Reverse-Engineer Undocumented Behavior

Background: Native [iOS / Android] app. There is a piece of behavior in
production nobody currently on the team understands, and we need to change
or replace it. Here is the code:

[Paste the class, function, or subsystem—include anything it calls that
you don't understand either]

Use case: Explain what this actually does, including the parts that look
like mistakes but might be deliberate.

Implementation details:
- Walk it line by line where the logic is non-obvious
- Separate what the code definitely does from what you're inferring
- Identify the edge cases it handles—these are usually the fossil record
  of production bugs, and they're the reason a "clean" rewrite regresses
- Call out anything version-, device-, or timing-dependent
- Flag any behavior that looks like a workaround for a platform bug

Deliverable: A plain-language explanation of the behavior, a table of the
edge cases it handles with your best guess at why each exists, an explicit
list of what you are unsure about, and the characterisation tests I should
write to lock in current behavior before I touch it.