Turn order in battle.

I think everyone knows the basic rule for turn order in battle – the higher the speed of a creature, the sooner its turn comes. When the speeds of creatures in battle are all different, everything is straightforward. But if the speeds of some creatures are the same (which happens very often), then it becomes a little more complicated, and factors such as the arrangement before the battle, the round of the battle, and possibly the side the creature is fighting for come into play... I admit, I've asked myself this question quite often, but I haven't objectively researched it. I looked for this information in the game files, but it's not there. In that case, we can assume that there is no pattern at all, but I don't think that's the case.
If anyone knows the exact pattern for turn order in battle, please share it.