[marcin-kordas-hoc]

Context

Adds the opt-in configuration option arrayFunctionResultOverwritesData (default false). When it is true, an array whose result spills onto occupied cells overwrites them instead of returning #SPILL!. When it is false, spills behave as before.

The overwrite reuses the step that places every spill: when the array formula is placed, it claims its whole spill range in the dependency graph, drops the occupants, and records their old values so the column index forgets them. A spill that would reach another array keeps #SPILL! and leaves that array intact, whatever the option says.

undo() restores the overwritten cells:

• entering an array formula with setCellContents snapshots the occupants before the spill;
• adding rows or columns inside the range an array reads makes the array grow; the cells it reaches are snapshotted, mapped back to their coordinates and references from before the added span (a formula =E1*2 comes back as =E1*2, not =F1*2), and written back right after undo removes the span.

The option does not overwrite in two places, on purpose:

• data declared in the same buildFromArray call as the array: the build path keeps #SPILL!, because claiming cells there would leave nothing to undo;
• moveRows and moveColumns: a move adds the moved rows or columns before it removes them, so an array that reads them grows for a moment. Overwriting in that moment cleared the cell next to the spill for good, even when the array ended at its old size. A move that makes an array grow now leaves the occupant in place and the array shows #SPILL!.

Also fixed here, because the option made it reachable more often: undoing one setCellContents call that enters an array formula and also writes a cell of its spill range brought the array formula back. This happened with the option off too. Undo now reverts the writes of one call in reverse order. It has its own CHANGELOG line under Fixed.

How did you test your changes?

Paired tests: handsontable/hyperformula-tests#45, branch feat/hf-305-overwrite-flag.

• unit/array-function-result-overwrites-data.spec.ts: 74 tests (configuration, the default, overwriting, array-vs-array collisions, undo and redo, undo of one call that writes an array and a cell of its spill, undo of added rows and columns, column index, structural operations including row moves, cycles, unrepresentable spill ranges).
• unit/undo-redo.spec.ts: one new test, with the option off, for undoing one call that writes an array formula and a cell of its spill range.
• The new undo tests fail on the previous head: 10 failures. The row-move tests fail when the move does not suspend overwriting: 3 failures. Dropping the coordinate and reference mapping of the snapshot fails 2 tests.
• Full local Jest run on this head with the paired tests: 521 suites passed, 6817 tests passed, 3 skipped.

Compatibility with Excel and Google Sheets

Not applicable to the default: with the option off, calculation results do not change. The option is opt-in and changes only what happens to cells an array spills onto.

Public API changes

• New configuration option arrayFunctionResultOverwritesData: boolean, default false.
• undo() of one setCellContents call reverts its writes in reverse order. The result differs only when a later write in the call lands in the spill range of an array an earlier write entered.

Out of scope and follow-ups

• Overwriting during moveRows and moveColumns, with undo, is not implemented: moves never overwrite. No follow-up ticket yet.
• Already on develop with the option off, not changed here: an array that shows #SPILL! keeps it after removing a column, or an undo, gives it room again; and undoing a row move inside the range an array reads changes that range (=TRANSPOSE(A3:A5) becomes =TRANSPOSE(A4:A5)). No follow-up ticket yet.

Types of changes

• Breaking change (existing functionality no longer works as before) – agreed with the maintainers
• New feature or improvement (a non-breaking change that adds functionality)
• Bug fix (a non-breaking change that fixes an issue)
• Additional language file, or a change to an existing language file (translations)
• Change to the documentation

Related issues

1. HF-305. No public issue.

Checklist

• My pull request meets the Definition of Done. External contributors: see also Contributing to HyperFormula.
• I have signed the Contributor License Agreement.
• I read my own diff end to end before requesting a review.
• I added a CHANGELOG.md entry, written by the changelog rules – or none is needed (docs, tests or tooling only).
• I updated the documentation and JSDoc to match the change.
• Breaking change only: I added a section to the migration guide.
• I updated the dev-docs/ pages this change affects – or none needed it.

---

Note

Medium Risk
Opt-in destructive cell clearing on array spill affects dependency graph, evaluation, structural ops, and undo; default false preserves Excel-compatible behavior.

Overview
Adds an opt-in engine flag arrayFunctionResultOverwritesData (default false). When enabled, array formulas that would normally hit #SPILL! on occupied non-array cells clear those occupants and spill instead; collisions with another array still return #SPILL!.

DependencyGraph wires the config in, gates overwrite via canOverwriteArrayResult / isOverwritingDataAllowed, suspends overwriting during row/column moves (withoutOverwritingData), and records content changes for cleared spill cells so the column index stays consistent. Initial buildFromArray placement is unchanged—occupants declared in the same build still win.

Operations snapshots spill victims before placing formulas, returns them from setCellContent and from addRows / addColumns when arrays grow over static data, and updates column search on formula restore.

Undo/redo restores snapshotted cells after reverting anchors or undoing row/column inserts that triggered growth overwrites; setCellContents undo order is fixed so a formula plus writes into its spill range no longer leaves a stray array.

