Implement sparse index for reordering records without affecting others

[isglass]

Today, reordering (and also inserting & removing) records leads to index & parentIndex of all records after the insertion / removal point being updated. If these are persisted to store the current order of records, it can lead to huge updates.

To have a better way of handing this, implement a sparseIndex field. Which holds a fractional index that is only updated for affected records.

By default, it returns the parentIndex of a record. But if a record is moved etc., it gets a value set. If we have 3 records at:

0 : A, 1 : B, 2 : C

And move C to between A & B we would get sparseIndex:

0 : A, 0.5 : C, 1 : B (C & B got it set, but no change for B)

parentIndex is still updated as today, since it is the actual index in an array:

0 : A, 1 : C, 2 : B

On a sort operation, all sparseIndex fields would need to be cleared.

By persisting sparseIndex, apps can store record order much more cheaply than by persisting parentIndex

Original request https://forum.bryntum.com/viewtopic.php?t=32720

[willydee]

Problem: When using StoreCRUD.insert() on a flat store, parentIndex is not continuous anymore. The inserted records get higher parentIndex values than their successor.

parentIndexes:
0 : A, 1 : B, 2 : C

parentIndexes after store.insert(0, D):
3 : D, 0 : A, 1 : B, 2 : C

When trying to calculate continuous sparseIndex values from parentIndex, they're not reflecting the order correctly.

[willydee]

Problem: When using StoreCRUD.insert() on a flat store, parentIndex is not continuous anymore.

Solved.