OdfTableCellRange() is unusably slow for spreadsheets with whole-sheet formatting

[Nnnes]

This issue is very closely related to #64. A fix for that issue would most likely also fix this one. The root problem is how LibreOffice handles setting formatting for an entire spreadsheet and how this library interacts with such spreadsheets.

Steps to reproduce:

1. Create a new spreadsheet document in LibreOffice Calc
2. Select all cells (Ctrl-A, or click to the left of the column letter list)
3. Set any kind of formatting (e.g. change the font)
4. Save

This creates a content.xml with the following inner content:

<table:table-column table:style-name="co1" table:number-columns-repeated="16384" table:default-cell-style-name="ce1" />
<table:table-row table:style-name="ro1" table:number-rows-repeated="1048575">
    <table:table-cell table:number-columns-repeated="16384" />
</table:table-row>
<table:table-row table:style-name="ro1">
    <table:table-cell table:number-columns-repeated="16384" />
</table:table-row>

getColumnCount() and getRowCount() then respectively return 16384 and 1048576. This is a problem for functions like getCellRangeByName(...) and getCellRangeByPosition(...) that eventually call getCellCoverInfos(), which will then attempt to iterate over every cell in the table multiple times.

It might be possible to fix this by patching getOwnerCellByPosition() without having to deal with the spreadsheet size issue. I'll try to do so and submit a pull request if I can figure out what a covered cell is.

[svanteschubert]

@Nnnes A pull request is very much welcome. Thanks in advance!
Regarding your question:
A covered cell is a cell that is "below" a spanned cell, see https://docs.oasis-open.org/office/OpenDocument/v1.3/os/part3-schema/OpenDocument-v1.3-os-part3-schema.html#element-table_covered-table-cell

[Nnnes]

I'm making progress. Laying out some of my thought process here, mostly for me to use as a reference for myself.

According to the spec, table:number-columns-repeated may only be used for <table:table-cell>/<table:covered-table-cell> if "[t]he cells are not merged horizontally or vertically" ("merged" I think means table:number-rows-spanned/table:number-columns-spanned is greater than 1, which excludes the covered cells - didn't see this clearly defined in v1.2 or v1.3 of the spec, but v1.1 mentioned it).

Similarly, table:number-rows-repeated may only be used if the rows "do not contain vertically merged cells". The spec doesn't mention horizontal merges, but if I try to collapse 6 identical rows with horizontally merged cells into a single <table:table-row table:number-rows-repeated="6"> by manually editing content.xml, LibreOffice seems to completely ignore the table:number-rows-repeated when I reopen the file, and only one row appears. So to avoid causing any breakage, I should not write anything that creates a horizontal merge in a repeated row.

As far as I can tell, all of this means I can handle repeated rows all at once in getCellCoverInfos() without affecting the result, similarly to how it's already being done here.

There are a few other places with patterns like for (int i = 0; i < table.getRowCount(); i++) (example 1, example 2) that I suspect could be improved in the same way. It might be worthwhile to write some kind of utility methods for iterating over rows/columns efficiently, but I'll save that work for a different pull request.

[svanteschubert]

There are usually existing test documents available, I took a quick look and found three promising ones:

1. https://github.com/tdf/odftoolkit/blob/master/odfdom/src/test/resources/test-input/MergedCells.ods
2. https://github.com/tdf/odftoolkit/blob/master/odfdom/src/test/resources/test-input/mergedCells.odt
3. https://github.com/tdf/odftoolkit/blob/master/odfdom/src/test/resources/test-input/mergeScenarios.ods

[svanteschubert]

Or https://github.com/tdf/odftoolkit/blob/master/odfdom/src/test/resources/test-input/_tableRepeated.ods
You may run a query (some bash that peeks into the XML within the ZIP looking for certain tags) or query/search the JSON of office operations within
https://github.com/tdf/odftoolkit/tree/master/odfdom/src/test/resources/test-reference/operations
to identify some functionality you require.
The allowed/possible operations (or document changes) are defined:
https://tdf.github.io/odftoolkit/odfdom/operations/operations.html

Godspeed!

[svanteschubert]

Cool! @Nnnes Thanks for the patch!
@mistmist Thanks for the review!