Docs and changelog document the option and the “Spilling onto occupied cells” behavior.

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[qunabu]

Task linked: HF-305 Array functions overwrite data instead of SPILL

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Performance comparison of head (7aecb9f) vs base (d143456)

                                     testName |    base |   head |  change
--------------------------------------------------------------------------
                                      Sheet A |  330.28 | 332.44 |  +0.65%
                                      Sheet B |   108.1 | 104.88 |  -2.98%
                                      Sheet T |   98.48 |  94.09 |  -4.46%
                                Column ranges |  486.79 | 501.55 |  +3.03%
                                Sorted lookup | 15813.4 |  17468 | +10.46%
Sheet A:  change value, add/remove row/column |   10.61 |   9.71 |  -8.48%
 Sheet B: change value, add/remove row/column |    94.2 |  84.93 |  -9.84%
                   Column ranges - add column |  122.62 |  107.8 | -12.09%
                Column ranges - without batch |  385.34 | 357.73 |  -7.17%
                        Column ranges - batch |   97.22 |     91 |  -6.40%

[marcin-kordas-hoc]

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
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[marcin-kordas-hoc]

Known-gap disclosure from writing the paired suite (tests #45), re-verified today on this head (3e2a099): with the flag ON, an array whose spill range cannot be represented silently loses #SPILL! and truncates to its anchor.

Repro: useArrayArithmetic: true, =2*(B:B) in A2 — flag OFF: #SPILL! (pinned by arrays.spec.ts); flag ON: A2 = 2, no error indicator anywhere. Same silent truncation on the buildFromArray / updateConfig path.

Root cause: overwriteWouldHitArray returns false for an undefined range, so canOverwriteArrayResult returns true and Evaluator skips setNoSpace(). Overwrite mode should bypass occupancy collisions only, not a range that cannot be spanned at all — suggested one-line fix: canOverwriteArrayResult returns false when arrayVertex.getRangeOrUndef() is undefined.

The paired suite deliberately pins only the stable half (occupants below are not clobbered), so it stays green whichever way this is resolved.

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[marcin-kordas-hoc]

Fixed in e410ed0f2 (tests 1116587, +4 cases → 60). The operative insight: every legitimate overwrite claims its spill range in the address mapping (exchangeOrAddFormulaVertex) before evaluation, so isThereSpaceForArray is already true when the array recomputes — the canOverwriteArrayResult bypass in Evaluator could only ever fire for arrays whose overwrite never happened, which is exactly the two silent-truncation paths. The bypass is removed; canOverwriteArrayResult additionally refuses an unrepresentable range as a defensive guard, and the build path keeps #SPILL! under the flag exactly as with it off. Fail-then-pass: the 3 new error assertions fail on the previous head, 60/60 after; changelog clause added. Verified independently on the pushed head: =2*(B:B) and the build-time collision both show #SPILL! with the flag on.

[Tobiadefami]

Confirmed the structural-undo issue noted in the description. With overwrite enabled, A1 contains =TRANSPOSE(D1:E1), D1:E1 contains 10, 20, and A3 contains 99.

State	Formula in A1	A1	A2	A3
Calculated sheet before insertion	=TRANSPOSE(D1:E1)	10	20	99
After inserting a column before E	=TRANSPOSE(D1:F1)	10	blank	20
After undo	=TRANSPOSE(D1:E1)	10	20	blank — expected 99

Could we preserve and restore the overwritten cells in the column-insertion undo path?

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Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

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[cursor]

Undo leaves stale column-index values

Medium Severity

Undoing addRows or addColumns that grew an array over a cell restores the occupant, but the vacated spill value can stay in the column index. With useColumnIndex on, MATCH and VLOOKUP can still hit that spill after undo().

Additional Locations (2)

• hyperformula/src/UndoRedo.ts#L622-L638
• hyperformula/src/Operations.ts#L971-L975

 

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[codecov]

Codecov Report

❌ Patch coverage is 99.13043% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.30%. Comparing base (d143456) to head (7aecb9f).

Files with missing lines	Patch %	Lines
hyperformula/src/Operations.ts	98.55%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1714      +/-   ##
===========================================
+ Coverage    97.28%   97.30%   +0.02%     
===========================================
  Files          207      207              
  Lines        16218    16304      +86     
  Branches      3556     3610      +54     
===========================================
+ Hits         15777    15865      +88     
+ Misses         433      432       -1     
+ Partials         8        7       -1     

Files with missing lines	Coverage Δ
hyperformula/src/Config.ts	94.69% <100.00%> (+0.04%)	⬆️
hyperformula/src/CrudOperations.ts	99.20% <100.00%> (+<0.01%)	⬆️
...yperformula/src/DependencyGraph/DependencyGraph.ts	98.84% <100.00%> (+0.03%)	⬆️
hyperformula/src/Evaluator.ts	100.00% <ø> (ø)
hyperformula/src/UndoRedo.ts	100.00% <100.00%> (ø)
hyperformula/src/Operations.ts	98.86% <98.55%> (-0.08%)	⬇️

... and 1 file with indirect coverage changes

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