An iterator that helps select old log files for deletion. It takes the input iterator of log files from the earliest file, and returns should-be-deleted files until the given maxTimestamp or maxVersion to delete is reached. Note that this iterator may stop deleting files earlier than maxTimestamp or maxVersion if it finds that files that need to be preserved for adjusting the timestamps of subsequent files. Let's go through an example. Assume the following commit history:

+---------+-----------+--------------------+

+---------+-----------+--------------------+

+---------+-----------+--------------------+

As you can see from the example, we require timestamps to be monotonically increasing with respect to the version of the commit, and each commit to have a unique timestamp. If we have a commit which doesn't obey one of these two requirements, we adjust the timestamp of that commit to be one millisecond greater than the previous commit.

Given the above commit history, the behavior of this iterator will be as follows:

• For maxVersion = 1 and maxTimestamp = 9, we can delete versions 0 and 1
• Until we receive maxVersion >= 4 and maxTimestamp >= 12, we can't delete versions 2 and 3. This is because version 2 is used to adjust the timestamps of commits up to version 4.
• For maxVersion >= 5 and maxTimestamp >= 14 we can delete everything The semantics of time travel guarantee that for a given timestamp, the user will ALWAYS get the same version. Consider a user asks to get the version at timestamp 11. If all files are there, we would return version 3 (timestamp 11) for this query. If we delete versions 0-2, the original timestamp of version 3 (7) will not have an anchor to adjust on, and if the time travel query is re-executed we would return version 4. This is the motivation behind this iterator implementation.

The implementation maintains an internal "maybeDelete" buffer of files that we are unsure of deleting because they may be necessary to adjust time of future files. For each file we get from the underlying iterator, we check whether it needs time adjustment or not. If it does need time adjustment, then we cannot immediately decide whether it is safe to delete that file or not and therefore we put it in each the buffer. Then we iteratively peek ahead at the future files and accordingly decide whether to delete all the buffered files or retain them.