (73 components) goes from 30m27s to 4m01s;
• the top-level-chain index of the HPRC whole-genome graph goes from 52m to 10m.

Checked against the serial build: same snarl tree, a one-to-one mapping of component numbers, and identical minimum_distance() on 2M random position pairs.

[adamnovak]

This looks like it should work.