Fix palette-packing decant logic to avoid UB from invalid iterator#2031
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Rangi42
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The "Iterate through the component's color sets, and transfer them" logic shifted `from.begin()`, which would make subsequent `std::advance` calls go past `_assigned.end()` and cause UB. This rewrite uses absolute numeric indexes into `_assigned` to avoid that potential problem. I haven't been able to craft a test case that actually *reaches* that piece of logic, and I suspect that it's unreachable given how we sort and process color sets before decanting, but cannot prove that; so it stays in. (Maybe if we added 4bpp 16-colors-per-palette support, it would become reachable?)
ISSOtm
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Jul 23, 2026
ISSOtm
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Logic looks sound, I only have some comments regarding the code style itself
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The "Iterate through the component's color sets, and transfer them" logic shifted
from.begin(), which would make subsequentstd::advancecalls go past_assigned.end()and cause UB. This rewrite uses absolute numeric indexes into_assignedto avoid that potential problem.I haven't been able to craft a test case that actually reaches that piece of logic, and I suspect that it's unreachable given how we sort and process color sets before decanting, but cannot prove that; so it stays in. (Maybe if we added 4bpp 16-colors-per-palette support, it would become reachable?)
This approach can be compared with the
pal_packing.rsport, which also uses absolute indexes (but changed how color sets work so there's no extra complexity of skipping over free slots).