To test this, simply create a beatmap which only contains circles with the same time (and nothing else).
osu! will return valid stars (e.g. 3.84), but oppai will return 0 stars.
The initial interval_end value change is mostly to stay 1:1 with osu code, the effect should be negligible.
See https://github.com/ppy/osu/blob/master/osu.Game.Rulesets.Osu/Difficulty/OsuDifficultyCalculator.cs
To test this, simply create a beatmap which only contains circles with the same time (and nothing else). osu! will return valid stars (e.g. 3.84), but oppai will return 0 stars.
The initial interval_end value change is mostly to stay 1:1 with osu code, the effect should be negligible.