Support for Tabixed BED

[sehilyi]

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Given that BED files are very common, we need to support Tabixed BED as well.

Reference

• https://github.com/GMOD/jbrowse-components/tree/4a35910bd2759ae5aa6174a15498953ff3e17f16/plugins/bed/src/BedTabixAdapter

Closely related issues

• https://github.com/gosling-lang/gosling.js/issues/764
• https://github.com/gosling-lang/gosling.js/issues/583

[manzt]

One thing to note is that TABIX supports separate types of indicies: traditional FAI and CSI. Since we make the index path explicit, for these file types, I think that we can determine which index is used from the index URL (i.e., .tbi vs .csi), but we will probably want to support both.

[manzt]

Another thing we could learn from jbrowse folks is coming up with a shared abstraction for retrieving "rows" (or features) or these abstract data formats:

• https://github.com/GMOD/jbrowse-components/blob/4a35910bd2759ae5aa6174a15498953ff3e17f16/plugins/bed/src/BedTabixAdapter/BedTabixAdapter.ts#L27
• https://github.com/GMOD/jbrowse-components/blob/4a35910bd2759ae5aa6174a15498953ff3e17f16/plugins/bed/src/BedTabixAdapter/BedTabixAdapter.ts#L90

That way rather than implementing N data-fetchers which wrap tabix, we can perhaps have a shared underlying object which performs fetching & parsing given a TabixIndexFile and some parser. So the adapters just need to initialize the TabixIndexedFile and parser.