Commit 8d9aee1
[gap-decorations] Assign flex gap decoration values for reversed flex
Assign listed and repeated flex gap-decoration values according to flex
placement order when `flex-direction` or `flex-wrap` reverses placement,
matching the CSSWG resolution in
w3c/csswg-drafts#14156.
By default, placement order is what we use to assign values in grid,
multicol, and grid lanes. The special case handling is needed for
reversed flex and soon for reversed column fill for grid-lanes.
Gap geometry remains in logical paint order. Layout records reversal
metadata, while paint maps each stored gap to its placement-order value
index.
So this CL adds an accessor class (`GapDataListValueAccessor`) that will
be used instead of the forward-only iterator when assigning gap
decoration values in the reversal cases. In that same file, we make some
of the logic previously used by `BuildRegions` into a helper function so
we can reuse it.
In the common case, time complexity for assigning values does not
change. Without reversal, the time complexity is O(A + G), where A is
the number of entries in the (non-expanded) list i.e. `rule-color: red,
green, blue` and G is the number of gaps in the axis.
With reversals, we add an extra logarithmic factor but ONLY for cases
where there are fixed integer repeaters `red, repeat(3, blue green),
blue`. In these cases the complexity is O(A+GlogA), but even then,
usually A will be small (in the aforementioned example it is 3).
Because of this seemingly small time complexity penalty, I will
investigate getting rid of the iterator entirely and just using the
accessor, if after profiling we see that the difference is not
significant.
We add WPT coverage for direction reversal, wrap reversal, RTL
interaction, uneven flex lines, and `repeat(auto, ...)`.
As a small refactor, `GapDataListIterator` now stores the shared
`RegionSlotCounts` directly, and we add a helper in `GapData` so the
iterator and accessor can reuse logic.
Change-Id: I559bcf0e294a233431636d554e29985476a26a70
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8248120
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Kurt Catti-Schmidt <kschmi@microsoft.com>
Auto-Submit: Javier Contreras <javiercon@microsoft.com>
Commit-Queue: Javier Contreras <javiercon@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1683498}1 parent 5d45ecb commit 8d9aee1
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