Fix incorrect ordering of items at song select when certain sort modes are used

[peppy]

This is quite an unfortunate one, but I can't see a good way of fixing beyond a "revert" for now. Which is to say, it reverts behaviour for the sort modes which will obviously fail due to oversights, but keeps the optimisation in place for the others. Unfortunately this affects most of the commonly used sort modes.

Basically, some sort modes rely on aggregating a "max" value for each CarouselBeatmapSet, which is then used to sort the full carousel. The value of this "max" will change as a constraining filter is applied, since it only aggregates visible beatmaps:

osu/osu.Game/Screens/Select/Carousel/CarouselBeatmapSet.cs

Lines 127 to 142 in 87d0bef

	/// <summary>
	/// All beatmaps which are not filtered and valid for display.
	/// </summary>
	protected IEnumerable<BeatmapInfo> ValidBeatmaps => Beatmaps.Where(b => !b.Filtered.Value || b.State.Value == CarouselItemState.Selected).Select(b => b.BeatmapInfo);
	private int compareUsingAggregateMax(CarouselBeatmapSet other, Func<BeatmapInfo, double> func)
	{
	bool ourBeatmaps = ValidBeatmaps.Any();
	bool otherBeatmaps = other.ValidBeatmaps.Any();
	if (!ourBeatmaps && !otherBeatmaps) return 0;
	if (!ourBeatmaps) return -1;
	if (!otherBeatmaps) return 1;
	return ValidBeatmaps.Max(func).CompareTo(other.ValidBeatmaps.Max(func));
	}

The optimisation applied in #25679 assumed that sorting was applied to all sets (even filtered ones), but the above smashes a hole in this theory. Here are two failing cases:

---

#25820

• Ruleset is changed while difficulty sort is enabled

In this case, we actually need to run a full sort, specifically for the difficulty sort type. I haven't special-cased this for now, but if we ever re-apply optimisation in the future it will need to be considered.

---

#25820 (comment)

• Difficulty sort is enabled
• Text criteria entered, filtering (hiding) some panels
• Current beatmap is updated, causing a Carousel.UpdateBeatmapSet – this is common when playing the beatmap as the LastPlayed will be updated

In this case, the beatmap will be added back to the carousel group using BinarySearch:

osu/osu.Game/Screens/Select/Carousel/CarouselGroup.cs

Lines 44 to 52 in 44efa2c

	if (lastCriteria != null)
	{
	i.Filter(lastCriteria);
	int index = items.BinarySearch(i, criteriaComparer);
	if (index < 0) index = ~index; // BinarySearch hacks multiple return values with 2's complement.
	items.Insert(index, i);
	}

But due to some items being hidden when the last sort was applied, the binary search will not run successfully. I believe this case can be improved with some further thinking (sort could be run more comprehensively on filtered-away sets), but another more amicable path may be to cache the aggregate max value on a BeatmapSet (and only invalidating when count of difficulties changes), reducing the overhead of these filter operations massively.

Closes #25820.

Note that I haven't added tests for now. I might get to it later or someone else might beat me. It's not going to be easy to test either of these – one needs fake star rating changes based on ruleset, which as far as i can tell won't be easy, the other needs a test crafted specifically to break BinarySearch, which I couldn't make happpen on a quick attempt.

[bdach]

Seems to work.

I gave writing tests a weak attempt but it is as annoying as you say, so in the interest of hotfixing this...