better-leftShift.md

Better leftShift

This all happens after resplitting. We almost certainly need better implementations of Slice#take/drop/takeRange

1. Partition columns by focus/unfocus
2. (Lazily?) inflate unfocused columns and build eager definedness bitsets
3. Build eager definedness bitsets for all focused paths
4. Remove shifted component of focused column paths
5. Group focused column paths by type
6. ModUnion ((CType, Array[(Int, Column)]) => Column) same-type columns ([1, 2, "foo", 3, "bar", true, 4, false]). The Int here represents the offset into the mod ring, since we can't just use the array index due to the example I gave.
7. (Lazily?) inflate non-union focused columns
8. Reassemble focused columns into map
9. Apply definedness bitsets
10. If OnUndefined.Omit, then union all definedness bitsets and use to mask unfocused columns
11. Otherwise, don't
12. Place unfocused columns under .0 and focused under .1

Definedness Bitsets

1. Assign every path (ignoring type) a position around the mod ring
2. For each definedness bitset, sparsen by mod offset (Bitset#sparsen(Int, Int): Bitset)